National Champion Test -GATE 2018 EE: Electrical Machines for ESE & GATE EE Exams

Question 1

In case of an armature controlled separately excited dc motor drive with closed loop speed control, an inner current loop is useful because it

Question 2

A dc series motor is rated 230V, 1000 rpm, 80 A (refer the following figure), the series field resistance is 0.11Ω, and the armature resistance is 0.14Ω. If the flux at an armature current of 20A is 0.4 times of that under rated condition, calculate the speed at this reduced armature current of 20A.

Question 3

A 100 kVA, 415 V(line), star-connected synchronous machine generates rated open circuit voltage of 415 V at a field current of 15 A. The short circuit armature current at a field current of 10 A is equal to the rated armature current. The per unit saturated synchronous reactance is

Question 4

A belt driven dc shunt generator runs at 1500 rpm delivering 10 kW, at 220 V bus bars. The belt breaks, following which the machine operates as a motor drawing 2 kW power. What will be its speed as a motor? The armature and field resistances are 0.25 ohms and 55 ohms respectively. Ignore armature reaction and assume the contact drop at each brush to be 1 V.

Question 5

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

Question 6

For an induction motor, operating at a slip s, the ratio of gross power output to air gap power is equal to:

Question 7

Assertion (A): The leakage reactance of a 3-phase induction motor should be small.

Reason (R): A small value of leakage reactance will increase the maximum power output of motor.

Question 8

A 400V/200V/200V, 50 Hz three winding transformer is connected as shown in the following figure. The reading of the voltmeter, V, will be
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Question 9

A three-phase, three-stack, variable reluctance step motor has 20 poles on each rotor and stator stack. The step angle of this step motor is

Question 10

A 1100 kVA, 11 kV/1.1 kV conventional two-winding transformer is connected as a 12.1 kV/11 kV step-down auto-transformer. The rating of the auto-transformer is

Question 11

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 6 A 1002 W
Blocked Rotor test: 90 V 15 A /62 W
Neglecting copper loss in no load test and core loss in Blocked Rotor test, estimate motor’s full load efficiency.

Question 12

A 3

, 10 MVA, 13.8 KV, 50 Hz synchronous generator yield the following test data.
OC test I_f = 842 A at rated voltage
SC test I_f = 226 A at rated armature current
Calculated its p.u. adjusted synchronous reactance. OC test revealed that the armature resistance per phase is 0.75

.

Question 13

A 500 V series motor of resistance 0.5

runs at 600 rpm when the current is 50 A. If an identical motor is mechanically coupled and joined in series,___________ A will be when the set runs at 200 rpm and exerts twice the original gross torque?

Question 14

For a single phase, two winding transformer, the supply frequency and voltage are both increased by 10%. The percentage changes in the hysteresis loss and eddy current loss, respectively, are

Question 15

The maximum efficiency of a 500 kVA, 3300/500 V, 50 Hz, single phase transformer is 97% and occurs at 3/4 full load, unity power factor. If the impedance is 10%, calculate the regulation at full load at 0.8 p.f. lagging