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151/23/007 > Electrical Machines > Flashcards

Electrical Machines Flashcards

1
Q

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

A

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

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2
Q

Explain 3 ways to increase the emf of an AC generator

A

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

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3
Q

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

A

Increase number of poles
Increase speed

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4
Q

What is the frequency equation?

A

F=Np/60

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5
Q

What is a bus bar made of?

A

Copper, brass or aluminium

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6
Q

What does a bus bar do?

A

Conducts electricity within a switchboard/ distribution board

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7
Q

Describe 2 types of 3 phase connections

A

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

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8
Q

Draw a graph of the output of a three phase generator

A

docket…

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9
Q

What are the advantages of 3-phase systems?

A

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

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10
Q

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

A

Line voltage = √3 phase voltage

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11
Q

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

A

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

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12
Q

Explain the purpose of using an unearthed electrical system

A

Improves equipment reliability not personal safety

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13
Q

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

A

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

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14
Q

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

A

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

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15
Q

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

A

Reduces maintenance requirements and improves overall efficiency.

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16
Q

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

A

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)

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17
Q

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

A

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

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18
Q

Describe the construction of the stator of an AC generator

A

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

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19
Q

What is the voltage regulation equation?

A

(VNL-VFL)/VFL x 100%

20
Q

Why is a voltage regulator necessary?

A

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

21
Q

How is hand control of a generator achieved?

A

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

22
Q

Explain the purpose of each main component of AVR

A

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

23
Q

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

A

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

24
Q

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

A

Controls the amount of current to the exciter field

25
Q

Explain 2 reasons for paralleling AC generators

A

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

26
Q

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

A

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

27
Q

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

A

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)

28
Q

Describe operation of a sychroscope

A

It indicates the frequency and phase difference between 2 AC supplies

29
Q

Explain the relationship between True, Reactive and Apparent power.

A

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)

30
Q

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

A

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

31
Q

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

A

Stable operating point for true and reactive power

32
Q

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

A

No droop

33
Q

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

A

0.8 lagging

34
Q

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

A

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

35
Q

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

A

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

36
Q

What is an application of a high frequency generator?

A

1600 Hz Permanent magnet generator

37
Q

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

A

decrease

38
Q

Describe two types of transformer core.

A

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

39
Q

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

A

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

40
Q

Explain the main difference between an autotransformer and a conventional transformer

A

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

41
Q

What are the advantages and disadvantages of an autotransformer?

A

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

42
Q

State an application of the autotransformer

A
43
Q

Explain how a current transformer works.

A

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.

44
Q

State 2 applications for a current transformer

A

Portable clamp on ammeter
?

45
Q

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

A

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

46
Q
A

151/23/007 (9 decks)

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