# Multiple choice question for engineering

## Set 1

1. The circuit depicting the equation V=Ea + I*Ra.

a)

b)

c)

d)

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2. The circuit depicting the equation V=Ea – I*Ra.

a)

b)

c)

d) None of the mentioned

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3. If the terminal voltage of 220-V dc generator having armature resistance of 1 ohms. The induced emf produced is 200-V. The armature current for the above machine is

a) 20 A

b) -20 A

c) -10 A

d) 10 A

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4. The voltage drop at brush-commutator contact is variable (1-2V) and dependent of armature current.

a) True

b) False

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5. The shaft power at the DC Generator is

a) sum of mechanical power and rotational losses

b) only mechanical losses

c) no-load losses and mechanical power

d) any of the mentioned

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6. If the electromagnetic torque in a DC shunt-generator is opposite, what can be further concluded?

a) Mechanical power is absorbed by the machine

b) Mechanical power is delivered by the machine

c) Electromagnetic torque is in same direction of prime mover

d) None of the mentioned

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7. The conductor EMF and current are in _____ direction and developed torque is in _____ for generating mode.

a) same, opposite

b) same,same

c) opposite, same

d) opposite, opposite

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8. If the armature terminal voltage is more that its induced EMF, the DC machine given is

a) motoring mode

b) generating mode

c) regenerative mode

d) none of the mentioned

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9. Consider a 200V, 25kW, 30A DC machine lap connected with armature resistance of 0.4 ohms. If the machine is later wave wound, then the developed power is

a) 25 kW

b) 12.5 kW

c) 20 kW

d) 50 kW

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10. If the DC machine is held constant at 3000 rpm. The DC voltage is 250V. If the field is held constant with 250V. Is this machine generator or motor?

a) Motor

b) Generator

c) None of the mentioned

d) Any of the mentioned

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11. A shunt generator has an induced voltage on open circuit of 127 V. When the machine is on load the terminal voltage is 120 V. The load current if the field resistance be 15 ohm and armature resistance be 15 ohm

a) 342 A

b) 350 A

c) 358 A

d) 8 A

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## Set 2

1. The armature mmf affects

a) commutation

b) generated voltage

c) torque

d) any of the mentioned

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2. The space distribution of flux produced in a DC machine is

a)

b)

c)

d) None of the mentioned

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3. Armature reaction in a dc machine is

a) cross magnetizing in nature

b) magnetizing in nature

c) demagnetizing in nature

d) none of the mentioned

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4. The air-gap flux density waveform, has decreased flux under one pole tip and measured under the other is due to

a) cross magnetization

b) magnetization

c) demagnetization

d) saturation of iron magnetic circuit

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5. The flux distortion caused by cross magnetizing armature reaction is more pronounced in a

a) dc shunt motor

b) dc series motor

c) any of the mentioned

d) differential Compound Motors

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6. The method needed to limit cross-magnetizing effect is

a) by increasing reluctance of cross flux path

b) by chamfering the pole faces

c) by compensating winding

d) any of the mentioned

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7. The brushes of a dc motor, are shifted by 5′, from the main field axis, then

a) demagnetizing or magnetizing effect may occur depending on machine

b) cross-magnetization will be pronounced

c) both cross-magnetization as well as magnetization and demagnetization will occur

d) none of the mentioned

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8. A 100KW, 250 V, 400 A, a long shunt compound generator has an armature resistance of 0.025 ohms. There are 1000 shunt fields turns per pole and 3 series field turns per pole. The series field is connected in a such a fashion that positive armature current produces direct-axis MMF which adds to that of the shunt field. Compute the gross MMF at the rated terminal current when shunt field current is 4.7A and speed is 1150 rpm.

a) 5.9AT

b) 3.5AT

c) 4.7AT

d) 1.2AT

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9. Compensating windings are embedded in pole faces and having the same polarity as that of adjoining armature winding.

a) True

b) False

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10. Compensating winding has advantage of

a) eliminating flux distortion

b) importing speed of response

c) protecting overloading

d) any of the mentioned

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11. The correct positioning of the windings in the below diagrams is

a)

b)

c)

d) Any of the mentioned

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12. A dc shunt motor is connected to the source through 3-point starter. If the field id kept open and starter handle is moved from off to on position, then

a) motor will not start

b) armature will draw large current from source

c) no sparking would occur

d) all of the mentioned

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13. A dc shunt motor is running at 1000 rpm at the rated load torque. If we reverse the filed winding terminals, then

a) direction of rotation reverses and commutation will be unaffected

b) direction of rotation will be same and commutation will be unaffected

c) direction of ration reverses and commutation will be affected

d) direction of ration will be same and commutation will be affected

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14. A dc shunt motor is running at 1000 rpm at the rated load torque. If we reverse the supply terminals, then

a) direction of rotation reverses and commutation will be unaffected

b) direction of rotation will be same and commutation will be unaffected

c) direction of ration reverses and commutation will be affected

d) direction of ration will be same and commutation will be affected

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15. A dc shunt motor is running at 1000 rpm at the rated load torque. If few of the field windings get shot circuited then

a) motor speed will increase and more armature current is drawn

b) motor speed will decrease and more armature current is drawn

c) motor speed will increase and lesser armature current is drawn

d) motor speed will decrease and lesser armature current is drawn

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## Set 3

1. Choose the correct representation, which depict a non-zero torque in the armature.

a)

b)

c)

d) any of the mentioned

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2. What is the principle of torque production in a dc machine?

a) Lorentz’s law

b) Lenz’s law

c) Faraday’s law

d) Self inductance

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3. The torque induced beyond the pole shoes in the DC machine is

a) 0

b) 2/pi *phi *i

c) 4/pi *phi *i

d) none of the mentioned

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4. The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms and magnitude strength = 0.25T is supplied with 120V. Suddenly the switch is closed at t=0, what is observed in the circuit?

a) Current will flow but zero induced EMF

b) Current will not flow and zero induced EMF

c) Current will not flow but EMF is induced

d) Current will flow and EMF will also be induced

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5. The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms and magnitude strength = 0.25T is supplied with 120V. What will be the magnitude of the following current at t=0+?

a) 400A

b) 200A

c) 0

d) any of the mentioned

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6. If the torque induced is zero in the dc machine, it can be said that

a) current is zero

b) flux can be zero

c) current or flux=0

d) any of the mentioned

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7. The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms and magnitude strength = 0.25T is supplied with 120V. What is the steady state angular velocity at no-load?

a) 480 rad/s

b) 960 rad/s

c) 320 rad/s

d) 490 rad/s

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8. If the torque is in the direction of rotation, the DC machine acts as

a) generator

b) motor

c) amplidyne

d) any of the mentioned

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9. The cogging torque is absent in the permanent magnet dc machine is due to

a) non-magnetic nature of rotor

b) magnetic nature of rotor

c) absence of ac supply

d) any of the mentioned

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10. DC Motor torque depends on

a) geometry

b) magnetic properties

c) any of the mentioned

d) both geometry and magnetic properties of the structure

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11. For a universal motor, the rated supply current is 10A giving a rated torque of 100 N-m. If a current of 5A is applied, what will be generated torque vary to?

a) 250 N-m

b) 400 N-m

c) 500 N-m

d) 1000 N-m

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12. Pulsating torque frequency in universal motor

a) 2f

b) f

c) f/2

d) 4f

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13. The variation of current in the armature can be best expressed by which of the below curves?

a)

b)

c)

d) An impulse signal

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14. The velocity of the dc machine at the final state is

a) E/Bl

b) Bl/E

c) EBl

d) None of the mentioned

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## Set 4

1. A starting resistance is inserted at the starting in an induction motor as well as dc motor.

a) Induction motor has to control starting torque whereas in dc motor, it is done to avoid large current

b) To limit starting current in both the machines

c) To limit starting speed

d) All of the mentioned

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2. Considering a human handed control system for the dc motor speed control, if the resistance wire cut out too slowly, then

a) starting resistance would burn

b) field winding would burn

c) speed will rise steeply

d) any of the mentioned

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3. A 100 hp, 250 V, 350 A shunt dc motor with an armature resistance of 0.05 ohms. To limit maximum starting current to twice the rated of its value, what will be the number of stages of starting resistances?

a) 3

b) 2

c) 4

d) 5

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4. Which of the following express the starting current nature of the dc motor.

a)

b)

c)

d)

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5. Following are the observations due to large starting current in dc motor.

(i) Sparking at brushes

(ii) Mechanical shock at shaft

(iii) Source current fluctuations

(iv) Burning of the field winding

a) (i), (ii), (iii), (iv)

b) (i), (iii)

c) (ii), (iii), (iv)

d) (i), (ii)

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6. The shunt motor starters that can be used is/are

a) 3-point and 4-point starter

b) 5-point starter

c) 4-point starter

d) 5-point and 3-point starter

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7. How does the speed build up takes place in a dc motor with time?

a)

b)

c)

d)

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8. The direct-on-line starter is used to start a small dc motor because it limits initial current drawn by armature circuit.

a) True

b) False

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9. Thyristor controlled starter is preferred over DOL starter due to

a) lesser losses

b) controlled direction

c) least resistance offered

d) all of the mentioned

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10. For a 7.46 kW, 200 V dc shunt motor with full load efficiency of 85% has armature resistance of 0.25 ohms. Calculate the value of starting resistance in ohms for a current 1.5 times of the full load current.

a) 2.788

b) 3.038

c) 2.688

d) 2.588

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11. The effect of fringing increases as we

a) increase air gap

b) decrease air gap

c) increase in flux density

d) introduce more ferric core material

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12. The post effects of the armature reaction is

a) main field distortion

b) shift in MNA

c) reduction in main field

d) none of the mentioned

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13. If the students give a forward shift of 10° to the dc generator, then it

a) reduces flux per pole

b) improves flux per pole

c) increases the flux density in core

d) none of the mentioned

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14. A dc machine is run at rated speed in forward direction and then in backward direction. It is observed that, speeds of the rotation are different, then

it leads to the conclusion of

a) incorrect brush placement

b) incorrect pole and core alignment

c) incorrect field supply

d) all of the mentioned

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15. For a dc machine , its commutator has a diameter of 50 c rotating at 1000 rpm. For a brush width of 1 cm, the time commutation taken by the machine will be

a) 0.382 ms

b) 0.456 ms

c) 0.573 ms

d) 0.312 ms

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## Set 5

1. The short circuit load losses is/are

a) direct load loss and stray load losses

b) direct load loss

c) stray load losses

d) field current loss

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2. The open circuit voltage and open circuit core loss variation is

a)

b)

c)

d)

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3. Rotational losses vary as following with respect to field current.

a)

b)

c)

d)

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4. A 100 KVA, 400V, 3-phase, star connected alternator due to following data

Friction and windage losses = 340W

Open circuit core loss = 480W

Rf = 180ohms, ra=0.02ohms.

The voltage applied to the field winding is 220V. The short circuit load loss at half full load is

a) 258W

b) 268W

c) 480w

d) 340W

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^{2})*ra = 3*((131.22/2)

^{2})*0.02 = 258.3 A.

5. A 100 KVA, 400V, 3-phase, star connected alternator due to following data

Friction and windage losses = 340W

Open circuit core loss = 480W

Rf = 180ohms, ra=0.02ohms.

The voltage applied to the field winding is 220V. The field current loss will be

a) 270W

b) 258W

c) 480W

d) 250W

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6. A 100 KVA, 400V, 3-phase, star connected alternator due to following data

Friction and windage losses = 340W

Open circuit core loss = 480W

Rf = 180ohms, ra=0.02ohms.

The voltage applied to the field winding is 220V. The efficiency of the machine at half load is

a) 96.7%

b) 95%

c) 94.2%

d) 97

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7. A 100 KVA, 400V, 3-phase, star connected alternator due to following data

Friction and windage losses = 340W

Open circuit core loss = 480W

Rf = 180ohms, ra=0.02ohms.

The voltage applied to the field winding is 220V. The load losses at full load is

a) 1033W

b) 500W

c) 540W

d) 940W

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^{2}*0.02 = 1033W.

8. A 100 KVA, 400V, 3-phase, star connected alternator due to following data

Friction and windage losses = 340W

Open circuit core loss = 480W

Rf = 180ohms, ra=0.02ohms.

The voltage applied to the field winding is 220V. Efficiency at full load is

a) 97%

b) 96.5%

c) 92%

d) 95%

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9. The given variable losses are 5kW fr a 500 KVA, 11 kV, 3-phase star connected alternator having armature resistance of 4 ohms. Calculate the current at which maximum efficiency occurs

a) 20.4A

b) 10.2A

c) 40A

d) 26A

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^{2}=5000/3*4 I = 20.4 A.

10. The given variable losses are 5kW fr a 500 KVA, 11 kV, 3-phase star connected alternator having armature resistance of 4 ohms. Calculate the

full load armature current per phase

a) 26.24 A

b) 20.41 A

c) 79 A

d) 40 A

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11. A synchronous machine with its field winding on stator and polyphase armature winding on rotor. At steady state, its air gap field is

I. stationary w.r.t. stator

II. rotating at double the speed Ns w.r.t. rotor

III. rotating in direction opposite to rotor

a) I,II,III

b) I

c) II,III

d) II,I

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12. A synchronous machine with its field winding on rotor and polyphase armature winding on stator. At steady state running condition, its air gap field is

a) rotating at synchronous speed w.r.t. stator

b) stationary w.r.t. rotor

c) rotating in the direction of the rotor rotor rotation

d) all of the mentioned

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13. Consider a 3-phase cylindrical-rotor alternator

A. E.m.f. generated by armature reaction lags armature current by 90°.

B. Air gap voltage leads the field flux by 90°

C. Air gap voltage lags the field flux by 90°

D. Armature reaction mmf lags the field flux by (90°+ internal p.f. angle)

a) A,B,D

b) A,C,D

C) B,D

D) C,D

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14. In a 3-phase cylindrical-rotor alternator, synchronous reactance is sum of

a) mutual and leakage reactance

b) magnetizing and leakage reactance

c) magnetizing and mutual reactance

d) mutual, magnetizing and leakage reactance

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15. The reactive power output of a synchronous generator is limited by

a) armature current and field current

b) field current and load angle

c) load angle and excitation

d) armature current only