The AM Forum

I have been searching for transformers and I am confused by some different terms and specs I have come across and was hoping that someone could help me understand.

I have seen and understand:
primary voltage
secondary voltage
secondary current

I have seen wattage rating and believe this is the power consumption and not the power production.  Is this correct?

I have seen the VA spec and believe that this is essentially the power production (secondary Volts x secondary Amps).  Is this correct?

Is there a typical efficiency factor one can use to guess when one knows the power comsumption and not the power production or vice-versa?

Are there other aspects of transformer specs I should be aware of?

The difference between volt-amperes and watts is that the V-A rating takes into account the  fact that with a.c. the voltage and current may be out of phase.

If voltage and current are exactly in phase, then the power factor is 1.0 and a V-A is exactly the same as a watt.  If there is a phase lag between voltage and current, the the power factor will be less than 1.0, usually expressed as a decimal fraction.  For example, if the power factor is 0.5, that means that one V-A would have the power equivalence of 1/2 watt even though the measured rms voltage multiplied by the measured rms current would be 1.0.

If the power factor is 0, the current and voltage are in quadrature, or 90 degrees out of phase, and the power equivalence is zero.  At the crest of the voltage the current is crossing the base line and vice versa.  This is what theoretically happens when a resonant circuit is excited with a.c. but there is no load on the circuit.  Of course, the power factor is never exactly zero, since there will be some loading of the circuit due to resistive and dielectric losses.

One of the best single references on iron power transfomer design that I know of is Chapter 5 of the Radiotron Designer's Handbook, 4th Edition.

In that chapter they state to assume 85 % efficiency on small power transformer calculations. 

Most calculations appear to be in VA.  Core loss and wire loss can be stated in Watts.  Ideally all power goes to the external load, none in the transformer itself.

I think that most transformer manufacturers catalogs today state the VA rating based on the total secondary outputs.

But looking at the Hammond catalog, they seem to state the input VA or higher yet because the value is much greater than the secondaries' VA's added up and accounting for typical efficiency.  Perhaps the VA stated is based on the capability of the core they chose.  So I guess there is no standard.

When I was designing iron we shot for 5% primary, 5 % secondary resistive losses and 5% core.

My immediate reaction to those 5% figures was "conservative CCS rating".
Yes?

           _-_-bear

yes.