# Efficiency in a transformer

1. Mar 25, 2012

### sgstudent

When I calculate efficiency of a transformer, I use output power/imput power x100%. Then what are these power values? Is the power the power in a load or the total power coming out of the transformer (meaning it includes the joule heating)? Then also what about the input power, which values do I use? Thanks for the help!

2. Mar 25, 2012

### Bob S

You need to consider any source of joule heating in the transformer, plus anything that creates a power factor <1. There are several classes of things you need to consider:

1) Self inductance and power factor (primary), and inductive (excitation current) losses.
2) mutual inductance (coupling efficiency to secondary circuit)
3) Core losses (eddy currents in laminations, hysteresis losses, magnetostriction*)
4) copper winding losses(I2R losses, primary and secondary, also eddy currents in copper)
5) secondary circuit losses (in load).

The overall efficiency is 100 x Pload/Pin, where Pin is measured at the wall plug. Are you talking about joule power (real) or reactive power (volt-amps)?

* If you can hear the transformer or feel vibrations, there are magneotstrictive losses.

3. Mar 25, 2012

### sgstudent

Oh so the cables getting heard is not included in the Pin or Pout? So its the power at the load, and the power from the generator?

4. Mar 25, 2012

### Hassan2

If we know the input and output current and voltages ( for example by measurement) then we can calculate the efficiency easily. All the losses are included in Pin-Pout. However , If we would like to calculate the efficiency analytically ( as in design process), we need to consider and calculate the losses as a function of the load ( and the input voltage). Core losses are not easy to calculate and just some approximations may be made.