# Power factor correction with a three phase alternator

• posted

Hi,

I want to know if some one can direct me where I can find information on how to implement a power factor correction on a three phase alternator with permanent magnets. I have a small alternator with high inductive winding and it's not efficient if I use only rectifiers to make my DC bus.

I don't know if I can use the same principles used with a three phase transformer when I have variable frequency due to the speed variation of the alternator.

Thanks

Pino

• posted

You really need to restate your problem in a clearer fashion.

Bill

-- Ferme le Bush

• posted

Power factor correction is easier done on the load side rather than the producing side.

Personally I have never done PF correction in DC. I will be interested in seeing how this is handled.

• posted

Well, Pino, I don't exactly follow you. But I will mention that vehicles have small alternators from which DC is derived. Maybe you can find information on techniques used in those alternators, to guide you.

hth

j
• posted
• posted

While I agree that the original question is nonsense, the use of rectifiers can lead to reduced power factors (form factor). The answer is harmonic filtering rather than conventional power factor compensation.

Bill

-- Ferme le Bush

• posted

Ok,

It seems no one have an idea about what I'm talking. So, I'll try to explain it in more details.

First, I have a small (just above 1 kVA) three phase permanent magnet alternator. The output of this alternator is of course AC. The rotation speed of the alternator is variable and produces sine wave from 50 to

200 Hz.

AC into DC and follow by a filtration stage.

Beside that my power source is an alternator, this configuration is exactly the same as if I was using a typical three phase transformer.

As it known from every one how work with that king of circuit, the power factor is usually better in three phase then single phase but it is less then unitary.

My question is : Can some one tell me where I can find some literature on the effects of the variation of the frequency and the reactance of the power source on the efficency of a VIENNA or a full bridge active power factor correction circuit?

p.s. By definition, PFC is an AC to DC converter whish is view from an AC power source as an almost resistive load (unitary power factor).

Sorry if I skip some details in first but I didn't think to make a general question.

Pino

• posted

This next paragraph makes absolutely no sense!

The rest of this post makes little sense, perhaps slightly more than none at all.

-- Ferme le Bush

• posted

OK

OK

OK

OK

OK (though I don't know the particulars of 'Gaetz' but I presume it's not material).

OK

A typical three phase transformer transforms AC to AC. You are converting AC to DC. This is not the same. Maybe you should try a word other than transformer.

You sure think highly of dc converters. The King of Circuits.

Until someone corrects me, I will accept this on your word. For my info, are you talking about bad power factor due to fundamental displacement or because of distortion?

Alright:

Darn, that started out sounding real promising. I don't know VIENNA. I would imagine source frequency and reactance effect active PFC circuits differently, depending on the type of active PFC circuit. I'm sure there are many types which would react differently to the changing parameters you mention. By the way, where did the full bridge active power factor correction circuit come from, I thought you were making a dc supply with a filtration stage.

P.S. That's not the definition of power factor correction. Not by a long shot. And if the goddamn thing has a unity power factor why did you tell me everyone how work with that king of circuit known that the power factors are less than unity? Or are you saying you have a circuit with (a) an alternator as a source, (b) a dc power supply feeding some load or no load, and (c) an active PFC circuit to fix the poor power factor of the dc power supply? Or maybe you have a circuit with (a) an alternator as a source and (b) a circuit (VIENNA, possibly?) that is a dc power supply but that also appears as a resistive load because it actively corrects the current waveform that it draws? No I don't know of any specific literature discussing the efficiency of active PFC with varying frequency / source reactance.

Right now you're not half as sorry as I am. You owe me 15 minutes.

• posted

If you mean you are using a three-phase alternator to feed your diode bridge and filtration instead of a three-phase transformer, then yes, I can see that. Except your alternator has a widely varying frequency and a three-phase transformer would be a fixed frequency at the local supply frequency.

The 'power factor' is actually two components. It can have a time-based component as is seen by inductive or capacitive loads on an AC system. These loads cause a phase shift between the applied voltage and the resulting current, resulting in a power factor equal to the cosine of the phase shift. But there can also be a harmonic component to power factor caused by applying AC to a non-linear load.

In your case, you have a diode bridge that will only conduct when the diodes are forward biased, and that only happens during that part of each phase's sine wave when the AC voltage exceeds the DC voltage on the output side of your diode bridge. This results in no current flow for parts of the sine wave when the instantaneous voltage is low, and then high current flow in that part of the sine wave when the instantaneous voltage is higher than the diode bridge output. The current waveform has a lot of harmonic content that produces a poor power factor.

Well I have to admit I hadn't heard of a Vienna Rectifier before, so I went and Googled to find this.

I must say, it seems like an ingenious circuit to rectify AC to DC without generating a lot of harmonics typically generated by conventional diode bridge rectifiers. Although a lot of complexity to avoid harmonic distortion. Since your alternator is supplying just this load, why is harmonic distortion so much of a concern? Getting too much heating in the alternator iron??

I haven't worked with this before (didn't even know it existed), but I notice that the PWM of the MOSFETS must be synchronized with the AC supply. Also the AC input has an inductor in each leg and so obviously the reactance will vary quite a bit with your varying supply frequency. In order to maintain the power factor, your 'fuzzy logic' control would have to compensate for the increased reactance as frequency rises. Exactly how, I couldn't say.

Sorry can't be more helpful, but perhaps if someone else knows more about Vienna Rectifier's they could contribute.

daestrom

• posted

Good,

Finaly some one how know about what I'm talking.

This is the question.

No apologize, you help me a lot by explaining my problem in a "clear fashion".

Thanks.

By the way, I know that VIENNA topology is not the only way to do active power factor correction in three phase. But it seem to be common when I searched for A.P.F.C..

So, I'll continue my search elsewhere.

Pino

• posted

----------- Agreed- and that is probably what he is looking for- His further explanation and Daestrom's reference help.

• posted

This is completely out of my field of expertise. However, the first two solutions to think about with engineering problems is to (1) buy something off the shelf, or (2) make sure it's a problem before you fix it.

I know that there are commerical power factors correction units available, so that's one thing you might want to look at. Second, it doesn't particularly matter, per se, if something is inefficient as long as it does the job, but if it doesn't do the job it might be simplier and cheaper just to get larger components. The power company, incidentally, won't charge residential customers anything extra for a low power factor. The power factor in my computers' power supply is probably only about .6 or .7 and I don't worry about it at all.

• posted

No but your paying the same price for less useful power

The power factor in my computers' power supply is

Perhaps you should examine that at a fundamental level - Why be happy with waste??

• posted

No you aren't. He was correct. Residential customers only pay for real power (the "useful" power). The reactive power which results in lower power factor is free. To be technically correct, you will have slightly more real power consumption due to greater line losses, but the difference is insignificant compared to the total.

Ben Miller

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