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Transformers & Energy Conversion(7)DC & AC Machines(22)

Transformers & Energy Conversion

7 Questions

GATE→2026→Electrical Machines→Transformers & Energy Conversion

Q1

MCQ[1 Mark]

A 15 kVA, 1100 V/220 V single-phase transformer is configured as a 1.32 kV/1.1 kV autotransformer. What is its rating?

(A)

60 kVA

(B)

75 kVA

(C)

90 kVA

(D)

100 kVA

Explanation

GATE→2026→Electrical Machines→Transformers & Energy Conversion

Q2

MSQ[1 Mark]

A 220V/12V, 100VA, 50Hz transformer is operated in the USA at 110V, 60Hz. Which statements are correct?

(A)

No-load current drawn would be smaller for operation in the USA compared to that in India

(B)

For the same load current, the losses would be higher in the USA compared to that in India

(C)

The peak magnetic flux density in the core would be higher while operating in the USA compared to that in India

(D)

The eddy current losses in the core would be approximately 44% higher while operating in the USA compared to that in India

Explanation

GATE→2026→Electrical Machines→Transformers & Energy Conversion

Q3

MCQ[2 Marks]

Three single-phase 11kV/3.3kV transformers form a 3-phase bank with connections shown. What is the vector group?

(A)

Dd0

(B)

Dd4

(C)

Dd6

(D)

Dd10

Explanation

GATE→2025→Electrical Machines→Transformers & Energy Conversion

Q4

MCQ[2 Marks]

For the given 3-phase transformer connection with 2:1 turns ratio, find the magnitude and phase relation between I_A and I_ad.

(A)

|I_A|/|I_ad| = 1/(2*sqrt(3)) and I_ad lags I_A by 30°

(B)

|I_A|/|I_ad| = 1/(2*sqrt(3)) and I_ad leads I_A by 30°

(C)

|I_A|/|I_ad| = 2*sqrt(3) and I_ad lags I_A by 30°

(D)

|I_A|/|I_ad| = 2*sqrt(3) and I_ad leads I_A by 30°

Explanation

GATE→2024→Electrical Machines→Transformers & Energy Conversion

Q5

MCQ[1 Mark]

Which option represents correct voltage polarities and induced EMFs in a single-phase two-winding transformer?

(A)

(A)

(B)

(B)

(C)

(C)

(D)

(D)

Explanation

GATE→2023→Electrical Machines→Transformers & Energy Conversion

Q6

MCQ[1 Mark]

For the rotating machine with stator DC conductors and rotor coils, rotating at omega rad/s, what can be said about induced EMFs E_{a-a'} and E_{b-b'}?

(A)

E_{a-a'} and E_{b-b'} have finite magnitudes and are in the same phase

(B)

E_{a-a'} and E_{b-b'} have finite magnitudes with E_{b-b'} leading E_{a-a'}

(C)

E_{a-a'} and E_{b-b'} have finite magnitudes with E_{a-a'} leading E_{b-b'}

(D)

E_{a-a'} = E_{b-b'} = 0

Explanation

GATE→2023→Electrical Machines→Transformers & Energy Conversion

Q7

NAT[2 Marks]

For the single-phase transformer with given core dimensions, B_max, and permeability, find the self-inductance of winding a-b in millihenry.

Explanation

DC & AC Machines

22 Questions

GATE→2026→Electrical Machines→DC & AC Machines

Q8

MCQ[1 Mark]

A synchronous motor-generator set converts 50 Hz grid power to 16.67 Hz. Which combination of poles for motor and generator is suitable?

(A)

2, 6

(B)

6, 2

(C)

4, 6

(D)

6, 4

Explanation

GATE→2026→Electrical Machines→DC & AC Machines

Q9

MCQ[1 Mark]

Which statement is correct regarding the typical double-squirrel-cage structure used in induction motors?

(A)

At starting, a larger portion of the rotor current flows in the outer cage

(B)

At starting, a larger portion of the rotor current flows in the inner cage

(C)

At rated speed, most of the rotor current flows in the outer cage

(D)

The purpose of the double-squirrel-cage structure is to lower the effective rotor resistance at starting

Explanation

GATE→2026→Electrical Machines→DC & AC Machines

Q10

MCQ[1 Mark]

A synchronous generator delivers power to an infinite bus at unity p.f. If field excitation is increased, what happens to stator current, power factor, and load angle?

(A)

Stator current increases, power factor becomes lagging, load angle remains the same

(B)

Stator current decreases, power factor becomes leading, load angle remains the same

(C)

Stator current increases, power factor becomes lagging, load angle decreases

(D)

Stator current increases, power factor becomes leading, load angle increases

Explanation

GATE→2026→Electrical Machines→DC & AC Machines

Q11

NAT[2 Marks]

A 3-phase induction motor runs at 1476 rpm with shorted slip rings and 1404 rpm with 0.27 ohm/phase added. Find per phase rotor resistance R_2.

Explanation

GATE→2025→Electrical Machines→DC & AC Machines

Q12

MCQ[2 Marks]

For the induction motor operating region and load torque curve, which trajectory from O to S takes the shortest time?

(A)

O-Q-R-S

(B)

O-P-S

(C)

O-E-S

(D)

O-F-S

Explanation

GATE→2025→Electrical Machines→DC & AC Machines

Q13

NAT[1 Mark]

A 3.3 kV synchronous motor has load angle deflection of 2° mechanical, X_s = 2 ohm. Find armature current magnitude in amperes.

Explanation

GATE→2025→Electrical Machines→DC & AC Machines

Q14

MCQ[2 Marks]

A DC series motor drives a load where T_L proportional to N^3. Fed from 400 V, draws 40 A. Find external resistance to reduce speed by half.

(A)

23.28

(B)

4.82

(C)

46.7

(D)

0

Explanation

GATE→2025→Electrical Machines→DC & AC Machines

Q15

MCQ[2 Marks]

For a 3-phase induction motor with R_r' = 1 ohm, X_eq = 4 ohm, find V and f for maximum starting torque under V/f control.

(A)

400 V and 50 Hz

(B)

200 V and 25 Hz

(C)

100 V and 12.5 Hz

(D)

300 V and 37.5 Hz

Explanation

GATE→2024→Electrical Machines→DC & AC Machines

Q16

MCQ[1 Mark]

A 3-phase, 50 Hz, 6-pole induction motor runs at 960 rpm with negligible stray losses. Find its percentage efficiency.

(A)

92

(B)

94

(C)

96

(D)

98

Explanation

GATE→2024→Electrical Machines→DC & AC Machines

Q17

NAT[2 Marks]

For a slip-ring induction motor with given parameters, find the extra rotor resistance per phase required for maximum starting torque.

Explanation

GATE→2024→Electrical Machines→DC & AC Machines

Q18

NAT[2 Marks]

For a 5 kW, 220 V DC shunt motor with R_a = 0.5 ohm, I_nl = 3 A, I_f = 1 A, find the armature current for maximum efficiency at rated load.

Explanation

GATE→2023→Electrical Machines→DC & AC Machines

Q19

MCQ[1 Mark]

Match the induction machine modes of operation (a. Generator, b. Motor, c. Plugging) with their slip ranges (p: 0 to 1, q: 1 to 2, r: -1 to 0).

(A)

a-r, b-p, c-q

(B)

a-r, b-q, c-p

(C)

a-p, b-r, c-q

(D)

a-q, b-p, c-r

Explanation

GATE→2023→Electrical Machines→DC & AC Machines

Q20

MCQ[1 Mark]

A 10-pole, 50 Hz single-phase induction motor runs at 540 RPM. Find the frequency of induced rotor currents due to the backward rotating field.

(A)

100 Hz

(B)

95 Hz

(C)

10 Hz

(D)

5 Hz

Explanation

GATE→2023→Electrical Machines→DC & AC Machines

Q21

NAT[1 Mark]

A separately excited DC motor drives constant torque load at 1500 RPM (400 V, 15 A, R_a = 1.2 ohm). If supply voltage drops by 10%, find the new speed in RPM.

Explanation

GATE→2023→Electrical Machines→DC & AC Machines

Q22

NAT[2 Marks]

A 3-phase synchronous motor with Z_s = 0.1 + j0.3 pu has a static stability limit of 2.5 pu. Find the corresponding excitation voltage E_f in pu.

Explanation

GATE→2023→Electrical Machines→DC & AC Machines

Q23

NAT[2 Marks]

A 3-phase induction motor running at 960 RPM has its supply voltage and frequency reduced by 20%. Find the new speed in RPM.

Explanation

GATE→2022→Electrical Machines→DC & AC Machines

Q24

MCQ[1 Mark]

Which single-phase induction motor type has the highest power factor during steady-state running conditions?

(A)

split phase (resistance start).

(B)

shaded pole.

(C)

capacitor start.

(D)

capacitor start, capacitor run.

Explanation

GATE→2022→Electrical Machines→DC & AC Machines

Q25

NAT[1 Mark]

A 3-phase 8-pole induction motor has stator frequency 40 Hz and rotor frequency 1 Hz. Find motor speed in rpm.

Explanation

GATE→2022→Electrical Machines→DC & AC Machines

Q26

NAT[2 Marks]

A separately excited DC motor with R_a = 1 Ω draws 30 A at 1000 RPM (V = 280 V) driving a torque proportional to speed load. Find the speed in RPM if an extra 10 Ω is added in series with armature.

Explanation

GATE→2022→Electrical Machines→DC & AC Machines

Q27

NAT[2 Marks]

A 4-pole induction motor with J = 0.1 kg-m^2 drives a constant load torque T_L = 2 Nm. Speed increases linearly from 1000 to 1500 RPM in 4 s. Find energy consumed in Joules.

Explanation

GATE→2022→Electrical Machines→DC & AC Machines

Q28

NAT[2 Marks]

A star-connected synchronous motor (400 V, 50 kVA, X_s = 1 Ω) has shaft load 10 kW, 2 kW losses, 0.8 leading p.f. Find per-phase excitation emf E_f in volts.

Explanation

GATE→2022→Electrical Machines→DC & AC Machines

Q29

NAT[2 Marks]

An induction motor draws 5x starting current. To reduce starting current to 2x using a 3-phase autotransformer, find the transformation ratio x (or tapping percentage).

Explanation