Electrical Machine Principles

Question 1

How does a magnetic field and current carrying conductor interact to produce a force in the conductor?

Answer 1

All at 90 degrees to each other
Fleming left hand rule

Question 2

How does a commutator enable a DC motor to produce rotation in a uniform direction?

Question 3

What is the purpose in laminating the armature core and pole pieces in a DC electric motor?

Question 4

What two quantities combine to produce torque in an electric motor?

Question 5

What is back EMF and how is it produced in a DC motor?`

Question 6

Explain why it is brushes on a DC motor are normally placed on the MNA

Question 7

Briefly explain why the MNA tends to move when a motor is placed on a load

Question 8

What are the two main effects of armature reaction in a DC motor?

Question 9

2 methods that can be used to decrease sparking in a DC motor affected by armature reactions?

Question 10

Why is a series motor not started on no load?

Question 11

How does a shunt field differ physically from a series field in a DC motor?

Question 12

What effect causes a curve to turn to a straight line?

Question 13

State 3 methods of speed control for DC motors

Question 14

Explain how field control is achieved for shunt and series motors

Question 15

Give 2 disadvantages of field control

Question 16

Advantages and disadvantages of armature resistance control

Question 17

Why should terminal voltage control only be applied to the armature of a dc motor?

Question 18

What effect on torque produced by a motor does an increase in its speed generally produce?

Question 19

Why are starters needed on DC motors?

Question 20

What are the basic requirements of a DC motor starter?

Question 21

What is the operation of the faceplate starter?

Question 22

Purpose of the ‘no volt’ release coil on the faceplate starter?

Question 23

How does the faceplate starter protect a motor from excessive current?

Question 24

Compare operating principles of modern starter with faceplate starter

Question 25

Explain why the armature of most AC generators is on the stator instead of the rotor

Question 26

Why does a DC machine have commutator and brushes but an AC machine has slip rings?

Question 27

Explain 3 ways to increase EMF on an AC generator

Question 28

Explain 2 ways of increasing the frequency output of an AC generator

Question 29

Describe 2 types of 3 phase connections

Question 30

Advantages of 3-phase systems?

Question 31

What is the relationship between phase and line voltages in a Star connected 3-phase system?

Question 32

Explain the one wattmeter and two wattmeter methods of measuring 3-phase power

Question 33

Why do you use an unearthed electrical system onboard ship?

Question 34

Explain why it is lethal to touch a bare conductor in an unearthed system

Question 35

What is the purpose of the DC exciter in an AC generator?

Question 36

Advantages of the brushless generator over the conventional generator?

Question 37

2 types of generator rotors and their application?

Question 38

How is field current applied to the rotor of the conventional and brushless generators?

Question 39

Describe the construction of the stator of an AC generator?

Question 40

Why is a voltage regulator necessary?

Question 41

How is hand control of a generator achieved?

Question 42

What are the two main operating parameters required of an AVR by the Royal Navy?

Question 43

What is the purpose of the thyristor bridge in an AVR?

Question 44

Explain two reasons for paralleling AC generators

Question 45

4 conditions that must be met before generators are paralleled?

Question 46

How do you parallel two AC generators using a synchroscope?

Question 47

Operation of a synchroscope?

Question 48

Relationship between True, Reactive and Apparent power?

Question 49

How does the speed of the prime mover and setting of the AVR affect the load sharing of an AC generator set?

Question 50

Explain why a generator requires a droop characteristic in order to operate in parallel with another generator?

Question 51

What is the normal voltage droop supply?

Question 52

Application for a high frequency generator?

Question 53

How does a 1600 Hz generator produce high frequency output at a relatively low prime mover speed?

Question 54

How does a rotating magnetic field be produced using 3 phase AC?

Question 55

Operation of a 3-phase synchronous motor?

Question 56

Difficulties when starting a synchronous AC motor?

Question 57

Application of synchronous motor?

Question 58

Construction of a squirrel cage rotor?

Question 59

Why can the rotor speed never equal the synchronous speed in a three-phase induction motor?

Question 60

What does the term 'slip' in an induction motor?

Question 61

How can the starting torque of an induction motor be improved?

Question 62

How is torque produced by the conducting bars in a squirrel cage rotor?

Question 63

How can synchronous speed of a motor be changed?

Question 64

Difference between stator switching and pole changing?

Question 65

How is pole changing achieved?

Question 66

What is the main disadvantage in rotor resistance control as a method of varying the speed of an induction motor?

Question 67

4 ways the speed of an AC induction motor can be altered?

Question 68

4 results of common motor faults?

Question 69

Symptoms exhibited by a single-phasing motor?

Question 70

How are motors protected from each type of fault?

Question 71

Purpose of the MTPU?

Question 72

3 possible causes of single-phasing in 3-phase motor?

Question 73

What are the requirements for the performance of an under-voltage protection unit?