Alternator Power Factor

Yeah! Sure! You are correct. Only load can tell how much power factor the Generators should have.

If there are atleast two generators are running in parallel and meets the load you are telling, then how can you tell that which Generator meets its reactive demand. How it is shared by both Generators, whether 50:50 or 0:100 or 29:71 or else?

Well, that is dcided by Excitation.

In first case (50:50) both Generators having same PF. But in second case Gen#1 has unity PF and Gen #2 has lagging PF.

rizwan; the power factor affects only the alternator, the lower the power factor the harder it is on the generator/alternator. perry

Relax Rizwan you are quite right. Power factor is a function of the load.

NEMA (US) and other agencies specify generators to be tested at 0.8 pf, purely for historical reasons. This permits the manufacturer to quote standardised figures e.g. a 600kW genset = 750 kVA @ 0.8 pf. In turn however, this infers (as per the standard) that the genset is capable of 750 kVA.

This does not work backwards. Example:

700 kVA load @ 0.9 pf = 630 kW; this is more than the kW power output of the genset. The load cannot be supported, even though the kVA rating is within.

500 kW load @ 0.5 pf = 1000 kVA; this is more than the kVA apparent power rating of the genset. The load cannot be supported, even though the kW rating is within.

Confused? The easy way out is to work in amps. The genset will have a maximum amperage output.

Phase angle, ie; voltage curve vs ampere curve. Unity is the point at which the peak of the voltage coincides with the peak of the amperage.

In general the load does determine the required rective power demand on a system. Read that carefully "on a system". The rub comes in on systems of interconnected generators as in a utility power system. The various generators can each supply a portion of the reactive power required. Short version adjust the torque on each generator shaft (Open the throttle) to adjust the real power supplied by each generator and adjust the voltage regulator on each generator to adjust the reactive power supplied by each generator.

Hi Rizwan,

are you asking why the gens are spec'd as kW and also kVA? If so my usual answer to this is that the kW rating relates to the prime mover and the kVA to the max current allowable in the windings of the alternator.

As outlined in babyguiness's post your load needs to be within both parameters so as not to overload the prime mover or the windings.

Chas

Of what I've understood by reading the posts above is:

1. Alternator itself has no power factor. The power factor mentioned on its nameplate is actually the value based on the amount of load it can handle. Right?
2. The KW rating of the alternator is actually the power rating converted from the HP the engine (prime mover) delivers to the alternator. Right?

Please correct me if I'm wrong. Thanks.

1. This is true that an alernator have internal power factor approx .8 . If this is wrong than we draw actual power(in Watt) which is denote its name plate(in VA) with Resistive load(unity Power Factor) that is not possible.
2. We denote the power of alternator=Vax.8(with resistive load)
3. In capacitive & inducitive it is depend on alternator PF+Load PF.

1. The alternator internal power factor is close to unity and is set by the field excitation.

2. The alternator is rated for the MVA/KVA/VA and in case of normal near unity power factor, no problem but for non unity lagging pf, the extra current will create more copper losses and this may create a problem in loading (due to probable associated insulation damage).

http://www.tpub.com/neets/book5/17a.htm

refer the last paragraph

3. It is not advised to go for a leading load to alternators. This has been covered in quite a few earlier threads.