How can I calculate how much power an isolation transformer itself consumes if no load is provided?

Question

Isolation transformers usually come with these measures:

• Voltamperes
• Voltage
• Current

In my case, I want to know estimate how much an isolation transformer will consume without a load using these metrics. As far as I know, an isolation transformer without a load on the secondary winding equals into a simple coil connected to the mains.

Given that my mains is 220 V / 50 Hz AC and my isolation transformer is 300 VA, how can I extract/calculate the power consumption in kilowatt-hours without a load on the secondary coil?

Answer 1

You can't calculate it as you don't have the numbers to calculate it.

You measure it. Or read the datasheet if manufacturer has an estimate for you.

Also note that even without load the losses are not purely resistive, so even if you get the power consumption in Watts or energy consumption in Wh, you need the power factor or VA measurement.

Answer 2

Calculating transformer losses is quite difficult, and prone to error, as you would need core parameters, geometry, flux level, to estimate the core and eddy current losses, and the winding parameters to estimate copper losses.

If you want to know core losses for estimating how much input power you need for a given output power, then don't bother. Assume a transformer is going to be much better than 90% efficient and you're done. If you are really trying to budget your input power to the nearest few percent, then you are asking for trouble.

If you want to know core losses for estimating how hot the isolation transformer is going to get, then don't bother. The manufacturer has already rated the VA of the transformer assuming a reasonable temperature rise.

If you want to know core losses for estimating the idle consumption of the transformer, then measure. Bear in mind that the idle VA and power consumption will rise steeply with applied voltage, they could change by a factor of many (maybe 3x, probably not 10x) for a 10% change in voltage. When you measure, make sure you are at the worst case input voltage.

Answer 3

Usually isolation transformers come with these measures

• Voltamperes
• Voltage
• Current

That’s true perhaps of isolation transformer as a finished product, and a rather poorly specified one! Unfortunately, manufacturers of isolation transformer “boxes” often do a poor job writing their spec sheets.

Transformers that are sold as component parts usually have quite a few more specifications in the datasheet.

One of them would be the magnetization current - the current drawn by the primary inductance of the transformer. This current is partly inductive, partly resistive, depending on how much core loss there is.

In the worst case, assume that the magnetization current is all resistive and that will be the upper bound for idle energy use calculation.

Another specification of interest would be the idle power in VA and in W. The latter is the real power wasted by the idling transformer. The former is partly real, just as magnetization current is partly resistive. If idle power is only given in VA, take it as the upper bound for real idle power in W.

Both the magnetization current and the idle power depend on the input voltage, and this dependence is nonlinear. An OK transformer datasheet will have magnetization current and/or idle power given for at least the nominal primary voltage and for primary voltage +5% or +10%. A very good datasheet will have some plots that approximate the typical current/power vs input voltage dependence.

Answer 4

Sure nowdays a power meter would do this but in the 1900s things were different .Measure the mains volts and measure the line current ,then calc the apparent power which will overestimate the actual watts which is wanted .What I did in the 90s was tune out the magnetising inductance with parallel X caps .I used 100n ,330n ,470n ,680n ,1uF etc to minimise my current reading .The minimum point was rather broad .Total cap value was not super critical.In other words I added more caps noting a slow fall in current then a valley and then a slow rise.In the day this scheme was considered for improving power factor at no load.