Induction Machines · Electrical Machines · GATE EE
Marks 1
1
A three phase, 50 Hz, 6 pole induction motor runs at 960 rpm. The stator copper loss, core loss, and the rotational loss of the motor can be neglected. The percentage efficiency of the motor is
GATE EE 2024
2
A 10-pole, 50 Hz, 240 V, single phase induction motor runs at 540 RPM while driving rated load. The frequency of induced rotor currents due to backward field is
GATE EE 2023
3
The frequencies of the stator and rotor currents flowing in the three-phase 8-pole induction motor are 40 Hz and 1 Hz, respectively. The motor speed, in rpm, is ________. (round off to nearest integer).
GATE EE 2022
4
A 3-phase, 4-pole, 400 V, 50 Hz squirrel-cage induction motor is operating at a slip of 0.02. The
speed of the rotor flux in mechanical rad/sec, sensed by a stationary observer, is closest to
GATE EE 2017 Set 2
5
A 4 pole induction machine is working as an induction generator. The generator supply
frequency is 60 Hz. The rotor current frequency is 5 Hz. The mechanical speed of the rotor in
RPM is
GATE EE 2017 Set 1
6
The direction of rotation of a single-phase capacitor run induction motor is reversed by
GATE EE 2016 Set 2
7
In a constant V/f induction motor drive, the slip at the maximum torque
GATE EE 2016 Set 1
8
The figure shows the per-phase equivalent circuit of a two-pole three-phase induction motor operating at 50 Hz. The 'air-gap' voltage, Vg across the magnetizing inductance, is 210Vrms, and the slip, s, is 0.05. The torque (in Nm) produced by the motor is ________.
GATE EE 2015 Set 2
9
In a constant V/f control of induction motor, the ratio V/f is maintained constant from 0 to
base frequency, where V is the voltage applied to the motor at fundamental frequency f.
Which of the following statements relating to low frequency operation of the motor is TRUE?
GATE EE 2014 Set 2
10
A three-phase, 4-pole, self excited induction generator is feeding power to a load at a
frequency $$f_1$$. If the load is partially removed, the frequency becomes $$f_2$$. If the speed of the
generator is maintained at 1500 rpm in both the cases, then
GATE EE 2014 Set 2
11
The torque-speed characteristics of motor ($$T_M$$) and load ($$T_L$$) for two cases are shown in the
figures (a) and (b). The load torque is equal to motor torque at points P, Q, R and S
The stable operating points are
The stable operating points areGATE EE 2014 Set 3
12
A single phase induction motor is provided with capacitor and centrifugal switch in series
with auxiliary winding. The switch is expected to operate at a speed of 0.7 Ns, but due to
malfunctioning the switch fails to operate. The torque-speed characteristic of the motor is
represented by
GATE EE 2014 Set 3
13
An 8-pole, 3-phase, 50 Hz induction motor is operating at a speed of 700 rpm. The frequency
of the rotor current of the motor in Hz is ________.
GATE EE 2014 Set 1
14
Leakage flux in an induction motor is
GATE EE 2013
15
The slip of an induction motor normally does not depend on
GATE EE 2012
16
A balanced three-phase voltage is applied to a star-connected induction motor, the
phase to neutral voltage being V. The stator resistance, rotor resistance referred
to the stator, stator leakage reactance, rotor leakage reactance referred to the
stator, and the magnetizing reactance are denoted by $$r_s,\;r_r,\;r_s,\;r_r\;and\;X_m$$,
respectively. The magnitude of the starting current of the motor is given by
GATE EE 2010
17
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?
GATE EE 2005
18
For an induction motor, operating at a slip 's', the ratio of gross power output to
air gap power is equal to:
GATE EE 2005
19
For a linear electromagnetic circuit, the following statement is true.
GATE EE 2004
20
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
GATE EE 2004
21
The type of single-phase induction motor having the highest power factor at full
load is
GATE EE 2004
22
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
GATE EE 2003
23
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
GATE EE 2003
24
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
25
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
26
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
27
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
28
If an induction machine is run at above synchronous speed, it acts as
GATE EE 1997
29
Unbalanced supply voltage given to a $$3-$$ phase, data-connected induction motor will cause
GATE EE 1996
30
When the supply voltage to an induction motor is reduced by $$10\% $$, the maximum torque will decrease by approximately
GATE EE 1996
31
In case of a split phase motor, the phase shift between currents in the two windings is around
GATE EE 1996
32
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
GATE EE 1996
33
In an induction motor, if the air gap is increased
GATE EE 1996
34
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
35
Skew is used in induction motors in order to reduce torque due to
GATE EE 1994
36
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
1
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
2
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
3
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
4
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
5
The equivalent circuit of a single phase induction motor is shown in the figure, where the
parameters are
R1 = R2' = Xl1 = Xl2' = 12 $$\Omega $$, Xm = 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).
R1 = R2' = Xl1 = Xl2' = 12 $$\Omega $$, Xm = 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
6
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
7
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
8
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 5th 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 5th harmonic component of voltage is _________.
GATE EE 2015 Set 1
9
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
10
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 fr 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
If the rotor winding is open circuited and the system is made to run at rotational speed fr 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 areGATE EE 2014 Set 2
11
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
12
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
13
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
14
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
15
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
16
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
GATE EE 2008
17
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:
$$R = 1.0\,\Omega ,{X_s} = X{'_r} = 1.5\,\Omega $$ 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:
GATE EE 2008
18
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
GATE EE 2008
19
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
$${W_1} = 1800\,W,\,\,{W_2} = - 200\,W.$$
Neglecting all losses of both the machines, the $$dc$$ generator power output and the current through resistance $$\left( {{R_{ex}}} \right)$$ will respectively be
GATE EE 2008
20
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
GATE EE 2008
21
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
GATE EE 2007
22
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
GATE EE 2007
23
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
GATE EE 2007
24
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
GATE EE 2007
25
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.
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
26
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
27
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)
The starting torque when the motor is started direct-on-line is (use approximate equivalent circuit model)
GATE EE 2006
28
Under no load condition, if the applied voltage to an induction motor is reduced
from the rated voltage to half the rated value,
GATE EE 2005
29
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
GATE EE 2005
30
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.
GATE EE 2005
31
In a single phase induction motor driving a fan load, the reason for having a high
resistance rotor is to achieve
GATE EE 2005
32
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
GATE EE 2004
33
A single-phase, 230 V, 50 Hz, 4 pole, capacitor-start induction motor has the
following stand still impedances
Main winding Zm =6.0 + j4.0Ω
Auxiliary winding Za = 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
Main winding Zm =6.0 + j4.0Ω
Auxiliary winding Za = 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
GATE EE 2004
34
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
GATE EE 2004
35
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 Fs and Fr 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.
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.GATE EE 2003
36
A single phase induction motor with only the main winding excited would exhibit
the following response at synchronous speed
GATE EE 2003
37
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
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
GATE EE 2003
38
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
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
GATE EE 2003
39
A 3-phase, 4-pole squirrel cage induction motor has 36 stator and 28 rotor slots.
The number of phases in the rotor is:
GATE EE 2000
40
The power input to a 415 V, 50 Hz, 6 pole, 3-phase induction motor running at
975 rpm is 40 kW. The stator losses are 1kW and friction and windage losses
total 2 kW. The efficiency of the motor is
GATE EE 2000
41
A three phase, wound rotor induction motor is to be operated with slip energy
recovery in the constant torque mode, when it delivers an output power P0 at slop
's'. then theoretically, the maximum power that is available for recovery at the
rotor terminals, is equal to
GATE EE 2000