Transformer load with bad power factor 2

[Eelcos]

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) ?

 

[waross]

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

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

 

[ScottyUK]

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.

 

[itsmoked]

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

 

[Eelcos]

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?)

 

[itsmoked]

Try this instead.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

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.