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Solution for Live Quiz Session: Electrical Machines-01 Feb 2018

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Question 1

A three-phase, salient pole synchronous motor is connected to an infinite bus. It is operated at no load at normal excitation.
The field excitation of the motor is first reduced to zero and then increased in the reverse direction gradually. Then the armature current

Question 2

The armature resistance of a permanent magnet dc motor is 0.8Ω. At no load, the motor draws 1.5 A from a supply voltage of 25 V and runs at 1500 rpm. The efficiency of the motor while it is operating on load at 1500 rpm drawing a current of 3.5 A from the same source will be

Question 3

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

Question 4

The power input to the rotor of a 3-phase, 60 Hz, 4 Pole induction motor is 50 kW. The rotor emf makes 150 complete alternations per minute. Find the mechanical power developed by the motor.

Question 5

Interpoles are placed mid-way between the main poles to reduce sparking at the commutator, as shown below.

For the given 2-pole dc motor, what is the polarity of the interpoles in order marked as A – B ?

Question 6

A long-shunt compound generator has a shunt field, series field, and armature resistance of 220 Ω, 0.06 Ω, and 0.02 Ω respectively. If the voltage generated in the armature winding is 557 V, determine the load current delivered by the generator at 550 V. Assume 1 V voltage drop per brush.

Question 7

A star connected synchronous motor at 100 kVA, 3-φ, 1400V, 23A, 60Hz, 126.5 rpm has an effective resistance of 0.5 ohm and a synchronous reactance of 10 ohm per phase. Determine internal power developed [in kW] by the motor when it is operating at rated current and 0.5 power factor leading.

Question 8

A 50 KVA, 1000/330 V transformer is undergone the short circuit test on the hv side. The meter readings obtained were:
Wattmeter reading = 900 W
Ammeter reading = 25 A
Voltmeter reading = 85 V
Determine the equivalent reactance referred to the primary.

Question 9

_______will be the ampere turns per pole of field excitation required for the single gap of a 2 pole generator with the following particulars; 200 V on no-load at 2000 rpm; armature length is 32 cm and effective gap length is 0.8 cm; pole arc 30 cm and 200 armature conductors.

Question 10

A 3 - ϕ, star connected alternator is rated at 1600 kVA, 13500 V. The armature effective resistance and synchronous reactance are 1.5 Ω and 30 Ω per phase respectively. Calculate the % regulation for a load of 1280 kW at p.f. at 0.8 leading.

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ESE & GATE EE Electrical Machines

Jun 24ESE & GATE EE