Alternators

Question 1

What are two other names for an alternator?

Answer 1

Synchronous generator,

Alternating-current generator.

Question 2

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

Answer 2

A rotor and Stator.

Question 3

What are the two main construction methods of alternators?

Answer 3

Rotating armature,

Rotating field.

Question 4

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

Answer 4

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

Question 5

What are four distinct advantages of rotating field alternators?

Answer 5

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.

Question 6

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

Answer 6

Cylindrical rotors.

Question 7

Which type of prime movers typically turn salient pole rotors?

Answer 7

Internal combustion engines;

Water turbines;

Electric motors.

Question 8

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

Answer 8

600V.

Question 9

Which types of losses does an alternator experience?

Answer 9

Mechanical;

Iron (eddy, hysteresis);

Copper;

Separately excited field circuit.

Question 10

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

Answer 10

Bearing friction;

Slip ring and brush friction;

Windage friction;

Cooling fan power loss.

{These losses are supplied by the prime mover}

Question 11

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

Answer 11

Because most losses are fixed for any value of load.

Question 12

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

Answer 12

Field excitation current.

Question 13

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

Answer 13

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

Question 14

What are the two main sources of field excitation?

Answer 14

Separately excited;

Self excited.

Question 15

What are two methods of self excitation?

Answer 15

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

Brushless excitation, recycling load current.

Question 16

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

Answer 16

Add an alternator in parallel.

Question 17

What four conditions must be met before paralleling alternators?

Answer 17

Phase sequence same;

Terminal voltage same;

Frequencies same;

Voltage phase position same.

Question 18

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

Answer 18

Three dark lamp method.

Question 19

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

Answer 19

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

Question 20

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

Answer 20

To prevent it from becoming a load on the system.

Question 21

Which tool is used to determine phase position synchronism?

Answer 21

A synchroscope.

Question 22

What does the vertical posiiton of a synchroscope mean?

Answer 22

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

Question 23

What is indicated when a synchroscope rotates clockwise?

Answer 23

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

Question 24

When is an alternator typically paralleled?

Answer 24

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

Question 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?

Answer 25

Total input power increases increasing speed frequency and voltage.

Question 26

How is reactive power transferred between alternators?

Answer 26

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

Question 27

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

Answer 27

By operating each alternator at the same power factor.

Question 28

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

Answer 28

By lowering both alternator’s field excitation current

Question 29

Which phenomenon helps to keep the alternators in synchronism?

Answer 29

Synchronizing power.

Question 30

What is hunting?

Answer 30

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

Question 31

How is hunting reduced?

Answer 31

Heavy flywheels;

Amortisseur windings in the rotor face.