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Electrical Machines Theory Period 4 SAIT-Alberta > Alternators > Flashcards

Flashcards in Alternators Deck (31)
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1

What are two other names for an alternator?

Synchronous generator,

Alternating-current generator.

2

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

A rotor and Stator.

3

What are the two main construction methods of alternators?

Rotating armature,

Rotating field.

4

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

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

5

What are four distinct advantages of rotating field alternators?

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.

6

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

Cylindrical rotors.

7

Which type of prime movers typically turn salient pole rotors?

Internal combustion engines;

Water turbines;

Electric motors.

8

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

600V.

9

Which types of losses does an alternator experience?

Mechanical;

Iron (eddy, hysteresis);

Copper;

Separately excited field circuit.

10

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

Bearing friction;

Slip ring and brush friction;

Windage friction;

Cooling fan power loss.

{These losses are supplied by the prime mover}

11

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

Because most losses are fixed for any value of load.

12

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

Field excitation current.

13

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

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

14

What are the two main sources of field excitation?

Separately excited;

Self excited.

15

What are two methods of self excitation?

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

Brushless excitation, recycling load current.

16

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

Add an alternator in parallel.

17

What four conditions must be met before paralleling alternators?

Phase sequence same;

Terminal voltage same;

Frequencies same;

Voltage phase position same.

18

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

Three dark lamp method.

19

Why must the three dark lamps be suitable for twice the phase voltage of the alternator?

because of the voltages are out of phase they may experience the "double" voltage.

20

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

To prevent it from becoming a load on the system.

21

Which tool is used to determine phase position synchronism?

A synchroscope.

22

What does the vertical posiiton of a synchroscope mean?

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

23

What is indicated when a synchroscope rotates clockwise?

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

24

When is an alternator typically paralleled?

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

25

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

Total input power increases increasing speed frequency and voltage.

26

How is reactive power transferred between alternators?

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

27

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

By operating each alternator at the same power factor.

28

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

By lowering both alternator's field excitation current

29

Which phenomenon helps to keep the alternators in synchronism?

Synchronizing power.

30

What is hunting?

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