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Flashcards in True Or False - DC Machines Review Deck (60):
1

The direction of rotation for a DC series motor may be reversed by interchanging only the field leads

True. The direction of current flow through the armature does not change, just the field polarity

2

In a cumulative compound motor, reversing the series field connections will change the direction of rotation

False. But it will change the motor to differential compound

3

The best method of reversing rotation for a shunt motor with interpoles is to interchange the leads at the brushes

False, this would change the polarity of the field and the armature as well, thus the same rotation

4

It is best to reverse the shunt field leads of a cumulative compound motor to reverse rotation.

False, this would convert the machine to a differential compound motor

5

If the supply lines to a series DC motor are reversed, the armature will rotate in the opposite direction.

False, both the field and armature current directions are reversed resulting in no change in rotation

6

A two pole motor has its field windings series connected, and then paralleled with the armature. This indicates a series motor.

False, parallel with the armature means a shunt motor

7

An under compounded generator has better voltage regulation than a straight shunt generator (of equal rating).

True, only differential compound has poorer voltage regulation than the shunt generator

8

A series DC motor may be connected as a shunt motor, and a shunt motor may be connected as a series motor, if necessary.

False, due to the physical differences of the shunt and series windings

9

As a general rule, to change rotation for a compound motor, it is best to reverse current flow through the armature and reverse both field windings.

False, results in same rotation

10

The starting torque of a cumulative compound motor, per amp of armature current, is considered higher than that for a shunt motor.

True. Series has good starting torque, shunt is poor, and compound is between them

11

Differential connected motors have lower starting torque than cumulative connected compound motors.

True, the series field opposes the shunt

12

The series motor has better speed regulation than a cumulative compound motor.

False, no load speed is very high

13

The load current of a self excited shunt generator is simply reversed by reversing the direction of rotation.

False, there would be no buildup of voltage without reversing the field connections

14

The polarity of a separately excited shunt generator can be reversed, simply by changing the direction of rotation, or interchanging the field leads

True

15

A shunt motor, started with resistance inserted in its field, will develop higher starting torque than without resistance.

False. High field resistance - low field current - low field flux - low torque

16

Differential compounded motors are used more extensively than cumulatively compound motors.

False

17

If the armature circuit voltage of a motor with a constant load is reduced, then the speed will increase.

False, results in lower armature current, lower torque and lower rpm

18

Less current through the field of a shunt motor will cause a speed increase.

True. Lower flux, lower CEMF, higher armature current, higher torque and rpm

19

If an over compounded motor is differentially connected, load changes may create unstable speed conditions.

True, poorer speed regulation closer to series motors. Reversing under high load could cause it to over speed.

20

The starting torque of a differential motor is much higher than the starting torque for a cumulative type motor.

False, fields oppose

21

If the series field of a cumulative compound motor is bypassed (shorted out), the speed of the motor will increase.

True. Less flux, less CEMF, higher torque

22

A cumulative motor is to be tested for proper connections. Rotation will be the same if run as a series motor (shunt field lead opened), or as a shunt motor (series field shorted out).

True, would reverse direction of differentially compounded

23

A differential motor started under heavy load may start up in a direction opposite to that desired.

True, the series flux may overpower the shunt flux due to high inrush current

24

A shunt generator may be either self or separately excited

True

25

The torque of a motor calculated by HP = TN/5252 is the full load torque.

True

26

If the speed of a shunt motor is increased, by a field current change, then the armature current will also increase momentarily.

True, a decrease in field current decreases the flux and CEMF, this would increase the armature current, torque and rpm

27

Very small HP rated (fractional) series motors will normally not reach destructive speeds if unloaded.

True, the residual flux is sufficient to maintain some CEMF

28

With suitable compounding, a generator can deliver a higher voltage at full load than at no load.

True, over compounding

29

The torque of a series motor always increases as the square of the armature current.

False, only up to the point of saturation

30

A generator is flat compounded when the series field current is equal to the shunt field current

False, when V(no load) = V(full load)

31

A shunt motor may have its field separately excited

True, good for speed control as it does not affect the armature voltage

32

In every DC generator, there is a fundamental motor action that occurs.

True

33

A DC motor can be used as a generator.

True

34

In a short shunt generator connection, the shunt field is directly across the terminal lines.

False

35

A DC series motor is considered to have a definite no load speed.

False

36

During starting, all DC motors have high CEMF

False, no CEMF at start

37

A DC generator can be operated as a motor

True

38

A DC shunt motor develops a higher starting torque, per amp of armature current, than a series motor.

False, series is best

39

A shunt generator supplies a constant output current under a varying load.

False, the terminal voltage drops due to armature reaction, IR drops and further field weakening

40

For constant speed drives, the best motor to select is a shunt motor

True, low speed regulation, called a constant speed motor

41

A cumulative compound motor has a definite no load speed.

True

42

A shunt motor, without load, develops a CEMF almost equal to the applied voltage.

True, limits the armature current

43

If any DC motor is started across the line, the inrush current, for all practical purposes, is determined by the armature circuit resistance.

True, this is why reduced voltage starting is used

44

If a series motor draws 50% of its rated current, then the torque produced is very nearly 25% of rated

True, torque is proportional to Iᴀ² (up to saturation)

45

Armature reaction is not as significant for DC motors as it is for DC generators

False, equally significant

46

In high torque applications, such as cranes and hoists, a shunt motor is much better suited than a series motor.

False, the opposite is true

47

Field strengthening in a shunt motor will cause its speed to decrease

True, increased flux increases the CEMF, decreases Iᴀ, decreases torque and rpm

48

If the field of a series motor under load is opened, a high speed may occur due to the flux reduction

False, open armature stops the motor

49

If the field of a shunt motor burns open, the motor will always stop.

False, decreases flux and CEMF, increases Iᴀ, torque and rpm

50

If connections remain unchanged, a cumulative compound motor used as a generator will be differentially compounded.

True, armature becomes a source causing Iᴀ to reverse

51

Compensating windings are connected in series with the armature and embedded in the main pole face. They are connected so that the current flow through them is in a direction opposite to the armature conductors immediately adjacent to them.

True, used to reduce armature reaction

52

A differential compound generator is best suited for a constant voltage output under varying loads

False, best used for DC arc welders

53

When the speed of a shunt motor drops, it is probably a sign that more resistance has been inserted in the field circuit.

False, more resistance would mean less flux, less CEMF, more armature current, more torque and rpm

54

The fundamental motor equation is that "Terminal voltage equals generated voltage plus the IR drop in the armature".

True. Vᴛ = CEMF + IᴀRᴀ

55

A reduction of CEMF, in a DC motor, permits more current to flow through the armature.

True

56

The DC motor that does not have a definite no load speed is the differential compound motor

False, series motor

57

Under load, the electrical neutral plane of a DC motor shifts in the direction of armature rotation.

False, motor shift is opposite to rotation

58

A compound DC motor has the following connections; L1 to F2 to A2; L2 to F1 to S2; and A1 to S1. It is connected cumulative compound for CW rotation

False

59

If the diverter rheostat on a cumulative compound DC generator is increased, then the terminal output will decrease.

False, the diverter rheostat is in parallel with the series winding. An increase in resistance would increase the current through the series winding, increase the flux and increase the generated voltage

60

Reversing the leads to the brushes of a DC motor will reverse the polarity of the interpoles

False