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Electrical Fundementals > Dc Motor/Generaton Theory > Flashcards

Dc Motor/Generaton Theory Flashcards

1
Q

In a DC motor, the stator’s magnetic field, due to armature reaction, moves.

A

in the opposite direction of rotation of the armature.

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2
Q

A small air gap between magnetic poles results.

A

in the same field as a large air gap, for the same magnetising force.

Magnetic field (number of flux lines) is the same regardless of distance between poles. However, the field density (flux density) will increase with a smaller air gap.

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3
Q

The electromagnetic brake coil in an actuator would be energised.

A

only when the actuator is running.

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4
Q

In a shunt wound direct current motor with a constant voltage field supply, the torque developed by the motor is.

A

directly proportional to armature current.

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5
Q

Decreasing the field current in a shunt motor will.

A

increase speed and increase torque.

Decreasing the field current in a shunt motor will reduce the back emf in the armature so the armature current will increase giving a rise in torque and the RPM will increase as a consequence.

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6
Q

To calculate generator output you need to know the.

A

armature speed and field strength.

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7
Q

In a shunt motor, if you reverse both field current and the armature current, the motor will.

A

continue to run in the same direction.

Prove it with Fleming’s left hand rule.

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8
Q

A starter generator has a.

A

low resistance series field and a high resistance shunt field.

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9
Q

If the brake coil on an actuator motor goes open circuit, the actuator will.

A

stop.

The brake coil is wired in series with the actuator motor coil. It withdraws a spring loaded lock to release the actuator when current is applied.

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10
Q

The diagram shows a 200 V long shunt generator. What is the voltage dropped across the series resistor?

A

0.12 V.

Don’t let the field configuration confuse you. The 0.04 and 100 ohm resistors are part of the generator and can be ignored. Just consider the 0.06 ohm (the series resistor) and the load resistor. Calculate current (about 2A) then calculate voltage across 0.06 ohm resistor.

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11
Q

If a generator speed is halved and the field strength is doubled, the voltage output will be.

A

unchanged.

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12
Q

How can the direction of rotation of a DC electric motor be changed?

A

reverse the electrical connections to either the field or armature windings.

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13
Q

The only practical method of maintaining a constant voltage output from an aircraft generator under varying conditions of speed and load is to vary the.

A

strength of the magnetic field.

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14
Q

Electric motors are often classified according to the method of connecting the field coils and armature. Aircraft engine starter motors are generally of which type?

A

Series.

Series motors have the highest starting torque.

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15
Q

If a heavy mechanical load is removed from a series motor.

A

the speed will increase and the armature current will decrease.

Speed increases and increases the back emf which decreases the armature current.

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16
Q

If the correct supply were connected to a DC shunt motor which had lost its residual magnetism it would.

A

run normally.

Its a motor, not a generator!.

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17
Q

One purpose of a growler test is to determine the presence of:

A

a shorted armature.

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18
Q

The purpose of an end-travel microswitch in a linear actuator is to.

A

remain closed and opens at end travel only.

See Pallett - Aircraft Electrical Systems, pg 140.

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19
Q

Linear actuators used in aircraft are of.

A

split field series wound type.

See Pallett - Aircraft Electrical Systems, pg 141.

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20
Q

If a generator sparks, a possible reason is.

A

the brush springs are loose.

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21
Q

If the field strength of a generator is doubled, and the speed is doubled, the output EMF will.

A

quadruple.

NIL.

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22
Q

In a DC motor the pole pairs are.

A

embedded within the field coils.

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23
Q

A series wound DC motor, the field has.

A

few turns of thick wire.

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24
Q

Doubling the running time of an electrical machine would.

A

double the joules used.

Power (watts) is the rate of using energy (Joules). So doubling running time will not change the power. Current is the rate of charge flow - so more charge (coulombs) will be used but not at a faster or slower ‘rate’.

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25
Q

On a linear actuator, the field cutoff coil will be energized when.

A

the actuator is running.

Field cutoff coil is believed to be the brake coil.

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26
Q

An electric motor produces a force of 5 N at a distance of 0.2m from the centre of rotation and rotates at a speed of 100 revs/sec. The motors output is.

A

628.4 watts.

Power in a motor = torque * 2pi * revs/sec. Torque = force * radius.

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27
Q

Starter motors are usually.

A

series wound.

Series wound motors have the greatest starting torque.

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28
Q

Generator brushes are normally made of.

A

carbon.

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29
Q

As the generator load is increased (within its rated capacity), the voltage will.

A

remain constant and the amperage output will increase.

The voltage regulator keeps the generator voltage output the same. Current will increase as the load resistance drops.

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30
Q

The voltage output of a generator is controlled by.

A

varying the current of the field.

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31
Q

The current flowing in the armature of a DC motor is equal to.

A

(applied volts-generated volts) / armature resistance.

Ohms law I = V/R. Voltage in the armature is the applied voltage - back emf (generated voltage).

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32
Q

What device is used to convert alternating current, which has been induced into the loops of the rotating armature of a DC generator into direct current as it leaves the generator?

A

A commutator.

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33
Q

Which of the following is not one of the purposes of interpoles in a generator?

A

Reduce field strength.

NOT’ one of the purposes. The purpose of interpoles is to overcome (in fact ‘prevent’) armature reaction. Armature reaction is partially responsible for arcing at the brushes.

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34
Q

To reverse the direction of a series motor.

A

the direction of the current through the field and the armature is reversed.

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35
Q

The current consumed by a DC starter motor will.

A

decrease as the engine speed increases.

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36
Q

How are generators rated?

A

Watts at rated voltage.

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37
Q

Due to armature reaction in a DC motor.

A

the trailing pole tips are magnetically weakened.

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38
Q

Due to armature reaction in a DC motor, what happens to the trailing pole tips?

A

The trailing pole tips are magnetically weakened.

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39
Q

Some electric motors have two sets of field winding wound in opposite directions so that the motor can be operated in which direction?

A

The motor can be operated in either direction.

Specifically series motors.

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40
Q

What polarity do interpoles take in a DC motor?

A

The same as the next main pole behind in the direction of rotation.

The interpole must pull back the magnetic field. In a motor, the magnetic field deflects in the opposite direction to the armature.

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41
Q

What is the principal advantage of the series-wound DC motor?

A

High starting torque.

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42
Q

Interpole windings fitted to DC series wound generators are in series with which component?

A

In series with the armature.

Interpole windings are always in series with the armature, so they carry the same current as the armature.

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43
Q

What polarity do interpoles take in a DC generator?

A

The same as the next main pole behind in the direction of rotation.

On a generator, the field is deflected in the same direction as the armature rotates. The interpoles must drag back the field.

44
Q

The method most often used in overcoming the effect of armature reaction is through the use of what?

A

Interpoles.

45
Q

The starting current of a series-wound DC motor produces what effect?

A

High starting torque.

At low speed (i.e starting) there is a very high current in the armature (due to no back-emf). Since the field is in series with the armature, the same high current flows in it, resulting in a very high torque.

46
Q

To increase the speed of a shunt motor, a resistance is placed where?

A

In series with the field.

An apparent anomaly. Increasing the field resistance decreases the field strength which reduces the back-emf in the armature. Armature current therefore increases and its speed increases.

47
Q

A 200 volt motor is taking 10 amperes armature current, with an armature resistance of 0.1 ohm. What is the Back-EMF under these conditions?

A

199 volts.

48
Q

What is a method used for restoring generator field residual magnetism?

A

Flash the fields.

49
Q

As the speed of a DC electric motor increases, what happens to the counter emf?

A

The counter emf builds up and opposes the applied emf, thus reducing the current flow through the armature.

50
Q

Aircraft generators are cooled by what method?

A

Ram air.

51
Q

On a combined DC starter - generator system, what happens to the voltage regulator after the start cycle is completed?

A

The voltage regulator is connected to the shunt field after the start cycle is completed.

52
Q

If the field current to a shunt wound DC motor is decreased when operating at a constant load, the motor speed will.

A

increase.

53
Q

If a motor is spinning at 50 cycles per second, how long is 1 cycle of the output?

A

0.02 seconds.

54
Q

In a DC motor, the rotation of the MNA.

A

is opposite to the rotation of the armature.

55
Q

In a wave wound generator with 4 poles, what would be the minimum number of brushes?

A

2.

56
Q

A wire is rotated through a magnetic field. To give DC it must be connected to.

A

a commutator.

57
Q

If the supply to a DC shunt motor was reversed.

A

it would rotate in the same direction.

58
Q

A series motor draws a high current on start, and then the current reduces. This is due to.

A

the back EMF.

59
Q

Interpoles in a DC generator are connected.

A

in series with the armature.

60
Q

In a DC generator, the effect of armature reaction may be reduced by moving the brush gear.

A

in the direction of armature rotation towards the MNA.

61
Q

The windings embedded in the pole faces of a DC generator are.

A

compensating windings.

62
Q

A 6 pole wave-wound generator has.

A

2 brushes.

63
Q

Armature reaction in a DC generator causes.

A

MNA to move in the direction of rotation.

64
Q

In a DC motor, back-EMF rises if the motor.

A

speed rises.

65
Q

A DC generator armature has 6 poles and is lap wound. The number of brushes required are.

A

6.

66
Q

Reactive sparking is reduced by.

A

interpoles.

67
Q

The back-EMF in a DC motor is.

A

less than the applied EMF.

68
Q

The flux density of a magnetic field is 1.5T. The length of the conductor in the field is 2 m and the current flowing through the conductor is 5 amps. The force on the conductor is.

A

15 Newtons.

69
Q

A DC shunt motor has 28 volts applied. The current taken from the armature of 2 ohms resistance is 1 amp. The Back-EMF is.

A

26 V.

70
Q

In a motor, armature reaction causes the MNA to move.

A

against the direction of rotation.

71
Q

The diagram shown is a.

A

long shunt compound wound machine.

72
Q

In a generator system, a stabilizing winding is used.

A

to prevent voltage overshoot.

73
Q

What part of a DC motor would you find the poles?

A

stator.

74
Q

The main advantage of a starter-generator over conventional units is.

A

for the same power output there is a weight reduction and the starter drive remains engaged.

75
Q

An A.C. shunt wound motor sometimes uses a volts dropping resistor. The resistor would be.

A

in series with the supply.

76
Q

Commutator ripple can be reduced by.

A

the use of a ripple filter.

77
Q

In a DC motor the interpoles are part of the.

A

field.

78
Q

Armature reaction is.

A

the MMF opposing rotation.

79
Q

A shunt motor.

A

is constant speed.

80
Q

In a DC shunt motor, if the field resistance is increased what happens to the back EMF?

A

decreases.

81
Q

To increase the voltage output of a generator you can.

A

increase speed.

82
Q

Generator voltage is.

A

dependant on speed of rotation.

83
Q

The output from a generator to a commutator is.

A

AC.

84
Q

What is the main disadvantage of a starter generator?

A

Will not supply output at low rpm.

85
Q

Back-EMF is the greatest when a motor is.

A

at operating speed.

86
Q

When a conductor carrying a current and placed in a magnetic field it experiences.

A

an electromagnetic force.

87
Q

The type of motor used in a linear actuator is.

A

series.

88
Q

If the speed of a coil in a magnetic field is doubled and the field strength is doubled the output will.

A

quadruple.

89
Q

How many brushes are used in an 8 pole wave wound machine?

A

8.

90
Q

If the speed of a coil in a magnetic field is doubled and the field strength is doubled, what will happen to the output?

A

Quadruple.

91
Q

How many brushes are used in an 8 pole wave wound machine?

A

2.

92
Q

In the shunt generator, the field is supplied by?

A

The armature.

93
Q

Increasing the speed of an aircraft generator results in an automatic?

A

Field circuit resistance increase.

94
Q

Which generators are usually used on aircraft?

A

Shunt.

95
Q

In a cumulative compound wound generator, the series and shunt fields?

A

Assist each other.

96
Q

The shunt wound generator is controlled by a variable resistance in?

A

Series with the field.

97
Q

The series wound generator’s terminal voltage will, when below saturation?

A

Decrease with an increase in load.

98
Q

The shunt wound generator is normally started?

A

Off load.

99
Q

How do you reduce ripple on the output of a DC generator?

A

Decrease the speed of the armature and add more series windings.

100
Q

Armature reaction in a DC motor causes the neutral axis to shift in?

A

The direction opposite to the direction of armature rotation.

101
Q

An indication of good commutation on a DC machine would be?

A

Little or no sparking at the commutator.

102
Q

If compound DC generators are operated in parallel they must?

A

Have an equalising or load sharing loop.

103
Q

The windings embedded in the pole faces of a generator are?

A

Compensating coils.

104
Q

Prolonged reactive sparking in a DC generator could damage the?

A

Commutator.

105
Q

The poles of a DC generator are laminated to?

A

Reduce hysteresis losses.

106
Q

One advantage of a DC motor over an AC motor is?

A

The DC motor will use less current.

107
Q

In a shunt wound direct current motor the torque is?

A

Proportional to the current in the armature.

Electrical Fundementals (19 decks)

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