Portfolia Part B

Question 1

Name the major components of a rotating DC machine

Answer 1

Armature, frame, field coil

Question 2

How many current paths does and 8 pole lap wound armature have

Answer 2

8 paths

Question 3

The basic operating principle of any motor is

Answer 3

electrical to mechanical

Question 4

When applying Flemings right hand rule for generators your thumb will point in the direction of

Answer 4

Motion of force

Question 5

Describe the physical difference between the coils used for series-field and the coils used for shunt-field in a compound DC machine

Answer 5

Series - windings are very thick and less turns

Shunt- wires are thin with lots of turns

Question 6

A 4-pole lap wound armature contains a total of 300 effective conductors. If the magnetic flux is 0.02 Webers per pole and the speed of rotation is 100RPM, what is the value of it’s generated voltage?

Answer 6

160V

Question 7

What is the most likely cause of a DC generator failing to generate a voltage when rotated

Answer 7

There is no residual magnetism

Question 8

2 methods used to determine losses in a D.C machine are by measurement and by calculation True or false

Answer 8

True

Question 9

What is the amount of torque developed by a motor which exerts a force of 80 N over a perpendicular distance of 200mm?

Answer 9

16Nm

Question 10

Why does the prime-mover for a DC generator have to work harder when the current drawn from the generator increases

Answer 10

It creates an opposing torque against the prime mover

Question 11

When testing a 220V DC electrical motor for insulation resistance between coils on what setting would you set the instrument?

Answer 11

500V

Question 12

Explain the purpose of a field Rheostat on a compound D.C motor

Answer 12

Allows you to change the resistance to control the motor

Question 13

Describe the most common method used to regulate the output voltage of a DC generator

Answer 13

Rheostat - regulates current to the field coils

Question 14

The methods used to create excitation in DC generators are

Answer 14

Separately excited, self excited

Question 15

How will an over-compounded generator perform under load, compared to a level compounded generator

Answer 15

Level - will maintain constant voltage under full loads

Over- higher voltage is produced under full loads

Question 16

What limits the current in a DC motor when it is running at normal operating speed

Answer 16

The opposing induced EMF

Question 17

Why is it that a series motor should not be operated under no load conditions

Answer 17

It will cause damage to itself as it speeds up

Question 18

A motor with an output of 5000 watts draws 25amps from 220volt supply. Calculate the motor efficiency

Answer 18

90.9%

Question 19

When reversing the direction of a cumulative compound motor you must

Answer 19

Swap the polarity of the armature or change the connections

Question 20

Name a method which can be used to reduce armature reaction

Answer 20

Compensating windings, inter-poles, brush shifts

Question 21

Name 1 way to start a DC motor

Answer 21

Progressively lower the resistance in series with the armature

Question 22

Which regulatory tool is used to ensure that Australians have efficient appliances and equipment

Answer 22

Minimum energy performance standards

Question 23

A four pole DC motor has a lap wound armature of 30 coils, each with 20 conductors. If the flux per pole is 0.02 Wb and the armature current is 19 Amps, how much torque is produced?

Answer 23

36.3Nm

Question 24

What is the most important safety factor associated with rotating machinery for electricians

Answer 24

No work shall be done on rotating machines if not locked out

Question 25

What do fingers indicate in Flemings left hand motor rule

Answer 25

Thumb is motion, pointer finger is flux and centre finger is current

Question 26

What electrical hand hel testing instrument can be used to test a DC armature for faults

Answer 26

Multimeter set to ohms

Question 27

What is the basic operating principle of a generator

Answer 27

Mechanical to electrical

Question 28

Which part of the DC generator produces the output voltage

Answer 28

The armature

Question 29

State the difference between a generator and a motor in terms of energy conversion

Answer 29

Motor - electrical to mechanical | Generator - mechanical to electrical

Question 30

Which electrical instrument can be used to compare the electrical measurements with nameplate details A: voltmeter B: watt-meter C: Ammeter D: all of the above

Answer 30

D

Question 31

What is the purpose of a prime-mover when related to a generator?

Answer 31

To turn the armature

Question 32

The voltage in a self excited generator may fail to build owing to

Answer 32

?

Question 33

Four safety precautions that should be observed when starting generators are

Answer 33

Properly earthed, emergency stop, must be guarded, coil to coil test

Question 34

A DC motor runs at 220v, draws a current of 22A, has a speed of 900 RPM and produces a torque of 50 Nm. calculate the output power

Answer 34

4712 watts

Question 35

Which DC motor has wide applications, especially within the traction industry?

Answer 35

Series excited motor

Question 36

What must DC motors have fitted to their windings to give better communications under all loads A: Inter-poles B: shunts C: Rheostats D: Permanent magnets

Answer 36

A

Question 37

Which of the following creates losses in a DC machine? A: Copper losses B: Iron losses C: Friction D: all of the above

Answer 37

D

Question 38

Does the speed characteristic of a shunt motor alter between no loads and full load?

Answer 38

Slightly on load

Question 39

Which component of a DC machine requires regular inspection and maintenance

Answer 39

Brushes