Alternators Flashcards
What is relative motion in relation to alternators?
Relative motion indicates that either the conductor is moving through a magnetic field, or the magnetic field is moving past the conductor.
What happens when there is relative motion between a magnetic field and a conductor?
A voltage is induced in the conductor/
What are alternators also called?
Synchronous generators or alternating-current generators.
When speaking about an alternator, which part is where the voltage is induced?
The armature.
Is the induced voltage generated by an alternator ac or dc?
ac
What waveform is the output of an alternator?
Sine wave or sinusoidal.
True or false:
An alternator can be constructed with either the armature windings or field windings mounted on the rotor
True.
Is a rotating armature or rotating field alternator more common?
Rotating Field Alternators are more common.
Why is a rotating field alternator more popular than the rotating armature variety?
A rotating armature means that you would have to transfer high KVA through slip rings and brushes. This isn’t practical so a stationary armature conductor or winding is better.
What are some advantages of a rotating field alternator?
- Slip rings and brushes are not required to connect the armature output to the external load.
- The field requires relatively low currents and voltages, so smaller brushes and slip rings are sufficient.
- The rotating field structure can be physically smaller, which results in mechanical advantages such as reduced bearing sizes and reduced vibrations.
- Most of the heat generated in an alternator comes from the armature windings which are easier to cool, because of their position.
What does salient mean?
Protruding, projecting, or standing out.
A salient pole rotor is typically (shorter/longer) in length and (smaller/larger) in diameter than cylindrical rotors.
Shorter in length and larger in diameter.
What two variables does the frequency of the generated voltage of an alternator depend on?
The number of poles on the rotor
The rotational frequency of the rotor in rpm
What is the formula for generated frequency
Frequency = (Poles x Speed) / 120
What types of losses can a synchronous alternator have?
- Mechanical losses
- Iron losses (core losses)
- Copper losses
- Losses in the separately-excited field circuit
What are the 2 major losses categorized as iron losses?
Hysteresis and Eddy currents
How do you calculate alternator efficiency?
(Power Out / Power In) x 100 = Percent Efficiency
True or False?
In an alternator, the brushes are stationary.
True
True or False?
In an alternator, the slip rings are stationary.
False. The slip rings are attached to the rotor.
True or False?
The excitation current that creates the magnetic field in an alternator is from an ac source.
False. The excitation is from a DC source.
True or False?
To establish a relative motion between the armature and the field, an alternator requires a rotor and a stator.
True
True or False?
The output voltage of an alternator is DC.
False. An alternator produces AC.
Alternators use rotating fields instead of rotating armatures because:
a) smaller bearings are required
b) the slip rings are smaller
c) it is easier to cool the stator
d) all of the above
d) All of the above
What are high speed rotors called?
Cylindrical rotors.
A rotor with projecting poles is called what?
Salient pole rotor. Salient means projecting or standing out.
A 60 Hz 4-pole alternator operates at how many RPM?
1800rpm
Formula: (Poles x Speed)/120 = frequency
Therefore, (frequency x 120)/poles gives you the speed.
How many poles does a 300rpm, 60Hz alternator have?
24 poles
f = (poles x speed)/120
A six-pole alternator rotating at 1200 rpm generates a frequency of:
60hz
f = (speed x poles)/120
For a given alternator, if a 30-turn winding of a stator generates 120v, how many volts will a 75-turn winding generate?
300v
Do laminations decrease hysteresis loss?
No, they only reduce eddy current losses.
Do some alternators have liquid cooled armatures?
Yes, you betcha they do.
What factors have an effect on the generated voltage of an alternator?
- The number of armature turns per winding
- The speed of the rotating field
- The amount of flux produced by the rotating field
Do most applications for synchronous alternators require a fixed frequency?
Yes! For example, we require 60Hz in most North American applications.
What is the only practical way to change the value of voltage generated by an alternator?
To alter the field flux. (Turns, type of core, amount of current in field winding)
How do you calculate the percent voltage regulation of an alternator?
% Voltage Reg = {(V(no load) - V(full load))/V(full load}x100
What are the 2 possible sources of field excitation in an ac alternator?
- Separately excited
- Self-excited
Using a stationary solid-state rectifier to supply current to an alternator’s field is an example of a __________ system.
Separately excited system
A relatively small shunt or compound dc generator mounted on the same alternator shaft that supports the main alternator rotor and field windings is an example of a __________ system.
Self-excited system.
In a brushless excitation system, the exciter output is connected to the alternator field via ________.
diodes
What conditions must be met before an incoming alternator is in synchronism with a running alternator in parallel and before they are connected together?
- The phase sequence must be the same
- The terminal voltages must be the same
- The frequencies of the alternators must be the same
- The voltage phase position must be the same.
What method is used to check phase sequence between 2 alternators?
The three dark lamp method.
How does the 3 dark lamp method work?
Each phase of the two alternators is connected in parallel with a lamp in the middle. If the lamps light up at all, you know that there is a potential difference between the two.
What does a synchroscope indicate?
Frequency and phase position.
Should an incoming alternator be running slightly faster when starting to synchronize?
Yes
What are 3 advantages of running alternators in parallel?
- Paralleling alternators can increase the output capacity of a system.
- A paralleled alternator can be disconnected from the system for maintenance and repair without power interruption.
- Paralleling alternators can allow for more efficient operation of the alternators over a wide range of loads
When the three dark lamp method is used and the lamps brighten and dim all at the same time, what does this indicate?
The same phase sequence but a different frequency
When the 3 dark lamp method is used and the lamps flash from dim to bright and back to dim, but not at the same time, what does this indicate?
A different phase sequence and a different frequency.
When the 3 dark lamp method is used and all 3 lamps stay bright all the time, this indicates that the two alternators have a 180deg phase displacement and what?
The same phase sequence and the same frequency.
When the 3 dark lamp method is used, what is the maximum voltage that can exist across a lamp?
Double the phase voltage.
If you increase the power input to an alternator that is connected in parallel with another alternator, what will result?
Increased total power of the system which will make an increased speed, frequency, and generated voltage of the system.
How can the system output voltage of two parallel alternators be increased?
Increase the field excitation of both alternators proportionally.
How can the system line frequency of two parallel alternators be increased without shifting the true power load?
Increase the speed of the prime movers proportionally.
How can the power load be shifted from a running alternator to an incoming alternator?
Increase the power input to the incoming alternator while decreasing the power input to the running alternator simultaneously.
How can the reactive power load be shifted from a running alternator to an incoming alternator?
Increase the field excitation to the incoming alternator while decreasing the field excitation to the running alternator.
Two paralleled alternators are observed speeding up and slowing down several times as they settle into a synchronous condition. The speeding up and slowing down is called what?
Hunting
How can hunting be reduced if the prime mover is a pulsating engine?
Hunting can be reduced with heavy flywheels and amortisseur windings
