Prime Mover
- Source of mechanical power for relative motion
- Two classes of Prime movers :
Low Speed : IC engines, Water turbines and DC motors
- Type of prime mover plays an important role in a generator installation. Many characteristics of alternator and its construction depend upon Speed at which the rotor is turned.
- Two types are used in rotating field alternators
- salient pole rotor( used with low speed prime movers)
Stator
Many types based on
- Power out put
- Voltage output
- Type of cooling
- Number of phases
Simplified schematic of 3 ph. Stator- Three single phase windings displaced by 120 degrees
- Star or Delta connection
- Neutral may or may not be brought out
- No. of terminals can be 3,4, or 6
Out-puts from a Generator
- Three phase ac power at :
-Rated Frequency
Common Ratings
- Small Generators : 25 kVA to 300 kVA
- Medium Size Generators : 500 kVA to 1 MVA
- Large size Generators : 1 MVA to 25 MVA
- Power Plant Generators: 110 MW to 500 MW
- Frequency depends up on the Speed of Rotation ( direct proportion)
- F = PN / 120 where F = frequency in Hz, P = No. of poles, N = speed of rotation in RPM, ( 120 is a constant to convert minutes to seconds and poles to pole pairs)
- Nominal frequency = 50/60 Hz
- Generator frequency can vary due to load fluctuations
- Frequency Regulation is mainly by adjusting the speed
- speed is controlled by varying the fuel input to the prime mover ( Governor control)
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