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

## 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.

Let,

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.
Here,
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.

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## 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.
Wsc=3(Isc)²R1e
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
excitation.
Rotor gets excited by the induced e.m.f
so separate source is not necessary.
The speed is always synchronous irrespective
The speed is always less than synchronous
but never synchronous.
Speed control is not possible. Speed control is possible though difficult.
speed constant at synchronous.
As load increases , the speed keeps on
decreasing.
By changing excitation , the motor p.f can be
It always operates at lagging p.f and p.f
control is not possible.
It can be used as synchronous condenser for p.f
improvement.
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.

## Shaded Pole Induction Motors-Working & Construction

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.

## Construction of Shaded Pole Induction Motor

### Stator

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.

### Rotor

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.

## 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.
• 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.

• Cheap
• These motors are commonly used in
• Small fans
• 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 :-( ]

## Braking Methods of 3 Phase Induction Motor

### Induction Motor Braking Regenerative Plugging Dynamic Braking of Induction Motor

Induction motors are most widely using motors in present days for domestic and industrial purpose.But controlling of AC motor is little bit difficult than DC motor due to alternating nature of voltage.Speed control of induction motors is difficult than dc motors.That’s why induction motor use was restricted.But efficiency wise,usage wise induction motors has lots of advantages.So some speed control,braking methods of induction motor  are invented.

Braking is the most important part of speed control.Braking is two types.They are,mechanical and electrical.In mechanical braking we have disadvantages like heat dissipation,mechanical brakes also depends on the skill of the operator.Electric braking is easy and reliable,it can stop the induction motors very quickly.So we go for electrical braking on induction motor.Though the motor is brought to rest electrically, to maintain its state of rest a mechanical brake is must.

### Dynamic or Rheostatic Braking On Induction Motor

In this braking a high resistance is inserted in the rotor circuit of induction motor, with the help of rheostat.In the below diagram braking on 3 phase induction motor is shown. Whenever we need braking one of the  supply line out of R, Y or B is disconnected from the supply.Depending upon the condition of this disconnected line,we classified two types they are,

1. Two lead connections : In this method, the line disconnected, kept open. This is shown in the Fig(a) and is called two lead connections of rheostatic braking.

2. Three lead connections : In this method, After disconnection of the line,it is connected directly to the other line of the machine. This is shown in the Fig(b).

Note : This method is effective only for slip ring or wound rotor induction motors.

As one of the motor terminal is disconnected from the supply, the motor continues to run as single phase motor. In this case the, breakdown torque i.e. maximum toque decreases to 40% of its original value and motor develops no starting torque at all. And due to high rotor resistance, the net torque produced becomes negative and the braking operation is obtained.

#### Which is preferred in rheostat braking on induction motor?

The braking torque is small in the case of two lead connections,high in the case of three lead connections.The braking torque is high at high speeds. But in three lead connections there is possibility of inequality between the contact resistances in connections of two paralleled lines. This might reduce the braking torque and even may produce the motoring torque again.So we prefer two lead connection over three lead connection.Even there is low braking torque in two lead system.

The torque-slip characteristics for motoring and braking operation of 3 phase induction motor is shown here,
Uses: We use dynamic or rheostat braking in cranes.

### Plugging On Induction Motor

Plugging induction motor braking is applied by just reversing the supply phase sequence.by interchanging connections of any two phases of stator we can attain plugging braking of induction motor.Due to the reversal of phase sequence, the direction of rotating magnetic field gets reversed. This produces a torque in the reverse direction and the motor tries to rotate in opposite direction. This opposite flux acts as brake and it slows down the motor.During plugging the slip is (2 - s), if the original slip of the running motor is s.
Disadvantages of plugging induction motor braking :
1. During the plugging operation very high I²R losses occur in the form of heat.This heat is more than produced when rotor is normally locked.
2. So we can't apply plugging frequently as due to high heat produced rotor which can damage or melt the rotor bars and even may over heat the stator as well.
Advantages of plugging induction motor braking :
1. It the quickest way.

### D.C.Braking On Induction Motor

This is also similar to dynamic braking of induction motor,two connections of stator are disconnected from the supply and connected to a dc source.When d.c. is supplied to the stator, stationary poles N, S are produced in stator. As rotor is rotating, rotor cuts the flux produced by the stationary poles. Thus the a.c. voltage gets induced in the rotor. This voltage produces an a.c. current in the rotor. The motor works as a generator and the R losses are dissipated at the expenditure of kinetic energy stored in the rotating parts. Thus D.C dynamic braking is achieved. This is the quick way to stop induction motor along with high load.

#### Advantages of d.c. dynamic braking of induction motor are,

1. Quick,Less heat produced as compared to the plugging method.
2. The energy dissipated in the rotor is not dependent on the magnitude of the d.c. current.
3. The braking torque is proportional to the square of the d.c. current.
4. Can be used for wound rotor,squirrel cage rotor induction motors.

### Regenerative Braking On Induction Motors

The input power to a three phase induction motor is given by, Pin = 3 Vph Iph cos Φ Where, Φ is the phase angle between stator phase voltage Vph and the stator phase  current  Iph.In motoring action This Φ is less than 90° for the motoring action.Φ > 90° for generating action.
When the induction motor runs above the synchronous speed, relative speed between the conductors and air gap rotating field reverses, as a result the phase angle because greater than 90° rotor produces torque in opposite direction to achieve the braking thus regenerative braking takes place.

Regenerative braking of induction motor can be applied only in case of constant frequency and speed must be above the synchronous speed.In generating action we get some power,we can use this to run other machine.

The torque-slip characteristics for motoring and generating action are shown in the above figure.

Tags:Induction motor braking methods wikipedia,ppt,advantages of electric braking over mechanical braking,regenerative braking of dc motor pdf

## Differences Between Induction Motor and Transformer

### Comparison of Induction Motor With A Transformer

An induction motor may be considered to be a transformer with a rotating short circuited secondary. The stator winding corresponds to transformer primary and rotor winding to transformer secondary. However, the following differences between the two are worth noting:

(i) Unlike a transformer, the magnetic circuit of a 3-phase induction motor has an air gap. Therefore, the magnetizing current in a 3-phase induction motor is much larger than that of the transformer. For example, in an induction motor, it may be as high as 30-50 % of rated current whereas it is only 1-5% of rated current in a transformer.

(ii) In an induction motor, there is an air gap and the stator and rotor windings are distributed along the periphery of the air gap rather than concentrated on a core as in a transformer. Therefore, the leakage reactances of stator and rotor windings are quite large compared to that of a transformer.

(iii) In an induction motor, the inputs to the stator and rotor are electrical but the output from the rotor is mechanical. However, in a transformer, input as well as output is electrical.

(iv) The main difference between the induction motor and transformer lies in the fact that the rotor voltage and its frequency are both proportional to slip s. If f is the stator frequency, E2 is the per phase rotor e.m.f. at standstill and X2 is the standstill rotor reactance/phase, then at any slip s, these values are:

Rotor e.m.f./phase,
E'2 = s E2Rotor reactance/phase,
X'2 = sX2Rotor frequency, f' = sf

Tags:difference between transformer and induction motor,Induction Motor and Transformer Comparisons

## 3 Phase Induction Motor Working Principle

### Working Principle of Three Phase Induction Motor

In the previous post we discussed construction of In this article we are going to discuss about "how induction motor works?".Consider a portion of 3-phase induction motor as shown in Fig. The operation of the motor can be explained as under.

(i) When 3-phase stator winding is energized from a 3-phase supply, a rotating magnetic field is set up which rotates round the stator at synchronous speed Ns (= 120 f/P).

(ii) The rotating field passes through the air gap and cuts the rotor conductors, which as yet, are stationary. Due to the relative speed between the rotating flux and the stationary rotor, e.m.f.s are induced in the rotor conductors. Since the rotor circuit is short-circuited, currents start flowing in the rotor conductors.

(iii) The current-carrying rotor conductors are placed in the magnetic field produced by the stator. Consequently, mechanical force acts on the rotor conductors. The sum of the mechanical forces on all the rotor conductors produces a torque which tends to move the rotor in the same direction as the rotating field.

(iv) The fact that rotor is urged to follow the stator field (i.e., rotor moves in the direction of stator field) can be explained by Lenz’s law. According to this law, the direction of rotor currents will be such that they tend to oppose the cause producing them. Now, the cause producing the rotor currents is the relative speed between the rotating field and the stationary rotor conductors. Hence to reduce this relative speed, the rotor starts running in the same direction as that of stator field and tries to catch it.

### Slip in Three Phase Induction Motor:-

We have seen above that rotor rapidly accelerates in the direction of rotating field. In practice, the rotor can never reach the speed of stator flux. If it did, there would be no relative speed between the stator field and rotor conductors, no induced rotor currents and, therefore, no torque to drive the rotor.
The friction and windage would immediately cause the rotor to slow down. Hence, the rotor speed (N) is always less than the suitor field speed (Ns). This difference in speed depends upon load on the motor. The difference between the synchronous speed Ns of the rotating stator field and the actual rotor speed N is called slip. It is usually expressed as a percentage of synchronous speed i.e.

slip, s= (Ns-N)/Ns

(i) The quantity Ns - N is sometimes called slip speed.
(ii) When the rotor is stationary (i.e., N = 0), slip, s = 1 or 100 %.
(iii) In an induction motor, the change in slip from no-load to full-load is
hardly 0.1% to 3% so that it is essentially a constant-speed motor.

Tags:Working Principle of 3 phase Induction Motor,three phase induction motor working operation theory,types of induction motor induction motor working principle pdf.

## Construction of Induction Motor

### Induction Motor Construction:

Induction motor is the most commonly used machine in domestic purpose and industrial purpose. Induction motor consists essentially of two main parts :

a) stator and b) rotor.

### Induction Motor Stator:-

1.The stator of an induction motor is almost same as that of a synchronous motor or generator. The main function of the stator core is to carry the alternating flux.

2.In order to reduce the eddy current  loss, the stator core is laminated. These laminated types of structure are made up of stamping which is about 0.4 to 0.5 mm thick. It is made up of a number of stampings, which are slotted to receive the windings.

3.The stamping is generally made up of silicon steel, which helps to reduce the hysteresis loss occurring in motor. The stator of induction motor carries a 3-phase winding and that is fed from a 3-phase supply.

 Stator core with winding

4.It is wound for a definite number of poles, the exact number of poles being detemined by the requirements of speed. Greater the number of poles, lesser the speed and vice versa.

5.When supplied to 3-phase windings, produce a magnetic flux, which is of constant magnitude but which revolves (or rotates) at synchronous speed (given by Ns=120f/P). This revolving magnetic flux also called as rotating magnetic field which induces an e.m.f. in the rotor by mutual induction.

### Induction Motor Rotor:

Induction motors are available with two types of rotors basically.
i) Squirrel-cage rotor : Motors employing this type of rotor are known as squirrel-cage induction motors.
ii)Phase-wound or wound rotor : Motors employing this type of rotor are variously known as 'phase-wound' motors or 'wound' motors or as 'slip-ring' motors.

### Squirrel-cage rotors :

Almost 90 percent of induction motors arc squirrel-cage type, because this type of rotor has the simplest and most rugged construction imaginable and is almost indestructible. The rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors which, it should be noted clearly, are not wires but consist of heavy bars of copper, aluminium or alloys. One bar is placed in each slot, rather the bars are inserted from the end when semi-closed slots are used. The rotor bars are brazed or electrically welded or bolted to two heavy and stout short-circuiting end-rings, thus giving us. what is so picturesquely called, a squirrel-case construction (Fig.).

It should be noted that the rotor bars are permanently short-circuited on themselves, hence it is not possible to add any external resistance in series with the rotor circuit for starting purposes.

The rotor slots are usually not quite parallel to the shaft but are purposely given a slight skew.This is useful in two ways :

i) it helps to make the motor run quietly by reducing the magnetic hum and

ii)it helps in reducing the locking tendency of the rotor i.e. the tendency of the rotor teeth to remain under the stator teeth due to direct magnetic attraction between the two.

In small motors, another method of construction is used. It consists of placing the entire rotor core in a mould and casting all the bars and end-rings in one piece. The metal commonly used is an aluminium alloy.

Another form of rotor consists of a solid cylinder of steel without any conductors or slots at all. The .motor operation depends upon the production of eddy currents in the steel rotor.

Advantages of squirrel cage induction rotor
• Its construction is very simple and rugged.
• As there are no brushes and slip ring, these motors requires less maintenance.

### Phase-wound or wound rotor :

This type of rotor is provided with 3-phase, double-layer, distributed winding consisting of coils as used in alternators. The rotor is wound for as many poles as the number of stator poles and is always wound 3-phase even when the stator is wound two-phase.

The three phases are starred internally. The other three winding terminal!: arc brought out and connected to three insulated slip-rings mounted on the shaft with brushes resting on them.

These three brushes arc further externally connected to a 3-phase star-connected rheostat. This makes possible the introduction of additional resistance in the rotor circuit during the starting period for increasing the starting torque of the motor, as shown in Fig. and for changing its  speed-torque/current characteristics.

 Phase-wound Induction Motor Circuit

When running under normal conditions, the slip-rings arc automatically short-circuited by means of a metal collar, which is pushed along the shaft and connects all the rings together. Next, the brushes are automatically lifted from the slip-rings to reduce the frictional losses and the wear and tear. Hence, it is seen that under normal running conditions, the wound rotor is short-circuited on itself just like the squirrel-case rotor.

Advantages of slip ring induction motor :
• It has high starting torque and low starting current.