Showing posts with label Induction Motor. Show all posts
Showing posts with label Induction Motor. Show all posts

Difference between Squirrel Cage and Slip Ring Induction Motor

Difference between Squirrel Cage and Slip Ring Induction Motor

On our electrical learning portal we have posted articles based on comparisons and differences.In this article we are going to discuss differences between squirrel cage and slip ring induction motor.Induction motors are classified based on the type of rotor they use.There are two types of rotors namely squirrel cage rotor and slip ring rotor.Lets have a comparison chart between slip ring and squirrel cage motor with key differences.
You can read here :

Types of induction motor at glance:

The squirrel cage induction motor is the most popular type of AC motor. It is very commonly used in industries because it is very cheap, robust, efficient, and reliable. The slip ring motor has very little application in industries. Rarely 5% – 10% slip ring motors are used in industries because it has several disadvantages like it required frequent maintenance, having a high copper loss, etc.

Slip Ring / Wound Rotor Induction Motor
Squirrel Cage Induction Motor
   Rotor consists of a three phase winding similar to the stator winding.
Rotor consists of bars which are shorted at ends with help of end rings.
Construction is complicated.
Construction is simple.
Slip rings and brushes are present to  add external resistance.
 Slip rings and brushes are absent.
Construction is delicate,due to brushes frequent maintenance is necessary.Only
5% of motors in industry use slip ring rotors.
Construction is simple and robust maintenance is necessary.
95% of motors in industry use squirrel cage rotor.
High starting torque can be obtained.Rotor resistance starter can be used.
Low starting torque..Rotor resistance starter can not be used.
Carbon brushes, slip rings etc are provided in the rotor circuit.
No moving contacts in the rotor.
Comparatively less efficiency.
Squirrel cage and slip ring induction motor gives higher efficiency than other.
High starting torque. It can be obtained by adding external resistance in the rotor circuit.
Low starting torque. It has 1.5 time full load torque.
Speed control by rotor resistance is possible.
Speed control by rotor resistance is not possible.
Less starting current compared to squirrel cage Induction Motor.
Starting current is 5 to 7 times the full load current.

Direct Online Starter Or DOL Starter

Direct Online Starter

In the previous post we have learned about Star Delta Starter. In this post we are going to learn about Direct Online Starter or DOL starter.

Why We Use Starter?

A starter is generally used to limit the high starting currents. An induction motor has high starting currents which may cause the winding's to damage. In order to prevent this we use starters while starting an induction motor. Direct online starter is one of the simplest motor starter which is used for starting the induction motor. This starter is generally used for starting of cage type induction motors.

Note: Starter action will be only at the start of induction motor i.e, until it gains a required amount of speed (generally until it  reaches 85% of synchronous speed) after that it will be isolated from the circuit automatically by few internal actions.

Principle Of DOL Starter:

1.We start by closing the contactor and by applying full voltage across the motor winding's. This causes the motor to draw very high amount of staring currents for a short duration.

2. Now the iron coil will get magnetized and the current will be limited to locked rotor current of the motor. Now the motor will develop locked rotor torque and accelerates to achieve full speed.

3. While it is accelerating the current will decrease slowly. Clear drop of current can be seen only after motor reaches 85% of synchronous speed.

4. Note that starting current curve depends only upon terminal voltage and design of motor, it doesn't depend upon the load applied on the motor.

5. Motor load only affects the time taken for the motor to accelerate to full speed but not magnitude of  starting current.

6. Only if the torque developed by the induction motor with DOL starter exceeds the load torque then only induction motor accelerates to full speed if not the induction motor must be replaced with a suitable capacity induction motor which can develop torque more than the load torque.

Key point: DOL starter has maximum starting current and maximum starting torque.

Main Parts Of DOL starter:

1. Contactors and coil.

2. Over load relay.

let us see use of these parts in detail now.

Contactors & Coils: 

The contactors that we are using are electromagnetic contactors. With the help of electromagnetic energy they can make or break the circuit.

It consists of a coil wound on a iron core. So when the electrical energy passes through this coil the coil makes the iron gets magnetized as a result it attracts the armature and hence the circuit is made that means a closed circuit is formed. When the current interrupts the iron core gets demagnetized as a result it releases the armature this breaks the circuit. The contactor has three main normally open(NO) contacts and auxiliary contact which has lower power ratings and it can be of normally open(NO) or normally closed(NC) type.

Over Load Relay:

From the name it suggests that it is used for over load protection. Generally over load relays have inverse-trip-time characteristics. It cannot provide protection against short circuit currents it only provides protection when the high currents are drawn for a longer period. This is the main advantage of this over load relay over fuses. As we have high inrush currents at the start of motor for a short period of time if we use a fuse it melts every time when we start a motor but this shouldn't happen, moreover if we use high capacity fuse to prevent this it cannot sense the currents which can damage the motor winding's. 
Unlike fuse over load relay is useful in this case since it permits high currents only for short time and high inrush currents at start of motor exists only for shorter duration the over load relay doesn't trip at the start. But later on when the current drawn exceeds the full load current for a longer duration then the over load relay trips the circuit. This means over load relay senses and reacts to the harmful over load currents only. We can classify over load relays based on the characters that are used to detect the over load conditions.

Types Of Over Load Relays:

There are 3 types of over load relays. They are as follows:

1. Thermal Relay:  

Based on the rising temperature caused by over load currents the thermal relay trips and breaks the circuit. This can be further classified into two types namely melting alloy and bimetallic.

2. Magnetic Relay:

When the current exceeds a certain limit the coil gets magnetized which helps to trip the relay and break the circuit. It will not be affected by temperature.

3. Electronic Relay:

These are very ideal relays and can trip the circuit precisely and in fast manner. They can be installed easily.

Working Of DOL Starter:

Now let us discuss the working of DOL starter or direct online starter in detail.

We can understand the working of DOL starter clearly by seeing the circuit diagram of DOL starter.

DOL Starter Circuit Diagram:

1. To switch on the induction motor first press the start push button,S1. Now the contactor gets energized from two lines L1,  L2 .

2. As a result three main contacts and auxiliary contact gets closed and ab is short circuited. So now the motor gets connected across the main supply.

3. Start button goes back due to spring action but the contactor gets energized through ab.

4. If the stop button is pressed supply through contactor is stopped and as a result the coil gets demagnetized and the main contacts and auxiliary contacts are opened as a result induction motor is disconnected from main supply.

5. Now to start the motor again we need to push the start push button.

How Under Voltage Protection Is Done?     

When the voltage falls  below a certain level or when there is fail in providing supply the coil of contactor gets de-energized as a result supply to induction motor stops. In this way under voltage protection is done.

How Over Load Protection Is Done?

When there is over load the over load coils in the circuit gets energized and the normally closed coil D will change to open position, contactor coil gets de-energized which causes the supply to  induction motor to stop. In this way induction motor winding's are saved from burning or over heating due to over loads.

Relation Between Starting Torque And Full Load Torque Of Induction Motor:

Ist be the starting current drawn from the main supply per phase.
Ifl is the full load current drawn from the main supply per phase.
Ʈest is the starting torque.
Sfl is the slip at full load.

we know,
 rotor copper loss = S x rotor input  (S means slip)

At start, S = 1, I2 = I2st, Ʈe = Ʈest. 
      So we get,
At full loads = sfl, I2 = I2fl, Ʈe = Ʈefl

If we neglect no load current, we have

Equating 6 and 7 equations we get,

Equating 5 and 8 equations we get,
If V1 is the stator voltage per phase equivalent

Ze10 is the standstill impedance per phase of the motor referred to the stator. 

Then the current at the starting is given by the equation as shown.

Starting current is equal to the short circuit current.
From equation 9 and 10 we get,
 This the relation between start torque and full load torque of induction motor.

Advantages Of DOL Starter:

1. It is the simplest form of starter.

2. It is more economical starter.

3. It gives high starting torque.

4. Control circuit is simple and can be easily troubleshooted.

5.Occupies less space.

6. Easy to find fault.

Disadvantages Of DOL Starter:

1. High starting currents.

2. High starting currents of motor causes the large voltage dip or drop of electrical supply which affects the other appliances connected to the supply.

3. High starting torque required by the load may cause increasing mechanical stresses on motor mechanical parts as well as the loads.

4. Not suitable for motors having higher ratings( above  10 KW).

In this post we have learnt about the working of DOL starter or direct online starter.

To download this post on DOL starter or direct online starter as PDF click here.

Star - Delta Starter Working

Star - Delta Starter 

To start a large induction motor we use star - delta starter because large induction motors(cage type) with delta connected stator when started directly on line it produces large starting current surges which causes fluctuations in voltage on supply line. To prevent this we run induction motor at reduced voltage at the start of induction motor and later on after getting require speed we operate induction motor at full supply voltage for this purpose we use star delta starter.

Working Principle Of Star - Delta Starter:

As discussed above to reduce fluctuations in voltage on supply line we need to reduce starting surge currents so we need to reduce the voltage at the start of induction motor for this purpose we firstly connect the stator windings of induction motor in star connection and later on we connect the stator windings in delta to operate at full supply voltage. See the below circuit diagram how the star - delta starter is connected to induction motor.

Connection diagram of star - delta starter:

star delta starter control circuit diagram
During start position of switch the stator windings are connected in star as shown in the following figure.

Now the induction motor gradually picks up it's speed. When the speed becomes 80 percent of its rated speed then the switch moves to run position as a result stator windings get connected in delta as shown in the below figure.

Theory And Calculations Of Star - Delta Starter:

To understand how voltage is reduced by star connection of stator windings see the below calculations.

In star connection the phase voltage is 1/√3 times of line voltage as the torque is directly proportional to square of voltage applied the torque is reduced to 1/3 times than the torque produced by starting with direct delta connection.


VL be the line voltage.

V1 is the phase voltage.

Istyp be  starting current per phase when the stator windings are connected in star.

Istyl is the starting line current when the stator windings are connected in star.

IstΔp is the starting current per phase by direct switching with the stator windings connected in delta

IstΔl is the starting line current by direct switching with the stator windings in the delta.

IscΔp is the short circuit phase current by direct switching with the stator windings in the delta.

Ze10 is the standstill equivalent impedance per phase of the motor, referred to the stator

In star connection,  line current will be equal to phase current.
In delta connection, the line current is equal to the √3 times of the phase current. so we get,

This shows that with star delta starter, the starting current from the main supply is one-third of that with direct switching in the delta.
This shows with star delta starter, the starting torque is reduced to one-third of the starting torque obtained with the direct switching in the delta.
IflΔp is the full load phase current with the winding connected in delta. But
So the above equations show how voltage and torques are reduced with star delta starter than direct delta starting.

Related searches:
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why star delta starter is used star delta motor connection


Torque equation and torque-slip characteristics of 3-phase induction motor.

Torque Equation And Torque - slip Characteristics Of Three Phase Induction Motor

Torque - slip characteristics shows the variation of torque of 3- phase induction motor with the change in slip of  that three phase induction motor.

To understand the torque - slip characteristics let us first derive torque equation under running conditions of three phase induction motor.

Torque equation under running conditions of 3 phase induction motor:

Let us see derivation for torque equation of 3 phase induction motor.

We know that,

Torque  T = Er Ir cosϕ2 or T ∝ ϕ Ir cosϕ2

Here Er = rotor e.m.f / phase under running conditions.
         Ir = rotor current / phase under running conditions.

And Er = s E2

       Ir = Er / Zr 

Zr can be calculated from the following figure by applying pythogerous theorem.

Now we get

From figure we get,


Where S is slip which is defined as ratio of difference between synchronous speed and actual speed of rotor.

When the speed changes slip changes as a result torque also changes. Also the equation clear shows that torque changes with respect to change in slip but is not always directly proportional it always depends on the value of slip that is whether the slip value is low , medium or high. Now let us see this in detail.

Relation between torque and slip of a 3 phase induction motor:

Now let us see how torque changes with low , medium and high values of slip in detail.

Slip less than zero ( s < 0):

If the slip is less than zero it will be in generating mode. As in this mode both torque and slip are negative it should be driven by prime mover so it acts as induction generator as is it used mechanical energy and converts into electrical energy. Generally this mode of operation is not used because if the induction machine is used as generator it requires high amount of reactive power to be supplied which is expensive again.

Slip is between zero and one ( 0 < s < 1 ):

From torque equation we have torque as
T =

At s=0, T will be zero so the curve starts from origin.

Now at speeds close to synchronous speed the value of sX2 is small and an be ignored. so now torque T becomes 

T ∝ s / R2 

If R2 is constant the we have

T ∝ s so the torque graph will be approximately a straight line for low values of slip.

As slip increases torque also increases and becomes maximum when slip s = R2 / X2. To know how at maximum torque slip is R2 / X2 see the following derivation.

Condition for maximum torque:

We know torque is given by,

Now differentiate above expression with respect to slip s and equate the expression to zero. 

Put Y = 1/T for making calculation simpler we get,

So we get at maximum torque slip is s = R2/X2.

The torque at this point is called breakdown torque or pull-out torque.

Now if the slip increases further by increasing the load then R2 can be ignored. So for large values of slip we get torque as

T ∝ 1/s.

So in this region the torque slip characteristics is a rectangular hyperbola. This can be seen in the following figure.

In this mode 3 phase induction machine will be in motoring mode. At no load slip will be zero and at full load slip will be 1.

Slip greater than one ( S > 1 ):

This mode is called breaking mode. In this mode the supply voltage polarities are changed as a result motor rotates in reverse direction an stops. This help to stops the induction machine in very short time.

The overall torque - slip characteristics of 3 phase induction motor is shown in the following figure.

Today we have learnt about torque equation of 3 phase induction motor and torque slip characteristics of 3 phase induction motor.

Blocked Rotor Test On Induction Motor

Blocked Rotor Test On Induction Motor

This test is also called as Locked Rotor test or short circuit test.In the previous articles we had discussions on construction of induction motor,working of induction motor.We know induction motor is functionally similar to transformer.Like short circuit test on transformer blocked rotor test on induction motor is conducted.
Quick Point: Blocked rotor test on induction motor is to find out stator copper loss, rotor copper loss without friction and windage losses. 
In this test, the rotor is blocked by a belt-pulley mechanism,so it is not allowed to rotate.So rotor speed will be zero (N= 0).So slip (s) =1 and RL' = R2' (1-s)/s is zero.If the induction motor whichever is using for this test is slip ring induction motor then the winding are short circuited at the slip rings. 

Procedure of  Blocked Rotor Test of Induction Motor

1.First block the rotor of induction motor by pulley-belt mechanism.
2.Apply 10 to 15 % of rated voltage to stator of induction motor.(Because if we apply even more than 30% rated voltage rotor will be short circuited.)
3.Now, slowly increase the voltage in the stator winding so that current reaches to its rated value. At this point, note down the readings of the voltmeter, wattmeter and ammeter to know the values of voltage, power and current.
4.Now the applied voltage Vsc, the input power Wsc and a short circuit current Isc are measured.

As RL' = 0, the equivalent circuit is exactly similar to that of a transformer and hence the calculations are similar to that of short circuit test on a transformer.
                     Vsc = Short circuit reduced voltage (line value)
                     Isc = Short circuit current (line value)
                     Wsc = Short circuit input power
       Now      Wsc = √3Vsc Isc cosΦsc ----------- Line values
cosΦsc=Wsc/√3Vsc Isc ( This gives us short circuit power factor of a motor.)
        Now the equivalent circuit is as shown in above figure.
        where Isc = Per phase value
R1e = Wsc/3(Isc)²
This is equivalent resistance referred to stator.
                       Z1e = Vsc (per phase)/ Isc (per phase) = Equivalent impedance referred to stator.

X1e = 1e-1e  =  equivalent reactance reffered to stator.
During this test, the stator carries rated current hence the stator copper loss is also dominant. Similarly the rotor also carries short circuit current to produce dominant rotor copper loss. As the voltage is reduced, the iron loss which is proportional to voltage is negligibly small. The motor is at standstill hence mechanical loss i.e. friction and windage loss is absent. Hence we can write,

Wsc = Stator copper loss + Rotor copper loss

But it is necessary to obtain short circuit current when normal voltage is applied to the motor. This is practically not possible. But the reduced voltage test results can be used to find current ISN which is short circuit current if normal voltage is applied.
If             VL = Normal rated voltage (line value)
                Vsc = Reduced short circuit voltage (line voltage)
then  ISN = (VL * ISC) / Vsc
where       Isc = Short circuit current at reduced voltage
Thus,        ISN = Short circuit current at normal voltage
Now power input is proportional to square of the current.
So   WSN = Short circuit input power at normal voltage
This cab be obtained as,
But at normal voltage core loss can not be negligible hence,
WSN = Wsc (ISN / ISC
       WSN = Core loss + Stator and rotor copper loss

Differences Between Synchronous and Induction Motor

Differences Between Synchronous and Induction Motor

We have two major types of AC motors.They are synchronous and induction motor,synchronous motor runs at synchronous speed where induction motor runs at less than synchronous speed.In this article we are going to discuss on comparisons between synchronous and induction motor. There are so many differences between synchronous motor and induction motor like excitation,speed control.etc.Most important points are listed below.

Synchronous Motor Induction Motor
Construction is complicated. Construction is simpler , particularly in
case of cage rotor.
Not self starting.(why read here) Self starting.
Separate DC source is required for rotor
Rotor gets excited by the induced e.m.f
so separate source is not necessary.
The speed is always synchronous irrespective
of the load.
The speed is always less than synchronous
but never synchronous.
Speed control is not possible. Speed control is possible though difficult.
As load increases, load angle increases,keeping
speed constant at synchronous.
As load increases , the speed keeps on
By changing excitation , the motor p.f can be
changed from lagging and leading.
It always operates at lagging p.f and p.f
control is not possible.
It can be used as synchronous condenser for p.f
It can not be used as synchronous condenser.
Motor is sensitive to sudden load changes and
hunting results.
Phenomenon of hunting is absent.
Motor is costly and requires frequent maintenance. Motor is cheap, especially cage rotors and
maintenance free.

Read Here: Differences B/W Induction Motor & Transformer
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Shaded Pole Induction Motors-Working & Construction

Shaded Pole Induction Motors

We know that 'single phase induction motors are not self-starting' .To make them self starting we use capacitors,shaded poles etc.In this article we going to see shaded pole induction motors-working, construction.Shaded pole induction motors are very popular for low ratings about 0.05 HP (~ 40 W) because of its extremely simple construction and high performance. 

Shaded pole induction motor consists of salient poles(projected poles) on stator and a squirrel cage rotor.Poles are shaded i.e. each pole carries a copper band on one of its unequally divided part called shading ban.You can observe 4 pole shaded poles in below figure.
Shaded Pole Induction Motors-Working & Construction

Construction of Shaded Pole Induction Motor

Shaded Pole Induction Motors-Working & Construction


The stator of shaded pole induction motors are also similar to normal motor's stator construction. Usually 2 to 4 salient poles are used. A coil is wounded on it and part of each pole is wrapped by a copper coil.These poles are shaded by copper band or ring which is inductive in nature. The poles are divided into two unequal halves. The smaller portion carries the copper band and is called as shaded portion of the pole.The slot is approximately one third distance from the edge of the pole. 

Speed of motor depends on number of poles for example at 50Hz supply 2 pole stator gives a synchronous speed of 3000 rpm where a 4 poled stator speed will be 1500rpm.


Squirrel cage type rotor is used in shaded pole induction motors.Rotor bars are skewed in 60 degrees to obtain efficient starting torque and for limiting the torque dip during run up.Air gap length will be around  0.25 to 0.5 mm between stator and rotor.It doenst have any extra make ups like commutator, brushes, collector rings etc.This is very cheap in cost, rugged in construction and reliable. Absence of centrifugal switch eliminates the possibility of motor failure due to faulty centrifugal switch mechanisms.
Shaded Pole Induction Motors-Working & Construction

Working of Shaded Pole Induction Motor

The working operation of  shaded pole motor can be easily understood.Our main aim is to produce rotating magnetic field with single phase AC supply this is possible with shaded poles.How ? lets see

Alternating flux is produced by input ac supply.We can observe waveform of the flux in below figure.The distribution of this flux in the pole area is greatly influenced by the role of copper shading band.Now assume our ac supply as sine wave as shown in 1.
Shaded Pole Induction Motors-Working & Construction
  • During the portion OA
In OA portion  the alternating-current cycle [1], the flux begins to increase and an e.m.f. is induced in the shading coil. The resulting current in the shading coil will be in such a direction (Lenz’s law) so as to oppose the change in flux. Thus the flux in the shaded portion of the pole is weakened while that in the unshaded portion is strengthened as shown in figure 2.
  • During the portion AB
During the portion AB of the alternating-current cycle, the flux has reached almost maximum value and is not changing. Consequently, the flux distribution across the pole is uniform [Fig 3] since no current is flowing in the shading coil. 
  • During the portion BC
As the flux decreases (portion BC of the alternating current cycle), current is induced in the shading coil so as to oppose the decrease in current. Thus the flux in the shaded portion of the pole is strengthened while that in the unshaded portion is weakened as shown in Fig 4.

The effect of the shading coil is to cause the field flux to shift across the pole face from the unshaded to the shaded portion. This shifting flux is like a rotating weak field moving in the direction from unshaded portion to the shaded portion of the pole.The rotor is of the squirrel-cage type and is under the influence of this moving field. Consequently, a small starting torque is developed. As soon as this torque starts to revolve the rotor, additional torque is produced by single-phase induction-motor action. The motor accelerates to a speed slightly below the synchronous speed and runs as a single-phase induction motor.

Advantages & Applications of Shaded Pole Induction Motor

  • Cheap
  • These motors are commonly used in
  • Small fans
  • Advertising displays
  • Film projectors
  • Record players
  • Gramophones
  • Hair dryers
  • Photo copying machines etc.
  • Low starting torque, low power factor and low efficiency, copper losses are high due to presence of copper band.[ Disadvantage :-( ]