I.
Title:
Introduction of Industrial Motor Control
a. Define of Motor Control
b. Advantages of Motor Control
II.
Objectives:
At the end of the lesson the student are expected to:
a. Identify the Industrial Motor Control
b. Importance of Industrial Motor Control
c. Advantage of Industrial Motor Control
d. Disadvantage of Industrial Motor Control
III.
Information
Basics of Industrial Motor Control: Part 1: BASIC COMPONENTS
FOR CONTROL CIRCUITS
Introduction
Industrial control, in its broadest sense, encompasses all the methods used to control the
performance of an electrical system. When applied to machinery, it involves the starting,
acceleration, reversal, deceleration, and stopping of a motor and its load. In this section we will
study the electrical (but not electronic) control of 3-phase alternating current motors. Our study is
limited to elementary circuits because industrial circuits are usually too intricate to explain briefly.
However, the basic principles covered here apply to any system of control, no matter how complex
it may appear to be.
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Control devices
Every control circuit is composed of a number of basic components connected together to achieve
the desired performance. The size of the components varies with the power of the motor, but the
principle of operation remains the same. Using only a dozen basic components, it’s possible to
design control systems that are very complex. The basic components are the following:

1. Disconnecting switches

2. Manual circuit breakers

3. Cam switches

4. Pushbuttons

5. Relays

6. Magnetic contactors

7. Thermal relays and fuses

8. Pilot lights

9. Limit switches and other special switches

10. Resistors, reactors, transformers, and capacitors
The ensuing list of Basic Components for Control Circuits illustrates these devices, and states their
main purpose and application. Fuses are not included here because they are protective devices
rather than control devices. The symbols for these and other devices are given below.
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BASIC COMPONENTS FOR CONTROL CIRCUITS
Disconnecting switches
A disconnecting switch isolates the motor from the power source. It consists of 3 knife-switches
and 3 line fuses enclosed in a metallic box. The knife-switches can be opened and closed
simultaneously by means of an external handle. An interlocking mechanism prevents the hinged
cover from opening when the switch is closed. Disconnecting switches (and their fuses) are
selected to carry the nominal full-load current of the mo tor, and to withstand short-circuit currents
for brief intervals.
Manual circuit breakers
A manual circuit breaker opens and closes a circuit, like a toggle switch. It trips (opens)
automatically when the current exceeds a predetermined limit. After tripping, it can be reset
manually. Manual circuit breakers are often used instead of disconnecting switches because no
fuses have to be replaced.
above: Three-phase circuit breaker, 600V, 100A.
Cam switches
A cam switch has a group of fixed contacts and an equal number of moveable contacts. The
contacts can be made to open and close in a preset sequence by rotating a handle or knob. Cam
switches are used to control the motion and position of hoists, callenders, machine tools, and so
on.
above: Three-phase surface-mounted cam switch..
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Push-buttons
A pushbutton is a switch activated by finger pressure.
Two or more contacts open or close when the button is depressed. Push-buttons are usually spring
loaded so as to return to their normal position when pressure is re moved.
--4. Mechanical-interlocked push buttons with NO (normally open) and NC (normally closed)
contacts; rated to interrupt an ac current of 6 A one million times.
Control relays
A control relay is an electromagnetic switch that opens and closes a set of contacts when the relay
coil is energized. The relay coil produces a strong magnetic field which attracts a movable armature
bearing the contacts.
Control relays are mainly used in low-power circuits.
They include time-delay relays whose contacts open or close after a definite time interval. Thus, a
time-delay closing relay actuates its contacts after the relay coil has been energized. On the other
hand, a time-delay opening relay actuates its contacts some time after the relay coil has been deenergized.
--5. Single-phase relays: 25A, 115/230V and 5A, 115V.
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Thermal relays
A thermal relay (or overload) is a temperature sensitive device whose contacts open or close when
the motor current exceeds a preset limit. The current flows through a small, calibrated heating
element which raises the temperature of the relay. Thermal relays are inherent time-delay devices
because the temperature cannot follow the instantaneous changes in current.
above: Three-phase thermal relay with variable current setting, 6A to 10A.
Magnetic contactors
A magnetic contactor is basically a control relay designed to open and close a power circuit. It
possesses a relay coil and a magnetic plunger, which carries a set of movable contacts. When the
relay coil is energized, it attracts the magnetic plunger, causing it to rise quickly against the force
of gravity. The movable contacts come in contact with a set of fixed contacts, thereby closing the
power circuit. In addition to the power contacts, one or more normally open or normally closed
auxiliary contacts are usually available, for control purposes. When the relay coil is de-energized,
the plunger falls, thereby opening and closing the respective contacts. Magnetic contactors are
used to control motors ranging from 0.5 hp to several hundred horsepower. The size, dimensions,
and performance of contactors are standardized.
Three-phase magnetic contactor rated 50 hp, 575V, 60 Hz. Width: 158 mm; height: 155 mm; depth:
107 mm; weight: 3.5 kg. (Siemens)
Pilot lights
A pilot light indicates the on/off state of a remote component in a control system.
--8. Pilot light, 120 V, 3 W mounted in a start-stop push button station. (Siemens)
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Limit switches and special switches
A limit switch is a low-power snap-action device that opens or closes a contact, depending upon
the position of a mechanical part. Other limit switches are sensitive to pressure, temperature, liquid
level, direction of rotation, and so on.
--9a Limit switch with one NC contact; rated for ten million operations; position accuracy: 0.5
mm. (Square D)
--9b Liquid level switch. (Square D)
Proximity detectors
Proximity detectors are sealed devices that can detect objects without coming in direct contact with
them.
Their service life is independent of the number of operations. They are wired to an external dc
source and generate an alternating magnetic field by mean" of an internal oscillator. When a metal
object comes within a few millimeters of the detector, the magnetic field decreases, which in turn
causes a dc control current to flow. This current can be used to activate another control device,
such as a relay or a programmable logic controller.
Capacitive proximity detectors, based on a similar principle but generating an ac electric field, are
able to detect nonmetallic objects, including liquids.
--10 Proximity detector to monitor the loading of a conveyor belt (Telemecanique, Groupe
Schneider)
In order to understand the sections that follow the legends should be read before proceeding
further.
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Normally open and normally closed contacts
Control circuit diagrams always show components in a state of rest, that is, when they are not
energized (electrically) or activated (mechanically). In this state, some electrical contacts are open
while others are closed. They are respectively called normally open contacts (NO) and normally
closed contacts (NC) and are designated by the following symbols: normally open contact (NO)
normally closed contact (NC)
Relay coil exciting current
When a magnetic contactor is in its de-energized or open position, the magnetic circuit has a very
long air gap, compared to when the contactor is closed.
Consequently, in the case of an ac contactor the inductive reactance of the relay coil is much lower
when the contactor is open than when it’s closed.
Because the coil is excited by a fixed ac voltage, the magnetizing current is much higher in the
open than in the closed contactor position. In other words, a considerable inrush current is drawn
by the relay coil at the moment it’s excited. This places a heavier than expected duty on auxiliary
contacts that energize the coil.
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Example +++1
A 3-phase NEMA size 5 magnetic contactor rated at 270 A, 460 V possesses a 120 V, 60 Hz relay
coil.
The coil absorbs an apparent power of 2970 VA and 212 VA, respectively, in the open and closed
contactor position. Calculate the following:
a. The inrush exciting current
b. The normal, sealed exciting current
c. The control power needed to actuate the relay coil compared to the power handled by the
contactor Solution
a. The inrush current in the relay coil is:
I=S/E = 2970/120 = 24.75 A
b. The normal relay coil current when the contactor is sealed (closed) is:
I= S/E 2121120 = 1.77 A
c. The steady-state apparent control power needed to actuate the relay coil is 212 VA. The apparent
power that the contactor can handle is
S = EI sqr-rt (3 ) = 460 X 270 sqr-rt (3 )
= 215 120 VA
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GRAPHIC SYMBOLS FOR ELECTRICAL DIAGRAMS
1. terminal; connection 2; conductors crossing 3. conductors connected 4. three conductors 5. plug;
receptacle 6. separable connector 7. ground connection; arrester 8. disconnecting switch 9.
normally open con tact (NO) 10. normally closed contact (NC) 11. pushbutton NO; NC 12. circuitbreaker 13. single-pole switch; three-way switch 14. double pole double throw switch 15. fuse 16.
thermal overload element 17. relay coil 18. resistor 19. winding, inductor or reactor 20. capacitor;
electrolytic capacitor 21. transformer 22. current transformer; bushing type 23. potential
transformer 24. dc source (general) 25. cell 26. shunt winding 27. series winding; commutating
pole or compensating winding 28. motor; generator (general symbols) 29. dc motor; dc generator
(general symbols) 30. ac motor; ac generator (general symbols) 32. 3-phase squirrel-cage induction
motor; 3-phase wound-rotor motor 33. synchronous motor; 3-phase alternator 34. diode 35.
thyristor or SCR 36. 3-pole circuit breaker with magnetic overload device, drawout type 37. dc
shunt motor with commutating winding; permanent magnet dc generator 38. magnetic relay with
one NO and one NC contact. 39. NPN transistor 40. PNP transistor 41. pilot light
Thus, the small control power (212 VA) can control a load whose power is 215 120/212 = 1015
times greater.
Control diagrams
A control system can be represented by four types of circuit diagrams. They are listed as follows,
in order of increasing detail and completeness:

• block diagram

• one-line diagram

• wiring diagram

• schematic diagram
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--11 Block diagram of a combination starter.
600 V / 3-phases --> fused disconnecting switch --> thermal overload relay
--12 One-line diagram of a combination starter.
--13 Wiring diagram of a combination starter.
--14 Schematic diagram of a combination starter.
A block diagram is composed of a set of rectangles, each representing a control device, together
with a brief description of its function. The rectangles are connected by arrows that indicate the
direction of power or signal flow. A one-line diagram is similar to a block diagram, except that the
components are shown by their symbols rather than by rectangles. The symbols give us an idea of
the nature of the components; consequently, one-line diagrams yield more information.
A list of typical symbols is displayed in Table 20A. The lines connecting the various components
represent two or more conductors. A wiring diagram shows the connections between the
components, taking into account the physical location of the terminals and even the color of wire.
These diagrams are employed when in stalling equipment or when troubleshooting a circuit.
A schematic diagram shows all the electrical connections between components, without regard to
their physical location or terminal arrangement.
This type of diagram is indispensable when troubleshooting a circuit or analyzing its mode of
operation ---14). In the sections that follow, this is the kind of diagram we will be using.
The reader should note that the four diagrams all relate to the same control circuit. The symbols
used to designate the various components are given in Table 20B.
Starting methods
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Three-phase squirrel-cage motors are started either by connecting them directly across the line or
by applying reduced voltage to the stator. The starting method depends upon the power capacity
of the supply line and the type of load.
Across-the-line starting is simple and inexpensive. The main disadvantage is the high starting
current, which is 5 to 6 times the rated full-load current.
It can produce a significant line voltage drop, which may affect other customers connected to the
same line. Voltage-sensitive devices such as incandescent lamps, television sets, and highprecision machine tools respond badly to such voltage dips.
Mechanical shock is another problem that should not be overlooked. Equipment can be seriously
dam aged if full-voltage starting produces a hammer-blow torque. Conveyor belts are another
example where sudden starting may not be acceptable.
In large industrial installations we can some times tolerate across-the-line starting even for motors
rated up to 10000 hp. Obviously, the fuses and circuit breakers must be designed to carry the
starting current during the acceleration period.
A motor control circuit contains two basic components: a disconnecting switch and a starter. The
disconnecting switch is always placed between the supply line and the starter. The switch and
starter are sometimes mounted in the same enclosure to make a combination starter. The fuses in
the disconnecting switch are rated at about 3.5 times full load current; consequently, they don’t
protect the motor against sustained overloads. Their primary function is to protect the motor and
supply line against catastrophic currents resulting from a short circuit in the motor or starter or a
failure to start up.
Under normal start-up conditions, the fuses don’t have time to blow, even though the initial current
is 6 to 7 times full-load current. The fuse rating, in amperes, must comply with the requirements
of the National Electric Code.
In some cases the disconnecting switch and its fuses are replaced by a manual circuit breaker.
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Manual across-the-line starters
Manual 3-phase starters are composed of a circuit breaker and either two or three thermal relays,
all mounted in an appropriate enclosure. Such starters are used for small motors (10 hp or less) at
voltages ranging from 120 V to 600 V. The thermal relays trip the circuit breaker whenever the
current in one of the phases exceeds the rated value for a significant length of time.
--15 Manual starters for single-phase motors rated 1 hp (0.75 kW); left: surface mounted; center:
flush mounted; right waterproof enclosure. (Siemens)
Single-phase manual starters are built along the same principles but they contain only one thermal
relay. The thermal relays are selected for the particular motor that is connected to the starter.
Magnetic across-the-line starters
--16a Three-phase across-the-line magnetic starter, 30 hp, 600 V, 60 Hz.
--16b Schematic diagram of a 3-phase across-the-line magnetic starter.
--17 Typical curve of a thermal overload relay, showing trip ping time versus line current. The
tripping time is measured from cold-start conditions. If the motor has been operating at full-load
for one hour or more, the tripping time is reduced about 30 percent.
--18 Three-phase across-the-line combination starter, 150 hp, 575 V, 60 Hz. The protruding knob
controls the disconnecting switch; the pushbutton station is set in the transparent polycarbonate
cover. (Klockner-Moeller)
Magnetic across-the-line starters are employed whenever a motor has to be controlled from a
remote location. They are also used whenever the power rating exceeds 10 kW. ---16 shows a
typical magnetic starter and its associated schematic diagram. The disconnecting switch is
external to the starter. The starter has three main components: a magnetic contactor, a thermal
relay, and a control station. We now de scribe these components.
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1. The magnetic contactor A possesses three heavy contacts A and one small auxiliary contact Ax.
As can be seen, these contacts are normally open.
Contacts A must be big enough to carry the starting current and the nominal full-load current
without overheating. Contact Ax is much smaller because it only carries the current of relay coil
A. The relay coil is represented by the same symbol (A) as the contacts it controls. Contacts A and
Ax remain closed as long as the coil is energized.
2. The thermal relay T protects the motor against sustained overloads.
* The relay comprises three individual heating elements, respectively connected in series with the
three phases. A small, normally closed contact T forms part of the relay assembly. It opens when
the thermal relay gets too hot and stays open until the relay is manually reset.
The current rating of the thermal relay is chosen to protect the motor against sustained overloads.
Contact T opens after a period of time that depends upon the magnitude of the overload current.
Thus, the tripping time as a multiple of the rated relay current. At rated current (multiple I), the
relay never trips, but at twice rated current, it trips after an interval of 40s. The thermal relay is
equipped with a reset button enabling us to re close contact T following an overload. It’s preferable
to wait a few minutes before pushing the button to allow the relay to cool down.
3. The control station, composed of start-stop push buttons, may be located either close to, or far
away from the starter. The pilot light is optional.
Referring to the schematic, to start the motor we first close the disconnecting switch and then
depress the start button. Coil A is immediately energized causing contacts A and Ax to close. The
full line volt age appears across the motor and the pilot light is on.
When the pushbutton is released it returns to its normal position, but the relay coil remains excited
be cause auxiliary contact Ax is now closed. Contact Ax is said to be a self-sealing contact.
To stop the motor, we simply push the stop button, which opens the circuit to the coil. In case of
a sustained overload, the opening of contact T produces the same effect.
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It sometimes happens that a thermal relay will trip for no apparent reason. This condition can occur
when the ambient temperature around the starter is too high. We can remedy the situation by
changing the location of the starter or by replacing the relay by another one having a higher current
rating. Care must be taken before making such a change, because if the ambient temperature
around the motor is also too high, the occasional tripping may actually serve as a warning.
---18 shows a typical combination starter.
---19 shows another combination starter equipped with a small step-down transformer to ex cite
the control circuit. Such transformers are always used on high-voltage starters (above 600 V)
because they permit the use of standard control components, such as pushbuttons and pilot lights
while reducing the shock hazard to operating personnel.
---20 shows a medium-voltage across-the line starter for a 2500 hp, 4160 V, 3-phase, 60 Hz
squirrel-cage motor. The metal compartment houses three fuses and a 3-phase vacuum contactor.
The contactor can perform 250 000 operations at full-load before maintenance is required. The
120 V holding coil draws 21.7A during pull-in, and the current drops to 0.4A during normal
operation.
Closing and opening times of the main contactor are respectively 65 ms and 130 ms.
---21 shows a special combination starter that can be reset remotely following a short-circuit. Its
distinguishing feature is that it’s programmable and requires no fuses. The sophisticated contactor
is designed to interrupt short-circuit currents in less than 3 ms, which is comparable to that offered
by HRC fuses. The contactor acts also as a disconnecting switch and consequently the overall size
is much smaller than more conventional combination starters.
--19 Three-phase across-the-line combination starter rated 1 00 hp, 575 V, 60 Hz. The isolating
circuit breaker is controlled by an external handle. The magnetic contactor is mounted in the
bottom left-hand corner of the waterproof enclosure. The small 600 V/120 V transformer in the
lower right-hand corner supplies low voltage power for the control circuit. (Square D)
--20 Three-phase 5 kV starter for a 2500 hp cage motor.
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The medium- and low-voltage circuits are completely isolated from each other to ensure safety.
The compartment is 2286 mm high, 610 mm wide, and 813 mm deep. The entire starter weighs
499 kg. (Square D, Groupe Schneider)
Inching and jogging
In some mechanical systems, we have to adjust the position of a motorized part very precisely. To
accomplish this, we energize the motor in short spurts so that it barely starts before it again comes
to a halt.
A double-contact pushbutton J is added to the usual start/stop circuit. This arrangement permits
conventional start-stop control as well as jogging, or inching. The following description shows
how the control circuit operates.
If the jog button J is in its normal position (not depressed) relay coil A is excited as soon as the
start button is depressed. Sealing contact Ax in the main contactor closes and so the motor will
continue to turn after the start button is released. Thus, the control circuit operates in the same
way.
Suppose now that the motor is stopped and we depress the jog button. This closes contacts 3, 4
and relay coil A is excited. Contact Ax closes, but contacts 1, 2 are now open and the closure of
Ax has no effect. The motor will pick up speed so long as the jog button is depressed. However,
when it’s re leased, coil A will become de-energized and contactor A will drop out, causing Ax to
open. Thus, when contacts 1, 2 are again bridged, the motor will come to a halt. Thus, by
momentarily depressing the jog button we can briefly apply power to the motor.
Jogging imposes severe duty on the main power contacts A because they continually make and
break currents that are 6 times greater than normal.
It’s estimated that each impulse corresponds to 30 normal start-stop operations. Thus, a contactor
that can normally start and stop a motor 3 million times, can only jog the motor 100000 times,
because the contacts have to be replaced. * Furthermore, jogging should not be repeated too
quickly, because the intense heat of the breaking arc may cause the main contacts to weld together.
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Repeated jogging will also overheat the motor. When is required, the contactor is usually selected
to be one NEMA size larger than that for normal duty.
Reversing the direction of rotation
We can reverse the direction of rotation of a 3-phase motor by interchanging any two lines. This
can be done by using two magnetic contactors A and B and a, manual 3-position cam switch as
shown in ---23.
When contactor A is closed, lines L1 , and L3 are connected to terminals A, B, C of the motor. But
when contactor B is closed, the same lines are connected to motor terminals C, B, A In the forward
direction, the cam switch engages contact 1, which energizes relay coil A, causing contactor A to
close. **
--21 Special self-protected starter rated at 40 hp, 460 V, 60 Hz. In addition to a short-circuit
capability of 42 kA at 460 V, it features adjustable thermal and magnetic trip settings. Overall
dimensions: 243 mm high, 90 mm wide, 179 mm deep. (Telemecanique, Groupe Schneider)
--22 Control circuit and pushbutton station for start-stop job operation. Terminals 8, L3 correspond
to terminals 8, L3 in ---13.
--23a Simplified schematic diagram of a reversible magnetic starter.
--23b Three-position cam switch in ---23a. (Siemens)
--23c Emergency stop pushbutton in ---23a. (Square D)
-A magnetic contact has an estimated mechanical life of about 20 million open/close cycles, but the
electrical contacts should be replaced after 3 million normal cycles.
The contacts and relay coils may be designated by any appropriate letters. Thus, the letters F and
R are often used to designate forward and reverse operating components. In this guide we have
adopted the letters A and B mainly for reasons of continuity from one circuit to the next.
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---24a Simplified schematic diagram of a starter with plugging control.
To reverse the rotation, we move the cam switch to position 2. However, in doing so, we have to
move past the off position (0). Consequently, it’s impossible to energize coils A and B
simultaneously.
Occasionally, however, a mechanical defect may prevent a contactor from dropping out, even after
its relay coil is de-energized. This is a serious situation, because when the other contactor closes,
a short circuit results across the line. The short-circuit current could easily be 50 to 500 times
greater than normal, and both contactors could be severely dam aged. To eliminate this danger, the
contactors are mounted side by side and mechanically interlocked, so as to make it physically
impossible for both to be closed at the same time. The interlock is a simple steel bar, pivoted at
the center, whose extremities are tied to the movable armature of each contactor.
During an emergency, pushbutton U, equipped with a large red bull's-eye, can be used to stop the
motor (---23c). In practice, operators find it easier to hit a large button than to turn a cam switch
to the off position.
--24b Typical zero-speed switch for use in ---24a.
Plugging
We have already seen that an induction motor can be brought to a rapid stop by reversing two of
the lines (Section 14.8). However, to prevent the motor from running in reverse, a zero-speed
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switch must open the line as soon as the machine has come to rest. The circuit of ---24a shows the
basic elements of such a plugging circuit. The circuit operates as follows:
1. Contactor A is used to start the motor. In addition to its 3 main contacts A, it has 2 small auxiliary
contacts Ax I and
2. The start pushbutton has one NO contact 1, 2 and one NC contact 3, 4 which operate together.
Thus, contact 3, 4 opens before contact 1, 2 closes.
3. Contactor B is used to stop the motor. It’s identical to contactor A, having 2 auxiliary contacts
Bx_1 and Bx_2 in addition to the 3 main contacts B.
4. The stop pushbutton is identical to the start pushbutton. Thus, when it’s depressed contact 7, 8
opens before contact 5, 6 closes.
5. Contact F-C of the zero-speed switch is normally open, but it closes as soon as the motor turns
in the forward direction. This prepares the plugging circuit for the eventual operation of coil B.
6. Contacts Ax 1 and Bx 1 are sealing contacts so that pushbuttons A and B have only to be pressed
momentarily to start or stop the motor.
7. Contacts Ax2 and Bx2 are electrical interlocks to prevent the relay coils A and B from being ex
cited at the same time. Thus, when the motor is running, contact Ax2 is open. Consequently, relay
coil B cannot become excited by de pressing pushbutton B until such time as contactor A has
dropped out, causing contact Ax_1 to reclose.
Several types of zero-speed switches are on the market and ---24b shows one that operates on the
principle of an induction motor. It consists of a small permanent magnet rotor N, S and a bronze
ring or cup supported on bearings, which is free to pivot between stationary contacts F and R. The
permanent magnet is coupled to the shaft of the main motor. As soon as the motor turns clockwise,
the permanent magnet drags the ring along in the same direction, thereby closing contacts F-C.
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When the motor stops turning, the brass ring returns to the off position. Because of its function
and shape, the ring is often called a drag-cup.
---24c shows another zero-speed switch that operates on the principle of centrifugal force.
Reduced-voltage starting
Some industrial loads have to be started very gradually. Examples are coil winders, printing
presses, conveyor belts, and machines that process fragile products. In other industrial
applications, a motor cannot be directly connected to the line because the starting current is too
high. In all these cases we have to reduce the voltage applied to the motor either by connecting
resistors (or reactors) in series with the line or by employing an autotransformer. In reducing the
voltage, we recall the following: I. The locked-rotor current is proportional to the voltage: reducing
the voltage by half reduces the current by half.
2. The locked-rotor torque is proportional to the square of the voltage: reducing the voltage by half
reduces the torque by a factor of four.
--25a Simplified schematic diagram of the power section of a reduced-voltage primary resistor
stator.
--25b Control circuit of ---25a.
--25c Control circuit of ---25a using an auxiliary relay RA.
--26a Typical torque-speed curves of a 3-phase squirrel-cage induction motor: (1) full-voltage
starting; (2) primary resistance starting with voltage reduced to 0.65 pu.
--26b Typical current-speed curves of a 3-phase squirrel-cage induction motor: (1) full-voltage
starting; (2) primary resistance starting with voltage reduced to 0.65 pu.
Primary resistance starting
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Primary resistance starting consists of placing three resistors in series with the motor during the
start-up period. Contactor A closes first and when the motor has nearly reached synchronous speed,
a second contact B short-circuits the resistors.
This method gives a very smooth start with complete absence of mechanical shock. The voltage
drop across the resistors is high at first, but gradually diminishes as the motor picks up speed and
the current falls. Consequently, the voltage across the motor terminals increases with speed, and
so the electrical and mechanical shock is negligible when full voltage is finally applied (closure of
contactor B). The resistors are short-circuited after a delay that depends upon the setting of a timedelay relay.
The schematic control diagram reveals the following circuit elements: A, B: magnetic contactor
relay coils A,: auxiliary contact associated with A RT: time-delay relay that closes the circuit of
coil B after a preset interval of time As soon as the start pushbutton is depressed, relay coils A and
RT are excited. This causes the contacts A and Ax to close immediately. However, the contact RT
only closes after a certain time delay and so the relay coil of contactor B is only excited a few
seconds later.
If the magnetic contactors A, B are particularly large, the inrush exciting currents could damage
the start pushbutton contacts if they are connected as shown. In such cases, it’s better to add an
auxiliary relay having more robust contacts.
Thus, the purpose of auxiliary relay RA is to carry the exciting currents of relay coils A and B.
Note that the start pushbutton contacts carry only the exciting current of relay coils RA and RT.
Other circuit components are straightforward, and the reader should have no difficulty in analyzing
the operation of the circuit.
How are the starting characteristics affected when resistors are inserted in series with the stator?
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---26a shows the torque-speed curve 1 when full voltage is applied to a typical 3-phase, 1800 RPM
induction motor. Corresponding curve 2 shows what happens when resistors are inserted in series
with the line. The resistors are chosen so that the locked-rotor voltage across the stator is 0.65 pu.
The locked-rotor torque is, therefore, (0.65)1 0.42 pu or only 42% of full-load torque. This means
that the motor must be started at light load.
---26b shows the current versus speed curve 1 when full voltage is applied to the stator. Curve 2
shows the current when the resistors are in the circuit.
When the speed reaches about 1700 rpm, the resistors are short-circuited. The current jumps from
about 1.8 pu to 2.5 pu, which is a very moderate jump.
Example 2 ______
A 150 kW (200 hp). 460 Y, 3-phase 3520 RPM, 60 Hz induction motor has a locked-rotor torque
of 600 N m and a locked-rotor current of 1400 A. Three resistors are connected in series with the
line so as to reduce the voltage across the motor to 0.65 pu.
Calculate:
a. The apparent power absorbed by the motor under full-voltage. locked-rotor conditions
b. The apparent power absorbed by the motor when the resistors are in the circuit
c. The apparent power drawn from the line, with the resistors in the circuit
d. The locked-rotor torque developed by the motor Solution
a. At full voltage the locked-rotor apparent power is
b. The voltage across the motor at 0.65 pu is
The current drawn by the motor decreases in proportion to the voltage:
I = 0.65 X 1400 910A
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The apparent power drawn by the motor is:
c. The apparent power drawn from the line is = 724 kVA
Thus, percentagewise, the apparent power is only 724 kVAlll14 kVA = 65% of the apparent power
under full-voltage conditions.
d. The torque varies as the square of the voltage: T = 0.652 X 600
0.42 X 600 = 252 N m (~186 ft·lbf) The results of these calculations are summarized.
Example 3: In Example 2, if the locked-rotor power factor of the motor alone is 0.35, calculate the
value of the series resistors and the power they dissipate.
Solution: We will solve this problem by considering active and reactive powers and using the
power triangle method.
The apparent power drawn by the motor at reduced voltage is Sm = 471 kVA (from Example
+++2)
The corresponding apparent power drawn by the line is SL = 724 kVA
The active power drawn by the motor is Pill = Sm cos e 471 X 0.35 165 kW The reactive power
absorbed by the motor is Q/n 441 kvar
The resistors can only absorb active power in the circuit. Consequently, the reactive power
supplied by the line must be equal to that absorbed by the motor: QL 441 kvar The active power
supplied by the line is PL 574 kW The active power absorbed by the three resistors is 574 - 165
409kW The active power per resistor is P PR/3 409/3 = 136 kW The current in each resistor is:
I=910 A)
The value of each resistor is: P = PR 136 000 = 910^2 x R
R = 0.164 ohm
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The three resistors must therefore each have a resistance of 0.164-ohm and a short-term rating of
136 kW. The physical size of these resistors is much smaller than if they were designed for
continuous duty.
This is an interesting example of the usefulness of the power triangle method in solving a relatively
difficult problem. The results are summarized.
Autotransformer starting
Compared to a resistance starter, the advantage of an autotransformer starter is that for a given
torque it draws a much lower line current. The disadvantage is that autotransformers cost more,
and the transition from reduced-voltage to full-voltage is not quite as smooth.
Autotransformers usually have taps to give output voltages of 0.8, 0.65, and 0.5 pu. The
corresponding starting torques are respectively 0.64, 0.42, and 0.25 of the full-voltage starting
torque.
Furthermore, the starting currents on the line side are also reduced to 0.64, 0.42, and 0.25 of the
full voltage locked-rotor current.
---29 shows a starter using two autotransformers connected in open delta. A simplified circuit
diagram of such a starter is given in ---30. It has two contactors A and B. Contactor A has five NO
contacts A and one small NO contact Ax. This contactor is in operation only during the brief period
when the motor is starting up.
--29 Reduced-voltage autotransformer starter, 100 hp, 575 V, 60 Hz. (Square D)
--30 Simplified schematic diagram of an autotransformer starter.
Contactor B has 3 NO contacts B. It’s in service while the motor is running.
The autotransformers are set on the 65 percent tap. The time-delay relay RT possesses three
contacts RT1 RT2, RT3. The contact RTI in parallel with the start button closes as soon as coil RT
ET 218 | 23
is energized. The other two contacts RT2, RT3 operate after a delay that depends upon the RT
relay setting.
Contactors A and B are mechanically interlocked to prevent them from closing simultaneously.
Contactor A closes as soon as the start button is depressed. This excites the autotransformer and
reduced voltage appears across the motor terminals.
A few seconds later, contact RT2 in series with coil A opens, causing contactor A to open. At the
same time, contact RT3 causes contactor B to close.
Thus, contactor A drops out, followed almost immediately by the closure of contactor B. This
action applies full voltage to the motor and simultaneously disconnects the autotransformer from
the line.
In transferring from contactor A to contactor B, the motor is disconnected from the line for a
fraction of a second. This creates a problem because when contactor B closes, a large transient
current is drawn from the line. This transient surge is hard on the contacts and also produces a
mechanical shock.
For this reason, we sometimes employ more elaborate circuits in which the motor is never
completely disconnected from the line.
--- compare the torque and line current when autotransformer starting (3) and resistance starting
(2) is used. The locked-rotor voltage in each case is 0.65 pu. The reader will note that the lockedrotor torques are identical, but the locked-rotor line current is much lower using an autotransformer
(2.7 versus 4.2 pu). However, when the motor reaches about 90 percent of synchronous speed,
resistance starting produces a higher torque because the terminal voltage is slightly higher than the
65 percent value that existed at the moment of start-up. On the other hand, the line current at all
speeds is smaller when using an autotransformer.
Because the autotransformers operate for very short periods, they can be wound with much smaller
wire than continuously rated devices. This enables us to drastically reduce the size, weight, and
cost of these components.
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--31 a Typical reduced voltage (0.65 pu) torque-speed curves of a 3-phase squirrel-cage induction
motor: (2) primary resistance starting; (3) autotransformer starting.
--31 b Typical reduced voltage (0.65 pu) current-speed curves of a 3-phase squirrel-cage induction
motor: (2) primary resistance starting; (3) autotransformer starting.
Example 4 ___
A 200 hp (150 k W), 460 V, 3-phase, 3520 rpm, 60 Hz induction motor has a locked-rotor torque
of 600 N·m and a locked-rotor current of 1400 A (same motor as in Example +++2). Two
autotransformers, connected in open delta, and having a 65 percent tap, are employed to provide
reduced-voltage starting.
Calculate:
a. The apparent power absorbed by the motor
b. The apparent power supplied by the 460 V line
c. The current supplied by the 460 Y line
d. The locked-rotor torque Solution
a. The voltage across the motor is E 0.65 X 460 299 Y
The current drawn by the motor is I 0.65 X 1400 910 A.
The apparent power drawn by the motor is Sm DEI X 299 X 910 471 kVA
b. The apparent power supplied by the line is equal to that absorbed by the motor because the
active and reactive power consumed by the autotransformers is negligible (Section 12.1).
Consequently, SL Sm 471 kVA
c. The current drawn from the line is I SL I (D El (8.9)
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= 471 000/( 1.73 X 460) 592 A Note that this current is considerably smaller than the line current
(910 A) with resistance starting.
d. The locked-rotor torque varies as the square of the motor voltage: T = 0.652 X 600
= 0.42 X 600 252 N m
The results of these calculations are summarized. It’s worthwhile comparing them with the results
in ---27.
--33 Part-winding starting of an induction motor.
Other starting methods
In addition to resistors and autotransformers, several other methods are employed to limit the
current and torque when starting induction motors. Some only require a change in the stator
winding connections. The part-winding starting method can be used when the induction motor has
two identical 3-phase windings that operate in parallel when the motor is running. During the
starting phase, only one of these 3-phase windings is used. As a result the impedance is higher
than if the two windings were connected in parallel. After the motor has picked up speed, the
second 3-phase winding is brought into service so that the two windings operate in parallel. ---33
shows how two 3-pole contactors A and B can be arranged for part-winding starting.
Contactor A closes first thus energizing windings 1, 2, 3. Shortly after, contactor B closes, bringing
windings 7, 8, 9 in parallel with windings 1,2,3.
There are many different types of part-winding connections and some larger motors have specially
designed windings so that the starting performance is optimized.
In wye-delta starting, all six stator leads are brought out to the terminal box. The windings are
connected in wye during start-up, and in delta during normal running conditions. This starting
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method gives the same results as an autotransformer starter having a 58 percent tap. The reason is
that the volt each wye-connected winding is only ( = 0.58) of its rated value.
Finally, to start wound-rotor motors, we progressively short-circuit the external rotor resistors in
one, two, or more steps. The number of steps depends upon the size of the machine and the nature
of the load.
--35 Schematic diagram of a cam switch permitting for ward-reverse and stop operation of a 3phase motor.
--34
a. Cam switch external appearance.
b. Detail of the cam controlling contact 1 in the stop position.
c. Table listing the on-off state of the five contacts; cam shown in off position
Cam switches
Some industrial operations have to be under the continuous control of an operator. In hoists, For
example, an operator has to vary the lifting and lowering rate, and the load has to be carefully set
down at the proper place. Such a supervised control sequence can be done with cam switches.
---34 shows a 3-position cam switch de signed for the forward, reverse, and stop operation of a 3phase induction motor. For each position of the knob, some contacts are closed while others are
open.
This information is given in a table, usually glued to the side of the switch. A cross (X) designates
a closed contact while a blank space is an open contact. In the forward position, For example,
contacts 2, 4, and 5 are closed and contacts 1 and 3 are open. When the knob is turned to the stop
position, all contacts are open.
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---34b shows the shape of the cam that controls the opening and closing of contact 1. The schematic
diagram shows how to connect the cam switch to a 3-phase motor. The state of the contacts (open
or closed) is shown directly on the diagram for each position of the knob.
The 3-phase line and motor are connected to the appropriate cam-switch terminals. Note that
jumpers J1, J2, 13, J4 are also required to complete the connections. The reader should analyze the
circuit connections and resulting current flow for each position of the switch. For example, when
the switch is in the forward position, contacts 2, 4, 5 are closed and L I is connected to T1, L2 to
T2, and L3 to T3.
Some cam switches are designed to carry several hundred amperes, but we often prefer to use
magnetic contactors to handle large currents. In such cases a small cam switch is employed to
control the relay coils of the contactors. Very elaborate control schemes can be designed with multi
contact cam switches.
--36 Electric drives can operate in four distinct quadrants, generator or brake; motor
Computers and controls
The control devices we have covered in this section are used throughout the industry. However,
with the advent of computers, it’s now possible to simulate the behavior of many relay coils and
relay contacts.
Furthermore, the connections between these devices can also be simulated. As a result, it’s possible
to make very complex control circuits by simply using a keyboard, a monitor; and a computer.
Thus, instead of using real relays, contacts, and time-delay dash pots, we simply program these
devices (and their wiring) on a computer. The computers used for this purpose are called
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Programmable Logic Controllers (PLCs). Their construction and basic principle of operation are
covered in Section 31.
Basics of Industrial Motor Control: Part 2: ELECTRIC DRIVES
Fundamentals of electric drives
In this section we have seen the basic control equipment that is used to start and stop induction
motors.
---- An electric drive is a system consisting of one or several electric motors and of the entire
electric control equipment designed to govern the performance of these motors.
--- However, some industrial drives require a motor to function at various torques and speeds,
both in for ward and reverse. In addition to operating as a motor, the machine often has to
function for brief periods as a generator or brake. In electric locomotives, for ex ample, the
motor may run clockwise or counter clockwise, and the torque may act either with or against the
direction of rotation. In other words, the speed and torque may be positive or negative.
In describing industrial drives, the various operating modes can best be shown in graphical form.
The positive and negative speeds are plotted on a horizontal axis, and the positive and negative
torques on a vertical axis. This gives rise to four operating quadrants, labeled respectively
quadrants 1, 2, 3, and 4.
If a machine operates in quadrant 1, both the torque and speed are positive, meaning that they act
in the same direction. Consequently, a machine operating in this quadrant functions as a motor.
As such, it delivers mechanical power to the load. The machine also operates as a motor in
quadrant 3, except that both the torque and speed are reversed.
A machine that operates in quadrant 2 develops a positive torque but its speed is negative. In
other words, the torque acts clockwise while the machine turns counterclockwise. In this
quadrant, the ma chine absorbs mechanical power from the load: consequently, it functions
basically as a generator.
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The mechanical power is converted into electric power and the latter is usually fed back into the
line.
However, the electric power may be dissipated in an external resistor, such as in dynamic
braking.
Depending on the way it’s connected, a machine may also function as a brake when operating in
quadrant 2. The mechanical power absorbed is again converted to electric power, but the latter is
immediately and unavoidably converted into heat. In effect, when a machine functions as a
brake, it absorbs electric power from the supply line at the same time as it absorbs mechanical
power from the shaft. Both power inputs are dissipated as heat--often inside the machine itself.
For example, whenever a machine is plugged, it operates as a brake. In larger power drives we
seldom favor the brake mode of operation because it’s very inefficient. Consequently, the circuit
is usually arranged so that the machine functions as a generator when operating in quadrant 2.
Quadrant 4 is identical to quadrant 2, except that the torque and speed are reversed:
consequently, the same remarks apply.
--37 Typical torque-speed curve of a squirrel-cage induction motor operating at fixed voltage and
frequency.
--38 Typical torque-speed curve of a DC motor.
Typical torque-speed curves
The torque-speed curve of a 3-phase induction motor is an excellent example of the motorgenerator-brake behavior of an electrical machine. We first examined it in Section 14, Section
14.16. The reader is encouraged to take a few moments to review this section.
Referring now to the solid curve, the machine acts as a motor in quadrant 1, as a brake in
quadrant 2, and as a generator in quadrant 4. If the stator leads are reversed, another torque speed
curve is obtained. This dash-line curve shows that the machine now operates as a motor in
quadrant 3, as a generator in quadrant 2, and as a brake in quadrant 4. Note that the machine can
ET 218 | 30
function either as a generator or brake in quadrants 2 and 4. On the other hand, it always runs as
a motor in quadrants 1 and 3.
To give another example: the complete torque-speed curve of a dc shunt motor when the
armature voltage is fixed. The motor-generator-brake modes are again apparent. If the armature
leads are reversed, we obtain the dotted torque-speed curve.
In designing variable-speed electric drives, we try to vary the speed and torque in a smooth,
continuous way to satisfy the load requirements. This is usually done by shifting the entire
torque-speed characteristic back and forth along the horizontal axis. For example, the torquespeed curve of the dc motor may be shifted back and forth by varying the armature voltage.
Similarly, we can shift the curve of an induction motor by simultaneously varying the voltage
and frequency applied to the stator.
To better understand the basic principles of variable speed control, we will first show how
variable frequency affects the behavior of a squirrel-cage induction motor.
Shape of the torque-speed curve
--39 Torque-speed curve of a 15 hp, 460 V, 60 Hz, 3-phase squirrel-cage induction motor.
--40 Torque-speed curve at three different frequencies and voltages.
--41 Stator excited by dc current.
The torque-speed curve of a 3-phase squirrel-cage induction motor depends upon the voltage and
frequency applied to the stator. We already know that if the frequency is fixed, the torque varies
as the square of the applied voltage. We also know that the synchronous speed depends on the
frequency. The question now arises, how is the torque-speed curve affected when both the
voltage and frequency are varied? In practice, they are varied in the same pro portion so as to
maintain a constant flux in the air gap. Thus, when the frequency is doubled, the stator voltage is
doubled. Under these conditions, the shape of the torque-speed curve remains the same, but its
position along the speed axis shifts with the frequency.
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Varying the voltage and frequency in the same proportion has given rise to the "volts per hertz
rule" of motor operation. By keeping the volts per hertz at the same level while the frequency is
varied, we ensure that the flux in the motor is always close to its rated value. However, at
frequencies be low about 20% of rated frequency, the volts per hertz ratio has to be progressively
increased to compensate for the IR drop in the stator.
The torque-speed curve of a 15 hp, (II kW) 3-phase, 460 V, 60 Hz squirrel-cage induction motor.
The full-load speed and torque are respectively 1725 RPM and 60 N·m; the breakdown torque is
160 N m and the locked-rotor torque is 80 N·m.
If we reduce both the voltage and frequency to one-fourth their original value (115 V and 15 Hz)
the new torque-speed curve is shifted toward the left. The curve retains the same shape, but
crosses the axis at a synchronous speed of 180014 = 450 RPM (Fig, 20040). Similarly, if we
raise the voltage and frequency by 50 percent (690 V. 90 Hz), the curve is shifted to the right and
the new synchronous speed is 2700 rpm.
Even if we bring the frequency down to zero (de), the torque-speed curve retains essentially the
same shape. Current can be circulated in any two lines of the stator while leaving the third line
open. The motor develops a symmetrical braking torque that in creases with increasing speed,
reaching a maximum in both directions. The dc current in the windings is adjusted to pro duce
the rated breakdown torque.
Because the shape of the torque-speed curve is the same at all frequencies, it follows that the
torque developed by an induction motor is the same when ever the slip speed (in RPM) is the
same.
Example +++5 _______
A standard 3-phase, 10 hp, 575 V, 1750 RPM, 60 Hz NEMA class 0 squirrel-cage induction
motor develops a torque of 110 N m at a speed of 1440 RPM.
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If the motor is excited at a frequency of 25 Hz, calculate the following:
a. The required voltage to maintain the same flux in the machine
b. The new speed at a torque of 110 N·m
Solution:
a. To keep the same flux, the voltage must be reduced in proportion to the frequency: E = (25/60)
X 575 = 240 V
b. The synchronous speed of the 4-pole, 60 Hz motor is obviously 1800 RPM. Consequently, the
slip speed at a torque of 110 N m is …
= 1800 - 1440 = 360 RPM
The slip speed is the same for the same torque, irrespective of the frequency. The synchronous
speed at 25 Hz is ns (25/60) x 1800 = 750 RPM
The new speed at 110 N·m is n = 750 360 = 390 RPM
Current-speed curves
--42 Current-speed curve at 60 Hz and 15 Hz. Also T-n curve at 460 V, 60 Hz.
--43 The starting torque increases and the current decreases with decreasing frequency.
The current-speed characteristic of an induction motor is a V-shaped curve having a minimum
value at synchronous speed. The minimum current is equal to the magnetizing current needed to
create the flux in the machine. Because the stator flux is kept constant, the magnetizing current is
the same at all speeds.
---42 shows the current-speed curve of the 15 hp, 460 V, 60 Hz squirrel-cage induction motor
mentioned previously. We have plotted the effective values of current for all speeds;
ET 218 | 33
consequently, the current is always positive. The locked-rotor current is 120 A and the
corresponding torque is 80 N·m.
As in the case of the torque-speed curve, it can be shown that if the stator flux is held constant,
the current-speed curve retains the same shape, no matter what the synchronous speed happens to
be.
Thus, as the synchronous speed is varied, the cur rent-speed curve shifts along the horizontal axis
with the minimum current following the synchronous speed. In effect, the torque-speed and
current speed curves move back and forth in unison as the frequency is varied.
Suppose that the voltage and frequency are reduced by 75% to 115 V, 15 Hz.
The locked-rotor current decreases to 80 A, but the corresponding torque increases to 160 N·m.
equal to the full breakdown torque. Thus, by reducing the frequency, we obtain a larger torque
with a smaller current. This is one of the big advantages of frequency control. In effect, we can
gradually accelerate a motor and its load by progressively increasing the voltage and frequency.
During the start-up period, the voltage and frequency can be varied automatically so that the
motor develops close to its breakdown torque all the way from zero to rated speed. This ensures
a rapid acceleration at practically constant current.
In conclusion, an induction motor has excellent characteristics under variable frequency
conditions.
For a given frequency the speed changes very little with increasing load. In many ways, the
torque speed characteristic resembles that of a dc shunt motor with variable armature-voltage
control.
--44 Effect of suddenly changing the stator frequency.
Example +++6 ______
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Using the information revealed by the 60 Hz torque-speed and current-speed curves, calculate
the voltage and frequency required so that the machine will run at 3200 RPM while developing a
torque of 100 N·m. What is the corresponding stator current?
Solution: We first have to find the slip speed corresponding to a torque of 100 N·m.
Accordingly, when the motor operates at 60 Hz and a torque of 100 N·m, the speed is 1650
RPM. Consequently, the slip speed is 1800 1650 150 RPM
The slip speed is the same when the motor develops 100 N·m at 3200 RPM. Consequently, the
synchronous speed must be 11, 3200 ISO 3350 RPM The corresponding frequency is, therefore,
f (3350/1800) x 60 111.7Hz The corresponding stator voltage is E (111.7/60) x 460 = 856V The
60Hz current-speed and torque-speed curves show that the stator current is 40A when the torque
is 100 N·m. Because the current-speed curve shifts along with the torque-speed curve, the
current is again 40 A at 3200 RPM and 100 N·m.
Regenerative braking
A further advantage of frequency control is that it permits regenerative braking. Suppose the
motor is connected to a 460 V, 60 Hz line. It’s running at 1650 RPM, driving a load of constant
torque TL 100 N m (operating point 1). If we suddenly reduce the frequency and voltage by 50
percent, the motor will immediately operate along the 30 Hz, 230 V torque-speed curve. Because
the speed cannot change instantaneously (due to inertia), we suddenly find ourselves at operating
point 2 on the new torque-speed curve. The motor torque is negative; consequently, the speed
will drop very quickly; following the 50% curve until we reach torque (operating point 4) The
interesting feature is that in moving along the curve from point 2 to point 3, energy is returned to
the ac line, because the motor acts as an asynchronous generator during this interval.
The ability to develop a high torque from zero to full speed, together with the economy of
regenerative braking, is the main reason why frequency controlled induction motor drives are
becoming so popular. These electronically-controlled drives are covered.
Definition of Motor Controls
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According Roller et al (2012) the production and control of human movement is a process
that varies from a simple reflex loop to a complex network of neural patterns that
communicate throughout the Central Nervous System (CNS) and Peripheral Nervous System
(PNS).
New motor patterns are learned through movement, interactions with rich sensory
environments, and challenging experiences that challenge a person to solve problems they
encounter. The knowledge about motor control and motor learning shape our understanding
of how individuals progress from novice to skilled motor performance throughout the
lifespan. This page provides an overview about Motor Control and Motor Learning.
Motor Control
Definition
Motor Control is defined as the process of initiating, directing, and grading purposeful
voluntary movement. Shumway-Cook has defined motor control as the ability to regulate
mechanisms essential to movement.
How does it work?
The Motor Control functions in the following way:
1. The task that needs to be completed is identified→ body gathers sensory information from
the environment→ perceives the information→ chooses a movement plan appropriate plan to
meet the goal of the task,
2. Plan is coordinated within the CNS → executed through motor neurons in the brain stem and
spinal cord → outcome communicated to the muscles in postural and limb synergies, and in
the head and neck→ motor units timed to fire in a specific manner.
3. Sensory feedback supplied to the CNS by the movement → decision taken to (1) modify the
plan during execution, (2) acknowledge the goal of the task to be achieved, and (3) store the
information for future performance of the same task-goal combination.
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Theories of Motor Control
The organization and production of movement is a complex problem, so the study of motor
control has been approached from a wide range of disciplines, including psychology,
cognitive science, biomechanics and neuroscience. The control of human movement has been
described in many different ways with many different models of Motor Control put forward
throughout the 19th & 20th Centuries.
Motor Control Theories include the production of reflexive, automatic, adaptive, and
voluntary movements and the performance of efficient, coordinated, goal-directed movement
patterns which involve multiple body systems (input, output, and central processing) and
multiple levels within the nervous system. Many textbooks and researcher recommend
adoption of a systems model of Motor Control incorporating neurophysiology, biomechanics
and motor learning principles (learning solutions based on the interaction between the patient,
the task and the environment). It is imperative to be aware of the effect this relationship
between the task and environment when planning our interventions so as to enable our patients
to achieve their goals.
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The Motor learning theories are:
MOTOR LEARNING THEORY
AUTHOR
DATE
Adams
1971

Adams Closed Loop Theory







Schmidt's Schema Theory
Schmidt
1975




Ecological Theory
Newell
1991 



PREMISE
Closed Loop - Sensory feedback is used for 
the ongoing production of skilled movement
Slow movements
Relies on sensory feedback (Sherrington) 
Blocked Practice

Errors = Bad! Needs to be accurate!
Memory Trace - Initiation of movement
Perceptual Trace - Built up over a period of
practice & is the reference of correctness.
Improvements = Increased capability of
performer to use the reference in closed loop
Open Loop

Schema - Abstract memory representation for
events → RULE

Generalized Motor Program - Rules that
allow for the generation of novel movements
Rapid, ballistic movements = recall memory
withmotor programs and parameters to carry 
out movement without peripheral feedback
Variability of Practice → Improve Motor
Learning
Based on Systems & Ecological Motor

Control Theories
Motor Learnining = Increases coordination
between perception and action thru task &
environmental constraints.
Perceptual-motor workspace - Identifies
mvmts and perceptual cues most relevant to
performance of task
Optimal task-relevant mapping of perception
& action → NO Rules!
CLINICAL IMPLICATIONS
Perform same exact movement
repeatedly to one accurate end
point
Increase Practice → Increase
Learning
Errors produced during learning
→ Increase strength of incorrect
perceptual trace
Optimal Learning → Task
practiced under many different
conditions
Positive benefits for error
production (learn from own
mistakes)
Schema has rules for all stored
elements, not just correct
elements
Patient learns to distinguish
relevant perceptual cues
important to action.
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Factors affecting Motor Learning:
1. Verbal instructions
2. Practice
3. Active participation and motivation
4. Possibility of errors
5. Postural control
6. Memory
7. Feedback
Clinical Significance of motor control and learning
Motor control and learning help therapists to understand the process behind
movements, motor tasks and skills. By acknowledging the theories of motor learning and
control and integrating them into day- to- day practice, therapists will have better chance of:
1. identifying issues in motor performance,
2. developing treatment strategies to help patients remediate performance problems, and
3. Planning programmers that include a new movement, or the reacquisition and/or modification
of movement to be taught in such a way that it is, consistent and transferrable (ability to
perform movement under different environments and conditions).
4. evaluating the effectiveness of intervention strategies employed
It is important that therapists identify the appropriate motor learning strategy and motor
control theory to get optimal and effective results.
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Advantages of Motor Controls
DC motors advantages and disadvantages over AC motors
The power of AC has turned out to be well known and advantageous on account of simple
generation, transmission with fewer losses to long distances due to stepping up voltage & stepping
low voltage to the desired level. In industries 70% of the motors and drives used are Induction
motors, hence AC motors in general means induction motors. Due to the advantages of its rugged
construction, Induction motors were preferred in industries, less maintenance and can be operated
in explosive atmospheres also. So these motors ruled out all the motors in industries in each
application. But DC motors were still used in where induction motors cannot fulfill the need. so,
DC motors have their importance in industries. This is because of some special characteristics they
possess.
Advantages of DC motors:
Speed can be control over a wide range both above and below the rated speed: The
attractive feature of the DC motor is that it offers the wide range of speed control both below &
above the rated speeds. This can be got in DC shunt motors by methods such as armature control
method and field control method. DC motors are widely used in fine speed applications such as in
rolling mills and paper mills.
Strong starting torque: DC arrangement engines are the most appropriate drives for
electrical footing applications utilized for driving overwhelming burdens in beginning conditions.
DC series motors will have a starting torque as high as 500% compared to normal operating torque.
Therefore DC series motors are used in electric trains and cranes.
Accurate steep less speed with constant torque: Constant torque drives are one such the
drives will have motor shaft torque constant over a given speed range. In such drives, shaft power
varies with speed.

Quick starting, stopping, reversing and acceleration

Numerous components which make DC engines progressively favorably contrasted with
air conditioning acceptance engines.

Reactive power consumption
ET 218 | 40
Disadvantages of DC motors:

High starting expense

Increased activity and upkeep cost because of the essence of commutator and brush gear

Can’t work in unstable and dangerous conditions due to starting to happen at brush (a
chance in replacement disappointment)
General comparisons
DC engines have the benefit of: higher beginning torque, snappy beginning, and ceasing,
reversing, variable paces with voltage information and they are simpler and less expensive to
control than AC.
Air conditioning engine favorable circumstances include: lower power request on begin
and negligible upkeep. While the market for AC engines is bigger than DC, it tends to challenge
discover AC arrangements with fractional horsepower appraisals.
DC applications
Demand is developing for 12 and 24V engines to help solar, marine and portable (truckmounted) hardware which DC innovation promptly bolsters. DC innovation is more financially
savvy than AC all in all for lower horsepower applications. DC engines are the more seasoned of
the two innovations, yet R&D has not finished. Engine companies keep on putting resources into
innovation to lessen upkeep expenses and broaden engine life.
ET 218 | 41
IV.
Series of Assessment/ Guide Questions
I.
Multiple Choices.
Read the items carefully and encircle the letter of the correct answer.
1. Are mainly used in low-power circuits.
a. Relays
b. Control Relays
c. Button
d. Cam Switches
2. Is a temperature sensitive device whose contacts open or close when the motor
current exceeds a preset limit.
a. Relays
b. Contactor
c. Thermal Relays
d. Control Relays
3. Is basically a control relay designed to open and close a power circuit.
a. Contactor
b. Relays
c. Control Relays
d. Magnetic Contactor
4. Indicates the on/off state of a remote component in a control system.
a. Lights
b. Pilot Lights
c. Indicator Lamp
d. Lamp
5. Is a low-power snap-action device that opens or closes a contact, depending upon
the position of a mechanical part.
a. Cam Switches
b. Limit Switches
c. Switches
d. Motor Control
6. Sealed devices that can detect objects without coming in direct contact with them.
a. Contactor
b. Magnetic Contactor
c. Proximity Detector
d. Detector
7. It opens and closes a circuit, like a toggle switch.
a. Breaker
b. Circuit Breaker
c. Manual Circuit Breaker
d. Manual Breaker
ET 218 | 42
8. Have a group of fixed contacts and an equal number of moveable contacts.
a. Switches
b. Manual Circuit Breaker
c. Cam Switches
d. Limit Switches
9. Is a switch activated by finger pressure.
a. Button
b. Push-Button
c. Limit Switches
d. Cam Switches
10. An electromagnetic switch that opens and closes a set of contacts when the relay
coil is energized.
a. Relays
b. Control Relays
c. Button
d. Cam Switches
11. In its broadest sense, encompasses all the methods used to control the performance
of an electrical system.
a. Industrial control
b. Motor control
c. Control Circuit
d. Motor
12. Every control circuit is composed of a number of basic components connected
together to achieve the desired performance.
a. Control Circuit
b. Control Device
c. Motor Control
d. Motor
13. Switch isolates the motor from the power source.
a. Disconnecting Switches
b. Switches
c. Cam Switches
d. Pilot Switches
14. Can be opened and closed simultaneously by means of an external handle.
a. Disconnecting Switches
b. Knife-Switches
c. Cam Switches
d. Switches
15. It prevents the hinged cover from opening when the switch is closed.
a. Interlocking Mechanism
b. Interlocking
c. Stop-Start
d. Jog-Start
ET 218 | 43
COLLEGE OF INDUSTRIAL TECHNOLOGY
II.
Give the meaning.
1. Industrial Motor Control
________________________________________________________
________________________________________________________
2. Magnetic Contactor
________________________________________________________
________________________________________________________
3. Control Device
________________________________________________________
________________________________________________________
4. Electric Drive
________________________________________________________
________________________________________________________
5. Cam Switches
________________________________________________________
________________________________________________________
III.
Write TRUE if the statement is correct but if it’s false, change the
underlined word or group of words to make the whole statement true.
____________ 1. A control relay is an magnetic contactor switch that opens and closes a
set of contacts when the relay coil is energized.
____________ 2. Three-phase magnetic contactor rated 50 hp, 575V, 60 Hz.
____________ 3. A schematic diagram is composed of a set of rectangles, each representing
a control device, together with a brief description of its function.
____________ 4. A control circuit can be represented by four types of circuit diagrams.
____________ 5. A thermal relays is basically a control relay designed to open and close a
power circuit.
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COLLEGE OF INDUSTRIAL TECHNOLOGY
V. KEY ANSWERS
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COLLEGE OF INDUSTRIAL TECHNOLOGY
Reference:
https://www.industrial-electronics.com/elec-mach-drvs-pwr-syst_200.html#:~:text=Industrial%20control%2C%20in%20its%20broadest,a%20motor%20and%20its%20load.
https://www.industrial-electronics.com/elec-mach-drvs-pwr-syst_20-1.html
www.physio-pedia.com › Motor_Control_and_Learning
https://www.everythingmro.com/blog/dc-motors-advantages-and-disadvantages-over-ac-motors/
ET 218 | 46