Losses in Induction Motor
The losses in induction motor are broadly categorized into two classes, namely constant losses and variable losses.
Constant losses in Induction Motor
These losses do not depend on the load hence they are known as constant losses. These can be further categorized as under:
- Core losses: These include eddy current and hysteresis losses in stator as well as in rotor magnetic core. Eddy current losses in rotor core are negligible since rotor current frequency is very small in the order of 0.5 to 2 Hz.
These losses of induction motor are constant since these depend upon voltage and frequency which is practically constant.
The hysteresis losses can be reduced by selecting a high permeability material for the core. The eddy current losses can be reduced by using the laminated cores instead of solid ones.
- Friction and windage losses: These losses are also constant losses as these losses depend upon the speed of the induction motor. The speed of the induction motor is approximately constant.
Variable Losses of Induction Motor
- I2R losses in the stator winding.
- I2R losses in the rotor winding.
These losses occur due to the resistance of rotor winding as well as the resistance of stator winding. These losses are also called copper losses.
These are proportional to the square of stator and rotor currents respectively. As these currents depend on the load, copper losses vary with the change in load.
Hence these are known as variable losses. The blocked rotor test is performed on induction motor to determine the variable losses.
Induction Motors — 4 | Objective Type Question Answers
#1 In induction motor, percentage slip depends on
copper losses in motor
#2 The efficiency of an induction motor ……….is than that of a transformer.
#3 If the rotor circuit of a squirrel cage induction motor is open, the rotor will
#4 The advantage of a slip-ring induction motor over a squirrel cage induction motor is that
it can be started with the help of rotor resistance starter
#5 The magnetising current drawn by induction motors and transformers is the cause of their………..power factor.
#6 A 3-phase slip-ring induction motor is always started with
full external resistance in rotor circuit
#7 Out of the following methods of starting a 3-phase induction motor, which one requires six stator terminals ?
#8 Each of the following statements regarding stator flux of an induction motor is true except
Its magnitude depends on the motor load
#9 The rotor of an induction motor cannot run with synchronous speed because
rotor torque would then become zero
#10 When a load on a squirrel-cage induction motor is increased from no-load to full-load, which of the following statement is incorrect ?
The magnetizing component of the stator exciting current is increased.
#11 The power factor of an induction motor under no-load conditions is about
#12 The principle of operation of a 3-phase induction motor is similar to that of
transformer with a shorted secondary
#13 The speed of a 3-phase wound motor can be increased by
decreasing external resistance in the rotor circuit
#14 The A.C. motor which would be best suited to drive a centrifugal pump for discharging a variable quantity of water against a fixed head is the
slip-ring Induction motor
#15 For starting a Schrage motor, 3-phase supply is connected to
rotor via slip rings
#16 The primary winding of a Schrage motor is located
in lower part of rotor
#17 When a stationary 3-phase induction motor is switched on with one phase disconnected
It is likely to bum out quickly unless immediately disconnected.
#18 The pull-out torque for a normal squirrel cage induction motor usually occurs at a percentage slip of about
20 to 25 per cent
#19 Speed variations or a squirrel cage induction motor are essentially similar to those of
D.C. shunt motor
#20 If air gap of the induction motor is increased, its
magnetizing current will increase
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