# Recent questions and answers in Induction Machine

1 answer 34 views

The ratio of rotor output to rotor input in an induction motor is 0.96. The slip of the motor is (a) 0.04% (b) 4% (c) 6% (d) 12%

1 answer 53 views

A 6-pole, 50Hz, 3-φ induction motor is running at 950 r.p.m. and has Cu loss of 5kW. The rotor input is (a) 10 kW (b) 95 kW (c) 100 kW (d) 9.5 kW

2 answers 89 views

In the Squirrel cage induction motors, the rotor slots are usually a slight skew to reduce (A) Windage losses (B) Eddy currents (C) Accumulation of dirt and dust (D) Magnetic hum

1 answer 16 views

The speed and torque of induction motors can be varied by which of the following means: 1) Stator voltage control 2) Rotor voltage control 3) Frequency control 4) All of these

1 answer 34 views

An 8 pole wound rotor induction motor operating on 60 Hz supply is driven at 1800 r.p.m. by a prime mover in the opposite direction of revolving magnetic field. The frequency of the rotor current is (a) 30 Hz (b) 60 Hz (c) 120 Hz (d) 180 Hz

1 answer 17 views

Full load current of an Induction motor is 20 Ampere. The no-load current may be expected to be: A) 25 Ampere to 28 Ampere B) 15 Ampere to 18 Ampere C) 11 Ampere to 14 Ampere D) 6 Ampere to 10 Ampere

1 answer 17 views

Synchronous wattage of induction motor means: A) rotor input in watts B) stator input in watts C) combined stator and rotor input in watts D) shaft output in watts

1 answer 14 views

Which statement out of the following is a wrong statement with regard to Induction motor? A) Auto-transformer starter is costlier to “direct on line” (DOL) starter B) DOL starter is used for motors up to 5 horse power only C) Auto transformer starter is used for motors up to 20 horse power only D) Star delta starter connects the motor first in delta at the time of starting then in star for running

1 answer 21 views

The stator of a 415V, 50 Hz, 4-pole slip ring Induction Motor is excited by rated frequency with slip rings open circuited. The rotor is made to rotate at a speed of 600 rpm in the opposite direction to the normal direction, The frequency of induced voltage across slip ring is A) 30 Hz B) 70 Hz C) 20 Hz D) 35 Hz

0 answers 24 views

The stator mmf and rotor mmf of a 3-phase induction motor (A) are equal (B) rotate with slip speed (C) are opposite but rotate with synchronous speed (D) rotate with the same rotor speed

1 answer 10 views

The rotor frequency for a 3-phase 6-pole, 1000 r.p.m. induction motor with a slip of 0.04 is (a) 2 Hz (b) 4 Hz (c) 6 Hz (d) 8 Hz

0 answers 87 views

The maximum torque of an induction motor is 200 kg-m at a slip of 12%. The torque at 6% slip would be (a) 100 kg-m (b) 160 kg-m (c) 50 kg-m (d) 40 kg-m

0 answers 54 views

The effect of increasing the length of air gap in an induction motor is to increase the (a) power factor (b) speed (c) magnetising current (d) air gap flux

0 answers 12 views

An induction motor operating at 0.8 pf lagging is consuming 300 kW. A zero real power consuming synchronous motor is connected across the induction motor to raise the pf to 0.92 lagging. The reactive power drawn by the synchronous motor is a. - 352 kVAR b. - 225 kVAR c. - 127.8 kVAR d. - 97.2 kVAR

1 answer 6 views

Electrical fault between two windings of same phase of a generator having double star winding can be detected by the following protection in a generator:

1 answer 4 views

Induction generator is stable at a:

1 answer 13 views

A 3-phase delta connected squirrel cage induction motor when started with a DOL starter has a starting torque of 600 NM. Its starting torque when star delta starter is used:

1 answer 13 views

In a 3-phase voltage source inverter used for speed control of induction motor, anti-parallel diodes are used across each switching device. The main purpose of diodes is to:

1 answer 13 views

A 3-phase induction motor is running at slip ‘s’. If its two supply leads are interchanged, then the operating slip at that instant will be:

1 answer 18 views

If a 3-phase, 400V, 50Hz, 4 pole induction motor is running at a slip of 5% then the relative speed of rotor field with respect to stator filed is:

1 answer 20 views

A three phase slip ring induction motor is fed from the rotor side with stator winding short circuited stator is: (1) Slip frequency (2) Supply frequency (3) Frequency corresponding to rotor speed (4) Zero

1 answer 14 views

Deep bar rotor construction is used in three phase induction motors to mainly: (1) Control speed (2) Control power factor (3) Increase starting torque (4) None of these

1 answer 321 views

in a three-phase induction motor, the rotor field runs at the following speed with respect to the stator structure:

1 answer 514 views

A single-phase induction motor with only the main winding excited would exhibit the following response at synchronous speed  (A) Rotor current is zero (B) Rotor current is non-zero and is at slip frequency (C) Forward and backward rotating fields are equal (D) Forward rotating field is more than the backward rotating field

1 answer 17 views

A 3-phase induction motor is driving a constant torque load at rated voltage and frequency. If both voltage and frequency are halved, following statements relate to the new condition if stator resistance, leakage reactance and core loss are ignored  1. The difference between synchronous speed and actual speed remains same 2. The airgap flux remains same 3. The ... . slip remains same Among the above, current statements are  (A) All (B) 1, 2 and 3 (C) 2, 3 and 4 (D) 1 and 4

1 answer 10 views

No-load test on a 3-phase induction motor was conducted at different supply voltage and a plot of input power versus voltage was drawn. This curve was extrapolated to intersect the y-axis. The intersection point yields  (A) Core loss (B) Stator copper loss (C) Stray load loss (D) Friction and windage loss

1 answer 16 views

A 400 V, 15 kW, 4-pole, 50 Hz, Y-connected induction motor has full load slip of 4%. The output torque of the machine at full load is  (A) 1.66 Nm (B) 95.50 Nm (C) 99.47 Nm (D) 624.73 Nm

1 answer 16 views

The synchronous speed for the seventh space harmonic mmf wave of a 3-phase, 8-pole, 50 Hz induction machine is  (A) 107.14 rpm in forward direction (B) 107.14 rpm in reverse direction (C) 5250 rpm in forward direction (D) 5250 rpm in reverse direction

1 answer 10 views

The direction of rotation of a 3-phase induction motor is clockwise when it is supplied with 3-phase sinusoidal voltage having phase sequence A-B-C. For counter clockwise rotation of the motor, the phase sequence of the power supply should be  (A) B-C-A (B) C-A-B (C) A-C-B (D) B-C-A or C-A-B

1 answer 26 views

The type of single-phase induction motor having the highest power factor at full load is (A) shaded pole type (B) split-phase type (C) capacitor-start type (D) capacitor-run type

1 answer 16 views

In a single phase induction motor driving a fan load, the reason for having a high resistance rotor is to achieve (A) low starting torque (B) quick acceleration (C) high efficiency (D) reduced size

1 answer 15 views

A three-phase cage induction motor is started by direct-on-line (DOL) switching at the rated voltage. If the starting current drawn is 6 times the full load current, and the full load slip is 4%, then ratio of the starting developed torque to the full load torque is approximately equal to (A) 0.24 (B) 1.44 (C) 2.40 (D) 6.00

0 answers 22 views

Under no load condition, if the applied voltage to an induction motor is reduced from the rated voltage to half the rated value,  (A) the speed decreases and the stator current increases (B) both the speed and the stator current decreases (C) the speed and the stator current remain practically constant (D) there is negligible change in the speed but the stator current decreases

1 answer 18 views

The speed of a 4-pole induction motor is controlled by varying the supply frequency while maintaining the ratio of supply voltage to supply frequency (V/f ) constant. At rated frequency of 50 Hz and rated voltage of 400 V its speed is 1440 rpm. Find the speed at 30 Hz, if the load torque is constant  (A) 882 rpm (B) 864 rpm (C) 840 rpm (D) 828 rpm

1 answer 19 views

A three-phase squirrel cage induction motor has a starting torque of 150% and a maximum torque of 300% with respect to rated torque at rated voltage and rated frequency. Neglect the stator resistance and rotational losses. The value of slip for maximum torque is  (A) 13.48% (B) 16.42% (C) 18.92% (D) 26.79%

1 answer 15 views

The slip of an induction motor normally does not depend on  (A) rotor speed (B) synchronous speed (C) shaft torque (D) core-loss component

1 answer 14 views

A 4-pole induction motor, supplied by a slightly unbalanced three-phase 50 Hz source, is rotating at 1440 rpm. The electrical frequency in Hz of the induced negative sequence current in the rotor is  (A) 100 (B) 98 (C) 52 (D) 48

1 answer 14 views

Leakage flux in an induction motor is  (A) flux that leaks through the machine (B) flux that links both stator and rotor windings (C) flux that links none of the windings (D) flux that links the stator winding or the rotor winding but not both

1 answer 14 views

An induction motor having full load torque of 60 Nm when delta connected develops a starting torque of 120 Nm. For the same supply voltage if the motor is changed to star connection, the starting torque developed will be  A) 40 Nm B) 60 Nm C) 90 Nm D) 120 Nm

1 answer 9 views

When the supply voltage to an induction motor is reduced by 10%, the maximum torque will decrease by about  A) 5% B) 10% C) 20% D) 40%

1 answer 13 views

For a single phase capacitor start induction motor, which of the following statements is valid?  A) The capacitor is used for power factor improvement B) The direction of rotation can be changed by reversing the main winding terminals C) The direction of rotation cannot be changed D) The direction of rotation can be changed by interchanging the supply terminals

1 answer 39 views

The frequency (fr) of the rotor current in an induction motor is given by the expression (s = slip, f = supply frequency)

2 answers 14 views

The actual speed of an induction motor is always • Less than synchronous speed • More than synchronous speed • Equal to synchronous speed • None of the above.

1 answer 8 views

Consider the following statements:  In a 3-phase induction motor connected to a 3-phase supply; if one of the lines suddenly gets disconnected, then the  1. Motor will come to a standstill  2. Motor will continue to run at the same speed with line current unchanged  3. Motor will continue to run at a slightly speed with increase in the line current  ... these statements:   (a) 1 and 4 are correct   (b) 1 and 2 are correct   (c) 3 and 4 are correct   (d) 2 and 3 are correct

1 answer 6 views

Consider the following statements: Star delta starter is used in 3-phase induction motor because it  1. Prevents heating of the motor winding  2. Ensures permissible minimum starting current  3. Is regulated by electricity authority  4. Ensures smooth run up to full load   Of these statements (a) 1, 2 and 3 are correct (b) 2, 3 and 4 are correct (c) 1, 3 and 4 are correct (d) 1 and 2 are correct

1 answer 22 views

A three phase slip ring induction motor with chopper controlled resistance has its torque proportional to   (a) Rotor current (b) Stator resistance (c) Square of rotor current (d) Square root of rotor current

1 answer 23 views

The power factor of an induction motor under no load conditions will be close to (A) 0.2 lagging (B) 0.2 leading (C) 0.5 lagging (D) Unity

1 answer 12 views

Crawling in an Induction motor is caused by (A) Low voltage supply (B) High loads (C) Harmonics developed in the motor (D) Improper design of the machine

1 answer 16 views

The number of poles in a phase induction motor at 50 Hz and running at 1500 rpm will be?

1 answer 27 views
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