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Generator Sizing

Generator Sizing for a Motor Load by rbulsara
Posted: 11 Sep 03 (Edited 8 Dec 06)

Generator Sizing for 3 phase Motor Loads.

Considerations for sizing:

Initial Condition: No load on the generator.

1.    Ability to provide real power required by motor = Motor rated kW.  The prime mover (the engine) only provides real power, kW. KW demand during a motor starting is very low as the power factor is about 0.2.

2.    Ability to provide starting kVA for the motor. This is the function of the alternator size.
a.    For full voltage (DOL) starting, starting kVA= 6*rated kVAod the motor . Use the nameplate LRA data or Code letter where available.
b.    For Star/delta starting, starting kVA = 2*rated kVA of the motor. (In otherwords, in star connection the starting current will be 1/3 of the DOL starting (LRA))
c.    For any other reduced voltage starting the starting current and torque will be reduced by the square of the voltage reduction.
d.    So obviously one must strive to use reduced voltage starting on a generator, as long as the starting torque requirements can be meet.

3.    Keeping the Voltage drop during motor starting to an acceptable value.

a.    While a motor by itself could tolerate 30% to 35% of voltage dip during the start, it could cause starter coil to drop out. Plus consideration should be given to effect on other systems on the generator such as lighting. Therefore more realistic value of acceptable voltage dip will be say about 15 to 20%.
b.    For finding the per unit voltage drop by calculation you need to know the transient reactances of the generator (usually 15-20%) and the motor (usually 20-25%).  

Rules of Thumb:

Most generators are capable of delivering 300% of the rated current for 10 seconds, which is sufficient time for most induction motors to get up to the rated speed. So for DOL starting a generator rated 2 to 3 times the kVA of the motor will easily supply the starting kVA of the motor and for a star/delta start even a generator sized just above the rated motor kVA will be sufficient. These rules of thumb generally result in generator size that will keep the voltage dip during the motor starting within acceptable limit.

IEEE red book have some curves indicating voltage at the motor terminals as percentage of the rated voltage for DOL starting of various size motors.

Of course for more accurate analysis you need to perform some detailed calculations.


Practical Issues:

Theoretically it is possible to show that for most cases a 600kW/750HP motor with a star/delta starter can satisfactorily be started on an 800kW/1000kVA generator set (800kW engine and 800kW/1000kVA generator).

Or for a DOL starting, an engine-generator set with 800kW engine and 2250kVA rated alternator will suffice. The problem is that it will be a non-standard set.  Most generators set manufacture will not customize their engine-generator set. So chances are most users will end-up with an 1800kW/2250kVA generator for a 600kW motor, technically way oversized.

Systems with initial loads:

For systems with initial loads, the generator set size needs to be augmented by the initial kW/kVA in addition to the largest motor sizing. Where practicable, arranging a starting sequence where the largest motor starts and comes to full speed before other loads are added to the generator, will result in most economical size of a generator.

Helpful Hints:

Make maximum use of æfreeÆ technical assistance provided by generator manufactures, often in form of a quick generator sizing software on a CD.


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