Electrical Machines Test 2

Question 1
2 / -0.33

A 230 V DC machine has an emf constant k = 212.21 V-s/Wb-rad and R_a = 0.278 Ω. The field is separately excited and produces 0.01 Wb per pole. Armature current, I_a = 36 A.

Which of the following is/are true?

A
The speed of the machine is 990 rpm if it is operated as a dc generator with a rated terminal voltage.

B
The speed of the machine is 1080 rpm if it is operated as a dc generator with rated terminal voltage

C
The speed of the machine is 990 rpm if it is operated as a dc motor with rated terminal voltage

D
The speed of the machine is 1080 rpm if it is operated as a dc motor with rated terminal voltage

Question 2
2 / -0.33

A 3600 rpm, 60 Hz, 13.8 kV synchronous generator has a synchronous reactance of 20 Ω. The generator is operating at rated voltage and speed with the excitation voltage. E_f (phase) = 11.5 kV and the torque angle is δ = 15°. Find the value of stator current (I_a) such that the maximum power can be converted from mechanical to electrical form without loss of synchronism if the field current is unchanged.

A
216.35 A

B
699.52 A

C
451.34 A

D
352.45 A

Question 3
2 / -0.33

A DC machine has total armature ampere conductors of 4500 and total flux in the machine is 0.14 wb. The torque developed in the machine is _________ (in N-m/rad)

A
99-101

B
994-1011

C
997-1016

D
999-1012

Question 4
2 / -0.33

A 10 kW, 200 V, 1200 RPM series DC generator has armature resistance of 0.1 Ω, and field resistance of 0.3 Ω. The frictional and windage loss of the machine is 200 W and brush contact drop is 1 Volt per brush. Find the efficiency (in %) of the machine

A
88-89

B
884-891

C
887-896

D
889-892

Question 5
2 / -0.33

A 1000 hp, 4 pole, 3 phase, 440 V, 50 Hz, Induction motor has a speed of 1440 rpm on full load. The number of cycles of the rotor voltage makes per minutes will be______

A
120-120

B
1204-1201

C
1207-1206

D
1209-1202

Question 6
2 / -0.33

A 250 V, 700 W, 50 Hz, 6 pole single phase induction motor is rotating is clockwise direction with a speed of 960 rpm. The rotor resistance at standstill is 10 Ω. What is the effective rotor resistance is the backward branch of the equivalent circuit in ohms?

A
2.45-2.65

B
2.454-2.651

C
2.457-2.656

D
2.459-2.652

Question 7
2 / -0.33

Two identical synchronous machines A and B running at same speed are connected to each other through an inductor. Machine A is supplying an active power to Machine B and Machine B is supplying reactive power to Machine A, then which among the following is correct?

A
|V_a| > |V_b|

B
|V_a| < |V_b|

C
|V_a| = |V_b|

D
None of the above

Question 8
2 / -0.33

A three-phase synchronous motor rated at 800 hp, 2.4 kV, 60 Hz operates at unity power factor. The line voltage suddenly drops to 1.8 kV, but the exciting current remains unchanged.

Which of the following statements is/are true?

A
The mechanical power will increase.

B
Power angle, δ increases

C
The stator poles fall slightly behind their former position.

D
The power factor will be less than unity and lagging.

Question 9
2 / -0.33

A factory has an overall load of 2.4 MVA at 0.707 lagging pf. The MVA rating of a synchronous motor that would bring the total plant to unity power factor for the following condition is ________
The motor is used to drive a new load of 0.405 MW and the motor is 91% efficient.

A
1.7-1.8

B
1.74-1.81

C
1.77-1.86

D
1.79-1.82

Question 10
2 / -0.33

The following data were obtained from testing a 48 kVA, 4800/240 V, 50 Hz transformer.
OC test from LV side
240 V
2 A
120 W
SC test from HV side
150 V
10 A
600 W

The maximum efficiency (in %) of the transformer for 0.8 lagging power factor load is ________ (up to two decimal places)

A
98.4-98.8

B
98.44-98.81

C
98.47-98.86

D
98.49-98.82

Question 11
2 / -0.33

A 6-pole, 50 Hz, 3 – phase induction motor running on full load with 4% slip develops a torque of 149.3 N-m at its pulley rim. The friction and windage losses are 200 W and the stator cu and iron losses equal to 1620 W. The efficiency at full load is ____

A
82.4%

B
85.9%

C
90.6%

D
88.4%

Question 12
2 / -0.33

A 3-phase, 4 pole 50 Hz, star connected induction motor, during short circuit test, took 100 A and 30 kW. In case stator resistance is equal to stand-still rotor resistance, the starting torque is

A
95.5 N-m

B
63.2 N-m

C
107.3 N-m

D
154.8 N-m

Question 13
2 / -0.33

A single phase 230V, 50Hz, 4 pole capacitor start IM has Z_m = 2.3 + 4j Ω and Z_a = 7 + 5j Ω value of starting capacitor is

A
2.64mf

B
3.26mf

C
4.2mf

D
3.6μf

Submit Test

OC test from LV side	240 V	2 A	120 W
SC test from HV side	150 V	10 A	600 W

Electrical Mach...

Question 1

The speed of the machine is 990 rpm if it is operated as a dc generator with a rated terminal voltage.

The speed of the machine is 1080 rpm if it is operated as a dc generator with rated terminal voltage

The speed of the machine is 990 rpm if it is operated as a dc motor with rated terminal voltage

The speed of the machine is 1080 rpm if it is operated as a dc motor with rated terminal voltage

Question 2

216.35 A

699.52 A

451.34 A

352.45 A

Question 3

99-101

994-1011

997-1016

999-1012

Question 4

88-89

884-891

887-896

889-892

Question 5

120-120

1204-1201

1207-1206

1209-1202

Question 6

2.45-2.65

2.454-2.651

2.457-2.656

2.459-2.652

Question 7

|Va| > |Vb|

|Va| < |Vb|

|Va| = |Vb|

None of the above

Question 8

The mechanical power will increase.

Power angle, δ increases

The stator poles fall slightly behind their former position.

The power factor will be less than unity and lagging.

Question 9

1.7-1.8

1.74-1.81

1.77-1.86

1.79-1.82

Question 10

98.4-98.8

98.44-98.81

98.47-98.86

98.49-98.82

Question 11

82.4%

85.9%

90.6%

88.4%

Question 12

95.5 N-m

63.2 N-m

107.3 N-m

154.8 N-m

Question 13

2.64mf

3.26mf

4.2mf

3.6μf

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Question Analysis

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|V_a| > |V_b|

|V_a| < |V_b|

|V_a| = |V_b|