I3 Install DC Motors and Generators

Question 1

What is the term for a rotating machine that converts mechanical energy into electrical energy?

Answer 1

Dynamo

Question 2

Define: armature

Answer 2

rotating part or rotor of the machine

Question 3

List the two types of armature windings

Answer 3

Lap used for high current, low voltage applications

Wave used for low current, high voltage

Question 4

What does the commutator do?

Answer 4

convert the induced AC within the armature within the armature coils to a uni-directional current in the external circuit.

Question 5

What is the function of the brushs

Answer 5

they conduct the current to or from the armature coils via the commutator from or to the external circuit.

Question 6

What are brushes commonly made of?

Answer 6

carbon
graphite
copper graphite

Question 7

How much pressure is on the brushes?

Answer 7

1 to 2 psi

Question 8

Define: field poles

Answer 8

they provide the magnetic field in which the armature will rotate.

Question 9

What type of bearing are used in DC Motors

Answer 9

sleeve bearings

ball bearings

Question 10

Define: magnetos

Answer 10

small dc generators that use permanent magnets to provide the magnetic field.

Question 11

what two ways can field poles be excited

Answer 11

DC source external to the machine

from the armature of the generator itself

Question 12

Define: Series field windings

Answer 12

connected in series with the armature and carry full armature current

Question 13

Define: Shunt field windings

Answer 13

connected in parallel with armature therfore are made up of large number of turns of small-guage wire.

Question 14

Self-exited generators must have what to produce voltage?

Answer 14

residual magnetism of the field core

Question 15

What is Faraday’s law of electromagnetic induction?

Answer 15

e = Blv

Question 16

Explain the generated voltage formula

Answer 16

Eg = k(flux)N
Eg = generated voltage in volts
k = constant
Flux 
N = rotational speed in rpm

Question 17

How can an absence of residual magnetism be rectified in a self exciting DC motor

Answer 17

by flashing the field

Question 18

If a DC motor is rotated backwards it generates a field current that ________

Answer 18

strips away the residual magnetism and voltage will fail to build-up

Question 19

Critical field resistance

Answer 19

for voltage build-up to occur, the field resistance curve must lie below the saturation curve therefore set field rheostats to their minimum value.

Question 20

Generated voltage is proportional to

Answer 20

the product of field flux and armature speed and some constant

Question 21

Define armature reaction

Answer 21

when an armature is delivering current to a load, a magnetic field surrounds the armature conductors

Question 22

how does armature reaction affect the operation of a generator

Answer 22

reduced terminal voltage of the load

impairs commutation by shifting the mechanical neutral plane to the electric plane (sparking brushes)

Question 23

what two ways can armature reaction be fixed

Answer 23

using interpoles

compensating windings in the field poles

Question 24

Interpole coils consist of a

Answer 24

small number of turns of large guage wire and in series with the armature

Question 25

List three types of losses in a DC generator

Answer 25

Mechanical Copper losses Core losses

Question 26

List three types of DC generators

Answer 26

Series Shunt Compound

Question 27

What do the letters A,F,S stand for

Answer 27

armature shunt field coils series field coils

Question 28

Numerical subscripts are used to indicate current direction in the windings. Flow from even to odd

Answer 28

F2 to F1 and from S2 to S1

Question 29

Define cumulative compound

Answer 29

when the shunt and series field coils work together

Question 30

Define: differential compound

Answer 30

when the shunt and series field coil oppose each other.

Question 31

how is a long shunt connected?

Answer 31

across the line

Question 32

how is a short shunt connected?

Answer 32

across the armature

Question 33

List the three types of cumulative compound DC generators

Answer 33

Flat-compound Over-compound Under-compound

Question 34

what is the main application for a DC differential compound generator.

Answer 34

DC arc welding

Question 35

what do the three finger represent in the Right-hand motor rule?

Answer 35

Thumb is thrust First finger is direction of flux from North to South Middle finger is direction of electron flow

Question 36

Define: Torque

Answer 36

twisting or turning force which is capable of producing rotation about an axis

Question 37

Torque is directly proportional to

Answer 37

the product of flux and armature current and some constant

Question 38

Define counter emf

Answer 38

the motion of the armature conductors cutting the field in a DC motor results in an induced emf that opposes the applied emf that is producing the armature current

Question 39

Define effective voltage

Answer 39

the product of armature current and armature resistance

Question 40

Describe mechanical loading of a DC motor

Answer 40

``` Speed decreases counter emf decreases increase in effective voltage increase in armature current increase in torque ```

Question 41

Describe how a DC motor is operated above base speed

Answer 41

by inserting rheostat in the shunt field cct. field current will decrease decrease in flux will cause counter emf to decrease armature current to increase armature current is greater than the decrease in excitation resulting in more torque.

Question 42

Describe how a DC motor is operated below base speed

Answer 42

reduce armature current by adding resistance in series with the armature ( not practical) Done with electronic devices

Question 43

which way does the electrical plane rotate in a generator?

Answer 43

CW

Question 44

which way does the electrical plane rotate in a motor

Answer 44

CCW

Question 45

Shunt motor is commonly referred to as a

Answer 45

constant speed motor

Question 46

To prevent run-away in a shunt motor an

Answer 46

over-speed senor (loss of field relay) is used

Question 47

Always mechanically couple a series motor

Answer 47

directly to its load

Question 48

common application of series DC motor

Answer 48

automotive starters locomotives mine haul trucks

Question 49

Main characteristics of the series motor are

Answer 49

high starting torque | poor speed regulation

Question 50

Describe compounding or degree of compounding

Answer 50

the degree of compounding can produce torque characteristics ranging from series to shunt. compounding is the ratio of resistances of the shunt coils to series coils. ( I think)

Question 51

Is it necessary to take precautions to prevent run-away of a compound motor?

Answer 51

No. shunt coils provide a no load base speed