GATE 2020: Electrical Machines Rapid Revision Quiz 2

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Question 1

If a 400V, 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?

• Areduces
• Bincreases
• Cremains the same
• Dincrease or reduces depending upon the rotor resistance

Question 2

A wound rotor induction motor runs with a slip of 0.03 when developing full load torque. Its rotor resistance is 0.25 ohm per phase. If an external resistance of 0.50 ohm per phase is connected across the slip rings. What is the slip for full torque?

• A0.03
• B0.06
• C0.09
• D0.1

Question 3

A 220 V, 20A, 1000 rpm, separately excited dc motor has an armature resistance of 2.5Ω. The motor is controlled by a step down chopper with a frequency of 1 kHz. The input dc voltage to the chopper is 250 V. The duty cycle of the chopper for the motor to operate at a speed of 600 rpm delivering the rated torque will be

• A0.518
• B0.608
• C0.852
• D0.902

Question 4

A field excitation of 20 A in a certain alternator results in an armature current of 400 A in short circuit and a terminal voltage of 2000 V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200 A is

• A1V
• B10 V
• C100 V
• D1000 V

Question 5

The emf induced in a conductor of machine driven at 600 rpm, the peak value of flux density is 1 Wb/m2, diameter of machine 2 m and length of machine 0.3 m is ______ V.

• A15.50
• B18.89
• C22.45
• D28.95

Question 6

Three single-phase transformers are connected to form a 3-phase transformer bank. The transformers are connected in the following manner:

The transformer connection will be represented by

• AYd0
• BYd1
• CYd6
• DYd11

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