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Transformer load with bad power factor
2

Transformer load with bad power factor

Transformer load with bad power factor

(OP)
A power transformer has an output rating in VA. If I load the transformer with a single side rectified circuit with a big electrolytic capacitor for smoothing and a DC load, the power factor of the transformer load will be very bad. Is it allowed in this case to load the transformer with an apparent power (rms current x rms voltage) equal to the power rating of the transformer? Or must a derating be applied because of the bad power factor (load current is no longer sinus shaped) ?

RE: Transformer load with bad power factor

Quote (OP)

Is it allowed in this case to load the transformer with an apparent power (rms current x rms voltage) equal to the power rating of the transformer?
Transformers are rated in Volts RMS x Current RMS. Hence VA, Volt amps.
With displacement power factor due to an inductive load, the apparent power in VA may equal the transformer rating in VA.
BUT

Quote (OP)

Or must a derating be applied because of the bad power factor (load current is no longer sinus shaped) ?
This is called distortion power factor. Distorted wave forms lead to harmonic currents which will cause extra heating.
Some derating may be adviseable.

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

RE: Transformer load with bad power factor

A single-side rectifier? Is that what the rest of us know as a half-wave rectifier?

A half-wave rectifier will cause DC to flow in the windings; bad things will happen if the core saturates due to the DC bias.

RE: Transformer load with bad power factor

Eelcos; What you are seeking is called "form factor" and refers to the shape of the resulting current distortion waveforms a transformer has to handle. There are a couple of places where there are formulas for sizing transformers when they're going to be used with weird fixed waveforms.

This link has a nice table for what you seek. It's half way down and they happen to be calling it "Rectifier transformers" which also works.

Wiki

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer load with bad power factor

(OP)
Thank you all for your useful comments and tips!
ScottyUK: yes, I mean half-wave rectified (bad translation from my native language)
itsmoked: thank you for the link; these are the formulas I was looking for. But I do not fully understand the parameters:
- IAC: Rms current the transformer needs to supply
- IDC: DC load current
- VA: Required apparent power rating of transformer
- But what is "watts"..? (sounds like a power figure, but how can a power figure be summed with a current figure?)

RE: Transformer load with bad power factor

You must apply a big de-rating to the VA rating of a general purpose transformer, especially using half-wave rectification. I would expect maybe 25% of transformer VA can be used as opposed to full-wave rectification where you can probably expect to use 75% of the transformer VA.

But, when you are building little power supplies you can buy transformers specifically rated for that use with the de-ratings already applied. So, in these cases you have to read the data sheets and tech info for the specific transformer you are using.

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