Electrical Machines

Question 1

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

Answer 1

Centrifugal force is no longer a problem
Much lighter so higher speeds and less wear on bearings
Brush gear not required

Question 2

Explain 3 ways to increase the emf of an AC generator

Answer 2

Increase field strength
Increase speed of armature
Add more loops to armature

Question 3

What are 2 ways of increasing the frequency output of an AC generator?

Answer 3

Increase number of poles
Increase speed

Question 4

What is the frequency equation?

Answer 4

F=Np/60

Question 5

What is a bus bar made of?

Answer 5

Copper, brass or aluminium

Question 6

What does a bus bar do?

Answer 6

Conducts electricity within a switchboard/ distribution board

Question 7

Describe 2 types of 3 phase connections

Answer 7

Star connected- the end of each phase are connected at one central point
Delta connected- 3 phases connected together forming a loop

Question 8

Draw a graph of the output of a three phase generator

Answer 8

docket…

Question 9

What are the advantages of 3-phase systems?

Answer 9

Produce more power
Greater economy, (3-4 conductors rather than 6) reduces cost and complexity of wiring
Provides a rotating magnetic field

Question 10

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

Answer 10

Line voltage = √3 phase voltage

Question 11

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

Answer 11

A single wattmeter can be used to measure power dissipated in a balanced 3 phase load.
Two wattmeters show the total power dissipated in a balanced or unbalanced 3 phase load
Measures current and voltage in reference to the third

Question 12

Explain the purpose of using an unearthed electrical system

Answer 12

Improves equipment reliability not personal safety

Question 13

Why is it leathal to touch a bare conductor in an unearthed system?

Answer 13

Because of the cabling to hull capacitance and low resistance of human skin, enough current can flow to kill a person

Question 14

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

Answer 14

Provided the DC excitation current to the generator rotor via the exciter commutator and a set of slip rings

Question 15

What is the advantage of the brushless generator over the conventional generator?

Answer 15

Reduces maintenance requirements and improves overall efficiency.

Question 16

Describe the two types of generator rotors and their applications in the RN

Answer 16

Salient pole rotor- all HM ships
Field coils wound around the laminated pole pieces which are fixed to the rotor shaft ( 6 pole machines in RN) (short and fat)
Cylindrical rotor- subs gas turbine T45 and QEC
Circular cross section with filed windings sited in slots on surface of rotor ( two pole rotor) (long and thin)

Question 17

Explain how the field current is applied to the rotor of the conventional and brushless generators

Answer 17

conventional- dc exciter directly to the generator rotor using slip rings
brushless- 3 phase ac generator converts to dc using a diode pack

Question 18

Describe the construction of the stator of an AC generator

Answer 18

Steel plate (yoke) with a soft iron laminated core
Windings are placed in slots in the stator core
Space between stator core and yoke for ventilation

Question 19

What is the voltage regulation equation?

Answer 19

(VNL-VFL)/VFL x 100%

Question 20

Why is a voltage regulator necessary?

Answer 20

To ensure the generator is supplied with a constant voltage, voltage regulator reduces droop

Question 21

How is hand control of a generator achieved?

Answer 21

increases and decreases the resistance to alter the field current of the DC exciter?

Question 22

Explain the purpose of each main component of AVR

Answer 22

TRU –> senses the output voltage of the generator and transforms and rectifies it
Reference voltage unit –> produces a DC voltage proportional to the desired output voltage
High frequency generator –> Provides a stable high frequency power supply
Comparator unit –> Compares the voltage signal from the TRU with the reference voltage unit
Thyristor bridge rectifier –> controls the amount of current to the exciter field.
Exciter generator –> output is designed to supply the rotor of the main AC generator

Question 23

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

Answer 23

no load to load–> maximum terminal voltage change less than 1%
suddenly applied load –> not change more than 15% and must be back within 1% within 1 second

Question 24

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

Answer 24

Controls the amount of current to the exciter field

Question 25

Explain 2 reasons for paralleling AC generators

Answer 25

Transfer load from one generator to another or to and from shore supply Enable 2 or more generators to share the ships electrical load

Question 26

List and explain 4 conditions that must be met before generators are paralleled

Answer 26

same terminal velocity frequency must be lightly higher than the running generators frequency same phase rotation Both generators must be in phase when the circuit breaker is shut

Question 27

Explain the procedure for paralleling two AC generators using a sychroscope including the reason for each action

Answer 27

Adjust frequency so sychroscope is slowly rotating clockwise (slightly higher frequency) Oncoming generator breaker is closed at 11:55 position as slight delay (breaker shuts when generators in phase) Oncoming generator picks up some load due to frequency difference (prevent motoring)

Question 28

Describe operation of a sychroscope

Answer 28

It indicates the frequency and phase difference between 2 AC supplies

Question 29

Explain the relationship between True, Reactive and Apparent power.

Answer 29

True- in phase with busbar voltage (power that can be used to do work by the machine) Reactive- at 90° to the true power (power that creates and maintains magnetic fields in the machine) Apparent- calculated from true and reactive using Pythagoras theorem (Power that must be generated to supply the machine)

Question 30

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

Answer 30

prime mover- controls frequency -true power AVR - voltage- reactive power

Question 31

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

Answer 31

Stable operating point for true and reactive power

Question 32

What is the normal voltage droop of the shore side supply?

Answer 32

No droop

Question 33

What is the most common power factor on board HM ships?

Answer 33

0.8 lagging

Question 34

Explain why the AC generators designed in chapter 7&8 will not work for a high frequency generator

Answer 34

When rotor speed is increased, causes problems with centrifugal force, prime mover cant run reliably at high speeds.

Question 35

Explain how the 1600 Hz generator produces such a high frequency output at a relatively low prime mover speed.

Answer 35

Inductor generator, rapidly varies the magnetic flux interacting with the stator winding as the rotor turns, induces a rapidly varying EMF

Question 36

What is an application of a high frequency generator?

Answer 36

1600 Hz Permanent magnet generator

Question 37

If a transformer is designed to a step-up voltage, how will the current change?

Answer 37

decrease

Question 38

Describe two types of transformer core.

Answer 38

Core- cheaper construction, for low voltage power transformers, and very large power transformers to save weight. Shell- less flux leakage, used in light current and higher voltage applications

Question 39

Describe an application of a single phase transformer with equal number of primary and secondary windings.

Answer 39

Isolation transformer, separate electrical networks into isolated zones, and to protect sensitive electronic equipment from external earths. used in radio and audio circuitry

Question 40

Explain the main difference between an autotransformer and a conventional transformer

Answer 40

Auto transformer as a single winding common to both primary and secondary circuits

Question 41

What are the advantages and disadvantages of an autotransformer?

Answer 41

adv - cheaper and lighter as less copper and only one metal core - can be configured as variable voltage transformer disadv - provide no electrical isolation between primary and secondary side

Question 42

State an application of the autotransformer

Question 43

Explain how a current transformer works.

Answer 43

The core of the transformer surround the bus-bar, the busbar forms a one turn primary coil. an ammeter is placed in series with the secondary windings, the ammeter is calibrated to measure secondary current and display actual busbar current.

Question 44

State 2 applications for a current transformer

Answer 44

Portable clamp on ammeter ?

Question 45

Why can a current transformer become dangerous if the ammeter or load is removed from the secondary side?

Answer 45

It will produce a large EMF in the secondary if left as an open circuit