Two Marks Questions with Answers

Q. 31 On which factors the torque produced in an induction motor depends ? State the torque equation.

Ans. :

The torque produced in the induction motor depends on the following factors :

1. The part of rotating magnetic field which reacts with rotor and is responsible to produce induced e.m.f. in rotor.

2. The magnitude of rotor current in running condition.

3. The power factor of the rotor circuit in running condition.

The torque equation is expressed as,

 

Q. 32 Obtain the expression of starting torque from the basic torque equation for an induction motor.

Ans. :

At start s =1 hence the starting torque can be obtained by substituting s = 1 in the torque equation.

 

Q. 33 State the ratios : i) Full load and maximum torques ii) Starting and maximum torques

Ans. :

i) The ratio of full load torque and maximum torque is,

ii) The ratio of starting torque and maximum torque is,

 

 

Q. 34 Draw the torque-slip characteristics of three phase induction motor and show the various regions of operation.

 

Q. 35 State the various losses occurring in an induction motor.

Ans. :

The various power losses in an induction motor can be classified as,

i) Constant losses : These are further divided as core losses in stator and rotor cores and mechanical losses. The core losses are hysteresis and eddy current losses while the mechanical losses are friction and windage losses at the bearing.

ii) Variable losses : These include the copper losses in stator and rotor winding due to current flowing in the winding. These are called I₂R losses.

 

Q. 36 Draw the power flow diagram for an induction motor.

(Refer section 5.17) pg 371

 

Q. 37 State the relation between rotor input, rotor copper losses and gross mechanical power developed.

Ans. :

The relation between rotor input (P₂), rotor copper losses (P_c) and the gross mechanical power developed (P_m) is,

P₂ : P_c : P_m is 1 : s : 1 - s

 

Q. 38 State the applications of various types of induction motors.

Ans. :

i) Squirrel cage type of motors having moderate starting torque and constant speed characteristics preferred for driving fans, blowers, water pumps, grinders, lathe machines, printing machines, drilling machine.

ii) Slip ring induction motors can have high starting torque as high as maximum torque. Hence they are preferred for lifts, hoists, elevators, cranes, compressors.

 

Q. 39 Why the reactance of three phase induction motor greatly varies between starting and running condition ?

Ans. :

At start the reactance of induction motor is high. In running condition the frequency of rotor becomes 'si' which is very small as slip s is very small. The reactance is directly proportional to the frequency. Thus it also reduces according to the frequency. Hence there is considerable variation between reactance of induction motor at start and in running condition.

 

Q. 40 A 6 pole, 50 Hz, 3 phase induction motor runs at 800 r.p.m. at full load. Determine the value of slip at this load condition.

 Ans. :

 

Q. 42 What is the speed of the rotor field in space ?

Ans. :

The speed of rotor field is 120fr /P = 120f / P × s. Thus it is slip times the synchronous speed.

 

Q. 43 A 6 pole, 3 phase induction motor operating on a 50 Hz supply has a rotor e.m.f. frequency as 2 Hz. Determine the rotor speed.

Ans. :

 

Q. 44 Why rotor core losses are negligible in three phase induction motor ?

Ans. :

The core losses depend on the voltage and the frequency. In the running condition the induced e.m.f. in the rotor is slip times the induced e.m.f. in the standstill condition while the rotor frequency is slip times the supply frequency. The slip of three phase induction motor is very small hence in running condition rotor induced e.m.f. and rotor frequency both are very small. Hence the rotor core losses are negligible in three phase induction motor.

 

Q. 45 State the principle of 3 phase induction motor.

Ans. :

When a 3 phase supply is given to the stator winding rotating magnetic field is developed which rotates at synchronous speed. The rotor at rest cuts this rotating magnetic field and e.m.f. is induced in the rotor conductors. As rotor is short circuited, the rotor current is established. Due to this, rotor produces its own flux called rotor flux. This rotor flux interacts with stator rotating magnetic field to produce necessary torque due to which the induction motor rotates. The initial rotor induced e.m.f. is due to induction principle hence the motor is called induction motor. 

 

Q. 46 State the expression for the power factor of an induction motor.

Ans. :

The power factor of induction motor at standstill and running condition can be obtained by the expressions,

 

Q. 47 Find the % slip of an induction motor with Ns = 2000 r.p.m. and motor is running at 1850 r.p.m.

Ans. :

Q. 48 Mention the advantages of squirrel cage induction motor.

Ans. :

1. Simple construction 2. Slip ring and brushes are absent hence construction is robust and maintenance free. 3. Rotor automatically adjusts itself for same number of poles as that of stator. 4. Rotor resistance is small hence less copper losses and high efficiency.

 

Q. 49 A 50 Hz, 6 pole, 3-phase induction motor runs at 970 r.p.m. Find its slip.

Ans. :

 

Q. 50 State the effect of rotor resistance on starting torque.

Ans. :

As the rotor resistance at start increases, the starting torque increases. The starting torque can be maximum when R₂ = X₂.

 

Q. 51 Is it possible to add an external resistance in the rotor circuit of a cage induction motor ? Justify your answer.

Ans. :

It is not possible to add an external resistance in the rotor circuit of a cage induction motor. In a cage induction motor, the rotor is in the form of bars which are permanently short circuited. Hence its self resistance is almost zero. For such a short circuited rotor, an external resistance can not be added.

 

Q. 52 Why three phase induction motor called singly excited motor ?

Ans. :

 Generally in an electric motor, electric supply is given to both stator and rotor. But in induction motor, only stator is excited by external three phase supply and e.m.f. gets induced in the rotor by induction principle. Such a machine which is excited only from one side is called singly excited hence the three phase induction motor is called singly excited motor. 

 

Q. 53 Why are most of the 3-phase induction motors constructed with delta connected stator winding ?

Ans. :

For the same power rating, the current drawn by the delta connected stator winding, from the supply is much less than the star connected stator winding, in running condition. Due to this, the stator copper losses are less and performance of induction motor is bettter. Hence most of the three phase induction motors are constructed with delta connected stator winding.

 

Q. 54 What is meant by single phasing.

Ans. :

For proper working of 3 phase induction motor, it must be connected to 3 phase a.c. supply. Once the 3 phase induction motor is started, it continues to rim even if one of the three phase supply lines get disconnected. The loss of current through one of the supply lines is called single phasing.

 

Q. 55 What are the effects of single phasing of an induction motor ?

Ans. :

1. Speed of the motor reduces.

2. A huming noise and vibrations are developed in the motor due to uneven torque production.

3. The current in the remaining two lines increases which is approxiemately 2-4 times the normal values.

4. The motor gets over heated and may bum if single phase protection is not provided.

5. If single phasing occurs when motor is at rest, it cannot start.

 

Q. 56 The starting torque of a squirrel cage induction motor cannot be altered when the applied voltage is constant . Why ?

Ans. :

The starting torque can be altered by changing rotor resistance for constant voltage. In squirrel cage induction motor, the rotor is short circuited and it is not possible to add external resistance in the rotor circuit. Hence starting torque of squirrel cage induction motor can not be altered for constant voltage.

 

Q. 57 How do change in supply voltage and frequency affect the performance of a 3 phase induction motor ?

Ans. :

The torque T in the induction motor is directly proportional to _sV² where s is slip and V is supply voltage under constant frequency. Thus as voltage changes, the torque changes as V². The slip at which T_max occurs remains same but value of T_max changes as voltage changes.

If frequency is changed keeping voltage constant, then air gap flux changes which affects the induction motor performance badly. It may cause core saturation leading to sharp increase in the current.

Practically (V/f) ratio is maintained constant to keep air gap flux constant though voltage and frequency are changed. 

 

Q. 58 What are the merits and demerits of double squirrel cage induction motors ? AU : Dec.-03, 14

Ans. :

Merits are,

i) Low starting current

ii) High starting torque

iii) Wide range of torque-slip characteristics can be obtained.

iv) Gives excellent running performance.

Demerits are,

i) Maximum torque is small.

ii) Power factor is low.

iii) For same rating, cost is higher than single cage motor.

iv) Effective rotor resistance is high hence at start rotor heating is large.

 

Q. 59 A 3 phase, 4 pole, induction motor operates from a supply whose frequency is 50 Hz. Calculate the frequency of the rotor current at standstill and the speed at which the magnetic field of the stator is rotating.

Ans. :

P = 4, f = 50 Hz

The frequency of the rotor current at standstill is same as that of supply frequency, which is 50 Hz.

The speed of stator magnetic field is Ns, given by

Q. 60 What is the effect of change in supply voltage on starting torque of induction motor ?

Ans. :

The torque equation of 3 phase induction motor is,

At start s = 1 and T_st ∝ E²₂ The voltage E2 is directly proportional to supply voltage V. Hence T_st ∝V² i.e. starting torque is proportional to the square of the supply voltage.

 

Q. 61 Explain why an induction motor, at no load, operates at very low power factor. AU : May -18

Ans. :

On no load, the magnetizing component of input current is the large part while load part is least. This is inductive component of current. As load increases, the in phase current supplied increases while the magnetizing component remains same for all the loads.

The higher magnetization current at no load is required to overcome air gap reluctance. Due to this large lagging current component, the p.f. of induction motor is low on no load.