True Or False - DC Machines Review

Question 1

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

Answer 1

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

Question 2

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

Answer 2

False. But it will change the motor to differential compound

Question 3

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

Answer 3

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

Question 4

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

Answer 4

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

Question 5

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

Answer 5

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

Question 6

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

Answer 6

False, parallel with the armature means a shunt motor

Question 7

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

Answer 7

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

Question 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.

Answer 8

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

Question 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.

Answer 9

False, results in same rotation

Question 10

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

Answer 10

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

Question 11

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

Answer 11

True, the series field opposes the shunt

Question 12

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

Answer 12

False, no load speed is very high

Question 13

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

Answer 13

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

Question 14

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

Answer 14

True

Question 15

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

Answer 15

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

Question 16

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

Answer 16

False

Question 17

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

Answer 17

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

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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

Question 20

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

Answer 20

False, fields oppose

Question 21

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

Answer 21

True. Less flux, less CEMF, higher torque

Question 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).

Answer 22

True, would reverse direction of differentially compounded

Question 23

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

Answer 23

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

Question 24

A shunt generator may be either self or separately excited

Answer 24

True

Question 25

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

Answer 25

True

Question 26

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

Answer 26

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

Question 27

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

Answer 27

True, the residual flux is sufficient to maintain some CEMF

Question 28

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

Answer 28

True, over compounding

Question 29

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

Answer 29

False, only up to the point of saturation

Question 30

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

Answer 30

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

Question 31

A shunt motor may have its field separately excited

Answer 31

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

Question 32

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

Answer 32

True

Question 33

A DC motor can be used as a generator.

Answer 33

True

Question 34

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

Answer 34

False

Question 35

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

Answer 35

False

Question 36

During starting, all DC motors have high CEMF

Answer 36

False, no CEMF at start

Question 37

A DC generator can be operated as a motor

Answer 37

True

Question 38

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

Answer 38

False, series is best

Question 39

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

Answer 39

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

Question 40

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

Answer 40

True, low speed regulation, called a constant speed motor

Question 41

A cumulative compound motor has a definite no load speed.

Answer 41

True

Question 42

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

Answer 42

True, limits the armature current

Question 43

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

Answer 43

True, this is why reduced voltage starting is used

Question 44

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

Answer 44

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

Question 45

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

Answer 45

False, equally significant

Question 46

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

Answer 46

False, the opposite is true

Question 47

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

Answer 47

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

Question 48

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

Answer 48

False, open armature stops the motor

Question 49

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

Answer 49

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

Question 50

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

Answer 50

True, armature becomes a source causing Iᴀ to reverse

Question 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.

Answer 51

True, used to reduce armature reaction

Question 52

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

Answer 52

False, best used for DC arc welders

Question 53

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

Answer 53

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

Question 54

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

Answer 54

True. Vᴛ = CEMF + IᴀRᴀ

Question 55

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

Answer 55

True

Question 56

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

Answer 56

False, series motor

Question 57

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

Answer 57

False, motor shift is opposite to rotation

Question 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

Answer 58

False

Question 59

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

Answer 59

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

Question 60

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

Answer 60

False