GENERATORS vs MOTORS power rating 2

[Alaa1992]

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I want to now why generators power is rated in KVA and motors in KW ?

Alaa Edine SMAALI
Energy (Oil&Gas) Project Engineer

 

[edison123]

Because generator's PF is fixed by the loads (which could be a mix of resistance and inductive loads) connected to it and hence it's KW will change according to the collective PF of its loads while the motor's PF is purely inductive and is predetermined during design and hence its KW is known.

Muthu

http://www.edison.co.in

 

[waross]

A motor is rated by the power out. Horse Power or mechanical kW.
A generator is rated by both electrical kW and by the heating limit of the windings.
The kW rating is limited by the power of the prime mover.
The heating is limite4d by the current through the windings.
The maximum current is determined by dividing the KVA rating by the rated voltage.
There are some interesting applications.
eg: A 100 KVA set rated for 240 Volts will have a maximum current of 100,000VA/240 Volts = 417 Amps.
The limit is the 417 Amps.
If the same set is rated for operation at 200 Volts, the 417 Amps is still a limit.
The KVA rating of the same set at 200 Volts will be 83 KVA.

Edison mentioned the PF. small sets are generally rated at a PF of unity or 1.0.
Sets above about 20 KVA are generally rated at a PF of 0.8 and the kW rating is 80% of the KVA rating.
Above about 20 KVA, single phase power is often supplied by reconnected three phase generator ends.
There may be some exceptions to the above information when single phase power is supplied by a reconnected three phase set.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Hoxton]

Simplistically because they are defined by what they do...

A (current) generator produces electric current (Amps. You need to know the power factor to reveal how much mechanical power is required to produce the required current.

Electrical motors produce mechanical power (torque) and hence are defined by the KW in the shaft,

Interestingly the efficiency / losses are treated differently.

A generator rated at 1000 kVA 0.8 pf can support a load of 800 kW at 0.8 pf but will require a drive power of more than 800 kW, since the prime mover will have to supply the generator losses.

Similarly a motor which supplies 800 kW mechanical power from its shaft will draw more than 800 kW from the supply.