Power Factor

In reply to your comments an why they are Off Topic:

From your comments, it shows that you do not know the definition of Power Factor in this context!

Here, the P.F. = Cos(phase angle between the Apparent Power S in kVA and the Real Power P in kW). It is not the ration between Dc and Ac Power as described by you.

Please revise whatever site you mentioned and I hope they do explain it properly because I did not check it.

Cheers.

Generally speaking, manufacturers like to rate their generators at a lower power factor because it makes them seem bigger than they really are. If you read the manufacturer's literature, you will find that they generally require derating the generator capacity for higher power factors. In other words, a 100 kW generator at 0.8 power factor will deliver only 80 kW at a power factor of 1.

I think it is other way. A 100 KW and unity p.f. power factor generator will give 100 KW at unity Power factor but at 0.8 p.f. it can give only 80 KW

A most unexpected bunch of answers to a rather simple question, IMHO.

Noudge, Power Factor is the cosine of the angle between the voltage and current vectors. You are only looking at AC sine wave voltage and a flat DC voltage wave. Please correct me if i am mistaken.

cwarner, the generator rated 100kW @ 0.8 pf can actually deliver 125 kW @ pf = 1. In other words, it is rated 125kVA.

Am i nuts ? Please feel free to tell me so if i am

"The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit,^[1][2] and is a dimensionless number between 0 and 1 (frequently expressed as a percentage, e.g. 0.5 pf = 50% pf). Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power." WIKIPEDIA

-I really wonder why Pf is not use for DC Power system and how would people try to understand above definition Made me feel I am nuts, but I preferred peanut

i am not sure what you are trying to say.

Inductance is there in almost every load. If it is in an AC circuit, the current lags the voltage by some angle depending on how much inductance and resistance are there in the circuit.

If a DC voltage is applied to the same circuit, the current does not rise immediately. There is a delayed rise, following the rule I = I_max {1-e^(-Rt/L)}.

i prefer salted peanuts with my single malt

kvsridhar-

Read the manual from the manufacturer. Evvery manual I have ever read says the capacity of the generator must be derated if the power factor is greater than 0.8. Because reactive power "returns" to the generator, it puts less load on the windings. When running at a power factor of 1, all of the power generated is consumed, causing more heating in the windings, thereby shortening the life. Further, reading the manufacturer's literature, one generally finds specifications for how long and how often one can exceed the rated load. Additionally, most generators are usually rated for "standby" service. If it is used as a primary source (i.e., 24/7 service) there is generally another derating factor.

Read the manufacturer's manuals before seeking advice here. Caterpillar manuals are especially helpful with this. You can find valuable information here:

http://www.catelectricpowerinfo.com/index.asp?rt=&SRC=11CAT37s#

Requires registration.

Note that manufacturers want to justify putting as big a number on there products as they can possibly justify. The power factor rating is one of those fudge factors that allow them to "rate" the gen set at higher capacity.

What determines the heat build up, the fuel consumption, bearing wear, etc. is the actual kW that is pulled from the system. The reactive part of the KVA just kind of circulates around, not doing a whole lot of good. Sort of like a politician...

My apologies, i bow to your superior knowledge. When i am a little freer, i will download and study the Cat literature.

However, i am foxed. The heat in the generator windings is due to current, I²R. It does not matter whether it is at 0.8 pf or 1. So, if the generator is rated 100 kW @ 0.8 pf, it is perfectly capable of carrying the current corresponding to 125kVA. So what is the problem in delivering 125kW @ pf =1? It is the same current.

Seems a GA..... but i remain foxed ..... is the machine capable of witstanding the current corresponding to 125 kVA or not? is not still clear to me, pardon my dimness..can you please give a oneline answer ?

Re: is the machine capable of witstanding the current corresponding to 125 kVA or not?

Just to be clear which machine we're talking about:

The machine rated at 100 kw at 0.8 pf is capable of generating 125 kva safely. (This also means it can generate 125 kw into a load with a 1.0 pf.)

The machine rated at 100 kw at 0.95 pf is capable of generating only ~105 kva safely. (This also means it can generate 105 kw into a load with a 1.0 pf.)

The machine rated at 100 kw at 0.8 pf is capable of generating 125 kva safely. (This also means it can generate 125 kw into a load with a 1.0 pf.)

That's what i have been thinking for the last 40 years, but got confused with the plethora of opinions (good, bad, indifferent and downright wrong).....

Yes it is capable of delivering the stator current corresponding to 125KVA but on lagging power factor side not on leading power factor side which is limited by end zone heating limit.

Yes, You have got most of it correct.

The confusion is that at 100 kW with p.f = 0.8, means that you are already at 125 kVA and at the maximum rated current:

125,000 VA / (400V x √3 ) = 184.2 A would be nameplate rating for this case

100,000 Watts / (400V x √3 x 0.8 pf) = 184.2 A Maximum kW at pf = 0.8

100,000 Watts / (400V x √3 x 0.9 pf) = 160.4 A here the generator is happier.

125,000 Watts / (400V x √3 x 1.0 pf) = 184.2 A which is the maximum kW and it must be at pf = 1.0

The KVA (load) increases as the PF decreases. This subjects the transformer (& generator) to more load and it heats more. Therefore as a standard, generators and transformers are rated for 0.8 PF. This is the safe limit to which these can be operated. The loading curve of the generator should be kept in view at other power factors.

Dear The 0.8 power factor is common practive of the manufacturer to make DG Sets as a standard and it is cheaper version. The manufacturer can make DG Sets for higher power factor but as a special requirement also which is costlier. As a practical, person generator can operate at any power factor. If operating power factor is less than the rated power power factor its thermal capability will be less because as it will be in the range of rotor heating. So we have to ensure the loading depending on the current capacity of ther generator to avoid overheating.

Who said a generator can not be run at a low power factor ?? Some generators are operating at ZERO power factor - known as >rotary condensers<, . . where the power factor is typically 0.00.

The problem is that all generators are rated at kVA - voltage and current - and if you operate at 0 power facor, you do not get any POWER out of it.

So, you have a choice. you can get some power out of a generator by operating it at a low power factor, . . . or you can get maximum power out of it by operating it a 1.0 power factor . . . take your pick!

Generators are designed for a particular KVA, power factor and KW in mind. For example. a 125KVA generator designed for 0.8 pf will deliver 100kW real power. A higher than 0.8 pf does not mean you get more than 100kW out of the same 125kVA generator. The windings are designed for 100kW load and will overheat or burn if you try to extract more real power out of it. Instead, a higher pf will mean you will need less than 125 KVA to get the same 100kW.

On the other hand, a load with pf lower than 0.8 will mean you will get less than 100kW real power out of the same 125kVA generator.

A 125 kVA Genset will deliver 125 kW at p.F. of 1.0 and will not overheat the windings!

What heats the windings is the current flow and since the generator is rated at 100 kVA, it will also be rated at a Voltage and frequency and will state the relative maximum current: for example, 500 kVA, 400V, 722A, 3phase, 50 Hz, and 400 kW at p.f.= 0.8

The 722A is from 500,000 = 400 x I x √(3) from which you can deduct the I = 722 A (neglecting the decimals).

Therefore, the maximum current should not exceed the 722A. At pf = 1.0, you will get the 500 kW output with 722A running into the windings. BUT if you try to get 500kW with a power factor less than 1.0, then the current will be higher than the 722A and you will overheat the windings.(also overload the Prime mover possibly).

Real power and apparent power are relative to the load. Apparent power is not the direct sum of real power and reactive power: it is a vectorial sum or the square root of the sum of their squares.

"A 125 kVA Genset will deliver 125 kW at p.F. of 1.0 and will not overheat the windings!"

Is there any generator exist with P.F.=1? Hmmm

Question: Is power factor a property of electrical equipment or circuit or just something you can adjust?

The 0.8 pf rating that is mentioned by Generator and transformer makers is not saying that the unit is a 0.8 pf unit. This is a general trade convention to make it easier to recognise like for like in the industry.

A rating of 0.8 does not mean it is a limit of operation for the equipment.

A generator rated 125kVA and 100 kW at pf = 0.8 is actually also a 125kW at pf = 1.0

I hope you get it. All this is theoretical and must be understood in a classroom before you can discuss it here. All the generated comments are now treating a different topic if I may say so.

Excess kw load might cause the generator to have high exhaust or cooling fluid temperature; insufficient fuel flow, which would slow the generator down; and/or overload currents.

Excess kva also leads to high current, and possible overheating of generator windings and conductors.

The dual rating helps to ensure that neither of these scenarios occurs.

See also rhkramer's well explained discussion in Post 2.

If PF is given as 0.8 in the name plate ,then you may check up the other details of the nameplate such as voltage, current and KVA rating then you can confirm by calculation weather the manufacturer has designed the generator for unity pf or for 0.8 for the rated KVA . Please google to get the following link . You will get a fairly good answer for your question.

http://www.generatorguide.net/watt-acpower.html

The link gives the following headlines.

AC POWER and ENERGY BASICS

WATT, KILOWATT, VA, and POWER FACTOR

Please read it to get a technical answer to your thread

Electrically a 125kva genset at 1.0 pf might be able to generate 125kw, but mechanically it might not be able to (fuel shortage, engine lugging, etc.) Hence the value of both ratings.

There are two major components in a generator.

First there is the engine which delivers Power or KW and second there is the alternator which delivers Current or KVA.

The rating of the engine and the alternator are not necessarily equal and are often matched to suit the actual load.

Power factor is the ratio of KW to KVA (real power to apparent power) and can be affected by the phase angle of the load (angle between applied voltage and current flow) and also by harmonics and distortion in the current as commonly caused by VFDs and solid state loads.

It is common for the power factor of connected loads to be less than 1.0, so generators are typically rated in KVA at a power factor of 0.8 This means that at a power factor of 0.8, the output power is equal to 80% of the KVA rating.

The output power at a power factor of 1.0 is dependent on the rating of the engine in KW. It is possible for the engine to be rated at 80% of the KVA rating of the alternator, and it is also possible for the engine to be rated at 100% (or higher) of the KVA rating of the alternator.

If the engine is rated at 80% of the KVA rating of the alternator, then the maximum power output at a power factor of 1 is 80% of the KVA rating. If the engine is rated at 100% of the KVA rating of the alternator, then the power output at a power factor of 1.0 is equal to the KVA rating of the alternator.

In this case, the KVA output is the same at a power factor of 1.0 and 0.8 but the KW drops to 80% at a power factor of 0.8 whereas in the case where the engine is rated at 80% of the KVA of the alternator, the KVA at a power factor of 1.0 is less than the KVA at a power factor of 0.8 and the KW is the same with a power factor of 1.0 and 0.8

You need to determine the rating of both the engine and the alternator to get the performance as pf changes.

Best regards,

Mark Empson

http://www.power-factor.co.nz