Electrical Machines - Electrical Circuits and Machines - Instrumentation Engineering Previous Year Questions

1

2024 · Instrumentation Engineering · Electrical Circuits and Machines · Electrical Machines

Instrumentation Engineering (IN) 2024

A 50 kVA transformer has an efficiency of 95% at full load and unity power factor. Assume the core losses are negligible. The efficiency of the transformer at 75% of the full load and 0.8 power factor is _____% (rounded off to one decimal place).

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2

2024 · Instrumentation Engineering · Electrical Circuits and Machines · Electrical Machines

Instrumentation Engineering (IN) 2024

A three-phase squirrel-cage induction motor has a starting torque of 100% of the full load torque and a maximum torque of 300% of the full load torque. Neglecting the stator impedance, the slip at the maximum torque is _____% (rounded off to two decimal places).

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3

2025 · Instrumentation Engineering · Electrical Circuits and Machines · Electrical Machines

Instrumentation Engineering (IN) 2025

A 2-pole, 50 Hz, 3-phase induction motor supplies power to a certain load at 2970 rpm. The torque-speed curve of this machine follows a linear relationship between synchronous speed and 95 % of synchronous speed. Assume mechanical and stray losses to be zero. If the load torque of the motor is doubled, the new operating speed of the motor, in rpm, is

A

2940

B

2812

C

2970

D

2850

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4

2021 · Instrumentation Engineering · Electrical Circuits and Machines · Electrical Machines

Instrumentation Engineering (IN) 2021

A single-phase transformer has a magnetizing inductance of 250 mH and a core loss resistance of 300 Ω, referred to primary side. When excited with a 230 V, 50 Hz sinusoidal supply at the primary, the power factor of the input current drawn, with secondary on open circuit, is _____ (rounded off to two decimal places).

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5

2021 · Instrumentation Engineering · Electrical Circuits and Machines · Electrical Machines

Instrumentation Engineering (IN) 2021

A single-phase transformer has maximum efficiency of 98 %. The core losses are 80 W and the equivalent winding resistance as seen from the primary side is 0.5 Ω. The rated current on the primary side is 25 A. The percentage of the rated input current at which the maximum efficiency occurs is

A

35.7%

B

50.6%

C

80.5%

D

100%

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6

2021 · Instrumentation Engineering · Electrical Circuits and Machines · Electrical Machines

Instrumentation Engineering (IN) 2021

A slip-ring induction motor is expected to be started by adding extra resistance in the rotor circuit. The benefit that is derived by adding extra resistance in the rotor circuit in comparison to the rotor being shorted is

A

The starting torque would be higher.

B

The power factor at start will be lower.

C

The starting current is higher.

D

The losses at starting would be lower.

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7

2022 · Instrumentation Engineering · Electrical Circuits and Machines · Electrical Machines

Instrumentation Engineering (IN) 2022

A 440 V, 8 kW, 4-pole, 50 Hz, star-connected induction motor has a full load slip of 0.04. The rotor speed (in rpm) at full load is ______ (round off to one decimal place)

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8

2023 · Instrumentation Engineering · Electrical Circuits and Machines · Electrical Machines

Instrumentation Engineering (IN) 2023

The no-load steady-state output voltage of a DC shunt generator is 200 V when it is driven in the clockwise direction at its rated speed. If the same machine is driven at the rated speed but in the opposite direction, the steady-state output voltage will be ______ V (rounded off to the nearest integer).

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9

2023 · Instrumentation Engineering · Electrical Circuits and Machines · Electrical Machines

Instrumentation Engineering (IN) 2023

A short-circuit test is conducted on a single-phase transformer by shorting its secondary. The frequency of input voltage is 1 kHz. The corresponding wattmeter reading, primary current and primary voltage are 8 W, 2 A and 6 V respectively. Assume that the no-load losses and the no-load currents are negligible, and the core has linear magnetic characteristics. Keeping the secondary shorted, the primary is connected to a 2 V (RMS), 1 kHz sinusoidal source in series with a \( \frac{1}{2\pi\sqrt{2}} \) mF capacitor. The primary current (RMS) will be __________ A (rounded off to two decimal places).

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10

2026 · Instrumentation Engineering · Electrical Circuits and Machines · Electrical Machines

Instrumentation Engineering (IN) 2026

A single phase step-down transformer has a high voltage primary winding and a low voltage secondary winding. The secondary winding is connected to a load impedance \[ Z_2 \]. The impedance \[ Z_2 \] when referred to the primary winding is denoted by \[ Z_2' \]. Which of the following conditions is true?

A

\(Z_2' = Z_2 \)

B

\(0.5 Z_2 < Z_2' < Z_2 \)

C

\(Z_2' > Z_2 \)

D

\(Z_2' \le 0.5 Z_2 \)

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11

2026 · Instrumentation Engineering · Electrical Circuits and Machines · Electrical Machines

Instrumentation Engineering (IN) 2026

A single phase transformer has a rating of 10 kVA, 50 Hz, 1100 V/ 220 V. In an open circuit test with the high voltage side open, the following are obtained:

• Voltage applied to the low voltage winding = 220 V
• Measured power = 363 W
• Measured current = 2.75 A

Magnetizing reactance \(X_m\) referred to the high voltage winding is ______ \(\Omega\).
(rounded off to the nearest integer)

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