Induction MotorBy

Ashvani ShuklaManager(C&I)BGR ENERGY

• INTRODUCTION• One of the most common electrical motor used in most applications which is

known as induction motor. This motor is also called as asynchronous motor because it runs at a speed less than synchronous speed. In this, we need to define what is synchronous speed. Synchronous speed is the speed of rotation of the magnetic field in a rotary machine and it depends upon the frequency and number poles of the machine. An induction motor always runs at a speed less than synchronous speed because the rotating magnetic field which is produced in the stator will generate flux in the rotor which will make the rotor to rotate, but due to the lagging of flux current in the rotor with flux current in the stator, the rotor will never reach to its rotating magnetic field speed i.e. the synchronous speed. There are basically two types of induction motor that depend upon the input supply - single phase induction motor and three phase induction motor. Single phase induction motor is not a self starting motor which we will discuss later and three phase induction motor is a self-starting motor. Now in general we need to give two supply i.e. double excitation to make a machine to rotate. For example if we consider a DC motor, we will give one supply to the stator and another to the rotor through brush arrangement.

• Working Principle of Induction Motor• But in induction motor we give only one supply, so it is really interesting to

know that how it works. It is very simple, from the name itself we can understand that there is induction process occurred. Actually when we are giving the supply to the stator winding, flux will generate in the coil due to flow of current in the coil. Now the rotor winding is arranged in such a way that it becomes short circuited in the rotor itself. The flux from the stator will cut the coil in the rotor and since the rotor coils are short circuited, according to Faraday's law of electromagnetic induction, current will start flowing in the coil of the rotor. When the current will flow, another flux will get generated in the rotor. Now there will be two flux, one is stator flux and another is rotor flux and the rotor flux will be lagging to the stator flux. Due to this, the rotor will feel a torque which will make the rotor to rotate in the direction of rotating magnetic flux. So the speed of the rotor will be depending upon the ac supply and the speed can be controlled by varying the input supply. This is the working principle of an induction motor of either type.

• Types Induction Motor

• SINGLE PHASE INDUCTION MOTOR • Split phase induction motor• Capacitor start induction motor• Capacitor start capacitor run induction motor• Shaded pole induction motor

• THREE PHASE INDUCTION MOTOR • Squirrel cage induction motor• Slip ring induction motor

• Construction of Three Phase Induction Motor• The three phase induction motor is the most widely used electrical motor. Almost 80% of

the mechanical power used by industries is provided by three phase induction motors because of its simple and rugged construction, low cost, good operating characteristics, absence of commutator and good speed regulation. In three phase induction motor the power is transferred from stator to rotor winding through induction. The Induction motor is also called asynchronous motor as it runs at a speed other than the synchronous speed.

• Like any other electrical motor induction motor also have two main parts namely rotor and stator

• Stator: As its name indicates stator is a stationary part of induction motor. A stator winding is placed in the stator of induction motor and the three phase supply is given to it.

• Rotor: The rotor is a rotating part of induction motor. The rotor is connected to the mechanical load through the shaft.

• The rotor of the three phase induction motor are further classified as Squirrel cage rotor,• Slip ring rotor or wound rotor or phase wound rotor.• Depending upon the type of rotor construction used the three phase induction motor are

classified as: Squirrel cage induction motor,• Slip ring induction motor or wound induction motor or phase wound induction motor.

• The construction of stator for both the kinds of three phase induction motor remains the same and is discussed in brief in next paragraph. The other parts, which are required to complete the induction motor, are: Shaft for transmitting the torque to the load. This shaft is made up of steel.• Bearings for supporting the rotating shaft. • One of the problems with electrical motor is the production of heat during its rotation.

In order to overcome this problem we need fan for cooling.• For receiving external electrical connection Terminal box is needed.• There is a small distance between rotor and stator which usually varies from 0.4 mm to

4 mm. Such a distance is called air gap.• Stator of Three Phase Induction Motor• The stator of the three phase induction motor consists of three main parts : Stator

frame,• Stator core,• Stator winding or field winding.• Stator Frame

It is the outer most part of the three phase induction motor. Its main function is to support the stator core and the field winding. It acts as a covering and it provide protection and mechanical strength to all the inner parts of the induction motor. The frame is either made up of die cast or fabricated steel. The frame of three phase induction motor should be very strong and rigid as the air gap length of three phase induction motor is very small, otherwise rotor will not remain concentric with stator, which will give rise to unbalanced magnetic pull.

Stator CoreThe main function of the stator core is to carry the alternating flux. 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. All the stamping are stamped together to form stator core, which is then housed in stator frame. The stamping is generally made up of silicon steel, which helps to reduce the hysteresis loss occurring in motor.

• Stator Winding or Field Winding• The slots on the periphery of stator core of the three phase induction motor carries

three phase windings. This three phase winding is supplied by three phase ac supply. The three phases of the winding are connected either in star or delta depending upon which type of starting method is used. The squirrel cage motor is mostly started by star – delta stater and hence the stator of squirrel cage motor is delta connected. The slip ring three phase induction motor are started by inserting resistances so, the stator winding of slip ring induction motor can be connected either in star or delta. The winding wound on the stator of three phase induction motor is also called field winding and when this winding is excited by three phase ac supply it produces a rotating magnetic field.

• Types of Three Phase Induction Motor• Squirrel cage three phase induction motor: The rotor of the squirrel cage

three phase induction motor is cylindrical in shape and have slots on its periphery. The slots are not made parallel to each other but are bit skewed (skewing is not shown in the figure of squirrel cadge rotor beside) as the skewing prevents magnetic locking of stator and rotor teeth and makes the working of motor more smooth and quieter. The squirrel cage rotor consists of aluminum, brass or copper bars (copper bras rotor is shown in the figure beside). These aluminum, brass or copper bars are called rotor conductors and are placed in the slots on the periphery of the rotor. The rotor conductors are permanently shorted by the copper or aluminum rings called the end rings. In order to provide mechanical strength these rotor conductor are braced to the end ring and hence form a complete closed circuit resembling like a cage and hence got its name as "squirrel cage induction motor". The squirrel cage rotor winding is made symmetrical. As the bars are permanently shorted by end rings, the rotor resistance is very small and it is not possible to add external resistance as the bars are permanently shorted. The absence of slip ring and brushes make the construction of Squirrel cage three phase induction motor very simple and robust and hence widely used three phase induction motor. These motors have the advantage of adapting any number of pole pairs. The below diagram shows squirrel cage induction rotor having aluminum bars short circuit by aluminum end rings.

Advantages of squirrel cage induction rotor-1.Its construction is very simple and rugged.2.As there are no brushes and slip ring, these motors requires less maintenance.Applications: Squirrel cage induction motor is used in lathes, drilling machine, fan, blower printing machines etc.

• Slip ring or wound three phase induction motor : In this type of three phase induction motor the rotor is wound for the same number of poles as that of stator but it has less number of slots and has less turns per phase of a heavier conductor. The rotor also carries star or delta winding similar to that of stator winding. The rotor consists of numbers of slots and rotor winding are placed inside these slots. The three end terminals are connected together to form star connection. As its name indicates three phase slip ring induction motor consists of slip rings connected on same shaft as that of rotor. The three ends of three phase windings are permanently connected to these slip rings. The external resistance can be easily connected through the brushes and slip rings and hence used for speed control and improving the starting torque of three phase induction motor. The brushes are used to carry current to and from the rotor winding. These brushes are further connected to three phase star connected resistances. At starting, the resistance are connected in rotor circuit and is gradually cut out as the rotor pick up its speed. When the motor is running the slip ring are shorted by connecting a metal collar, which connect all slip ring together and the brushes are also removed. This reduces wear and tear of the brushes. Due to presence of slip rings and brushes the rotor construction becomes somewhat complicated therefore it is less used as compare to squirrel cage induction motor.

1.Advantages of slip ring induction motor - It has high starting torque and low starting current.2.Possibility of adding additional resistance to control speed.Application: Slip ring induction motor are used where high starting torque is required i.e in hoists, cranes, elevator etc.

Slip ring or phase wound Induction motor Squirrel cage induction motorConstruction is complicated due to presence of slip ring and brushes Construction is very simple

The rotor winding is similar to the stator winding The rotor consists of rotor bars which are permanently shorted with the help of end rings

We can easily add rotor resistance by using slip ring and brushes

Since the rotor bars are permanently shorted, its not possible to add external resistance

Due to presence of external resistance high starting torque can be obtained Staring torque is low and cannot be improved

Slip ring and brushes are present Slip ring and brushes are absentFrequent maintenance is required due to presence of brushes Less maintenance is requiredThe construction is complicated and the presence of brushes and slip ring makes the motor more costly

The construction is simple and robust and it is cheap as compared to slip ring induction motor

This motor is rarely used only 10 % industry uses slip ring induction motor

Due to its simple construction and low cost. The squirrel cage induction motor is widely used

Rotor copper losses are high and hence less efficiency

Less rotor copper losses and hence high efficiency

Speed control by rotor resistance method is possible

Speed control by rotor resistance method is not possible

Slip ring induction motor are used where high starting torque is required i.e in hoists, cranes, elevator etc

Squirrel cage induction motor is used in lathes, drilling machine, fan, blower printing machines etc

Difference between Slip Ring and Squirrel Cage Induction Motor

• We had mentioned above that single phase induction motor is not a self starting and three phase induction motor is self starting. So what is self starting? When the machine starts running automatically without any external force to the machine, then it is called as self starting. For example we see that when we press the key the fan starts to rotate automatically, so it is self starting. Point to be note that fan used in home appliances is single phase induction motor but it is self starting. How? We will discuss it how. Why is Three Phase Induction Motor Self Starting?• In three phase system, there are three single phase line with 120° phase

difference. So the rotating magnetic field is having the same phase difference which will make the rotor to move. If we consider three phases a, b and c, when phase a is magnetized, the rotor will move towards the phase a winding, in the next moment phase b will get magnetized and it will attract the rotor and than phase c. So the rotor will continue to rotate.

Working Principle of Three Phase Induction Motor• Why Single Phase Induction Motor is not Self Starting?• But what about single phase. It will be having only one phase still it makes the rotor to rotate, so it is

quite interesting. Before that we need to know why single phase induction motor is not a self starting motor and how the problem is overcome. We know that the ac supply is a sinusoidal wave and it produces pulsating magnetic field in uniformly distributed stator winding. Since pulsating magnetic field can be assumed as two oppositely rotating magnetic fields, there will be no resultant torque produced at the starting and due to this the motor does not run. After giving the supply, if the rotor is made to rotate in either direction by external force, then the motor will start to run. This problem has been solved by making the stator winding into two winding, one is main winding and another is auxiliary winding and a capacitor is fixed in series with the auxiliary winding. This will make a phase difference when current will flow through the both coils. When there will be phase difference, the rotor will generate a starting torque and it will start to rotate. Practically we can see that the fan does not rotate when the capacitor is disconnected from the motor but if we rotate with hand it will start to rotate. So this is the reason of using capacitor in the single phase induction motor. There are several advantages of induction motor which makes this motor to have wider application. It is having good efficiency up to 97%. But the speed of the motor varies with the load given to the motor which is an disadvantage of this motor. The direction of rotation of induction motor can easily be changed by changing the sequence of three phase supply, i.e. if RYB is in forward direction, the RBY will make the motor to rotate in reverse direction. This is in the case of three phase motor but in single phase motor, the direction can be reversed by reversing the capacitor terminals in the winding.

• Working Principle of Three Phase Induction Motor• An electrical motor is such an electromechanical device which converts electrical

energy into a mechanical energy. In case of three phase AC operation, most widely used motor is Three phase induction motor as this type of motor does not require any starting device or we can say they are self starting induction motor. • For better understanding the principle of three phase induction motor, the basic

constructional feature of this motor must be known to us. This Motor consists of two major parts: Stator: Stator of three phase induction motor is made up of numbers of slots to construct a 3 phase winding circuit which is connected to 3 phase AC source. The three phase winding are arranged in such a manner in the slots that they produce a rotating magnetic field after AC is given to them. Rotor: Rotor of three phase induction motor consists of cylindrical laminated core with parallel slots that can carry conductors. Conductors are heavy copper or aluminum bars which fits in each slots & they are short circuited by the end rings. The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed because this arrangement reduces magnetic humming noise & can avoid stalling of motor.

• Working of Three Phase Induction Motor

• Production of Rotating Magnetic Field

• The stator of the motor consists of overlapping winding offset by an electrical angle of 120°. When the primary winding or the stator is connected to a 3 phase AC source, it establishes a rotating magnetic field which rotates at the synchronous speed.

• Secrets behind the rotation:• According to Faraday’s law an emf induced in any circuit is due to the rate of change

of magnetic flux linkage through the circuit. As the rotor winding in an induction motor are either closed through an external resistance or directly shorted by end ring, and cut the stator rotating magnetic field, an emf is induced in the rotor copper bar and due to this emf a current flows through the rotor conductor.

• Here the relative velocity between the rotating flux and static rotor conductor is the cause of current generation; hence as per Lenz's law the rotor will rotate in the same direction to reduce the cause i.e. the relative velocity.

• Thus from the working principle of three phase induction motor it may observed that the rotor speed should not reach the synchronous speed produced by the stator. If the speeds equals, there would be no such relative velocity, so no emf induction in the rotor, & no current would be flowing, and therefore no torque would be generated. Consequently the rotor can not reach at the synchronous speed. The difference between the stator (synchronous speed) and rotor speeds is called the slip. The rotation of the magnetic field in an induction motor has the advantage that no electrical connections need to be made to the rotor. Thus the three phase induction motor is: • Self-starting. • Less armature reaction and brush sparking because of the absence of commutators and brushes that may cause sparks. • Robust in construction. • Economical. • Easier to maintain.

• Classification of Squirrel Cage Induction Motor.• NEMA in United States and IEC in Europe have classified the design of

the squirrel cage induction motors based on their speed-torque characteristics into some classes. These classes are Class A, Class B, Class C, Class D, Class E and Class F.• In Class A Design • A normal starting torque. • A normal starting current. • Low slip. • In this Class, pullout torque is always of 200 to 300 percent of the full-

load torque and it occurs at a low slip (it is less than 20 percent). • For this Class, the starting torque is equal to rated torque for larger

motors and is about 200 percent or more of the rated torque for the smaller motors.

• In Class B Design • Normal starting torque, • Lower starting current, • Low slip. • Induction Motor of this Class produces about the same starting torque as the

class A induction motor and this starting torque is with about 25 percent less current. • Pullout torque is always greater than or equal to 200 percent of the rated load

torque. But it is less than that of the class A design because it has increased rotor reactance. • Again Rotor slip is still relatively low (less than 5 percent) at full load. • Applications of Class B design are similar to those for design A. But design B is

preferred more because of its lower starting-current requirements.

In Class C Design• High starting torque. • Low starting currents. • Low slip at the full load (less than 5 %). • Up to 250 percent of the full-load torque, the starting torque is in this

class of design.• The pullout torque is lower than that for class A induction motors.• In this design the motors are built from double-cage rotors. They are

more expensive than motors of Class A and B classes.• Class C Designs are used for high-starting-torque loads (loaded pumps,

compressors, and conveyors).

• In Class D Design• In this Design of Class motors has very high starting torque (275

percent or more of the rated torque).• A low starting current. • A high slip at full load. • Again in this class of design the high rotor resistance shifts the peak

torque to a very low speed. • It is even possible at zero speed (100 percent slip) for the highest

torque to occur in this class of design.• Full-load slip (It is typically 7 to 11 percent, but may go as high as 17

percent or more) in this class of design is quite high because of the high rotor resistance always.

• In class E Design• Very Low Starting Torque. • Normal Starting Current. • Low Slip. • Compensator or resistance starter are used to control starting

current.• In Class F Design• Low Starting Torque, 1.25 times of full load torque when full voltage is

applied.• Low Starting Current. • Normal Slip.

Circle Diagram of Induction MotorThe “CIRCLE DIAGRAM” means that it is figure or curve which is drawn has a circular shape. As we know, the diagrammatic representation is easier compared to theoretical and mathematical descriptions. Actually, we do not have that much time or patience to go through the writings so we prefer diagrammatic representation. Also, it is very easy to remember the things which are shown in picture. As we know, “A PICTURE IS WORTH 1000 WORDS”. This also holds good here and we are to draw circle diagram in order to compute various parameters rather than doing it mathematically. Importance of Circle DiagramThe diagram provides information which is not provided by an ordinary phasor diagram. A phasor diagram gives relation between current and voltage only at a single circuit condition. If the condition changes, we need to draw the phasor diagram again. But a circle diagram may be referred to as a phasor diagram drawn in one plane for more than one circuit conditions. On the context of induction motor, which is our main interest, we can get information about its power output, power factor, torque, slip, speed, copper loss, efficiency etc. in a graphical or in a diagrammatic representation.

• Test Performed to Compute Data Required for Drawing Circle Diagram• We have to perform no load and blocked rotor test in an induction

motor. In no load test, the induction motor is run at no load and by two watt meter method, its total power consumed is calculated which is composed of no load losses only. Slip is assumed to be zero. From here no load current and the angle between voltage and current is required for drawing circle diagram and calculated. The angle will be large as in the no load condition induction motor has high inductive reactance. In block rotor test, rotor is blocked which is analogous to short circuit secondary of a transformer. From this test, we need to calculate short circuit current and the lag angle between voltage and current for drawing circle diagram. Also, we need rotor and stator copper loss.

• Procedure to Draw the Circle Diagram• We have to assume a suitable before drawing it. This assumption is done according to

our convenience.

• The no load current and the no load angle calculated from no load test is plotted. This is shown by the line OA, where Ɵ0 is the no load power factor angle.

• The short circuit current and the angle obtained from block rotor test is plotted. This is shown by the line OC and the angle is shown by ƟB.

• The right bisector of the line AC is drawn which bisects the line and it is extended to cut in the line AE which gives us the Centre.

• The stator current is calculated from the equivalent circuit of the induction motor which we get from the two tests. That current is plotted in the circle diagram according to the scale with touching origin and a point in the circle diagram which is shown by B.

• The line AC is called the power line. By using the scale for power conversion that we have taken in the circle diagram, we can get the output power if we move vertically above the line AC to the periphery of the circle. The output power is given by the line MB.

• The total copper loss is given by the line GM.• For drawing the torque line, the total copper loss should be separated to both the rotor

copper loss and stator copper loss. The line DE gives the stator copper loss and the line CD gives the rotor copper loss. In this way, the point E is selected.

• The line AD is known as torque line which gives the torque developed by induction motor.

• Maximum Quantities from Circle Diagram

• Maximum Output Power• When the tangent to the circle is parallel to the line then output power will be

maximum. That point M is obtained by drawing a perpendicular line from the center to the output line and extending it to cut at M. Maximum Torque• When the tangent to the circle is parallel to the torque line, it gives maximum

torque. This is obtained by drawing a line from the center in perpendicular to the torque line and extending it to cut at the circle. That point is marked as N. Maximum Input Power• It occurs when tangent to the circle is perpendicular to the horizontal line. The point

is the highest point in the circle diagram and drawn to the center and extends into S. That point is marked as R. Conclusion of Circle Diagram• This method is based on some approximations that we have used in order to draw

the circle diagram and also, there is some rounding off of the values as well. So there is some error in this method but it can give good approximate results. Also, this method is very much time consuming so it is drawn at times where the drawing of circle diagram is absolutely necessary. Otherwise, we go for mathematical formulas or equivalent circuit model in order to find out various parameters.

• Torque Slip Characteristics of Induction MotorThe torque slip curve for an induction motor gives us the information about the variation of torque with the slip. The slip is defined as the ratio of difference of synchronous speed to the speed at any mechanical load to the synchronous speed of the machine. The variation of slip can be obtained with the variation on speed that is when speed varies the slip will also vary and the torque corresponding to that speed will also vary. The curve can be described in three modes of operation-

• Motoring Mode In this mode of operation, supply is given to the stator sides and the motor always rotates below the synchronous speed. The induction motor torque varies from zero to full load torque as the slip varies. The slip varies from zero to one. It is zero at no load and one at standstill. From the curve it is seen that the torque is directly proportional to the slip. That is, more is the slip, more will be the torque produced and vice-versa. The linear relationship simplifies the calculation of motor parameter to great extent. Generating Mode In this mode of operation induction motor runs above the synchronous speed and it should be driven by a prime mover. The stator winding is connected to a three phase supply in which it supplies electrical energy. Actually, in this case, the torque and slip both are negative so the motor receives mechanical energy and delivers electrical energy. Induction motor is not much used as generator because it requires reactive power for its operation. That is, reactive power should be supplied from outside and if it runs below the synchronous speed by any means, it consumes electrical energy rather than giving it at the output. So, as far as possible, induction generators are generally avoided.

• Braking Mode In the breaking mode, the two leads or the polarity of the supply voltage is changed so that the motor starts to rotate in the reverse direction and as a result the motor stops. This method of breaking is known as plugging. This method is used when it is required to stop the motor within a very short period of time. The kinetic energy stored in the revolving load is dissipated as heat. Also, motor is still receiving power from the stator which is also dissipated as heat. So as a result of which motor develops enormous heat energy. For this stator is disconnected from the supply before motor enters the breaking mode.• If load which the motor drives accelerates the motor in the same direction

as the motor is rotating, the speed of the motor may increase more than synchronous speed. In this case, it acts as an induction generator which supplies electrical energy to the mains which tends to slow down the motor to its synchronous speed, in this case the motor stops. This type of breaking principle is called dynamic or regenerative breaking.

• Torque Slip Characteristics of Single Phase Induction Motor

• From the figure, we see that at a slip of unity, both forward and backward field develops equal torque but the direction of which are opposite to each other so the net torque produced is zero hence the motor fails to start. From here we can say that these motors are not self starting unlike the case of three phase induction motor. There must be some means to provide the starting torque. If by some means, we can increase the forward speed of the machine due to which the forward slip decreases the forward torque will increase and the reverse torque will decrease as a result of which motor will start.• From here we can conclude that for starting of single phase induction

motor, there should be a production of difference of torque between the forward and backward field. If the forward field torque is larger than the backward field than the motor rotates in forward or anti clockwise direction. If the torque due to backward field is larger compared to other, then the motor rotates in backward or clockwise direction.

Torque Equation of Three Phase Induction MotorThe torque produced by three phase induction motor depends upon the following three factors:Firstly the magnitude of rotor current, secondly the flux which interact with the rotor of three phase induction motor and is responsible for producing emf in the rotor part of induction motor, lastly the power factor of rotor of the three phase induction motor. Combining all these factors together we get the equation of torque as-

Where, T is the torque produced by induction motor, φ is flux responsible of producing induced emf, I2 is rotor current, cosθ2 is the power factor of rotor circuit. The flux φ produced by the stator is proportional to stator emf E1. i.e φ ∝ E1 We know that transformation ratio K is defined as the ratio of secondary voltage (rotor voltage) to that of primary voltage (stator voltage).

Rotor current I2 is defined as the ratio of rotor induced emf under running condition , sE2 to total impedance, Z2 of rotor side,

and total impedance Z2 on rotor side is given by

• Putting this value in above equation we get,

We know that power factor is defined as ratio of resistance to that of impedance. The power factor of the rotor circuit is

Putting the value of flux φ, rotor current I2, power factor cosθ2 in the equation of torque we get,

• Combining similar term we get,

Removing proportionality constant we get,

Where ns is synchronous speed in r. p. s, ns = Ns / 60. So, finally the equation of torque becomes,

• Derivation of K in torque equation. In case of three phase induction motor, there occur copper losses in rotor. These rotor copper losses are expressed as Pc = 3I2

2R2 We know that rotor current,

Substitute this value of I2 in the equation of rotor copper losses, Pc. So, we get

The ratio of P2 : Pc : Pm = 1 : s : (1 - s) Where P2 is the rotor input, Pc is the rotor copper losses, Pm is the mechanical power developed.

Substitute the value of Pc in above equation we get,

On simplifying we get,

• The mechanical power developed Pm = Tω,

Substituting the value of Pm

We know that the rotor speed N = Ns(1 - s) Substituting this value of rotor speed in above equation we get,By calculating and substituting the all values we get the equation.

• Equation of Starting Torque of Three Phase Induction Motor• Starting torque is the torque produced by induction motor when it is

started. We know that at start the rotor speed, N is zero.

So, the equation of starting torque is easily obtained by simply putting the value of s = 1 in the equation of torque of the three phase induction motor,