Alternators Flashcards
What are two other names for an alternator?
Synchronous generator,
Alternating-current generator.
Which two mechanical components are required in an alternator to establish relative motion between a magnetic field and a conductor?
A rotor and Stator.
What are the two main construction methods of alternators?
Rotating armature,
Rotating field.
Why is a rotating armature alternator limited to lower KVA capacities?
Because high AC voltages are difficult to transfer through slip rings and brushes.
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.
Which rotors are capable of being driven at high speeds, like that produced from gas or steam turbine prime movers?
Cylindrical rotors.
Which type of prime movers typically turn salient pole rotors?
Internal combustion engines;
Water turbines;
Electric motors.
If a single turn winding in an alternator stator generates 30 V, how much would a 20 turn winding generate?
600V.
Which types of losses does an alternator experience?
Mechanical;
Iron (eddy, hysteresis);
Copper;
Separately excited field circuit.
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}
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.
What is the only factro that can be altered after an alternator is constructed to change generated voltage?
Field excitation current.
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.
What are the two main sources of field excitation?
Separately excited;
Self excited.
What are two methods of self excitation?
A shunt or compound DC generator mounted on the same alternator shaft;
Brushless excitation, recycling load current.