Polyphase Induction Motors Model Test(1st)

The efficiency of an induction motor can be expected to be nearly

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99 %
60 to 90 %
80 to 90 %
95 to 98 %

The injected e.m.f. in the rotor of induction motor must have

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The same frequency as the slip frequency
The same phase as the rotor e.m.f.
Zero frequency
High value for the satisfactory speed control

If an induction motor with certain ratio of rotor to stator slots, runs at ¹/₇ of the normal speed, the phenomenon will be termed as

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Crawling
Cogging
Hunting
Humming

The purpose of using short-circuit gear is

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To short circuit the stator phase of motor to form star
To short circuit the rotor at slip rings
None of the above
To short circuit the starting resistances in the starter

The torque developed in the cage induction motor with auto-starter is

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K × torque with direct switching
k2/torque with direct switching
K2 × torque with direct switching
k/torque with direct switching

Slip ring motor is preferred over squirrel cage induction motor where

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High starting torque is required
All of the above
Heavy pull out torque is required
Load torque is heavy

A 3-phase slip ring motor has

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Any of the above
Double cage rotor
Wound rotor
Short-circuited rotor

The starting torque of a 3 phase squirrel cage induction motor is

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Twice the full load torque
Equal to full load torque
1.5 times the full load torque
Any of the above

The number of slip rings on a squirrel cage induction motor is usually

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Four
None
Three
Two

Which type of bearing is provided in small induction motors to support the rotor shaft?

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Cast iron bearings
Ball bearings
Bush bearings
None of the above

Slip ring motor is recommended where

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Frequent starting, stopping and reversing is required
High starting torque is needed
Speed control is required
All above features are required

The frame of an induction motor is usually made of

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Bronze
Silicon steel
Aluminium
Cast iron

The speed characteristics of an induction motor closely resemble the speed-load characteristics of which of the following machines

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D.C. shunt motor
Universal motor
D.C. series motor
None of the above

Reduced voltage starter can be used with

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Squirrel cage as well as slip ring induction motor
None of the above
Slip ring motor only but not with squirrel cage induction motor
Squirrel cage induction motor only but not with slip ring motor

An induction motor can run at synchronous speed when

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e.m.f. is injected in the rotor circuit
It is run in reverse direction
It is run on load
It is run on voltage higher than the rated voltage

Insertion of resistance in the stator of an induction motor

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None of the above
Increases the load torque
Decreases the starting torque
Increases the starting torque

In an induction motor the relationship between stator slots and rotor slots is that

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None of the above
Stator slots are not exact multiple of rotor slots
Stator slots are exact multiple of rotor slots
Stator slots are equal to rotor slots

Insertion of reactance in the rotor circuit

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Reduces starting torque as well as maximum torque
Increases starting torque but maximum torque decreases
Increases starting torque but maximum torque remains unchanged
Increases starting torque as well as maximum torque

For which motor the speed can be controlled from rotor side?

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Slip-ring induction motor
None of the above
Both (A) and (B)
Squirrel-cage induction motor

The torque of an induction motor is

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Directly proportional to slip
Proportional to the square of the slip
Inversely proportional to slip
None of the above

The low power factor of induction motor is due to

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All of the above
Stator reactance
The reactive lagging magnetizing current necessary to generate the magnetic flux
Rotor leakage reactance

Size of a high speed motor as compared to low speed motor for the same H.P. will be

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Any of the above
Same
Bigger
Smaller

The torque of a rotor in an induction motor under running condition is maximum

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At the zero value of slip
At the value of the slip which makes rotor reactance per phase equal to the resistance per phase
At the value of the slip which makes the rotor reactance half of the rotor
At the unit value of slip

Short-circuit test on an induction motor cannot be used to determine

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Power scale of circle diagram
Copper losses
Transformation ratio
Windage losses

In case of the induction motors the torque is

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Directly proportional to (slip)2
Inversely proportional to slip
Directly proportional to slip
Inversely proportional to (V-slip)

In squirrel cage induction motors, the rotor slots are usually given slight skew in order to

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Reduce windage losses
Reduce accumulation of dirt and dust
Reduce magnetic hum
Reduce eddy currents

A squirrel cage induction motor is not selected when

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Higher starting torque is the main consideration
All above considerations are involved
Maintenance cost is to be kept low
Initial cost is the main consideration

An induction motor is identical to

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D.C. compound motor
D.C. series motor
Asynchronous motor
Synchronous motor

Step-less speed control of induction motor is possible by which of the following methods?

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None of the above
e.m.f. injection in rotor eueuit
Cascade operation
Changing the number of poles

In a three-phase induction motor

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Power factor at starting is high as compared to that while running
Power factor at starting in the same as that while running
None of these
Power factor at starting is low as compared to that while running

In an induction motor, no-load the slip is generally

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Less than 1 %
1.5 %
4 %
2 %

The speed of a squirrel-cage induction motor can be controlled by all of the following except

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Reducing supply voltage
Changing supply frequency
Changing number of poles
Changing winding resistance

The crawling in the induction motor is caused by

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Improper design of the machine
Harmonics developed in the motor
High loads
Low voltage supply

Which is of the following data is required to draw the circle diagram for an induction motor?

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Block rotor test and no-load test
Block rotor test only
Block rotor test, no-load test and stator resistance test
No load test only

The auto-starters (using three auto transformers) can be used to start cage induction motor of the following type

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Both (A) and (B)
Delta connected only
None of the above
Star connected only

Slip of an induction motor is negative when

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None of the above
Rotor speed is more than the synchronous speed of the field and are in the same direction
Magnetic field and rotor rotate in opposite direction
Rotor speed is less than the synchronous speed of the field and are in the same direction

Blocked rotor test on induction motors is used to find out

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All of the above
Power factor on short circuit
Leakage reactance
Short-circuit current under rated voltage

In a star-delta starter of an induction motor

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Resistance is inserted in the stator
Reduced voltage is applied to the stator
Applied voltage per stator phase is 57.7% of the line voltage
Resistance is inserted in the rotor

The torque developed by a 3-phase induction motor least depends on

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Rotor current
Rotor power factor
Shaft diameter
Rotor e.m.f.

What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero?

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The rotor will not run
The slip of the motor will be 5%
The torque produced will be very large
The rotor will run at very high speed

The starting torque of a three phase induction motor can be increased by

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Increasing current
Increasing slip
Both (A) and (B)
None of the above

In an induction motor if air-gap is increased

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Windage losses will be more
Bearing friction will reduce
Copper loss will reduce in an induction motor
The power factor will be low

5 H.P., 50Hz, 3-phase, 440 V, induction motors are available for the following r.p.m. Which motor will be the costliest?

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960 r.p.m.
730 r.p.m.
1440 r.p.m.
2880 r.p.m.

When the equivalent circuit diagram of double squirrel-cage induction motor is constructed the two cages can be considered

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In series-parallel
In parallel
In parallel with stator
In series

If any two phases for an induction motor are interchanged

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The motor will run at reduced speed
The motor will not run
The motor will burn
The motor will run in reverse direction

In three-phase induction motors sometimes copper bars are placed deep in the rotor to

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Improve power factor
Improve efficiency
Improve starting torque
Reduce copper losses

Slip rings are usually made of

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Aluminium
Phosphor bronze
Copper
Carbon

Lubricant used for ball bearing is usually

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Molasses
Graphite
Mineral oil
Grease

The shape of the torque/slip curve of induction motor is

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Parabola
Straight line
Hyperbola
Rectangular parabola

The term 'cogging' is associated with

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D.C. series motors
Three-phase transformers
Compound generators
Induction motors

Temperature of the stator winding of a three phase induction motor is obtained by

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Thermometer method
All above methods
Embedded temperature method
Resistance rise method

In a three-phase induction motor, the number of poles in the rotor winding is always

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More than the number of poles in stator
Zero
Less than number of poles in stator
Equal to number of poles in stator

An induction motor with 1000 r.p.m. speed will have

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2 poles
8 poles
4 poles
6 poles

The rotor of an induction motor runs at

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Synchronous speed
Below synchronous speed
Above synchronous speed
Any of the above

A 3-phase induction motor stator delta connected, is carrying full load and one of its fuses blows out. Then the motor

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Will stop and carry heavy current causing permanent damage to its winding
Will continue running without any harm to the winding
Will continue running burning its two phases
Will continue running burning its one phase

Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor?

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Rotor rheostat control
By operating two motors in cascade
By injecting e.m.f. in the rotor circuit
By changing the number of stator poles

For driving high inertia loads best type of induction motor suggested is

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Squirrel-cage type
Slip ring type
None of the above
Any of the above

The 'cogging' of an induction motor can be avoided by

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Proper ventilation
Using DOL starter
Having number of rotor slots more or less than the number of stator slots (not equal)
Autotransformer starter

The 'crawling' in an induction motor is caused by

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Harmonics developed in the motor
High loads
Improper design of machine
Low voltage supply

A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6-poles. The slip of the machine will be

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0.03
0.02
0.04
0.01

A double squirrel-cage induction motor has

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Two rotors moving in opposite direction
Two parallel windings in rotor
Two series windings in stator
Two parallel windings in stator

As load on an induction motor goes on increasing

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Its power factor goes on decreasing
Its power factor remains constant
Its power factor goes on increasing up to full load and then it falls again
Its power factor goes on increasing even after full load

Star-delta starting of motors is not possible in case of

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High speed motors
Variable speed motors
Low horse power motors
Single phase motors

Less maintenance troubles are experienced in case of

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Both (A) and (B)
Slip ring induction motor
None of the above
Squirrel cage induction motor

DOL starting of induction motors is usually restricted to

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Low horsepower motors
High horsepower motors
Variable speed motors
High speed motors

A 3-phase induction motor delta connected is carrying too heavy load and one of its fuses blows out. Then the motor

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Will continue running burning its one phase
Will continue running without any harm to the winding
Will continue running burning its two phase
Will stop and carry heavy current causing permanent damage to its winding

It is advisable to avoid line starting of induction motor and use starter because

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Starting torque is very high
It will pick up very high speed and may go out of step
Motor takes five to seven times its full load current
It will run in reverse direction

The complete circle diagram of induction motor can be drawn with the help of data found from

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All of the above
Stator resistance test
No-load test
Blocked rotor test

The shaft of an induction motor is made of

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Cast iron
Stainless steel
Carbon steel
High speed steel

Insertion of resistance in the rotor of an induction motor to develop a given torque

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Increases the rotor current
Rotor current remains same
Decreases the rotor current
Rotor current becomes zero

Low voltage at motor terminals is due to

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None of the above
Inadequate motor wiring
Poorly regulated power supply
Any one of the above

If a 3-phase supply is given to the stator and rotor is short circuited rotor will move

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In the same direction as the direction of the field
In the opposite direction as the direction of the rotating field
In any direction depending upon phase sequence of supply
None of these

In the circle diagram for induction motor, the diameter of the circle represents

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Line voltage
Slip
Running torque
Rotor current

In medium sized induction motors, the slip is generally around

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0.04 %
0.4 %
4 %
14 %

In the squirrel-cage induction motor the rotor slots are usually given slight skew

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To increase the tensile strength of the rotor bars
To ensure easy fabrication
To reduce the magnetic hum and locking tendency of the rotor
None of the above

A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be

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0.2 Hz
200 Hz
2 Hz
50 Hz

An induction motor is

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Self-starting with high torque
Self-starting with low torque
Non-self starting
Self-starting with zero torque

In induction motor, percentage slip depends on

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Supply voltage
Copper losses in motor
Supply frequency
None of the above

The stating torque of the slip ring induction motor can be increased by adding

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External resistance to the rotor
Both resistance and inductance to rotor
External inductance to the rotor
External capacitance to the rotor

In three-phase squirrel-cage induction motors

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Rotor conductors are kept open
Rotor conductors are short-circuited through end rings
Rotor conductors are connected to insulation
Rotor conductor ends are short-circuited through slip rings

A change of 4% of supply voltage to an induction motor will produce a change of approximately

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12% in the rotor torque
16% in the rotor torque
8% in the rotor torque
4% in the rotor torque

Rotor rheostat control method of speed control is used for

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Slip ring induction motors only
Both (A) and (B)
None of the above
Squirrel-cage induction motors only

The power scale of circle diagram of an induction motor can be found from

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No-load test only
None of the above
Short-circuit test only
Stator resistance test

The maximum torque in an induction motor depends on

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All of the above
Square of supply voltage
Rotor inductive reactance
Frequency

A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen? It will

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Stall immediately
Continue to run at lower speed without damage
Get heated and subsequently get damaged
Stall after sometime

Which of the following component is usually fabricated out of silicon steel?

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None of the above
Bearings
Shaft
Stator core

In a squirrel cage motor the induced e.m.f. is

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Slip times the stand still e.m.f. induced in the rotor
Dependent on the shaft loading
None of the above
Dependent on the number of slots

In N_s is the synchronous speed and s the slip, then actual running speed of an induction motor will be

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(l-s) N_s
s.N
(N_s-l) s
N_s

In case the air gap in an induction motor is increased

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Speed of motor will increase
The windage losses will increase
The power factor will decrease
The magnetizing current of the rotor will decrease

The vafcie of transformation ratio of an induction motor can be found by

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Open-circuit test only
Stator resistance test
Short-circuit test only
None of the above

The good power factor of an induction motor can be achieved if the average flux density in the air gap is

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Large
Small
Absent
Infinity

The circle diagram for an induction motor cannot be used to determine

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Frequency
Power factor
Output
Efficiency

Polyphase Induction Motors Model Test(1st) : Question 1 of 92