INDUCTION MOTOR

Since the motors work on the principle of electromagnetic induction, it is called as induction motors. When the motor having three phase winding runs on AC 3-phase supply on induction principle it is said to be an AC three phase induction motor.

Principle: When a short circuited conductor is placed within a rotating magnetic field, due to electromagnetic induction a current is set up in the conductor and tends to move.

Current taken by a 3-phase induction motor is calculated by the formula:

                                                  H.P. x 746

Current(I) in amps = ------------------------------------

                                             √3 x V x Cosφ x η

Where V = Line voltage,  Cosφ = Power factor,  η = Efficiency.

Types of Induction motor: (a) Squirrel case induction motor

                                                               (i) Single cage, (ii) Double cage

                                                (b) Wound rotor or Slip ring induction motor.

Difference between Squirrel cage type rotor and phase wound type rotor:  Squirrel cage type rotor has no winding. A large number of copper bars area arranged on end rings at each end, riveted and soldered forming the whole system a short circuited body appears to be the cage of a squirrel.  

Phase wound rotor has second winding like stator accommodated in the slots of rotor core the terminals of which are brought out and connected to slip rings mounted on the same shaft.

Type of slot: (a) Semi closed type slots & (b) Closed type slots   

Semi closed type slots are preferred in wound rotors to reduce the magnetic reluctance.

Rotor slots made skewed by a small angle to the shaft axis:

(a) To prevent magnetic locking of rotor teeth with stator teeth.  

(b) To reduce humming noise when running

(c) To be more uniform torque when running

(d) To avoid crawling of an induction motor.

Advantage of using large number of slots in an induction motor: Variable air gap reluctance is reduced thus resulting in reduction of pulsating exciting current, irregular torque, tooth losses and noise.

 Disadvantage of using large number of slots in an induction motor: Tooth thickness is reduced which increases the cost of manufacture.

Advantage of decreasing the air gap length of a three phase induction motor: It reduces the requirement of magnetizing current to set up the air gap flux and also improves power factor.

Disadvantage of decreasing the air gap length of a three phase induction motor: It increases the possibility of mechanical difficulties motor noise and tooth losses.

Rotating magnetic field: When an AC source is applied to the stator circuit of the motor a uniform magnetic field of constant value is created on the stator which rotates in space at synchronous speed and when impressed on the rotor circuit produces the rotation of the rotor, then this field is said to be rotating magnetic field.

Working of Induction motor: The stator winding when fed by a 3-phase supply creates a rotating magnetic field revolving at synchronous speed. As this rotating magnetic field cuts the short circuited copper bars of the squirrel cage rotor winding a heavy current flows in the rotor bars and creates another field.

The effect of the rotor field is to oppose the main rotating field. Since the rotor is free to move it tends to follow the rotating field in the same direction. As a result the rotor starts and picks up speed until it approaches the synchronous speed. But the rotor actually never reaches the synchronous speed, runs always just below the synchronous speed.

Synchronous speed of an AC motor: In an AC machine for a fixed number of poles, the speed at which the rotating magnetic flux rotates is called the synchronous speed. It is denoted by N_s.

Slip of an AC induction motor: The difference between synchronous speed and rotor speed expressed in terms of synchronous speed is called the slip of the motor. So slip of the motor:

       N_S - N

S = ----------         It is also known as fractional slip.

          N_S        

                                     N_S - N

Percentage slip, S = ----------  x 100%        It is also known as fractional slip.   

                                        N_S        

The difference between the synchronous speed and the rotor speed is called the slip speed.

The Induction motor runs always below the synchronous speed.

Slip speed: Is a difference between the synchronous speed and the rotor speed.

The slip speed can be measured by (i) Synchronous speed and actual speed measurement method and (ii) Stroboscopic method.

The synchronous speed of an induction motor depends on the frequency and number of poles. If the number of poles are doubled the speed will be half.

Frequency of a rotor current: If ‘f₂’ is the frequency of rotor current then:

                                                                                                              f₂      N_S - N

f₂ is proportional to Ns – N. But f is proportional to Ns    so    ---- = ---------  = S     so  f₂ = S.f.

                                                                                                               f            N_S

Therefore the rotor frequency depends upon the slip and supply frequency both.

Rotor frequency at the time of starting and running conditions: At the time of starting the rotor frequency is same as the supply frequency but decreases with the increasing of the speed of the motor and in running condition the rotor frequency is equal to the multiplication of slip and supply frequency.

 If the motor reaches the synchronous speed the relative motion between stator field and rotor will be zero. As a result there will be no emf induced on rotor conductors and hence no current or torque will be created. The rotor therefore can never reach the synchronous speed.

Slip of an induction motor becomes zero: The motor will stop.

Torque: Torque means the turning or twisting moment of a force about an axis which is measured by the product of the force and the radius at which this force acts.

Torque equation of an induction motor:

Torque developed by the rotor, T = K₁ E₂I₂cosφ₂

Where K₁ = q donw5qn5,

             E2 = Standstill emf induced in the rotor/phase

             I2 = Rotor current and

      cosφ₂ = Rotor power factor.

The torque of an induction motor is depends upon the flux produced by the stator, rotor current per phase and rotor power factor.

There are two types of torque taken into consideration e.g. (a) Starting torque & (b) Running torque.

Torque is proportional to the slip.

Since the speed of an induction motor slows down slightly when the torque is increased therefore the drooping nature of speed torque curve known as shunt characteristics is applicable to the induction motor.

Torque is directly proportional to the square of the voltage.

The starting torque will be the maximum when the rotor winding resistance per phase (R₂) is equal to the rotor winding reactance per phase at standstill (X₂).

Normally the rotor winding resistance per phase (R₂) is about 1 or 2 percent of the rotor winding reactance per phase at standstill(X₂). For getting good starting torque a large rotor resistance with minimum standstill reactance and reduced supply voltage are preferable. For getting good running torque a low rotor resistance and constant (steady) supply voltage are preferable.

Breakdown torque of an induction motor: The maximum torque developed by an induction motor at which the motor becomes unstable with increase in slip is called as breakdown torque. It is nearly double the full load torque.

The squirrel cage motor has poor starting torque because the rotor of the squirrel cage motor has a little resistance as compared to its reactance. So the starting current of rotor becomes large and lagging behind the rotor emf with a large angle resulting poor rotor power factor. Due to poor rotor power factor starting torque becomes poor.

 Relation between (i) Full load torque and maximum torque & (ii) Starting torque and maximum torque of an induction motor:

                  Full load torque(T₁)                     2aS

(i)             --------------------------------        =   ---------

                 Maximum torque(T_max)              a² + 1

  

                  Starting torque(T_S1)                       2a

(ii)             --------------------------------        =   ---------

                 Maximum torque(T_max)              a² + 1

  

                      R₂

Where a =  ------   and S = full load slip of the motor.

                      X₂

Relation between the number of poles of stator and rotor windings of a three phase induction motor: The number of poles are same for both stator and rotor. By adding external resistance in the rotor circuit rotor power factor is improved and accordingly high starting torque is produced.

Slip ring induction motor has a wound rotor. By adding external resistance in the rotor circuit rotor power factor is improved and accordingly high starting torque is produced. The slip ring motor is used where high starting torque is required.

Advantage of slip ring motor over a squirrel cage motor:

a) The stating current of a slip ring motor is less than that of a squirrel cage motor.

b) Due to the presence of resistance in rotor circuit the starting torque of a slip ring motor is higher than that of a cage motor.

c) Due to the presence of starter in the rotor circuit speed control to some extent is possible in slip ring motor. But no speed control is possible in cage motor.

Disadvantage of continuous running of a slip ring motor at reduced speed with external resistance in rotor circuit:  The disadvantages due to external resistance in rotor circuit in continuous running operation at reduced speed are (i) the I²R losses increase which decrease the operating efficiency of the motor. The loss is directly proportional to the reduction in the speed, (ii) Double dependence of speed not only on load but also on external resistance.

Double cage rotor: Double cage rotor is used to overcome the disadvantage of a poor starting torque of one cage rotor without resorting rotor winding, thus giving a higher starting torque.

High torque induction motor: High torque induction motors are those motors which are intended to give a high starting torque of 60% to 75% of full load torque with a comparatively low starting current of 1.5 to 1.75 times full load current. These motors incorporate two distinct sets of squirrel cage bars placed one over another inside the rotor slots in such a manner of high resistance low reactance bars for starting at upper end and low resistance high reactance bars for running at lower end. They look like ordinary squirrel cage motor but perform as slip ring motors.

Advantage of Double cage induction motor over single cage induction motor: Comparatively high starting torque with less starting current is obtained for which ‘direct on’ starting can be adopted.

Effect of variation of supply voltage in an induction motor:

Increase in supply voltage decreases power factor, secondary copper loss and slip but increases torque, magnetic density, magnetizing current and iron loss.

Decrease in supply voltage increases power factor, secondary copper loss and slip but decreases torque, magnetic density, magnetizing current and iron loss.

Variation of supply frequency in an induction motor: Increase in supply frequency increases power factor but decreases the torque while decrease in supply frequency decreases power factor but increases the torque. Variation of supply frequency has no effect on percent slip.

Slip ring motor can be used as a variable speed motor if it is designed specially for this purpose.

Single phasing of a 3-ph induction motor: Single phasing means the opening of one wire eigher by blown of fuse or any disconnection) during running of three phase motor when other two wires will act as single phase operation.

Losses in three phase induction motors:

(i) Constant losses:-

a) Stator iron losses : Eddy current loss and Hysteresis loss,

b) Wind age loss &

c)  frictional loss

(ii) Variable losses:

a) Stator copper loss & b) Rotor copper loss.

Different methods of speed control of three phase induction motor:

a) By varying the frequency

b) By changing the number of poles

c) By inserting resistance in the rotor circuit

d) By injecting emfs in the rotor circuit and

e) By cascade connection

Ø  If frequency will increase, the speed of the motor will be increased as the speed is directly proportional to the frequency.

Ø  In the number poles will increase, the speed will be decreases as the speed in inversely proportional to the number of poles.

Ø  The speed is controlled by inserting resistance in the rotor circuit to control the speed below normal.

Ø  Rheostatic control causes more copper losses(I²R). This is the disadvantage.

Cogging of squirrel cage induction motor: With certain ratios of stator slots of rotor slots an induction motor may exhibit a tendency to run stably at low speed e.g. one seventh of normal speed on account of the presence of a pronounced seventh harmonics in the field form. This phenomenon of running at low speed is known as crawling or balking. This phenomenon is also observed with other harmonics. This crawling can be avoided by skewing the rotor slots.

Advantages and disadvantages of AC induction motors as compared with DC motors:

Advantages:

i) Like DC motor AC induction motor has no commutator and brushes and therefore it is simpler and robust in construction.

(ii) It has long life, high efficiency and more or less constant speed.

(iii) Due to simplicity its cost is low and  it is very reliable.

iv) It requires minimum maintenance.

v) Its starting arrangement is very simple

Disadvantage:

(i) Speed variation is not possible.

(ii) With the increase of load its speed decreases like DC shunt motor.

(iii) Its starting torque is comparatively inferior to that of a DC motor

Comparison of a squirrel cage motor with a slip ring motor:

a) The construction of the squirrel cage motor is robust and compact. It has no winding on the rotor like slip ring motor.

b) There is no chance of fire risk due to sparking in cage motor as it needs no slip rings or brushes like slip ring motor.

c) The starting current of cage motor is larger than that of slip ring motor.

d) The starting torque of a slip ring motor is higher than that of a cage motor.

e) Speed control is not possible in cage motors. But speed control in slip ring motor is possible due to presence of starter resistance in the rotor circuit.

f) The efficiency of cage motors is higher than that of slip ring motors.

g) Power factor is comparatively low in cage motor.

h) Maintenance is comparatively low in cage motor.

 Effect of single phasing of(i) working (ii) non working motor:

i) If single phasing occurs when a three phase motor is working the motor will continue to run as a single phase machine with a peculiarly squeaking noise resulting large current increases in the running two lines and also in all the three windings till it is isolated by the over load device or the single phasing preventer. As single phasing in working motor of course reduces the capacity of the motor and also places extra duty to the windings if this condition persists for a long time the ultimate result will be the burnt out of the coils.

99) If single phasing occurs in case of a non working three phase motor, the motor will fail to start and give a humming sound when it is connected to the supply lines. Current drawn under this condition will be about 87% of the starting current under the normal condition. As a result the motor winding may be burnt out if the supply voltage is applied for a long time.

A three phase motor continues to run even if the fuse on one phase is blown: Current will continue to be induced in the short circuited rotor winding. For the same loading condition the stator winding will be heavily overloaded and get unduly heated up resulting a probable burnt out of the phase winding.

I) The phase connected across the live or operative lines is likely to burn out carrying nearly 300% of its normal current.

ii) The two phases falling in series combination connected across the live or operative lines are likely to burn out carrying nearly 250% of its normal full load current.

Different types of duties of motor power ratings:

i) Continuous duty (S₁) : Continuous rating is that amount of power which it can deliver continuously for sufficiently long interval of time so that it reaches the final steady state temperature.

ii) Short time duty (S₂): Short time rating is the maximum power which it can deliver at a constant load for a short period of time followed by a long period of rest so as to cool down to the initial temperature.

iii) Intermittent periodic duty (S₃): Intermittent periodic rating is the maximum power which it can deliver with a constant load for a fixed short period of time (1 hour or half an hour or 15 minutes) followed by a short period  of rest according to the duty cycle so that the motor does not reach a steady temperature during working periods and cools down to ambient temperature within short period of rests.

iv) Intermittent periodic duty with starting (S₄)

v) Intermittent periodic duty with starting and braking (S₅)

vi) Continuous duty with intermittent periodic loading (S₆)

vii) Continuous duty with starting and braking (S₇)

viii) Continuous duty with periodic speed changes (S₈).

Difference between contactor and circuit breaker: A contactor is basically capable of making, carrying and breaking electric current under normal and overload conditions, whereas a circuit breaker must be capable of making, carrying and breaking short circuit currents as per the assigned ratings.

Breaking capacity of a contactor used in AC circuit: In AC system the rated breaking capacity of a contactor is the r.m.s. value of symmetrical component of current which the contactor can break without excessive erosion of contacts or, display of flame.

Inching: It means the energization of motor once or for repeat periods to obtain small movements for mechanisms. It is also called jogging. For inching and plugging application of a squirrel cage induction motor AC-4 category of contactor is used.

 If rotor bars of induction motor will break: If rotor bars are broken the induction motor will run at abnormal speed below normal even with rated voltage and frequency resulting noise, low starting torque and variation of current and the rotor will be overheated due to abnormal current.

Power factor correction: It means improvement of power factor by the uses of devices which utilize reactive power with a leading power factor thus reducing the overall reactive component in the system.

Three phase delta connected capacitor bank unit is connected in parallel with the supply lines to the motor in such a position as close as possible to the motor on the load side of its controlling apparatus.