A Glance on
Three Phase Induction Motor
By
Contents
Three Phase Induction Motor Construction……………………………………..4
Principle of Operation of 3-Phase Induction Motor………………………….6
Motor Performance under Load……………………………………………………….8
Starting Methods of Induction Motor……………………………………………….9
Speed Control of Three Phase Induction Motor……………………………….12
Three Phase Induction Motor Protection………………………………………….15
3 Phase Induction Motor
Due to the robust construction and ease of control, three-phase asynchronous motors
are widely preferred over many other motors for the AC-motor-driven applications. This
three-phase motor is accountable for larger load operations in several applications like
goods and lift hoists, conveyors, compressors, pumps, ventilation systems, industrial fan
controllers, etc.
With the invention of adjustable speed drives and several other types of motor starters,
three-phase motors have become favorable drives for variable-speed applications. As
these motors are important in load driving, it is also important to ensure their safety &
protection against starting inrush currents, overloads, single phasing, overheating and
other faulty conditions. Here you can get more information on three-phase induction
motor.
Three Phase AC Motors
The three-phase induction motor is also
called as an asynchronous motor, and it is
the most commonly used type of motor in
industrial applications. Three-phase or
poly-phase motors are of mainly two types:
induction or asynchronous motors and
synchronous motors. Synchronous
motors are special types of motors used in
constant speed applications, whereas
most of the motors used in the industrial
applications are of induction type.
Three Phase Induction Motor Construction
These motors are squirrel and slip-ring type induction motors. Three-phase induction
motor consists of a stator and a rotor, and there is no electrical connection between these
two. These stator and rotors are made up of high-magnetic core materials with less
hysteresis and eddy current losses.
Stator frame can be constructed using cast iron, aluminum or rolled steel. Stator frame
provides necessary mechanical protection and support for stator laminated core,
windings and other arrangements for ventilation. The Stator consists of three-phase
windings overlapped with one another at 120 degree phase shift fitted into slotted
laminations. The six ends of the three windings are brought out and connected to the
terminal box so that these windings are excited by three-phase main supply.
These windings are of copper wire insulated with varnish fitted into insulated slotted
laminations. At all working temperatures, this impregnated varnish remains rigid. These
windings have high-insulation resistance and high resistance to saline atmosphere,
moisture, alkaline fumes, oil and grease, etc. Whichever suits the voltage level, these
windings are connected in either star or delta connections.
This three-phase AC motor rotor is different for the slip-ring and squirrel-cage induction
motors. In a squirrel-cage motor, the rotor consists of heavy aluminum or copper bars
that are shorted on both ends of the cylindrical rotor. The shaft of the induction motor is
supported on two bearings at each ends to ensure free rotating within the stator and to
reduce the friction. It consists of stack of steel laminations evenly spaced slots that are
punched around of its circumference into which un-insulated heavy aluminum or copper
bars are placed.
In a slip-ring-type induction motor, the rotor consists of three-phase windings that are
internally starred at one end, and the other ends are brought outside and connected to
the slip rings mounted on the rotor shaft, as shown in the figure. With the help of carbon
brushes, rheostat is connected to these windings for developing a high-starting torque.
This external resistors or rheostat is used at the starting period only. Once the motor
attains the normal speed, the brushes are short circuited, and the wound rotor works as
squirrel cage rotor.
Principle of Operation of 3-Phase Induction Motor
Ü Whenever three-phase supply is given to a three-phase stator winding, a rotating
magnetic field with 120 displacements at constant magnitude and rotating at a
synchronous speed is produced in it. This changing magnetic field travels over to the
rotor conductor through air gap causing to induce a current in the rotor conductors
according to the Faradays laws of electromagnetic induction. Since the rotor
conductors are shorted, the current starts to flow through these conductors as shown
in the above figure b.
Ü In the presence of magnetic field of stator, rotor conductors are placed, and therefore
according to the Lorenz force principle, a mechanical force acts on the rotor
conductor. Thus, all the rotor conductors force, i.e., the sum of the mechanical forces
produces torque in the rotor which tends to move it in the same direction of rotating
magnetic field.
Ü This rotor conductor’s rotation can also be explained according to the Lenz’s law which
tells that the induced currents in the rotor oppose the cause for its production, i.e.,
rotating magnetic field. As a result, the rotor starts rotating in the same direction of
the rotating magnetic field. However, the rotor speed must be less than the stator
speed – otherwise no currents are induced in the rotor because the relative speed of
the magnetic fields of the rotor and the stator is the reason for rotor motion. This
difference between the stator and the rotor fields is called slip. Due to this relative
speed difference between the stator and the rotors, this 3-phase motor is called
asynchronous machine.
As we discussed above, the relative speed between the stator field and the rotor
conductors causes to rotate the rotor in a particular direction. Hence, for producing the
rotation, the rotor speed Nr must always be less than the stator field speed Ns, and the
difference between these two parameters depends on the load on the motor.
The difference of speed or the slip of the induction motor is given as
l When the stator is stationary, Nr=0; so the slip becomes 1 or 100%.
l When Nr is at synchronous speed, the slip becomes zero; so the motor never runs at
synchronous speed.
l The slip in the induction motor from no load to full load is about 0.1% to 3%; that’s why
the induction motors are called as constant-speed motors.
Motor Performance under Load
When the power supply is given to the
induction motor, it starts rotating up to rated
speed. It will begin to slow down once the
mechanical load is connected to the shaft of
the motor. So the rotor conductors try to cut
the rotating magnetic field at higher rate this
causes to induce the voltage in the rotor
conductors. The resulting current due to
induced voltage in the rotor conductors will
increase further, and thereby produce a high
torque.
When this motor torque is equal to the load torque, mechanical load and motor reach a
state of equilibrium – and, at this condition – the motor runs at constant speed. Since the
rotor impedance is low, decrease in speed varies large rotor current so the drop of the
speed is while increasing the load. This increase in load is met by the increased rotor current
which produces a higher torque on the motor.
This can also be described as this: by adjusting
the slip, we can meet the load on the motor.
The slip increases when the load on the motor
increases, i.e., there is a greater relative speed
between the rotating flux and the rotor
conductors due to drop in the rotor speed.
This high slip increases the rotor that
produces a higher torque to meet the load. In
case of decreasing the load on the motor, the
reverse will happen.
In this way a load torque is controlled internally in the induction motor; that’s why this
motor is called as self-stabilized motor.
Starting Methods of Induction Motor
It is so important to use three-phase motor starter
to limit the staring current of the induction motor.
As we know – in every electrical machine,
particularly in induction motor –when supply is
provided, there is an opposition to this supply by
an induced EMF – which is called back EMF. This
limits the current drawing by the machine, but at
the beginning, the EMF is zero because it is directly
proportional to the speed of the motor. And
therefore, the zero back EMF’s huge current will be
drawn by the motor at the start, and this will be 812 times the full-load current as shown in the
figure.
To protect the motor from the high-staring current, there are different staring methods
available like reduced voltage, rotor resistance, DOL, star-delta starter, auto transformer,
soft starter, etc. some of these are discussed below:
Stator Resistance Starter: This is also called stator
reduced voltage method. It consists of three resistors in
series with each phase of the stator windings, which
causes a voltage drop across each resistor – and, as a
result – a low voltage is applied to each phase. High
power is dissipated in this type, so it is not
implemented in practice.
Direct On line Starter: It consists of simple push
button as the controller. When the start button is
pressed, the switch connecting the motor and the main
supply is closed, and the motor gets the supply current.
In case of over current, the stop button is pressed and
the bypass auxiliary contact is opened.
Thermal overload protection is included in this DOL starter to protect the motor against
over temperatures. This DOL starter is used for small motors up to 5KW rating to avoid
voltage fluctuations; and it is most economical, simple for maintenance and
establishment.
Auto Transformer starter:
It consists of an auto transformer (A Transformer with a
single winding tapped at different points to supply
percentage of its primary voltage across secondary) in star
connection that reduces the voltage applied to the motor
terminals. It consists of 3 tapped secondary coils connected
to the three phases. In the starting period, the transformer
allows application of lower voltages to the three windings.
Star Delta Starter: The 3 windings are first connected in star connection, and
then – after some time (decided by the timer or other controller circuit) – the
windings are connected in delta connection. In star connection, the current
drawn is 0.58% of the normal current and also the phase voltage is reduced to
0.58%. Thus, the torque is reduced at the time of starting. During normal
operation, the motor gets connected to delta wherein the line voltage is equal to
the phase-voltage, and therefore, the motor runs at rated voltage to meet the full
load torque.
Soft Solid Starters Soft start provides a reliable and economical solution to
these problems by delivering a controlled release of power to the motor, thereby
providing smooth, steeples acceleration and deceleration. The damage to
windings and bearings are reduced, resulting in an extended motor life.
A soft starter is a type of reduced-voltage
starter for AC induction motors. The soft
starter is similar to a primary resistance or
primary reactant starter in that it is in series
with the supply to the motor. It consists of
solid state devices like SCRs, as shown below
to control the current flow and the voltage
applied to the motors. The soft starters can be
connected in series with the line voltage, or
connected inside the delta loop. These are
most efficient and highly used starters
nowadays.
Speed Control of Three Phase Induction Motor
Many industrial applications like fan and pump applications require constant
speed; hence an ideal suit for these applications is induction motor. Some of the
applications require variable speeds depending on the type of load. There are
different methods available to control the speed of the induction motor over
different ranges which are given below:
Ÿ
Ÿ
Ÿ
Ÿ
Pole Changing
Stator or Applied Voltage Control
Supply Frequency Control
Rotor Resistance Control
1.
Pole Changing Method of Speed Control
The pole changing method is only suitable for cage type of induction motor since
the cage rotor automatically develops rotor number of poles exactly equals to the
stator winding poles. The number of stator poles can be changed by different
methods like multiple stator windings, method of consecutive poles, and pole
amplitude modulation (PAM).
In multiple stators, the winding stator is
provided with two windings for two different
pole numbers. Only one winding is connected
to the supply at a time. It is more costly and
less efficient method of speed control. Due to
the change in poles' speed is also changed; 4pole winding generates 1500 rpm, whereas 6poles give 1000 rpm.
In the method of consecutive poles, stator
winding is divided into coil groups. These entire
group terminals are brought out so that the
numbers of the poles are changed by changing
the connections of the coils. Speed ratio of 2:1
only is possible with this type of method. Other
than this, the ration flexible method of changing
the poles is Pole Amplitude Modulation (PAM). In
PAM, for obtaining the desired modulation, two
methods are used - which are coil conversion
and coil inversion. Each phase winding is divided
into two parts in both these methods.
2.
Stator Voltage Control
Since the torque developed by the induction
motor is proportional to the square of the stator
voltage - change in the stator voltage
appropriately changes the speed of the
induction motor. In the adjacent figure, we can
observe that the torque comes down with the
decrease in applied voltage. It is therefore
considered that the starting torque is low for too
reduced voltage levels so it tells that even the
applied voltage is sufficient to achieve the running torque – motor may not start. Thus, this
method is suitable for the loads that require low-starting torque. By reducing the voltages,
we can get the different torques and speeds of the induction motor in this method.
3.
Stator or Supply Frequency Control
The synchronous speed expression, i.e., Ns= 120f/P indicates that the change in frequency
also changes the speed. But this change in frequency also changes the air gap flux at the
constant voltage. So, in order to avoid saturation and minimizing the losses – air gap flux
must be constant; this is achieved by maintaining the V/F ratio constant. It is the most
efficient method of speed control, which allows wide range of speed control with improved
starting torque.
Therefore, this type of speed
control system requires variable
voltage and variable frequency
power source. This supply is
generally obtained by voltage
source inverter, current source
inverter and cyclo converter.
4.
Rotor Resistance Control
This type of speed control is used only for slip ring induction motor, and not applicable for
cage motors. In this system, motor is controlled by connecting external resistors to the
rotor through slip rings. As we can observe from the figure, an increase in resistance
reduces the speed of the motor while maintaining the high torque values. In the figure,
R3>R2>R1>R0 – as we are placing the resistance in series with the rotor –the starting torque
is improved further due to the reduced rotor current and speed. This is why this method is
used for high-starting torque applications. But this type of induction motor is inefficient
due to the heat dissipation in resistors.
Three Phase Induction Motor Protection
Three-phase induction motors are accountable for 85 percent of the installed capacity of
the industrial driving systems. Therefore, the protection of these motors is necessary for
reliable operation of loads. Motor failures are mainly divided into three groups: electrical,
mechanical and environmental. Mechanical stresses cause overheating resulting in the
rotor bearings’ wear and tear, whereas the over mechanical load causes heavy currents to
draw, and thus results in increasing temperatures. Electrical failures are caused by various
faults like Phase-to-phase and phase-to-ground faults, single phasing, over and under
voltage, voltage and current unbalance, under frequency, etc.
Protection is needed for the induction motor against the following
abnormalities:
Thermal Overload Protection: To detect overload, high ambient temperature, failure to
start, etc.
Loss of Load Supervision: To detect the sudden loss of load.
Negative- sequence Overcurrent Protection: To detect phase loss, unbalanced loads,
etc.
Phase Reversal: To detect the phase reversed connections.
Short Circuit Protection: To protect from excessive currents, phase to phase and grounds
faults, etc.
Low and Overvoltage: To maintain the motor to operate on rated voltage levels.
Abnormal Frequency: For operating motor within 5% variations of frequency.
Thus, the induction motor is a special type of motor having its own typical characteristics
and performance in terms of starting, speed control, protections, and so on. The
performance of this motor over a wide range of applications makes it predominant in
industries compared to other motors. We hope that the furnished information on this topic
gives a basic idea about this motor. We appreciate your presence and attention for this
document and awaiting your feedback on it. So kindly write your comments and any other
technical doubts in the below given comments section.
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