ACTUATORS IN INDUSTRIAL AUTOMATION
2. Electromechanical actuators
2.1. Electrical drive
An electrical drive is an assembly of an electronic system, an electrical motor and mechanical
transmission joined to drive a mechanical load by electrical energy. Schematics of typical
drives are diverse. A generalize block diagram of an electrical drive Fig. 2.1.
Electrical drive
Energy E
source
U, I
Power
converter
Motor
Transmission
T, n Load
Gate circuits
Regulator
Controller
Feedback data
from sensors
Inputs
Reference node
Fig. 2.1 Block diagram of an electrical drive
Its upper part represents the drive power system whereas the lower part is the control system.
In response to an input command, direct current (dc) and alternating current (ac) drives
efficiently control the speed n, torque T and the position of a mechanical load. By comparing
the input command for speed, torque or position with the actual values measured through
sensors, the controller, and appropriate reference node provides signals to the regulator and
then to the gate circuit, which controls the power converter. The power converter is energized
from the utility source with single- or three-phase sinusoidal voltages of a fixed frequency and
amplitude. It converts electrical energy into an output power of the appropriate form (number
of phases frequency, voltage U and current I) with values that are optimally suited for
operating the motor.
In many general-purpose applications electric drives operate in an open-loop manner without
any feedback. This large group of simple drive systems with energy supply by mains is used
in a variety of industrial and domestic machines. Different power converters can be found in
drive systems with a battery supply such as forklift trucks, starter generators and automotive
auxiliary drives. The converters can feed dc motors, induction and synchronous motors.
Electrical drives are mainly used in industrial applications, electric transport and households.
Nowadays 60 % of total electrical energy is consumed by electrical drives.
2.2. Conversion of electrical energy to mechanical energy
Electrical energy can be easily converted into mechanical energy, heat and light.
Electromechanical conversion process finds state in the air gap of an electric motor, where
ACTUATORS IN INDUSTRIAL AUTOMATION
alternating magnetic field of the stator induces an electromotive force in the rotor. This
process is called electromagnetic induction.
The changing magnetic field can be achieved by: a) moving a wire in a static magnetic field,
b) moving a magnetic field relatively to wire, c) periodical change of current in a wire and
magnetic flux density around it. Electromagnetic induction is illustrated in Fig. 2.2.The
magnetic field is generated by a permanent magnet, where N is the north pole and S is south
pole.
If the wire is moved in magnetic field with the flux density B with speed v, then the
electromotive force (voltage) E will be induced in the wire. The higher is the flux density and
the speed, the higher will be the induced electromotive force. Electromotive force is
calculated by
E = B l v sin α
,
where electromotive force E is measured in volts (V), magnetic flux density B is measured in
tesla (T), l is the length of the wire measured in meters (m), v is the moving speed of the wire
measured in m/s, α is the angle between the moving speed direction and the magnetic field.
If the moving wire, where electromotive force is induced is connected to an electrical circuit,
the current will flow through it. The current i can be calculated with Ohm’s law i = E / R.
N
N
e
i
B
B
F
e
v
i
v
F
S
S
a)
b)
Fig. 2.2 Effect of electromagnetic induction. a – force occurrence; b – occurrence of
electromagnetic force
If the current flows in the conductor, it is always surrounded by a magnetic field, as shown in
figure x. In order to achieve higher magnetic fields the wire is wounded around a core, where
the magnetic field of each winding is summed. The magnetic field of a core is illustrated in
Fig. 2.3 a, b.
ACTUATORS IN INDUSTRIAL AUTOMATION
a)
b)
Fig. 2.3 Magnetic field around wire a) and iron core b)
If the current flows through the conductor that is situated in the magnetic field, a force F is
occurs that deflects the conductor. The force occurs due to counteraction between outer
magnetic field and the magnetic field that surrounds the conductor. The force can be
evaluated as follows:
F = B I l sin α
where F is force measured in Newtons (N), B is magnetic flux density measured in tesla (T), I
is the amperage measured in amperes (A), l is the length of the wire in meters (m), α is the
angle between the current vector and the magnetic flux vector.
In case of wire, the force causes deflection of it, in case of a motor, the force causes rotation
of rotor. These effects are reversible. If the wire is moved in the magnetic field with an outer
mechanical force, the electrical energy is generated, that can be conducted to the electrical
circuit. In this case the mechanical energy is converted into electrical (generator of electrical
energy). If the current is lead through the wire that is situated in the magnetic field, the force
will be induced, so the mechanical energy is generated. In this case, if the motion of the wire
is possible, the electrical energy is converted into mechanical (electric motor).
Electromagnetic actuators
Electromagnetic actuators convert electrical energy into mechanical using magnetic options of
materials. An example is solenoid that is actuated with direct current. The solenoid is shown
in Fig. 2.4.The winding is wounded onto U-shape ferromagnetic iron core. On the ends of the
plate of ferromagnetic material is situated, that is connected to the housing via spring. If the
current is led through the winding, the magnetically flux is induced in the iron core that
penetrates into the plate through the air gap. The force that pulls the plate against the ends of
U-shaped magnetic core is induced. If the winding is not excitated any more, the spring pulls
the plate back to previous position.
ACTUATORS IN INDUSTRIAL AUTOMATION
Fig. 2.4 Solenoid
2.3. Electrical motors
2.3.1. Design of electric motors
Electric motors are electromechanical actuators that convert electrical energy into mechanical
to run the operating machine. Electric motors are the most spread electromechanical actuators
nowadays.
Electrical motors consist of a stator and a rotor. In the stator the rotating magnetic field is
created that is necessary to make the rotor move. The rotor rotates on the shaft that is
connected to the housing via the bearing. The working machine is connected to the shaft.
Between the stator and the rotor an air gap that exists is necessary to transfer energy from the
stator to the rotor. The design of an electric motor is shown in Fig 2.5, a.
In order to make the motor move a torque must be applied to the rotor. To create a torque a
magnetic field and a powered wire are necessary. If an electrically closed frame is in the
placed magnetic field and it is powered from a current source, then the force F will act to the
frame (Fig. 2.5 b). Torque is the multiplication of the force F and an the angle of the frame D
evaluated as follows
M = F ⋅ D ⋅ sin α
ACTUATORS IN INDUSTRIAL AUTOMATION
Rotating axis
Contacts
Brushes
Contacts
Bearing
Contacts b
a
Fig. 2.5 Force and torque estimation in a dc motor
Depending on the supply voltage electric al motors can be classified into three groups:
• dc motors
• ac motors
• pulse controlled motors
2.3.2. DC motors
Direct current motors consist also from a stator and a rotor. On the stator the magnetic poles
are situated that are used to create a magnetic field. The magnetic field can be induced either
with permanent magnets or with an electromagnet. Electromagnet consists of a coil that is
wound around the iron core. When the coil is excited with current then magnetic field will
appear. Rotor consists of many windings that are excited with direct current. Direct current is
lead to the rotor via brushes (called also commutator). In order to keep a rotor’s rotation
constant the rotor current must be reversed after each half-cycle.
Based on the dependence of the design of the excitation coil, the dc motors can be classified
to a) motor with separate excitation, where the excitation coil is fed from separate current
source; b) motor with serial excitation, where the excitation coil is connected in series with
the main supply (serial with rotor circuit); c) motors with parallel excitation, where the
excitation coil is connected in parallel with rotor circuit. Also, permanent magnets are often
used to create constant homogenous magnetic field (1).
In a stable working mode, the following equation is valid for dc motors
U = E + I a Ra ,
where U is supply voltage, E is the electromotive force induced in rotor, IaRa is the voltage
drop in the rotor winding.
A proportional relationship between speed n, magnetic flux Φ and voltage U exists
E U − I a Ra
n∝ ∝
Φ
Φ
,
ACTUATORS IN INDUSTRIAL AUTOMATION
The foliowine relationship exists between torque M, current I and magnetic flux Φ is as
follows
T ∝ I aΦ ,
From equations above it can be concluded that the rotational speed of a DC motor can be
changed with an increase of the voltage or decrease of the magnetic flux (with a decrease of
the excitation coil voltage supply). The torque can be increased with in increase of the current
or magnetic flux. So, the control of magnetic flux influences both, the speed and torque with
reversed proportional dependence. DC motors are started with an additional resistance in
order to limit the starting current. With an increase of speed the value of resistance is
decreased.
Earlier DC motors were often used in variable speed drives. Despite good efficiency, which is
usually over 90 %, dc motors are not frequently used nowadays. The technology of power
electronics and microcontrollers allows to control after noting current (AC) motors much
more effectively. AC motors require less maintenance and are cost less. In addition to that it is
not allowed to use dc motors in rooms with high risk of an explosion, because when using a
mechanical commutator the estimation of electric sparks is inevitable (2)
2.3.3. AC motors
AC motors use alternating current for their supply. They are in turn divided into one- and
three-phase motors. One-phase motors are frequently used in working tools and household
machines. Three-phase motors are more frequently used in powerful industrial applications.
To start up a one-phase ac motor an additional starting circuit is required. In this book only
three-phase motors are analyzed. There are two main groups of AC motors: asynchronous
motors and synchronous motors.
Asynchronous motor is due to its low price and simple design the most widely used motor in
industrial applications nowadays. The design and operating principles of an asynchronous
motor an i described n details later in this book.
Synchronous motor. The operating principles of a synchronous motor are similar to an
asynchronous motor. A three - phase supply creates a rotating magnetic field around stator. In
rotor another magnetic field is created with an electromagnet of a permanent magnet. So, two
different magnetic fields are present in the motor. The rotating stator field “grips” the rotor
field and the rotor starts to rotate with the speed of the magnetic field of stator. The current of
excitation coil is led to the rotor via brushes when using the electromagnet. (3) Different
designs of a synchronous motor are shown in Fig. 2.6
Fig. 2.6 Different designs of a synchronous motor (3)
A synchronous motor achieves the torque only in a synchronous regime (when achieved the
nominal speed). So, it is very problematic to start a synchronous motor directly from the
ACTUATORS IN INDUSTRIAL AUTOMATION
mains. That is used for the asynchronous start and later on the motor is switched over to the
mains. The anguber speed control of the synchronous motor can be realized with the control
of the frequency. Mechanical characteristics of the synchronous motor are shown in Fig. 2.7.
ω0
f = var
ω
1
ω0
2 Characteristic of
starting winding
0
Mst Mn
Mm
M
Fig. 2.7 Characteristics of a synchronous motor: 1 – in a synchronous mode; 2 – during start
up
Reluctance motor. Reluctance motor is similar to a synchronous motor but it has neither
excitation coil nor permanent magnets. Here, the magnetic resistance varies with the change
in position of the rotor.
Synchronous motors are mainly used in a powerful compressor, pumps, winders, ship
controls. Synchronous motors are also used in industrial Computer Numerical Control (CNC)
machines and industrial robots. Synchronous motors are more expensive and complicated
than asynchronous motors.
2.3.4. Pulse controlled electrical motors
Stepper motors. In stepper motors the rotating magnetic field is created instead of threephase sinusoidal voltage with sequential pulses. They are suitable for use in low power
applications with position control, where the number of pulses is proportional to motor
rotation (motor position). Because of low efficiency the use of stepper motors in high power
applications is not rational. Stepper motors are detailed described later in this book.
2.3.5. Losses in electrical motors
In the operation of every machine the losses are always present. Losses can be caused by the
following reasons (3):
• When the current flows through the motor windings. As the windings have active
resistance the heat energy will emit. As almost all windings are made of copper, these
losses are also called copper losses
• Losses in the magnetic field more due to alternating magnetic field, hysteresis and Eddy
currents. These losses are called iron losses. Iron losses depend on the size of the
magnetic core, loop of hysteresis and magnetizing frequency
• Friction between machine parts and air, called cooling losses
• Friction in the bearings, called frictional losses
ACTUATORS IN INDUSTRIAL AUTOMATION
Efficiency of the motor can be increased with the reduction of the losses. To reduce the
copper losses the wires with low resistance must be used. To reduce iron losses special
ferromagnetic materials are used, also sheet steel is used to reduce Eddy currents. As the air
gap is also part of the magnetic circuit it is made as small as possible. Friction in bearings can
be reduced with the use of a high quality lubricant.
2.3.6. Motor drive duty modes
For different applications different machines with different options are required. In order to
meet all requirements the actuators must be chosen properly. In dependence of surrounding
environment and variable conditions the load, speed and rotating direction can vary in time.
Devices must be able to cope with these changes and continue reliable operation.
Different duty modes of motors can be as follows (3):
• continuous constant speed rotation (fan, saw, electric al vehicles)
•
variable speed rotation (pump, hard disk drive)
•
rotation with variable speed in both directions (hoist, robot)
•
linear movement with constant speed (conveyor)
•
periodical movement (printing devices)
•
non- periodical movement (positioning devices)
Rotation of an electrical motor without a load is called a no-load operation. In this case a
small torque is only applied a machine consumes less energy. Alas the amount of reactive
power remains the same and the power factor cos φ is low.
During the work of each mechanism heat energy is always emitted. Heat energy raises the
temperature of the machine parts. The most temperature sensitive part of the electrical motor
is the winding insulation that is made of a synthetic material. Overheating can cause melting
of insulation and damage to the whole mechanism. Different insulation classes and different
duty modes are specified in standard EN 60034. Duty modes are specified in Table 2.1.
Electrical drives can work in continuous or intermittent mode. Usually the relative runtime is
marked on in percent the motors data label. Relative runtime is the ratio between load
operation and the period of one cycle T. In data sheets the power for different relative
runtimes of 15 %, 25 %, 40 % and 60 % is given (3). Relative runtime is calculated as
follows:
t
q= k
T .
Usually the motor is chosen by the power in continuous operation (S1), but many electrical
drives work in other duties. For example, hoisting machines rise and lower the weight, in
some periods they have to wait for load. Also the boring machines that are loaded only for the
period of boring, in the pause the motor is switched off. It means that the loading of the motor
can vary and must be taken into consideration when designing an electrical drive.
Table 2.1.Motor drive duty modes EN 60034
Type Duty
Description
ACTUATORS IN INDUSTRIAL AUTOMATION
Continuous running
Operation at constant load of sufficient duration for the thermal
equilibrium to be reached.
S2
Short-term
Operation at constant load during a given time, less than
required to reach thermal equilibrium, followed by a rest and a
de-energizing period of sufficient duration to reestablish
machine temperatures within 2 °C of the coolant.
S3
A sequence of identical duty cycles, each including a period of
operation at constant load and a rest as a de-energizing period.
Intermittent periodic
In this duty type, the cycle is such that the starting current does
not significantly affect the temperature rise.
S4
Intermittent periodic A sequence of identical duty cycles, each cycle including a
with a high startup significant period of starting, a period of operation at constant
torque
load and a rest as a de-energizing period.
S5
Intermittent periodic
with a high startup
torque and electric
braking
A sequence of identical cycles, each cycle consisting of a
period of starting, a period of operation at constant load, a
period of rapid electric braking and a rest as a de-energizing
period.
S6
Continuousoperation periodic
A sequence of identical duty cycles, each cycle consisting of a
period of operation at constant load and a period of operation at
no load without de-energizing period.
S7
Continuousoperation periodic A sequence of identical duty cycles, each cycle consisting of a
with a high startup period of starting, a period of operation at constant load and a
torque and electric period of electric braking without a de-energizing period.
braking
S8
A sequence of identical duty cycles, each cycle consisting of a
Continuousperiod of operation at constant load corresponding to a
operation periodic
predetermined speed of rotation, followed by one or more
with related loadperiods of operation at other constant loads corresponding to
speed changes
different speeds of rotation without a de-energizing period.
S1
Careful assessment of duty types S2 to S8 reveals that there exist two distinct groups: first –
duties S2, S3 and S6 permit uprating of motors relative to the output permissible in
continuous running duty S1 because during the load the motor does not achieve maximal
permissible temperature; second – duties S4, S5, S7 and S8 requiring derating relative to the
output permissible in continuous running duty S1, because during the operation they warm up
more than in the continuous running duty S1.
2.3.7. Protection classes
Table 2.2. Motor drive classification by cooling protection
IP
X- protection against accidental contact
Y- protection against penetration of water
ACTUATORS IN INDUSTRIAL AUTOMATION
0
1
2
3
4
5
No protection
Large surface and solid objects
exceeding 50 mm in diameter
Fingers and solid objects exceeding 12
mm in diameter
Tools and solid objects exceeding 1 mm
in diameter
Any object and harmful dust deposits,
which can interfere with operation
Any contact and any kind of dust
No protection
Dripping water (vertical falling drops)
Water drops falling up to 15° from the
vertical
Spray water up to 60° from the vertical (rain)
Deck water (splash water from all directions)
Jet water from all directions
6
Temporary flooding (deck of a ship)
7
Water proof not deeper than 1 m
8
Pressurized water, water deeper than 1 m
Design and operating conditions of actuators depend on the environment where they are
installed. To suit the demands of the prevailing ambient conditions – high humidity,
aggressive media, splash-water, dust accumulation etc. – equipment is available in the
corresponding enclosure class according to EN 60529 with Ingress Protection coding standard
IP XY.
In addition to table above, special care of protecting electrically excited actuators must be
taken. The housings must be either properly grounded, insulation must be improved,
separation transformer can be used or total touch safety must be guaranteed.
2.4. Asynchronous motors
2.4.1. Design and operating principles of an asynchronous motor
An asynchronous motor is the most frequently used electrical motor in industry. It consists of
a stator and a rotor. The stator is the stationary part and the rotor is the rotating part of an
electrical motor. The stator and the rotor are separated by the air gap between them. The
width of air gap can vary from 0,1…1 mm. design of an asynchronous motor is shown in Fig.
2.8Error! Reference source not found..
a) Cross section
b) view of the components
Fig. 2.8 Design of an asynchronous motor
1)motor
housing
2) bearings
3)bearing
holder
4) fan
5) fan cover
6)electrical
connections
7) iron core of
stator
8)stator
winding
9) rotor
10) shaft
ACTUATORS IN INDUSTRIAL AUTOMATION
The stator of an asynchronous motor consists of three copper coils that are geometrically
shifted to each other and supplied from the three-phase grid. The coils can be star or delta
connected. Such configuration creates a rotating magnetic field around the stator that
penetrates through the air gap in the rotor and causes the current flow in rotor. The current a
magnetic cause in turn field in the rotor. Interaction between the stator and rotor the magnetic
fields causes a force that accelerates the rotor. The rotational speed of a motor depends on the
rotational speed of the magnetic field that in turn depends on the number of pole pairs and
supply frequency. Fig. 2.9 shows two asynchronous motors with one - and two - pole pairs.
Four poles
n0 = 1500 p/min
Two poles
n0 = 3000 p/min
Stator
N
Squirrel- cage
rotor
N
S
S
N
Air gap
S
Fig. 2.9 An asynchronous motor with one and two pole pairs
Nowadays two types of rotors are used in asynchronous motors: slip - ring rotors and squirrelcage rotors. In slip - ring rotors the speed of the rotor is controlled by changing the resistance
of the rotor that in turn lowers the current. For that special brushes are used that transfer
electrical current from the rotor to the resistances that are placed on the stator, however, they
wear down very quickly and need frequent maintenance. In a squirrel - cage rotor the coils are
short circuited and the whole energy flow from stator to rotor takes place through the air gap.
Design of a slip - ring rotor and squirrel - cage rotor is shown in Fig. 2.10.
a
b
Fig. 2.10 Rotor designs used in asynchronous motors. a – squirrel - cage rotor; b – slip - ring
rotor
The speed of the stator magnetic field is called synchronous speed and can be calculated as
follows
60 ⋅ f
n =
s
p ,
ACTUATORS IN INDUSTRIAL AUTOMATION
where ns is the synchronous speed of the motor (rev/min), f is the supply frequency and p is
the number of pole pairs. The larger is the number of pole pairs the slower the synchronous
speed is, however the higher torque the motor can achieve. Different synchronous speeds
correspond to different numbers of pole pairs. In table 2.3 these values are shown for the
supply frequency 50 Hz.
Table 2.3 Synchronous speeds that correspond to the number of pole pairs
No. of pole pairs
Synchronous speed rev/min
1
2
3
4
5
3000
1500
1000
750
600
The speed of the magnetic field can also be described as an angular speed ω that shows the
rotational speed in radians per second. Angular speed can be evaluated as follows
ω = 2π ⋅ f ,
The real rotational speed of an asynchronous motor is lower than the rotating magnetic field
of stator. That is described by slip s that shows the difference between the real motor speed n
and the synchronous speed ns, be evaluated as follows
n − n ωs − ω
s= s
=
ns
ωs ,
With the increase of the load the slip also increases. Normal value of the slip is between 1…5
% of synchronous speed. The torque M that the motor creates on the shaft can be calculated as
follows
P
P
M = mech = mech
ωs
2πf ,
where ωs is synchronous angular velocity and Pmech is the mechanical power on the shaft, the
power given on the motors label data. After the torque is applied the motor starts to accelerate
with an angular velocity ε. Angular acceleration can be evaluated as follows
n
ε=
t .
In the operation of each machine losses are present. This means that part of the whole
consumed electrical energy will be converted into heat, lost in magnetizing processes, in the
friction between the motor shaft and bearings etc. The ratio between the useful output of an
energy conversion machine and the input is called efficiency η. The useful output is
mechanical work. The efficiency can be obtained as follows
P
η = mech
Pel ,
where Pmech is the mechanical power on the motor shaft (output power) and Pel is the
consumable electrical energy from the grid. The higher is the efficiency the more power goes
to useful output. The efficiency of asynchronous motors varies between 0,8…0,95.
The dependence between the torque and the angular velocity of the motor is described with
its mechanical characteristic, as shown in Fig. 2.11, a. In order to start the motor, the starting
ACTUATORS IN INDUSTRIAL AUTOMATION
torque must be applied that is 1…3 times higher than the nominal motor torque. The highest
torque that the motor can achieve is called the breakdown torque. When the nominal operating
mode is achieved, the motor operates at the nominal torque and nominal speed. The working
area of the motor can vary by the order of slip (2…8 %).
a)
b)
Fig. 2.11 Mechanical characteristics of an asynchronous motor. a – M/n characteristics;
b – I/n characteristics
The acceptable overload for the motor can be 1,6…1,8 times higher than nominal torque.
Higher overloads can cause motor to achieve breakdown torque. Operation in breakdown
torque region is very dangerous because the speed decreases rapidly and motors coils start to
hot up quickly. Overheat of the motor causes the destruction of whole motor.
Figure Fig. 2.11 b shows the dependence between the current and the angular velocity during
startup. As can be seen, the starting current of an asynchronous motor can be 4…8 times
higher than nominal current In.
2.4.2. Label data of an asynchronous motor
Each motor has a label data that is connected to the housing of the motor. The label data gives
information about the motor characteristics. Using the label data the user can decide which
motor is appropriate for certain applications. The symbol and designations of stator windings
of a squirrel cage asynchronous motor are shown in figure Fig. 2.12.
ACTUATORS IN INDUSTRIAL AUTOMATION
U
V
W
Symbol of
asynchronous motor
U1
V1
W1
U2
V2 W2
Designations of
stator windings
Fig. 2.12 Symbol and designations of stator
windings of an asynchronous motor with squirrel
cage rotor
2.4.3.
Label data of anasynchronous
motor
Manufacturer
Type of motor, serial number
Nominal power Pn
Nominal voltage and connections Un
Synchronous speed (no. of poles) n0
Slip sn
Efficiency ηn
Power factor ϕn
Ways of connection of an asynchronous motor
There are two ways of connecting an asynchronous motor (but also synchronous motors)
called the star connection and the delta connection.
2.4.4. Star connection
When the motor windings are connected so that ends of the windings are connected together
in one point (called, zero point) and the beginnings of each winding are connected to a
separate phase, the connection is called a star connection. A star connection is denoted by Y.
Star connection is shown in Error! Reference source not found..
UL
v1
IL
UF
v2
IF
u2
w2
u1
w1
b)
a)
Fig. 2.13 Star connection of as an asynchronous motor. (a) schema; (b) connecting cables to
motor terminals
ACTUATORS IN INDUSTRIAL AUTOMATION
The following electrical relationships are valid in a star connection:
I L = I F – phase current is equal to the line current.
U L = 3 ⋅ U F – line voltage is by the order of
3 higher than the phase voltage
S = 3 ⋅ U L ⋅ I = 3 ⋅ U F ⋅ I – complex power
P = S ⋅ cos ϕ = 3 ⋅ U L ⋅ I ⋅ cos ϕ = 3 ⋅ U F ⋅ I ⋅ cos ϕ – active power
Q = S ⋅ sin ϕ = 3 ⋅ U L ⋅ I ⋅ sin ϕ = 3 ⋅ U F ⋅ I ⋅ sin ϕ – reactive power
2.4.5. Delta connection
When motor windings are connected so, that the beginning of one winding is connected to the
end of another winding, and each of these connection points is connected to separate phase,
then the motor is delta connected. A delta connection is denoted by Δ. Star connection is
shown in Fig. 2.14.
UL
IL
u2
v1
u1
IF
v2
w2
w1
b)
a)
Fig. 2.14 Delta connection of an asynchronous motor. (a) schema; (b) connecting cables to
motor terminals
The following electrical relationships are valid in a delta connection:
I F = 3 ⋅ I L – phase current is by the order of 3 higher than the line current
U L = U F – line voltage is equal to the phase voltage
S = 3 ⋅ U ⋅ I L = 3 ⋅ U ⋅ I F – complex power
P = S ⋅ cos ϕ = 3 ⋅ U ⋅ I L cos ϕ = 3 ⋅ U ⋅ I F cos ϕ – active power
Q = S ⋅ sin ϕ = 3 ⋅ U ⋅ I L sin ϕ = 3 ⋅ U ⋅ I F sin ϕ – reactive power
ACTUATORS IN INDUSTRIAL AUTOMATION
A delta connected motor consumes three times more power from the grid than a star
connected motor. Motors can be delta connected only when the windings are suitable for the
grid voltage.
When connecting a motor to the grid an attention must be paid to the motors label data. When
the labeling data has the nominal voltage Δ/Y 230/400 V, the motor in the European
electrical network can only in star connection. When connecting in star, only the voltage of
230 V will be applied to each winding, when connecting in delta, the voltage of 400 V will be
applied to each winding. Overvoltage can damage the motor. Such motor can be connected in
delta only through a voltage lowering transformer that lowers the line voltage to 230 V. If
motor has the following data Δ/Y 400/690 V, then it can be connected in delta for best
performance, because in this case 400 V are applied to each winding. If this motor is
connected to the same grid, then only 230 V will be applied to the windings and motor does
not reach its nominal operation mode. This motor must be connected in star to an industrial
electrical grid with line voltage of 690 It is illustrated below by an example.
2.4.6. Example
A three-phase induction motor has the following label data (see table). Motor windings are
delta connected to three-phase grid, with the line voltage of 400 V and frequency of 50 Hz. (a)
the following values are to be determined (see table). (b) Question how much power will the
motor consume from the same electrical grid if connected in star?
Label data
power Pmech = 5,5 kW
voltage U = 400/ 690 V Δ/ Y
current
I = 11/ 6,4 A Δ/ Y
frequency f = 50 Hz
rotational speed n = 1460 min-1
power factor cos φ = 0,84
Values to be determined
electrical powers S, P, Q
efficiency η
no. of pole pairs p
slip s
torque M
consumable energy during 1,5 h
price of electrical energy if 1 kWh costs 0,20 EUR
Solution (a). ∆ connection of the motor:
Consumable complex power
S = 3 ⋅ UI = 3 ⋅ 400 ⋅ 11 = 7621 VA.
∆
Consumable active power
P = S ⋅ cos ϕ = 7621 ⋅ 0,84 = 6402 W.
∆
∆
Consumable reactive power
Q = S ⋅ sin ϕ = S ⋅ sin(arccosϕ ) = 7621 ⋅ 0,543 = 4135 VAr.
∆
∆
Efficiency
P
5500
η = mech =
= 0,86
6402
Pel
.
Number of pole pairs
nel . field 3000
p=
=
=2
1500
ns
.
ACTUATORS IN INDUSTRIAL AUTOMATION
Although current is given on the label it can also be determined as follows:
Pmeh
5500
I=
=
≈ 11 A.
3 ⋅ U ⋅η ⋅ cos ϕ
3 ⋅ 400 ⋅ 0,86 ⋅ 0,84
Slip, difference between stator field and rotor fields
n − n 1500 − 1460
=
= 0,027
s= s
ns
1500
.
Torque on the shaft
P
P
5500
= 17,5 Nm.
M = meh = meh =
ω
2πf 2π ⋅ 50
Consumable active energy during 1,5 h
Wa = Pel ⋅ t = 6402 ⋅ 1,5 = 9603 Wh = 9,603 kWh.
Price of electrical energy
Wa ⋅ 0,2 = 9,603 ⋅ 0,2 = 1,92 EUR.
Solution (b). Y connection of the motor:
When star connected, smaller voltage is applied to the windings
400
U
= 230 V.
UF = L =
3
3
To determine the current that flows through one winding, its resistance must be calculated.
Nominal current in star connection is 6,4 at the line voltage 690 V but the phase voltage is
400 V.
U
400
Z= F =
= 62,5 Ω .
IF
6,4
At voltage 230 V the phase current of the motor is
U 230
I= =
= 3,68 A.
Z 62,5
Consumable complex power
S = 3 ⋅ UI = 3 ⋅ 400 ⋅ 3,68 = 2550 VA.
Y
As can be seen, if delta connected, the motor consumes three times more power (and achieves
higher power) in comparison to star connection.
S ∆ 7621
=
≈3
SY 2550
.
2.4.7. Motor and generator operation
During the operation of electrical drives the rotating speed, torque and in certain cases
direction can vary. If electrical machine converts electrical energy into mechanical, then a
machine operates in a motor regime. If an electrical machine converts mechanical energy into
electrical, then a machine operates in a generator regime. These regimes are divided into four
quadrants (see figure Fig. 2.15). In the motor regime the torque and direction are applied in
same direction (quadrants I and III). An example is a hoist machine when lifting a weight,
when the torque must be applied in the same direction with the motor rotation. In a generator
regime the torque is a applied in the opposite direction to the rotation (quadrants II and IV).
An example is hoist machine when lowering the weight. In this case the torque must be
ACTUATORS IN INDUSTRIAL AUTOMATION
applied in opposite direction to slow down the speed (act against gravitational force). This
torque is called a braking torque. During lowering the mechanical energy is converted into
electrical. The electrical drives must have a built in converter to operate machine in several
quadrants.
In short, the following circumstances can cause the generator regime of an electrical machine:
• When an external machine is turning the motor (e.g. steam turbine, internal
combustion engine) i.e. when the rotational speed increases over the synchronous
speed, the motor achieves the braking torque.
• When an motor operates in a regenerative braking mode, i.e. electrical drive is stopped
at a constant torque.
Generator
Motor
Generator
Motor
Fig. 2.15 Four quadrant operations of electrical drives
2.4.8. Start of an asynchronous motor
Starting of high power asynchronous motors is very problematic. In certain cases starting
current can be in certain cases seven times higher than nominal motor current that causes big
current peaks in electrical lines. Starting current does not depend on the motor load rather it
has a certain value that is given in the motors label data as Istart/In. Start up of an asynchronous
motor due to a load and a high moment of inertia can last quite long, tres lowering the voltage
in the whole electrical system. Long start up can cause overheating of motor windings and
also damage them.[4]
There are many ways to start a squirrel cage - asynchronous motor:
Direct on-line start is the simplest way of starting an asynchronous motor. In this case the
motor is connected directly to the grid through the main switch and overload protection. This
method is simple and there is no necessity for complicated control systems, however, it causes
very high starting currents that can be seven times higher than nominal current. As the motor
is not excited at the beginning, the starting current peak can reach 14 times higher value than
the nominal motor current. In addition to high current, the motor achieves also high starting
torque that is also several times higher than the motor nominal. High starting torque causes
unnecessary forces and stress in gears. In spite of everything this method is still very
frequently used.
Star-delta start up method allows to reduce starting current to 30 % and starting torque to 25
% of the nominal. A control system consists of switches, overload protection and timer. The
timer is programmed to switch the motor connection from star to delta at a certain moment.
ACTUATORS IN INDUSTRIAL AUTOMATION
The motor is started in star connection (in star connection lower currents flow in motor) and
later switched over to delta connection. The motor must be connected to delta at the start. If
the motor is heavily loaded then this method is not suitable to start this application. Star-delta
start up is suitable to start up fans and pumps.
Start up with a soft starter is a comfortable way of starting an asynchronous motor. This
method uses an AC voltage converter to adjust the RMS value of voltage thers reducive the
starting current and torque. A soft starter is described in more detail in section 2.5.
Start up with frequency converter is the best way of starting an asynchronous motor and
controlling its speed. Nowadays electrical drives with frequency converters are used in many
industrial applications. A frequency converter is described in more detail in section 2.5..
Table 2.44 gives an overview of the starting methods and problems related to start up and stop
of asynchronous motors.
Table 2.4 Problems related Fo different starting methods at motor start up and stop
Problem
Slipping
belts
/
bearing wear
High starting current
Wear and tear of gear
boxes
Damaged
goods
during stop
Water hammering in
pipe system when
stopping
Transmission peaks
Star-delta
start
Frequency
converter
Yes
Medium
No
No
Yes
No
No
No
Yes
Yes
No
No
Yes
Yes
No
No
Yes
Yes
No, best solution
Yes, reduced
Yes
Yes
No
No
Direct start
Soft starter
As can be seen from table above, the most problematic is the direct on line start and star-delta
start of an asynchronous motor, however, the best solutions are to use soft starters and
frequency converters
2.4.9. Stop of an asynchronous motor
There are many different ways of stopping an electrical machine. Most frequently used
method is by helo of mechanical brakes, where the whole saved energy is converted into heat
because of friction. Nowadays the electrical braking method is more frequently used. During
electrical braking the motor dissipates the energy into the environment. In this case the
machine operates in a generator mode. Each electrical machine has energy, that has to be
dissipated somewhere, to stop the motor. Mainly two types of energy are saved in the
machine:
a) Kinetic energy is saved in all rotating or linearly moving machines and can be evaluated in
a straightforward way as follows
mv 2
Wkin =
2 ,
where m is the mass of body and v is the speed. In case of a rotating machine it can be
evaluated as follows
ACTUATORS IN INDUSTRIAL AUTOMATION
Jω 2
2 ,
where J is the moment of inertia of the body and ω is the angular velocity.
b) Potential energy is saved in hoisting machines and elevators. When the hoist is in the
upper position, the potential energy of it can be evaluated as follows
W pot = mgh
,
where m is the mass of the body, g is gravitational acceleration and h is the height from the
surface.
When lowering the hoist the braking torque must be applied to the motor in order to keep the
lowering speed of hoist constant. The drive losses, mechanical resistance and transmission
efficiency work in favor of deceleration reducing the braking power demand. When machine
is decelerating the kinetic or/ and potential energy are converted into electrical energy. To
enable the motor to decelerate, this additional energy must be dissipated. There are some
possibilities for this [6]:
• energy feedback to the mains (electrical energy becomes accessible to other
consumers)
Wkin =
• energy conversion into heat by braking resistor connecting via a brake chopper
• energy exchange in multi-motor applications (electrical energy feeds other motors
connected to the same converter)
• dc injection braking, when the kinetic energy of the motor load system is converted to
heat in the motor
Mains energy feedback. The advantage of this mains energy feedback is that energy is fed
back into the supply network and therefore remains available as electrical energy. For this
form of braking the additional converter is needed. Due to the higher cost of this design it is
usually found in drives specifically designed to operate within application areas which
typically display regenerative characteristics.
However, in view of the intermittent operation and the small size of motors, regeneration is
not worthwhile unless a battery of limited capacity serves as a power source.
Dynamic braking. In contrast to mains energy feedback, in dynamic braking, the energy of
the braking resistor is not fed back into supply. If only small braking energy is produced, it
may be less expensive to use a brake chopper with an external resistor rather than the
additional braking converter. In dynamic braking the energy being extracted from the load is
transformed into the thermal form.
DC braking. The idea of dc braking is to disconnect the motor from the line and to pass
direct current through the windings. Using the inverter, a controlled dc voltage is applied to
the motor, therefore a braking and holding torque is produced in the rotor without
regeneration back to the supply. However, because no frequency is applied (frequency of dc is
f = 0 Hz) there is no control over motor speed and it is not possible to predict the stopping
time of the load. The torque on the rotor is maintained even at standstill, so the dc braking can
be used to hold the rotor and the load for short periods, if required. Continued use of dc
braking will cause overheating in the motor and suitable protection must be considered (2)
A motor can also be stopped with the freewheeling method or stop ramp. When stopping the
motor with the freewheeling motor, the supply is disconnected from the motor and the motor
ACTUATORS IN INDUSTRIAL AUTOMATION
continues to run until the frictional forces cause it to stop. When stopping the motor with the
stop ramp the frequency is lowered to certain values, afterwards the dynamical braking is
applied (2).
2.4.10.
Example
A three-phase asynchronous motor accelerates in 1,5 seconds to the nominal speed n = 2850
rev/min. Determine the number of pole pairs , slip , angular velocity , angular acceleration .
How fast must the stator field rotate for rotor to the rotate at n = 1000 rev/min.
Solution:
For instances the synchronous speed of the motor may be 3000 rev/min. So, the motor has 1
pole pair.
p =1
Slip of the motor
n − n 3000 − 2850
s= s
=
= 0,05
ns
3000
.
Angular speed of the motor
2π ⋅ n 2π ⋅ 2850
=
= 298,5 s-1 .
ω=
60
60
Angular acceleration of the motor
ω 298,5
= 199 s-2.
ε= =
t
1,5
To achieve the speed of n = 1000 rev/min the stator field must rotate by an order of a slip
faster.
nstaator = n ⋅ (1 + s) = 1000 ⋅ (1 + 0,05) = 1050 rev/min.
2.5. Electrical drive with a frequency converter
2.5.1. Design and operating principles of a frequency converter
A frequency converter is the major part of modern electrical drives used for speed control.
Traditionally, a frequency controller was used for smooth adjusting of a motors supply
voltage and frequency. Nowadays a frequency converter integrates a frequency conversion
inverter, sensors, control block that allow full control of the motor and the working machine.
A network interface allows the implementation of frequency converters into complicated
automated control systems.
Many different types of frequency converters are used today, most common being a frequency
converter
with
a
DC
link,
shown
in
Fig.
2.16.
ACTUATORS IN INDUSTRIAL AUTOMATION
Starting circuit
Braking
circuit
Inverter
Output
Power
Control
Fig. 2.16 Design of the frequency converter with a DC link
A frequency converter consists of an uncontrollable rectifier, a DC link and an inverter.
A rectifier consists of six diodes (2 diodes per one phase) that are connected in a bridge. The
aim of the rectifier is to change an alternating current into direct current. The output of the
rectifier is a pulsating direct current with a voltage Uz that has a value U Z = 565 VDC in a
three phase 400 V system. The working principle of a three phase B6 rectifier is shown in Fig.
2.17.
U~
UZ
UZ
U~
Fig. 2.17 Operating principle of an uncontrolled three - phase rectifier
A DC Link consists in turn of a smoothing capacitor, starting and braking circuits. The
smoothing capacitor is used for smoothing the voltage pulsations. When switching on the
frequency converter a large current pulse occurs due to the charge of the capacitor. In order to
limit the starting current a thyristor controlled starting circuit is used in DC link. The resistor
limits the starting current, when the capacitor is charged then the thyristor switches the
resistance off the circuit. The DC link voltage frequency converter is usually uncontrollable.
The braking circuit is used in dynamic braking mode in order to dissipate the heat energy
produced while braking the motor. The braking circuit is controlled by a transistor. When
ACTUATORS IN INDUSTRIAL AUTOMATION
switching off the frequency converter a hazardous high voltage remains for several minutes
on the capacitor, so it should be always considered when working with a frequency converter.
A inverter is used for converting the direct current into alternating current with variable
voltage and frequency. Inverter consists of six transistors with anti - parallel connected
diodes. The transistors are controlled by a microcontroller with a pulse width modulation
(PWM) principle. The output of the inverter is connected to the motor (2).
Type
Interface
Grounding of EMC
filter
Flasm memory
Grounding of Varistor
Potentiometer
Analog signal
Connection of supply, motor
and braking resistor
Inputs-Outputs
Fig. 2.18 ABB frequency converter ACS 150
2.5.2. Frequency control
The most common way of controlling the motor’s speed is using the frequency control
method, where the voltage is a function of the frequency U = f ( f ) . In the simplest way, the
ratio between the voltage and the frequency is kept constant U / f = const . The necessity to
raise voltage U is due to the inductance losses with the rise of frequency f. The losses are
compensated with a higher voltage. The block schematics of frequency control is shown in
Fig. 2.19, where the units with star (*) mean the reference values.
Voltage
inverter
Fig. 2.19 Block schematics of the frequency control method
The frequency control method is an open loop control method, where the control system has
no feedback from the manufacturing process. So it is not always clear for the frequency
ACTUATORS IN INDUSTRIAL AUTOMATION
control, at which speed the motor is rotating, or if it is rotating at all. Thus, overload
protection at current 150% from the nominal value will be activated (4). Sudden change in the
motor load can cause breakdown of the motor. This will result in stop ping the motor or
operation on low revolutions, which in turn makes the cooling of the motors windings worse.
It can be concluded, that applying the correct torque for the motor with the frequency control
method is complicated (5). The voltage-frequency control method is suitable for applications
that operate at constant speed, when the voltage and the frequency are changing slowly (3).
2.5.3. Torque characteristics in the field weakening mode
Overload area
Maximal allowable torque
Torque
Fig. 2.20 Torque characteristics in the field weakening mode
2.5.4. Constant torque mode at frequency 87 Hz
In certain cases the frequency can be raised up to 87 Hz for the motors with the nominal
frequency 50 Hz. The frequency converter must have built in function to achieve it. In this
mode the motor is supplied with the phase voltage 230 V at frequency 50 Hz, when rising the
frequency to 87 Hz the voltage will rise to 400 V. 87 Hz operation is possible in the motors
that can be supplied with 400 V in delta connection (4).
It is important that even at the frequencies higher than 230 V the ratio between the supply
voltage and frequency must remain constant U / f = const .
The motors with the nominal voltage 230 / 400 V (Δ/Y) are suitable for 87 Hz mode. Many
high power motors (> 4kW) are usually designed for the supply voltage 400/690 V (Δ/Y) that
are unsuitable for 87 Hz operation.
With the rise of voltage and frequency the constant torque area rises also, hence the motor
power rises 3 times. When the frequency goes higher than 87 Hz, then the motor will
operate in the flux weakening mode. With the rise of frequency the cooling conditions of the
motor will also improve. Thus, in a continuous duty mode (S1) the permissible power can be
35 % higher. For example, a motor with rated power 3 kW can operate in the delta connection
87 Hz mode with a power of 4 kW [14]. The 87 Hz mode is described in Fig. 2.21.
ACTUATORS IN INDUSTRIAL AUTOMATION
Fig. 2.21 87 Hz operating mode of an electrical motor
2.5.5. Pulse width modulation
Pulse width modulation (PWM) is a control principle that is used for the generation of
variable voltage and frequency. The PWM method is used in voltage inverters for controlling
the transistors. The output of the PWM is a sequence of electrical pulses with constant
amplitude and variable duration (width). With the pulse width, the rms value of the voltage
can be controlled as shown in Fig. 2.22. The output frequency of the PWM can be from 1
kilohertz for the motor control up to several megahertz for the control of certain converters.
ton
Fig. 2.22 Pulse width
toff
U
t
Relative switching duration can be obtained as follows
ton
t=
ton + toff
,
The rms value of the voltage during this period can be evaluated as follows
t on
U eff = U ⋅
t on + t off
,
As can be seen, the longer the relatively duration ton of a pulse the higher is the rms value of
the voltage during that period.
Many different PWM techniques exist, the most common is sinusoidal PWM. So let us
elaboratel on it.
The aim of sinusoidal PWM is to form the sinusoidal output voltage. The PWM signals are
generated by comparing the sinusoidal voltage signal and the triangular voltage as shown in
ACTUATORS IN INDUSTRIAL AUTOMATION
Fig. 2.23. The signals are compared in an electronic element – comparator. When the actual
value of the sinusoidal voltage is higher than the actual value of the triangular signal, then the
transistor conducts current to the motor and vice versa.
ton
U
toff
Uk Usin
t
T
Us
t
Fig. 2.23 Generation of PWM signals for one phase
To generate a three - phase AC voltage, three sinusoidal signal generators are necessary. The
higher is the frequency of the triangular frequency the more similar is the output signal to
ideal sinus. With the control of the reference signal, the output voltage and frequency are
controlled. This PWM control method is used mainly for the speed control of the induction
and synchronous motors however it can also be used to control the speed of DC motors. The
rms value of the voltage is changed with the pulse width.
2.5.6. Direct torque control
The DTC control of an induction motor is open - loop control that resembles the DC motor
control. In the PWM mythology the frequency and voltage are controlled, but many
mathematical blocks in the controller have to be passed before. The DTC method uses direct
control of the torque and magnetic flux that that depend directly on the motor. There is no
necessity for a feedback transducer.
A major advantage of the DTC is the control possibility at very low speeds (below 0,5 Hz)
with the nominal torque achieved (Danger! Motor cooling). Usually the speed in open loop
controlled drives varies 10% of the nominal slip that satisfies the requirements of 95 % drives
used in industry. With the DTC control the system reacts to changes in 1-2 ms but in PWM
controlled drives this time is 100 ms. DTC provides torque linearity which is very important
in precise drives, e.g. paper winders.
The use of DTC method is restricted in the parallel operation of many motors. In this case the
control block has no information about the status of the motor. In the parallel operation of
motors the frequency control method is most suitable (6).
2.5.7. Motor loads and their characteristics
Motor is usually loaded with a loading machine. The load can have various behaviors and
options. The chosen motor must correspond to the load characteristics. The most important
issue is that the starting torque of the motor must be higher than the starting torque of the
ACTUATORS IN INDUSTRIAL AUTOMATION
load. Otherwise, the motor will fail to start. Also, the load must not overload the motor and
stop the rotor.
The loads are divided into four main categories:
• constant relationship between the speed and the load
• linearly increasing relationship between the speed and the load
• square proportional relationship between the speed and the load
• inverse proportional relationship between the speed and the load
Examples of different loads, the relationship between speed, torque and power, also the
mechanical characteristics are shown in Table 2.5.
Table 2.5 Different load characteristics
hoist,
conveyor,
robot.
M = const
P~n
rolling mill,
mill,
calender.
M~n
P ~ n2
pump,
fan,
centrifuge.
M ~ n2
P ~ n3
borer,
winder,
press.
M ~ n-1
P = const
At constant relationship between the speed and the load the load torque is independent of the
rotational speed (M = const). Examples of such mechanisms are hoists, conveyors and robots
that require high starting torque. So, the motor and the frequency converter must be capable of
working under short overload conditions. When high loaded devices are working on small
revolutions then the cooling conditions are also worse. In order to improve cooling an external
cooling must be provided to protect devices from overheating. Nowadays a temperature
sensor (thermistor) is mounted into the motor, to switch the devices off when critical
temperature is achieved.
Examples of linearly increasing relationship between the speed and load machines are rolling
mills, mills and calenders. The starting torque of these applications is usually small. Power
ACTUATORS IN INDUSTRIAL AUTOMATION
increases with a square proportional characteristic, so at two time higher speed than nominal a
four times higher power is consumed from the grid.
Examples of square proportional relationship between the speed and load machines are fans,
pumps and centrifuges, i.e. devices where the main resistance is caused by air or fluids. The
starting torque is usually very small. Usually these applications operate on high revolutions,
so good cooling conditions are guaranteed. The power increases with a cube proportional
characteristic. If we decrease the speed of a fan from 100 % to 90 %, then consumable power
decreases to 0,93·Pn, almost to 70%.
Examples of inverse proportional relationship between the speed and load machines are
borers, winders, presses. These applications operate at constant speed and the torque is
usually small because they loaded at nominal speed conditions. The consumable power
remains constant.
These four characteristics are ideal, however, in practice many deviations from ideal
characteristics occur.
2.6. Functions of a frequency converter
2.6.1. Starting and stopping of an electrical drive
Frequency converters include a control system. The starting and stopping of a motor is done
by start and stop ramps. The duration of both ramps can be adjusted from milliseconds to
tenths minutes. With the increase of the ramp frequency and voltage also increase. A motor is
decelerated either with free wheel operation or with a stop ramp. When stopping with a ramp,
the supply frequency is lowered until a certain level. After achieving this level, a dynamical
braking mode is used.
Speed
frequency
Starting
frequency
Braking
frequency
Dynamic
braking
Braking time
Braking voltage
Forward or
reverse
Fig. 2.24 Programmable start and stop ramps, also duration of dynamical braking.
Start and stop ramps must be chosen in accordance with the motor data. For example, very
fast start ramp cannot be set for motors with high moment of inertia as it will cause high
ACTUATORS IN INDUSTRIAL AUTOMATION
consumption of power that in turn will activate the overload protection and stop the motor.
When stopping the motor with the freewheeling method the motor must be first stopped
before applying starting ramp, as it will cause activation of the overload protection.
2.6.2. Slip compensation
Slip compensation function can improve the dynamical behavior of an electrical drive. This
function is used mainly to control high power frequency controlled induction motors. When
the slip compensation function is turned on, the motor speed is held constant (with adjustment
of supply frequency) in case the load varies. So, this function can only be used in closed loop
control systems. Usually the slip compensation is chosen between 0…5 %. Overcompensation
can cause unstable operation of the motor. The slip compensation principle is shown in fig
2.24.
frequency
load
Fig. 2.25 Principle of slip compensation
2.6.3. IR compensation
IR compensation function is used to overcome the voltage drop ΔU in the stator, especially at
low speeds. IR compensation provides appropriate magnetic flux and a better start of the
motor. Voltage rise does not start from zero, but from an appropriate value ΔU = IR that is
usually set between 0…20 % [4]. The principle of IR compensation is shown in Fig. 2.26.
U
UN
IR
0
fN
f
Fig. 2.26 Compensation of voltage drop in a stator (IR compensation)
.
The following table gives an overview of possible IR compensation values for the ABB
frequency converter ACS 400 that controls 400 V three-phase motors Table 2.6.
ACTUATORS IN INDUSTRIAL AUTOMATION
Table 2.6 Suggested IR compensation values for 400V rotating machines
Power [kW]
3
7,5
15
22
37
IR comp. [V]
21
18
15
12
10
2.6.4. Torque compensation
Torque compensation function allows the frequency converter to choose a shape of a start
ramp that corresponds to the load type. If the relationship between the motor speed and torque
increases linearly, then a linear start ramp is used. In case of square proportional relationship
between the speed and torque, a parabolic start ramp is used. The shapes of start ramps are
shown in Fig. 2.27.
U
U
High load
Low load
IR
IR
f
f
b)
a)
Fig. 2.27 Start ramps a) Torque compensation, b) torque automatic compensation
2.7. An electrical drive with a soft starter
2.7.1. What is a soft starter?
An electrical soft starter is a thyristor - based AC voltage converter, used for soft starting of
induction motors by temporarily reducing the motor load and torque. This can be achieved by
controlling the rms value of voltage. Also, the soft starter allows dynamical braking and short
operation on low revolutions (up to 120 s). The acceleration and deceleration ramps can be
chosen by the user. In the heavy start conditions (high inner friction of machine) it is also
possible to perform a kick start. A soft starter of the company ABB is shown in Fig. 2.28.
ACTUATORS IN INDUSTRIAL AUTOMATION
Fig. 2.28 An ABB soft starter
The AC voltage converter consists of bidirectional (e.g. triac) or in antiparallel connected
semiconductor elements (e.g. thyristor). The rms value of a voltage is adjusted by controlling
the firing angles α of the semi- conductive switches. The larger the firing angle is, the lower
the output voltage value will be. A soft starter does not change the frequency, so the soft
starter is not suitable for rotational speed control. The principal of voltage regulation is shown
in Figure x. A dashed line shows the sine voltage and the electrical current in the grid. When
the thyristor is switched on with the firing angle α, then only a certain part of the half-period
voltage will be led to the motor (continuous line). The current will also differ from the perfekt
sine and flow only when voltage is applied (thyristors are switched on).
uv
uv
γ
α
α
t
λ
E
iv
t
λ
iv
φ
t
t
Fig. 2.29 Voltage and current diagrams of a voltage converter with different firing angles.
There are one - and three - phase voltage converters.
A one phase voltage converter (Fig. 2.30, a) consists of two antiparallel connected thyristors.
The rms value of voltage is adjusted by changing the firing angles of the thyristors. When
using fully controlled electrical switches like transistors, the a pulse width modulation
principle can be used to adjust the voltage level. One-phase voltage converters are used in
many home appliances and instruments to control the speed of washing machines, vacuum
cleaners, borers and other applications with universal motors. The same principal is also used
in the control of lighting (7).
A three-phase voltage converter (Fig. 2.30, b) consists of three one-phase voltage convertes. If
the load is connected with a neutral conductor N, then the operating principle is identical to
ACTUATORS IN INDUSTRIAL AUTOMATION
one phase voltage converter. If there is no connection to neutral (usually motors do not have
it), then the thyristors must be switched on in pairs that makes the control of a device more
complicated. Some soft starters use only two phase control and the third phase is switched
directly to the network. Danger: in this case when connecting a soft starter to the network
the directly connected phase can also become energized in the output of a soft starter.
N
~U
L
N
L1
L2
α
α
α
~3 U1
α
L3
α
α
α
α
R
L
3~
M
i2
E1
Rs Ls Es
Fig. 2.30 AC voltage converter: a) one-phase, b) three-phase
2.7.2. Connecting a soft starter to the load
There are two possible ways of connecting a soft starter: In Line connection, which is the
most common method, and In Delta connection. Not all soft starters support the In Delta
connection.
In Line connection The most common way to connect a soft starter is In line connection (Fig.
2.30 a, b). All three phases are sequentially connected to the overload protection, the
contactor and other devices. In this case the data of a soft starter must correspond to motor
data. For example, a motor that operates with a current of 100 A should be connected through
100 A soft starter and a 100 A switch.
In Delta connection In Delta connection allows the motor to be connected in delta, so that a
star-delta starting method can be used. Being in delta connected, only 58 % (1/
√3) of a soft
starter´s power is used. This means that a smaller (and cheaper) soft starter can be chosen for
the application. For example, a motor that operates with a current of 100 A can be connected
through a 58 A soft starter and 58 A switch. The six-wired cable is necessary for this solution.
ACTUATORS IN INDUSTRIAL AUTOMATION
a)
b)
Fig. 2.31 In Line and In Delta connection
2.7.3. Starting an induction motor with a soft starter
Start of an induction motor is an extremely important process that causes the ramp up and
acceleration of the motor that in turn causes increased torque and current consumption from
grid. Every start up process can be described by voltage- current or frequency-torque
characteristics. The soft starter allows user to choose the desired start up ramp. If a motor
must be run up in heavy conditions then it is possible to apply a short full voltage pulse to the
motor. The startup of a squirrel-cage induction motor requires 3-4 times higher current than
nominal, in heavy duty start 4-5 times higher. In comparison with the direct start, star-delta
start and start with a soft starter have many differences. Different startup methods are shown
in Fig. 2.32. The duration of a start up ramp must be chosen accordance to load, to rapid start
of a high load can cause effectuation of overload protection.
ACTUATORS IN INDUSTRIAL AUTOMATION
Star- delta start
Direct start
U
U
100 %
100 %
Start up with soft starter
U
Start up pulse
100 %
70 %
58 %
30 %
0
0
Area of ramp
control
0
t
t
t
Fig. 2.32 Voltage- time characteristics with different startup methods
The current and torque characteristics while the start up are shown in Fig. 2.33. As can be
seen, the characteristics are much smoother by the start up method with soft starter.
Current
I∆
I
T
6
5
4
Direct start
Soft starter
2
2
0
T∆
3
3
1
Torque
1
Star-delta
IY
Direct start
Soft starter
TY
Star-delta
ω
0
ω
Fig. 2.33 Current and torque characteristics depending on the rotational speed
One soft starter can be used to drive many motors simultaneously. The connection of
induction motors and a soft starter is shown in Fig x. Both motors are supplied from a
common grid through overcurrent protection, a contactor and a soft starter. Each motor has
separate overload protection with thermorelays. The motors can be stopped using free
wheeling of a motor, stopping with a ramp, stopping with a DC current or using dynamical
braking.
ACTUATORS IN INDUSTRIAL AUTOMATION
Electrical grid
Fuse
or
protective switch
Contactor
Soft starter
Overload
protection,
thermorelay
Motors
Fig. 2.34 Schematics of connecting
two induction motors with one
common soft starter
2.7.4. Protective functions of a soft starter
Nowadays soft starters are able to protect the motors and working machines that they are
controlling. Most common protections are: maximum current limit, brake in the input or
output phase, short current of thyristors, overheating, wrong frequency of supply voltage,
breakdown of a processor.
Some soft starters have also additional protective features, like a sudden stop of a rotor,
sudden disappearance of load, too long startup or with low-speed duty. These features are
very important, for example, a smooth control of a pump can help to avoid hydraulic kicks in
the pipes.
2.7.5. Selection of a soft starter
Normally a soft starter has to be selected according to the motor rated power. In certain cases
like heavy duty start or frequent startup, a larger soft starter must be chosen. Some selection
criteria are listed in the Table 2.7.
Table 2.7 Selection criteria of a soft starter
Normal start
Typical
Compressor,
Heavy duty start
bow
thruster, Conveyor (long), crusher, mixer,
ACTUATORS IN INDUSTRIAL AUTOMATION
applications
centrifugal pump, elevator
mill
Selection
criteria
Select the soft starter according to For soft starters designed for normal
start, select one size larger that the
the motor rated power
rated motor power
For soft starters designed for heavy
duty start, select according to the
rated motor power
2.7.6. Implementation of a soft starter: centrifugal fan
A centrifugal fan driven by a three-phase induction motor with a squirrel-cage rotor is shown
in Fig. 2.35.
Fig. 2.35 Centrifugal fan with an induction motor
Some applications are run up with a reduced starting torque in no load operation. The start up
process of a centrifugal fan can be simple because the startup is usually carried out in a closed
environment, however, the moment of inertia of a fan is large enough, so the startup time can
be quite long.
Centrifugal fans are very often driven by one or more drive belts. During the direct on line
start these belts have a tendency to slip, which is caused by the high moment of inertia of fans
(comparable to flywheel). The belts slip depending on whether the starting torque from the
motor is too high during the start sequence and the belts are not able to transfer these forces.
This typical problem gives high maintenance costs but also production losses when you need
to stop production to change belts and bearings.
The star-delta starter gives a lower starting torque but because of the load torque increasing
with the square of the speed, the motor torque will not be high enough in the star position to
accelerate the fan to the rated speed. When switching over to the delta position it will be both
a high transmission and current peak, often equal to values when making a direct on line start
or even higher, with a slipping belt as a result. It is possible to reduce the slip by stretching the
belts very hard.
One possible way to reduce the starting torque of the motor is to use a soft starter. If the soft
starter is selected correctly, the voltage is decreased to a low value at the beginning of the
start, low enough to avoid slip but high enough to start up the fan. The soft starter provides
the ability to adjust to any starting condition, both unloaded and fully loaded starts.
ACTUATORS IN INDUSTRIAL AUTOMATION
When selecting an appropriate soft starter the motor rated power and start up type have to be
considered. At normal start conditions the soft starter according to motor rated power has to
be chosen. At heavy duty start conditions an appropriate soft starter must be chosen with the
same parameters. It is also possible to select a soft starter for normal start, select a unit with
one size larger power rating than the motor and use an overload relay. Recommended initial
settings are: start ramp: 10 sec; stop ramp: 0 sec; initial voltage: 30 %; current limit is
recommended for use.
2.8. Stepper motor drive
2.8.1. Construction and principle of operation
The stepper motor is an electric machine which transforms dc energy pulses into the
mechanical energy of the motor shaft. Depending on the construction (bipolar or unipolar),
the stepper motors have 4, 6 or 8 connection terminals. Their assembly resembles
synchronous motors with the rotors rotating in accordance with the driving pulses given to the
stator windings and the rotating angle is determined by the number of pulses.
As the stepper motors are controlled digitally, they are ideally matched to discrete control
systems such as microprocessors. A certain step angle α corresponds to every impulse, and to
n impulses the turning angle γ = n · α. Accordingly, a stepper motor can be applied for
position control with an open loop, i.e. in systems without feedback. An advantage of the
stepper motors is that there is no need for feedback sensors during positioning. To increase
positioning accuracy, the motors can be built with increased number of poles. As the motor is
driven by a pulse train, then, at lower speeds, its motion can be jerky.
The stepper motors are feasible up to ~1 kW; they are also manufactured as linear motors.
The step angle α is determined by
α=
360°
N ph ⋅ m ⋅ Z
,
is the number of poles per phase, m the number of phases and Z the number of
where Nph
teeth.
A stepper motor together with the impulse generator and the power stage comprises the
stepper motor drive (Fig. 2.36). The driving impulses are generated on the basis of work
reference values (velocity, position, acceleration) and used for driving the power stage, based
on transistor switches. The transistor switches, in turn, commutate the stepper motor windings
so that the given actuator trajectory is followed as precisely as possible.
ACTUATORS IN INDUSTRIAL AUTOMATION
Fig. 2.36 Structure of a stepper motor drive
2.8.2. Types of stepper motors
According to construction, the stepper motors are divided into reluctance (reactive rotor) and
permanent magnet (active rotor) motors, a combined solution is also possible. The rotor of a
reluctance stepper motor consists of monolithic toothed electrotechnical steel core, after
cutting of the stator voltage the residual a magnetism disappears. Therefore, after applying
voltage to the motor, the magnetic flux can move through the core without any obstacles. The
rotor of the reluctance motor starts to move towards the minimum magnetic resistance, i.e.
towards the smallest air gap between the next tooth and the winding.
The stator of permanent magnet stepper motors (Fig. 2.37 a) consists of electrotechnical steel
and the rotor of alternating permanent magnet poles. The rotor is rotated by the magnetic field
produced in the stator.
As the reluctance stepper drives (Fig. 2.37 b) have no permanent magnets, they do not have
holding torque in the currentless state either. Their number of poles and accordingly the
positioning accuracy are limited. In hybrid stepper motors (Fig. 2.37 c), characteristics of both
constructions are present, with the rotors having both permanent magnets and the tooth track.
The term high-torque motors is used to describe the stepper motors, where extremely strong
rare earth magnetic materials are used to manufacture the rotors. Such magnets produce field
densities higher than usual.
Stator
a) with active rotor
b) with reactive rotor
Rotor
c) with hybrid rotor
Fig. 2.37 Types of stepper motors
2.8.3. Principle of operation
Unipolar stepper motors with 5 or 6 wires are usually wired as shown in the schematic in Fig.
2.38 a. The motor cross section shown belongs to a 30° step motor. Motor winding number 1
is distributed between the top and bottom stator pole, while motor winding number 2 is
distributed between the left and the right motor poles. The rotor is a permanent magnet with 6
poles, 3 south and 3 north, arranged around its circumference. For higher angular resolutions,
the rotor must have proportionally more poles. If the power to winding 1 is removed and
winding 2 is energized, the rotor will turn 30 degrees, or one step. To rotate the motor
continuously, power is applied to the two windings in sequence.
ACTUATORS IN INDUSTRIAL AUTOMATION
a) unipolar
b) bipolar
Fig. 2.38 Stepper motor connections
Bipolar permanent magnet (Fig. 2.38 b) and hybrid motors are constructed with exactly the
same mechanism as is used on unipolar motors (Fig. 2.38 a), but the two windings are wired
more simply, with no center taps. Thus, the motor itself is simpler but the drive circuitry
needed to reverse the polarity of each pair of motor poles is more complex. If the windings
are not switched over, the energized stepper permanent magnet or hybrid motor sustains the
holding torque, avoiding the unwanted movement caused by external forces.
2.8.4. Control of stepper motors
The simplest stepper motor control is the so-called WaveDrive mode, where only one winding
is energized at any instant. The disadvantage of this method is smaller produced torque, the
possible rotor positions of a two-phase two-tooth motor lay at 0°, 90°, 180° and 270°. By
energizing both windings simultaneously, the motor starts to operate in two-phase full-step
mode (Fig. 2.39 a) and produces more torque. The possible rotor positions lay between the
position angles of the WaveDrive mode, being accordingly 45°, 135°, 225° and 315°. By
combining both modes, i.e. by the alternate energizing of both windings, 8 positions are
possible. This is called a half-step mode (Fig. 2.39 b).
By supplying both windings with sine-cosine signal, correspondingly, the result is a nearly
ideal rotating magnetic field and rotational motion. The efficiency of a stepper motor increase
with the number of steps per turn, the energy consumption becomes more levelled, the danger
of step loss is mitigated and the motor operates more swiftly.
Supply of the stepper motor windings with discrete impulses based on sine-cosine rule results
in the so-called micro-step mode. An advantage of such a mode is the swiftness of motion, but
the disadvantage is decreased accuracy, bringing along the need for speed and position
sensors.
a)
ACTUATORS IN INDUSTRIAL AUTOMATION
b)
Fig. 2.39 Possible rotor positions in the full- (a) and half-step mode (b)
The stepper motor drive calculates the covered angle by counting the impulses. The acquired
value is incremental, to determine the absolute position, the counter must be initially reset by
defining the reference, which is usually implemented by limit switches.
The control part of a stepper motor drive consists of a microcontroller-based impulse
generator and the power stage (Fig. 2.40). To energise 4 winding terminals, the
microcontroller must issue a 4-bit train. With every bit train, the rotor rotates stepwise; a
constant rotation is implemented by a program cycle, where the number of steps is given as a
reference. An advantage of a stepper motor is the accuracy by determining the rotor position.
For example, if a wheel circumference is known, the step/cm value, or the number of steps for
covering 1 cm can be calculated. Accordingly, a work subroutine can be written, which uses
length as a reference, calculates the number of steps and transmits it over the ports to the
power stage as a cycle (Fig. 2.40).
ACTUATORS IN INDUSTRIAL AUTOMATION
Fig. 2.40 Power stage of a stepper motor drive
The input terminals of the power stage can be connected directly to the output port of the
microcontroller or to the parallel port of a computer:
Designation Explanation
CW/CCW
Gives the direction of the rotation (CW = ClockWise, CCW =
CounterClockWise).
Clock
By applying a short pulse, the rotor moves by a step. During the control cycle,
only this terminal is energized.
Half/Full
Grounded by default. By applying +5 V, the motor starts to operate in half-step
mode.
Enable
By grounding this terminal, the motor supply is cut off.
Vref
Vref (0...3 V) determines the maximal motor current. Vref = I m ⋅ RS
For example: if the maximal current Im = 0.5 A and selected Rs = 1 Ω, 0.5 V
must be applied to the terminal Vref. This can be accomplished by a
potentiometer.
RESET
Resets the stepper motor. To operate the motor, the +5 V voltage must be
sustained.
Control
To change the chopping mode. By active input, the current changes slowly
(phase chopping), for rapid changes, the terminal is grounded (inhibit
chopping).
Sync,
Home
These terminals usually remain unconnected.
As it becomes evident, only three terminals are necessary to operate a stepper motor in
desired direction and at a desired speed.
2.8.5. Loading a stepper motor
The torque produced by a stepper motor depends on the stepping frequency and accordingly
on the rotor speed. The motors provide the torque, defined on the nameplate only at relatively
low speeds. As the speed increases, the available torque decreases, which may lead to pullout, characterized by falling out of synchronism, or the step loss. Such a torque-speed
characteristic is caused by the winding inductances, where constant switchovers produce
counter-electromotive force, acting against the supply voltage; thus the current and torque
diminish. The modern stepper motors are usually fed from controlled current sources,
ensuring constant current and torque in a wider speed region. While keeping the current
constant, a 12 V nameplate motor terminal voltage may rise up to 30 V...40 V.
Certain limitations must be kept in mind while loading a stepper motor: (Fig. 2.41).
1. The holding torque Th is the load that can be applied to a statically energized standing
motor without making the rotor turn.
ACTUATORS IN INDUSTRIAL AUTOMATION
2. The start-stop frequency fs is the highest step frequency to start and stop an unloaded
stepper motor without falling out of the synchronism.
3. The motor speed is limited to the maximum slew rate fmax.
T [N·m]
4. The region, where the motor can start and stop without falling out of the synchronism, is
limited by the pull-in curve. The pull-out curve limits the region where the loaded motor
can operate without falling out of the synchronism.
Pull-out curve
Th
Pull-in curve
fs
fmax
f [Hz]
Fig. 2.41 Stepper motor speed-torque curve
2.8.6. Calculation example
The stator of a hybrid stepper motor consists of two poles and 100 teeth.
1. How big is the step angle in the full-step mode?
2. What must be the impulse frequency to make the motor rotate at n = 500 min-1 ?
Solution:
The step angle α =
360°
360°
=
= 0.9°
N ph ⋅ m ⋅ Z 2 ⋅ 2 ⋅ 100
,
n ⋅ 360° 500 ⋅ 360°
Pulse frequency f =
=
= 3.33 kHz
0.9 ⋅ 60
α ⋅ 60
.
2.8.7. Application example of a stepper drive
The gripper of an industrial robot is controlled by a stepper motor. The characteristic of the
whole system is shown in Fig. 2.42. The maximum aperture of an open gripper is 60 mm. The
width of the gripped object is 35 mm. How many pulses must be given to the motor to grip
this object with 6 N force if the gripper’s initial position is 53 mm?
ACTUATORS IN INDUSTRIAL AUTOMATION
N
Fig. 2.42 Characteristics of the industrial robot gripper
Solution:
As seen from the figure, the curve crosses the x-axis at 750 pulses and the maximal force of
10 N is reached by 1000 pulses. To make a 60 mm open gripper to grip a 35 mm wide object,
750
= 225 pulses are necessary.
60
1000 − 750
To reach the gripping force of 6 N, n force = 6 ⋅
= 150 pulses are necessary.
10
nclose = (53 − 35) ⋅
The total number of pulses nΣ = nclose + n force = 225 + 150 = 375 .
ACTUATORS IN INDUSTRIAL AUTOMATION
2.9. Servo drives
2.9.1. Construction and principle of operation
A servo drive consists of a servo motor and a servo amplifier (Fig. 2.43and Fig. 2.44). The
task of a servo amplifier (also known as servo converter) is to control the phase currents of a
servo motor. The word “servo” itself has a Latin origin, where “servus” means a servant, a
slave or a helper. In workbenches, the servo drives are usually used as auxiliary drives.
Fig. 2.43 Structural layout of a servo drive
Fig. 2.44 Servo amplifiers and motors
A servo amplifier controls the current led to the motor windings to achieve the torque and
speed defined by the specified work controller. A servo amplifier is comprised of the power
stage and controller as the main components.
ACTUATORS IN INDUSTRIAL AUTOMATION
The controller of the servo amplifer drives the power stage and ensures the accurate operation
of the drive at varying loads by a constant comparison of the reference and actual values
(current, position, speed).
The power stage is in principle a modulator based on power electronics, forming the servo
motor supply currents so that the given trajectory of the actuator becomes accurately
followed.
As different from the conventional electrical drives operating mostly at constant speed, the
operation of a servo drive is uneven as a rule. The acceleration to the rated speed often lasts
only a few milliseconds, shortly followed by a fast deceleration; during which the positioning
accuracy must remain within the limits of a few hundredths of millimetres.
The servo drives are in many cases subjected to the following requirements:
1) good positioning accuracy
2) good speed accuracy
3) wide control area
4) torque stability
5) sufficient overloadability
6) fast response
The advantages of servo drives over the other types of controlled drives are good dynamic
properties, accuracy and zero speed torque (i.e. large torque at near-zero torque) and a
compact construction with good power density. Under the dynamic properties, one means the
response time, the growth of which results in the increased speed, number of working cycles
and ultimately the productivity of the machines.
The required accuracy of a drive is often determined by the working machine. A modern fast
response drive must be adjustable to fulfill the requirements of different working machines.
The servo drives are the accurate and fast response drive systems, operating in a wide speed
region and performing well even during temporary overloads.
2.9.2. Motors of the servo drives
The servo motors are electrical motors, constructed under the priority of the dynamic
properties, i.e. fast acceleration and deceleration. The servo motors can be manufactured
either for ac (synchronous and induction motors) or for dc (brushed and brushless).
The permanent magnet synchronous motor (PMSM) is the best motor today to fulfill the
requirements set for the motors of the servo drives. High power density is achieved by the use
of permanent magnets such as neodymium-iron-boron (NdFeB), samarium-cobalt (SmCo)
and ferrite materials. To a less extent, induction servo motors are applied. The servo motors
are manufactured with a built-on position sensor, providing also speed feedback for the
controller (Fig. 2.45). There can also be a built-in mechanical brake, avoiding the shaft
movements of a de-energized drive due to external forces.
The construction of a brushless dc motor (BLDC) resembles the PMSM, the only difference
lying in the control method.
ACTUATORS IN INDUSTRIAL AUTOMATION
Signal
connections
Power
connections
Shaft
Position sensor
Nameplate
Mounting flange
Fig. 2.45 Exterior view of a typical servo motor
2.9.3. Synchronous motors
The synchronous motors are polyphase motors, where the magnetic fields of the stator and the
rotor rotate synchronously. The rotating magnetic field is produced by the proper spatial
layout of the stator windings and the temporal sequence of their currents. The rotational speed
of the magnetic field is
nd =
60 ⋅ f
,
p
where f is the ac supply voltage frequency and p the number of stator pole pairs.
The stator of a synchronous motor is comprised of the motor body, laminated magnetic
circuits and stator windings. The rotor is comprised of the shaft, laminated magnetic circuit
and the permanent magnets attached to it. To enhance the dynamic properties, the rotor’s
magnetic circuit is perforated. Thereby, the motor’s moment of inertia and the response time
become decreased.
If a synchronous motor is supplied with a constant frequency ac voltage, the rotational speed
of its rotor is equal to the rotational speed of the magnetic field. By applying load to the
motor, a delay occurs between the rotating fields of the rotor and the stator, characterised by
the so-called load angle α. The greater the delay, the higher the torque delivered by the motor.
If the load angle becomes equal to 90°, then the rotor poles are located between the two poles
of the stator and the torque delivered by the motor becomes maximal. If the delay continues to
increase, i.e. when the motor is overloaded, the torque starts to decrease again and the motor
becomes instable with the danger of stalling (Fig. 2.46).
ACTUATORS IN INDUSTRIAL AUTOMATION
T
-180 °
α
-90 °
0
+90 °
+180 °
T = f(sinα)
Fig. 2.46 Mechanical torque-angle characteristic of a constant-frequency ac-fed synchronous
motor
The speed-torque characteristics of a synchronous servo motor show three limitations, which
must be kept in mind while designing the drive (Fig. 2.46 Mechanical torque-angle
characteristic of a constant-frequency ac-fed synchronous motor Fig. 2.46).
1) The maximal torque of the motor is, besides other parameters, limited to the magnetic flux
of the permanent magnets. If the motor is overloaded and the stator current increases over
the allowable limit, then the magnets demagnetize, resulting in the torque loss. If the
motor and the servo amplifier are properly selected, then there is no danger of
demagnetization.
2) At higher speeds, the motor torque is limited to the maximal terminal voltage. The latter,
in turn, depends on the voltage of the power stage and the voltage drop in the connection
cable. Due to the counter-electromotive force provoked by the rotation, the motor is not
able to produce full torque at higher speeds. Accordingly, when the speed increases, the
counter-emf grows, the motor current and torque decrease.
3) The motor torque is limited to the thermal load as well. The average load torque is
calculated during the motor design process. The calculated torque must be less than the
zero-speed torque T0. Crossing the allowable thermal load (temperature) level results in
the demagnetization of the magnetic circuit or the isolation failures of the windings.
ACTUATORS IN INDUSTRIAL AUTOMATION
U = UN
Decrease of torque by U = 0.9 UN
80
Tmax
1)
70
[Nm]
T
2)
60
50
40
30
2)
2)
S3 (25 % ED)
S3 (40 % ED)
S3 (60 % ED)
S3 (100 % ED)
20
3)
10
nN = 2000 min-1 nN = 3000 min-1 nN = 4500 min-1
1000
2000
3000
4000
[min-1]
Fig. 2.47. Typical mechanical characteristics of a synchronous servo motor
2.9.4. Brushless dc motors
The construction of a BLDC resembles that of a PMSM. The stator windings are similar to the
polyphase ac machine and the rotor is comprised of one or more permanent magnet pole pairs.
As differently from a brushed machine, the load angle is not kept at 90° with the help of a
mechanical commutator, but with the precisely timed switching of the power stage
semiconductor switches. For precise switching, the exact position of the rotor is necessary,
determined either by the Hall or optical sensors. The electronic commutator is a part of the
servo amplifier power stage.
Electronic commutator
M
3~
DC
Position sensor
Logic circuit
Fig. 2.48 BLDC = PMSM + electronic commutator
ACTUATORS IN INDUSTRIAL AUTOMATION
2.9.5. Control and feedback in the servo drives
2.9.5.1. Servo amplifiers
A servo amplifier (also known as the servo converter or servo controller) controls the motor
currents to achieve the defined movement reference values such as speed, torque and position.
The advantages of compact servo amplifiers are their small dimensions and mass together
with a compact construction. There is no need for additional connections, which is the case of
the modular controllers.
The modular digital servo amplifiers have one supply module and several coordinate servo
modules (Fig. 2.49). Their advantages become obvious when servo drives with multiple
degrees of freedom are necessary; in this case several coordinate modules are fed from one
supply module. The rated power of the supply module is selected on the bases of the sum
power of the modules and their duty ratio. Other components of such a modular servo system
can be the braking and regeneration circuits. Another advantage of a multi-coordinate servo
drive is that several modules connected to a common dc link can exchange energy
independently, thus saving the supply energy from the mains.
A servo drive gets its work references from a control program, driving the operation of the
actuator. The control program can be uploaded to the internal memory of the servo amplifier.
The servo references are principally the data samples of the actuator movement, i.e. of the
position, speed acceleration and sometimes of the torque. The movement trajectory is
subjected to several hardware limitations, therefore it must be “conditioned” before
transferred to the actuator. For example, the actuator’s position must not exceed certain limits,
the maximal speed and acceleration/deceleration rate of a motor are limited by its construction
(Fig. 2.50).
ACTUATORS IN INDUSTRIAL AUTOMATION
Work
reference
Servo amplifier
Controller
Supply module
400 V / 230 V
50Hz
Coordinate
module „X”
Coordinate
module „Y”
Coordinate
module „Z”
M
3~
M
3~
M
3~
Fig. 2.49 A modular three-coordinate servo amplifier
Limitations by software
Position reference
Trajectory generator
Speed reference
Max speed
Accel reference
Max accel
Fig. 2.50 Generation of the actuator trajectory reference
Actuator trajectory reference
ACTUATORS IN INDUSTRIAL AUTOMATION
2.9.5.2. Feedback
As a standard, the servo motors are supplied together with the resolvers, determining the
shaft’s absolute position over one rotation (360°). Sincos sensors (one- or multi-turn absolute
sensors) and incremental encoders are also compatible to the digital servo controllers. Such
sensors ensure enhanced accuracy and response.
A resolver’s operation is based on the principle of a rotary transformer (Fig. 2.51) comprised
of the rotor and stator windings. The two windings on the stator are spatially shifted by 90°
and their sinusoidal output voltages ud and uq are used to determine the rotor’s speed and
position increment. For a contactless signal transmission, additional windings may exist on
the resolver’s stator and the rotor.
Construction
Formulae
Signals
^
u = U ⋅ sin ϖ ⋅ t
u d = u ⋅ cos γ
u q = u ⋅ sin γ
Fig. 2.51 Principle of operation of a resolver
The signals of the four output channels of an incremental encoder (Fig. 2.52) are used for the
accurate positioning of a drive. The outputs are the signals A and B together with their
inversions A and B. The incremental encoder issues a certain number of pulses per rotation,
the signals A and B being 90° shifted in time. The motor rotates clockwise, when the rising
edges of the channel A precede the rising edges of the channel B and vice versa. While the
incremental encoders detect the position increment rather than its absolute value, a reference
position to calculate the motion must be defined initially. The reference might be set by an
index on a separate code track.
ACTUATORS IN INDUSTRIAL AUTOMATION
A
A
180°
360°
90°
B
B
90°
C
C
Fig. 2.52 Signals of an incremental encoder
An incremental encoder consists of a light source and an opaque code disc with perforated
code tracks. The photo sensor catches the light, which has penetrated through the perforations
and outputs a sinusoidal signal, which must be converted into a squared pulse train (Fig.
2.53). The shaft’s speed is determined by the encoder output pulse frequency (Fig. 2.54).
Index
Code track
Code disc
Squaring circuitry
Photosensor
Light source
Mask
Fig. 2.53 Construction of an encoder
Speed
Encoder output
Fig. 2.54 The dependency between the encoder’s output and the speed
ACTUATORS IN INDUSTRIAL AUTOMATION
2.9.6. Servo application examples
Positioning is a classical task of a servo drive. Within a fraction of a second, the drive must
obtain its rated speed and decelerate rapidly after a moment for reaching the reference
position with the utmost accuracy.
The speed control is used to guarantee a smooth operation of a drive. The acceleration and
deceleration rates are defined by corresponding ramps. The subroutines inside an industrial
controller enable to apply several speed curves according to the stage of the process.
To position a spindle, the motor must be halted for a while and the axis moved to a separate
position for changing the tool or the workpiece.
The synchronization of axes is necessary in the applications, where the movement in one
coordinate depends on the other.
The electronic cam disc is used in the mechanisms with variable transmission, replacing the
older mechanical solution with a cam disc on the main shaft and the following axis.
The flying saw is a function used for processing the moving material, like on a conveyor:
cutting, printing, welding etc. During processing, the axis must follow the workpiece and
return to the initial position after processing.
The torque control is implemented by an accurate and dynamic current regulation. A servo
drive is capable of providing full torque at low speeds, being therefore applied in the winding
machines
and
presses.
ACTUATORS IN INDUSTRIAL AUTOMATION
2.10.
Self check
5. What is an electric drive?
1. Electro-mechanical system
2. Driven equipment
3. Electrical motor with a regulator
6. What surrounds a wire when an electrical current flows through it?
1. Magnetic field
2. Electrical field
3. Energy field
7. What is used in electrical motors in order to change the direction of armature current?
1. Mechanical commutator
2. Semiconductor commutator
3. Frequency commutator
8. What kind of an AC motor is most often used in the industry?
1. Asynchronous motor
2. Reluctance motor
9. What are iron losses?
1. Losses occurring in the magnetic material of an electrical motor
2. Losses occurring in the coil of an electrical motor
3. Losses caused by friction inside electrical motor
10. What does the code IP 45 mean according to the EN 60529 standard?
1. Protected against solid foreign objects: Tools and objects with the diameter over 1
mm; protection against water: Projected water jets from any direction
2. Protected against solid foreign objects: Fingers and objects with the diameter over 12
mm; protection against water: Projected water jets from any direction
3. Protected against solid foreign objects: Tools and objects with the diameter over 1
mm; protection against water: Running water
ACTUATORS IN INDUSTRIAL AUTOMATION
11. What is shown in the picture?
1. Connecting a power cable to the terminals of an asynchronous motor using star
connection
2. Connecting a power cable to the terminals of an asynchronous motor, using delta
connection.
3. Terminals for a three phase transformer
12. How many times, as compared to the rated current, will the starting current of an
asynchronous electric motor increase during direct starting?
1. 14 times
2. 6 times
3. 2 times
13. What are the most common methods used for stopping an asynchronous electric motor?
1. Converting electrical energy to heat by leading the electric current through the
stopping resistor.
2. Giving the electrical energy back to the electrical grid where the energy can be used by
other electrical equipment. This is called electrical recuperation.
3. Mechanical braking
14. What does a frequency converter consist of?
1. Uncontrolled tree phase rectifier, DC link and inverter
2. Uncontrolled tree phase rectifier, exciter and inverter
3. Uncontrolled tree phase rectifier, DC link and load resistor
15. What is shown in the picture?
ACTUATORS IN INDUSTRIAL AUTOMATION
1. Block diagram of voltage - frequency control of asynchronous motor
2. Block diagram of voltage commutation
3. Block diagram of braking an asynchronous motor
16. What kinds of devices have a load characteristic as shown below?
1. Devices such as fans, pumps and centrifuges which have a square proportional
relationship between the speed and the load.
2. Devices such as mills, rolling mills and calenders with linearly increasing relationship
between the speed and the load.
3. Devices such as borers, milling cutters and winders with constant relationship between
the speed and the load.
17. What is the slip compensation used for?
1. Slip compensation function can improve the dynamical behavior of an electrical drive
2. Slip compensation is used for braking small induction motors
3. Slip compensation is used to improve the selectivity of electrical drives
18. Why are soft starters used with induction motors?
1. To achieve a soft start (lower inrush current)
2. For braking and stopping and to save energy when used a with varying load.
3. To change the rotation angle of a motor.
ACTUATORS IN INDUSTRIAL AUTOMATION
19. In Line is a…
1. Way to connect a soft starter.
2. Connection where 3 phases are connected in parallel with overload protection relay.
3. Connection that enables to connect the motor in delta.
20. What kind of starting is it
A
U
U
100 %
100 %
B
C
U
Käivitusimpulss
100 %
70 %
58 %
30 %
0
0
0
t
Rambi
reguleerimisala
t
t
1. A-direct on-line startup; B – star-delta startup; C- start up with a soft starter
2. A- star-delta startup; B – direct on-line startup; C- start up with a soft starter
3. A- startup with a soft starter; B star-delta startup; C- direct on-line a start up
21. A stepper motor can be either?
1. A brushless dc motor or a servo motor
2. A shunt wound or an induction motor
3. A permanent magnet, a hybrid or a reluctance motor
4. A hollow rotor or a perforated rotor motor
5. An induction motor or a synchronous motor
22. How does speed influence the stepper motor torque?
1. The torque increases with the speed
2. The torque decreases with the speed
3. The torque is proportional to the speed squared
4. The torque is reciprocal to the speed
5. The torque does not depend on the motor's speed
23. What is the basic servo motor type?
1. Permanent magnet synchronous motor
ACTUATORS IN INDUSTRIAL AUTOMATION
2. Stepper motor
3. Single-phase induction motor
24. The mostly used speed/position feedback devices in servo systems are?
1. Resolvers and incremental encoders
2. Cam switches
3. Analogue multi-turn potentiometers
4. Tachogenerators
5. Single-turn analogue potentiometers