Alternators Flashcards

1
Q

What are two other names for an alternator?

A

Synchronous generator,

Alternating-current generator.

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2
Q

Which two mechanical components are required in an alternator to establish relative motion between a magnetic field and a conductor?

A

A rotor and Stator.

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3
Q

What are the two main construction methods of alternators?

A

Rotating armature,

Rotating field.

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4
Q

Why is a rotating armature alternator limited to lower KVA capacities?

A

Because high AC voltages are difficult to transfer through slip rings and brushes.

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5
Q

What are four distinct advantages of rotating field alternators?

A

No need for discontinuity in the load circuit;

The field requires relatively low current, therefore smaller brushes can be used;

Structure can be physically smaller, reduced bearing size and vibration;

Easier to cool, due to position of armature.

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6
Q

Which rotors are capable of being driven at high speeds, like that produced from gas or steam turbine prime movers?

A

Cylindrical rotors.

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7
Q

Which type of prime movers typically turn salient pole rotors?

A

Internal combustion engines;

Water turbines;

Electric motors.

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8
Q

If a single turn winding in an alternator stator generates 30 V, how much would a 20 turn winding generate?

A

600V.

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9
Q

Which types of losses does an alternator experience?

A

Mechanical;

Iron (eddy, hysteresis);

Copper;

Separately excited field circuit.

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10
Q

What are some causes of mechanical losses in a rotating machine?

A

Bearing friction;

Slip ring and brush friction;

Windage friction;

Cooling fan power loss.

{These losses are supplied by the prime mover}

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11
Q

Why does the overall efficiency of an alternator decrease rom a full load to a half load condition?

A

Because most losses are fixed for any value of load.

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12
Q

What is the only factro that can be altered after an alternator is constructed to change generated voltage?

A

Field excitation current.

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13
Q

Why is the terminal voltage sometimes different then the generated voltage in an alternator?

A

Because the power factor of the load affects the internal voltage drop due to armature reaction.

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14
Q

What are the two main sources of field excitation?

A

Separately excited;

Self excited.

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15
Q

What are two methods of self excitation?

A

A shunt or compound DC generator mounted on the same alternator shaft;

Brushless excitation, recycling load current.

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16
Q

What is the solution to higher demand in an electrical system?

A

Add an alternator in parallel.

17
Q

What four conditions must be met before paralleling alternators?

A

Phase sequence same;

Terminal voltage same;

Frequencies same;

Voltage phase position same.

18
Q

What is the most common method to check phase sequence synchronism?

A

Three dark lamp method.

19
Q

Why must the lamps be suitable for twice the phase voltage of one alternator when using the Three-Dark Lamp method?

A

When the phase voltages of the Alternators are 180° out of phase, the voltage across the lamps is double that of the generated voltage of one alternator.

20
Q

Why are the frequency and output voltage of the incoming alternator slightly higher?

A

To prevent it from becoming a load on the system.

21
Q

Which tool is used to determine phase position synchronism?

A

A synchroscope.

22
Q

What does the vertical posiiton of a synchroscope mean?

A

The phase voltages of the two alternators are peaking at the same time.

23
Q

What is indicated when a synchroscope rotates clockwise?

A

The frequency of the incoming alternator is higher than the existing one.

24
Q

When is an alternator typically paralleled?

A

When the synchroscope pointer is moving slowly in the “fast” direction and approaching 12 o clock.

25
Q

What happens if two alternators are in parallel and the input power to one is increased without simultaneously decreasing input power to the other?

A

Total input power increases increasing speed frequency and voltage.

26
Q

How is reactive power transferred between alternators?

A

Increasing the field excitation of the alternator which “desires” reactive power load, while simultaneously decreasing the field excitation of the other alternator.

27
Q

What is the best way to reduce circulating current in alternators?

A

By operating each alternator at the same power factor.

28
Q

How is proper output voltage maintained after frequency has been increased in a two alternator system?

A

By lowering both alternator’s field excitation current

29
Q

Which phenomenon helps to keep the alternators in synchronism?

A

Synchronizing power.

30
Q

What is hunting?

A

A back and forth oscillation as a result of the alternators producing and consuming synchronizing power.

31
Q

How is hunting reduced?

A

Heavy flywheels;

Amortisseur windings in the rotor face.

32
Q

What kind of connection is this?
How many lead Alternator?

A

Series Wye Connection

Twelve-Lead Alternator

33
Q

What is Phase Sequence? What is another name for it?

A

It’s the direction of the magnetic field rotation. (ABCABC or CBACBA)

Phase Rotation