AC Generators Flashcards
2 main parts to AC generators
Stator which carries 3 pairs of field windings - stationary
Rotor which carries an electromagnetic field
Why do we use 3 pairs of windings - AC generator
Allows us to get a 120* spilt
Advantages of 3 phase AC
Higher efficiency
Fewer conductors required for transmission and distribution
3 phase motors have better characteristics than single phase
Can use the same power supply for single and 3 phase
2 different types of connections for AC
Star/Y connection
Delta or mesh connection
Star/ Y connection
Centre goes the the a/c unipole system
Allow us to supply single phase or 3 phase
Delta or mesh connection - only 3 phase
Phase voltage
Between star point or neutral point to one of the phases (Vp)
Line voltage
Potential difference across any 2 lines/phases in the star connection (VL)
When is the star connected system balanced
When it is supplying equal loads in each line then the magnitude of the phase voltage for each line is the same
Having the neutral point in the star connection system wired to earth allows
Generator to feed unbalanced loads
Easy to access phase voltages
Majority of the a/c AC generators are wired in star connection
Freq formula for an AC generator
Freq = NP/60
N = speed (RPM)
P = number of poles
Phase rotation - implications of having a negative phase sequence
Results in the 3 phase motors running in the wrong direction
3 phase power supplies must have a positive phase sequence - ABC
Brushless generator
No arcing/sparking
Instead of using battery - uses a small magnet driven from the engine - AC is created and then to DC via a voltage regulator
In order to produce 400Hz the generator needs to be spun at
8,000RPM
Constant speed drive unit what is it
Hydraulic pump - swash plate constant pressure pump
Drives a hydraulic motor to dive the output to the generator
Self contained system
