Induction Machines · Electrical Machines · GATE EE

On the torque/speed curve of induction motor shown in Fig, four points of operation are market as W, X, Y and Z. Which one of them represents the operation at a slip greater than 1?

For a linear electromagnetic circuit, the following statement is true.

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

The type of single-phase induction motor having the highest power factor at full load is

No-load test on a 3-phase induction motor was conducted at different supply voltages and a plot of input power versus voltage was drawn. This curve was extrapolated to intersect the y-axis. This intersection point yields

Group I lists different applications and Group II lists the motors for these applications. Match the application with the most suitable motor and choose the right combination among the choices given there after:

Group I
P. Food mixer
Q. Cassette tape recorder
R. Domestic water pump
S. Escalator

Group II
1. Permanent magnet dc motor
2. Single phase induction motor
3. Universal motor
4. Three phase induction motor
5. DC series motor
6. Stepper motor

If a 400 V, 50 Hz, star connected, 3 phase squirrel cage induction motor is operated from a 400 V, 75 Hz supply, the torque that the motor can now provide while drawing rated current from the supply?

GATE EE 2002

Starting torque can be obtained in the case of a single phase induction motor with identical main and auxiliary windings by connecting

GATE EE 1999

The following starting method for an induction motor is inferior view of the poor starting torque per ampere of the line current drawn:

GATE EE 1999

A 3-phase squirrel cage induction motor has a full load efficiency of 0.8 and a maximum efficiency of 0.9. It is operated at a slip of 0.6 by applying a reduced voltage. The efficiency of the motor at this operating point is:

GATE EE 1998

If an induction machine is run at above synchronous speed, it acts as

GATE EE 1997

Unbalanced supply voltage given to a $$3-$$ phase, data-connected induction motor will cause

When the supply voltage to an induction motor is reduced by $$10\% $$, the maximum torque will decrease by approximately

In case of a split phase motor, the phase shift between currents in the two windings is around

An induction motor having full load torque of $$60$$ $$Nm$$ when delta-connected develops a starting torque of $$120$$ $$N$$-$$m.$$ For the same simply voltage, if the motor is changed to star-connection, the starting torque developed will be

In an induction motor, if the air gap is increased

An induction motor is fed from a balanced three phase supply at rated voltage and frequency through a bank of three single phase transformers connected in delta-delta. One unit of the bank develops fault and is removed. Then

GATE EE 1995

Skew is used in induction motors in order to reduce torque due to

GATE EE 1994

A three phase slip ring induction motor is fed from the rotor side with stator winding short circuited. The frequency of the currents flowing in the short circuited stator is

GATE EE 1993

Marks 2

A 3-phase star connected slip ring induction motor has the following parameters referred to the stator:

$R_s = 3\, \Omega,\; X_s = 2\, \Omega,\; X_r' = 2\, \Omega,\; R_r' = 2.5\, \Omega$

The per phase stator to rotor effective turns ratio is 3:1. The rotor winding is also star connected. The magnetizing reactance and core loss of the motor can be neglected. To have maximum torque at starting, the value of the extra resistance in ohms (referred to the rotor side) to be connected in series with each phase of the rotor winding is _______ (rounded off to 2 decimal places).

GATE EE 2024

A three phase 415 V, 50 Hz, 6-pole, 960 RPM, 4 HP squirrel cage induction motor drives a constant torque load at rated speed operating from rated supply and delivering rated output. If the supply voltage and frequency are reduced by 20%, the resultant speed of the motor in RPM (neglecting the stator leakage impedance and rotational losses) is ___________ (Round off to the nearest integer).

GATE EE 2023

A 4-pole induction motor with inertia of 0.1 kg-m² drives a constant load torque of 2 Nm. The speed of the motor is increased linearly from 1000 rpm to 1500 rpm in 4 seconds as shown in the figure below. Neglect losses in the motor. The energy, in joules, consumed by the motor during the speed change is _________. (round off to nearest integer).

GATE EE 2022

A 3-phase, 415 V, 50 Hz induction motor draws 5 times the rated current at rated voltage at starting. It is required to bring down the starting current from the supply to 2 times of the rated current using a 3-phase autotransformer. If the magnetizing impedance of the induction motor and no load current of the autotransformer is neglected, then the transformation ratio of the autotransformer is given by ________. (round off to two decimal places).

GATE EE 2022

The equivalent circuit of a single phase induction motor is shown in the figure, where the parameters are

R₁ = R₂^' = X_l1 = X_l2^' = 12 $$\Omega $$, X_m = 240 $$\Omega $$

and s is the slip. At no-load, the motor speed can be approximated to be the synchronous speed. The no-load lagging power factor of the motor is___________ (up to 3 decimal places).

GATE EE 2018

A star-connected, 12.5 kW, 208 V (line), 3-phase, 60 Hz squirrel cage induction motor has following equivalent circuit parameters per phase referred to the stator. R₁ = 0.3 Ω, R₂ = 0.3 Ω, X₁ = 0.41 Ω, X₂ = 0.41 Ω. Neglect shunt branch in the equivalent circuit. The starting current (in Ampere) for this motor when connected to an 80 V (line), 20 Hz, 3- phase AC source is __________.

GATE EE 2017 Set 2

A 375W, 230 V, 50 Hz capacitor start single-phase induction motor has the following constants for the main and auxiliary windings (at starting): $$Z_m=\left(12.50\;+\;j15.75\right)\;\Omega$$ (main winding), $$Z_a=\left(24.50\;+\;j12.75\right)\;\Omega$$(auxiliary winding). Neglecting the magnetizing branch the value of the capacitance (in µF ) to be added in series with the auxiliary winding to obtain maximum torque at starting is _______.

GATE EE 2017 Set 1

A 3-phase 50 Hz square wave (6-step) VSI feeds a 3-phase, 4 pole induction motor. The VSI line voltage has a dominant 5^th harmonic component. If the operating slip of the motor with respect to fundamental component voltage is 0.04, the slip of the motor with respect to 5^th harmonic component of voltage is _________.

GATE EE 2015 Set 1

A 220 V, 3-phase, 4-pole, 50 Hz inductor motor of wound rotor type is supplied at rated voltage and frequency. The stator resistance, magnetizing reactance, and core loss are negligible. The maximum torque produced by the rotor is 225 % of full load torque and it occurs at 15% slip. The actual rotor resistance is 0.03 Ω/phase. The value of external resistance (in Ohm) which must be inserted in a rotor phase if the maximum torque is to occur at start is ______.

GATE EE 2015 Set 2

A three-phase slip-ring induction motor, provided with a commutator winding, is shown in the figure. The motor rotates in clockwise direction when the rotor windings are closed.

If the rotor winding is open circuited and the system is made to run at rotational speed f_r with the help of prime-mover in anti-clockwise direction, then the frequency of voltage across slip rings is f₁ and frequency of voltage across commutator brushes is f₂. The values of f₁ and f₂ respectively are

GATE EE 2014 Set 2

A 3 phase, 50 Hz, 6 pole induction motor has a rotor resistance of 0.1 Ω and reactance of 0.92 Ω. Neglect the voltage drop in stator and assume that the rotor resistance is constant. Given that the full load slip is 3%, the ratio of maximum torque to full load torque is

GATE EE 2014 Set 1

A 4-pole induction motor, supplied by a slightly unbalanced three-phase 50Hz source, is rotating at 1440 rpm. The electrical frequency in Hz of the induced negative sequence current in the rotor is

GATE EE 2013

The locked rotor current in a 3-phase, star connected 15 kW, 4-pole, 230 V, 50 Hz induction motor at rated conditions is 50 A. Neglecting losses and magnetizing current, the approximate locked rotor line current drawn when the motor is connected to a 236 V, 57 Hz supply is

GATE EE 2012

A three-phase 440V, 6 pole, 50Hz, squirrel cage induction motor is running at a slip of 5%. The speed of stator magnetic field to rotor magnetic field and speed of rotor with respect to stator magnetic field are

GATE EE 2011

A $$200$$ $$V,$$ $$50$$ $$Hz,$$ single-phase induction motor has the following connection diagram and winding orientations shown. $$MM'$$ is the axis of the main stator winding $$\left( {{M_1}{M_2}} \right)$$ and $$AA'$$ is that of the auxiliary winding $$\left( {{A_1}{A_2}} \right).$$ Directions of the winding axis indicate direction of flux when currents in the windings are in the directions shown. Parameters of each winding are indicated. When switch $$S$$ is closed, the motor

GATE EE 2009

A $$230V,$$ $$50Hz,$$ $$4$$ pole, single phase induction motor is rotating in the clockwise (forward) direction at a speed of $$1425$$ $$rpm.$$ If the rotor resistance at standstill is $$7.8\,\,\Omega ,$$ then the effective rotor resistance in the backward branch of the equivalent circuit will be

A $$400$$ $$V,$$ $$50$$ $$Hz$$, $$4$$ pole, $$1400$$ $$rpm$$, star connected squirrel cage induction motor has the following parameters referred to the stator:
$$R = 1.0\,\Omega ,{X_s} = X{'_r} = 1.5\,\Omega $$ Neglect stator resistance and core and rotational losses of the motor. The motor is controlled from a $$3$$-phase voltage source inverter with constant $$V/f$$ control. The stator line-to-line voltage $$(rms)$$ and frequency to obtain the maximum torque at starting will be:

A $$400$$ $$V,$$ $$50$$ $$Hz,$$ $$30$$ $$hp,$$ three-phase induction motor is drawing $$50$$ A current at $$0.8$$ power factor lagging. The stator and rotor copper losses are $$1.5kW$$ and $$900$$ $$W$$ respectively. The friction and windage losses are $$1050$$ $$W$$ and the core losses are $$1200$$ $$W.$$ The air-gap power of the motor will be

A $$3$$-phase, $$440$$ $$V,$$ $$50$$ $$Hz,$$ $$4$$ pole, slip ring induction motor is feed from the rotor side through an auto transformer and the stator is connected to a variable resistance as shown in the figure.

The motor is coupled to a $$220$$ $$V$$, separately excited $$d.c.$$ generator feeding power to fixed resistance of $$10\Omega .$$ Two watt-meter method is used to measure the input power to induction motor. The variable resistance is adjusted such that motor recorded
$${W_1} = 1800\,W,\,\,{W_2} = - 200\,W.$$

The Speed of rotation of stator magnetic field with respect to rotor structure will be

Neglecting all losses of both the machines, the $$dc$$ generator power output and the current through resistance $$\left( {{R_{ex}}} \right)$$ will respectively be

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 three phase squirrel cage induction motor has a starting current of seven times the full load current and full load slip of $$5\% $$

If an auto-transformer is used for reduced voltage starting to provide $$1.5$$ per unit starting torque, the auto-transformer ratio $$\left( \% \right)$$ should be

A three phase squirrel cage induction motor has a starting current of seven times the full load current and full load slip of $$5\% $$

If a star-delta starter is used to start this induction motor, the per unit staring torque will be

A three phase squirrel cage induction motor has a starting current of seven times the full load current and full load slip of $$5\% $$

If a starting torque of $$0.5$$ per unit is required then the per unit starting current should be

A $$3$$ phase, $$10$$ $$kW$$, $$400$$ $$V,$$ $$4$$ pole, $$50$$ $$Hz$$, star connected induction motor draws $$20$$ A on full load. Its no load and blocked rotor test data are given below.
No Load Test: $$400V$$ $$6A$$ $$1002W$$
Blocked Rotor Test: $$90V$$ $$15A$$ $$762W$$
Neglecting copper loss in No Load Test and core loss in Blocked Rotor Test, estimate motor's full load efficiency.

GATE EE 2006

A $$3$$ phase, $$4$$ pole, $$400$$ $$V,$$ $$50$$ $$Hz,$$ star connected induction motor has following circuit parameters $${r_1} = 1.0\Omega ,\,{r_2} = 0.5\Omega ,$$ $${\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {x_1} = x{'_2} = 1.2\Omega ,{\mkern 1mu} {x_m} = 35\Omega $$
The starting torque when the motor is started direct-on-line is (use approximate equivalent circuit model)

GATE EE 2006

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

GATE EE 2006

Assertion [a]: Under V/f control of induction motor, the maximum value of the developed torque remains constant over a wide range of speed in the subsynchronous region.

Reason [r]: The magnetic flux is maintained almost constant at the rated value by keeping the ratio V/f constant over the considered speed range.

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%, the ratio of the starting developed torque to the full load torque is approximately equal to

Under no load condition, if the applied voltage to an induction motor is reduced from the rated voltage to half the rated value,

In a single phase induction motor driving a fan load, the reason for having a high resistance rotor is to achieve

A rotating electrical machine having its self-inductances of both the stator and the rotor windings, independent of the rotor position will be definitely not develop

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 single-phase, 230 V, 50 Hz, 4 pole, capacitor-start induction motor has the following stand still impedances
Main winding Z_m =6.0 + j4.0Ω
Auxiliary winding Z_a = 8.0 + j6.0Ω
The value of the starting capacitor required to produce 90° phase difference between the currents in the main and auxiliary windings will be

When Stator and Rotor windings of a 2-pole rotating electrical machine are excited, each would produce a sinusoidal mmf distribution in the air gap with peak values F_s and F_r respectively. The rotor mmf lags stator mmf by a space angle $$\delta\;$$ at any instant as shown in Fig. Thus, half of stator and rotor surfaces will form one pole with the other half forming the second pole. Further, the direction of torque acting on the rotor can be clockwise or counter-clockwise.

The following Table gives four sets of statements as regards poles and torque. Select the correct set corresponding to the mmf axes as shown in Figure.

Following are some of the properties of rotating electrical machines
P. Stator winding current is dc, rotor-winding current is ac
Q. Stator winding current is ac, rotor-winding current is dc
R. Stator winding current is ac, rotor-winding current is ac
S. Stator has salient poles and rotor has commutator
T. Rotor has salient poles and sliprings and stator is cylindrical
U. Both stator and rotor have poly-phase windings

DC machines. Synchronous machines and Induction machines exhibit some of the above properties as given in the following table. Indicate the correct combination from this table

A single phase induction motor with only the main winding excited would exhibit the following response at synchronous speed

A 3-phase Inductor 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.
P. The difference between synchronous speed and actual speed remains same
Q. The air-gap flux remains same
R. The stator current remains same
S. The p.u. slip remains same
Among the above, correct statements are