[Amps] current in primary of microwave oven transformer

[David Ackrill]

Angel Vilaseca wrote:

> I have a microwave oven transformer that I plan to use for my next HV 
> power supply.
> 
> With the secondary unloaded and 220 V into the primary, I measured  670 
> mA through it.
> 

Others have answered the question of phase angle, so I wont repeat that 
information again.

To address the root of your question, all transformers have losses, due 
to 'copper' losses in the windings, which will have some resistance 
(this will be an 'in phase' loss as it's purely resistive), iron losses 
due to eddy currents set up in the iron core of the transformer, 
although these will be kept low due to the act of laminating the core 
and putting a thin layer of insulation between each sheet of iron, and 
hysteresis losses as the AC wave swaps over every cycle and the magnetic 
lines of flux increase and decrease arround the windings.

So, 147 Watts may well be quite normal for your transformer.

Many years ago, as a trainee engineer, I was given the task of 
calculating the losses of having large 132/11kV transformers energised, 
but not carrying any load, as standby units should the main transformer 
have to be switched out.  This ran into thousands of pounds per year 
across the area that the electricity company I worked for covered. 
However, having done the excersize, the chief engineer said they would 
not switch the transformers off as the time taken to switch them back on 
in case of a failure was not acceptable and having them on 'hot standby' 
meant that there was less danger of them being faulty, and the fault not 
being noticed until they were needed.  But I'm going off topic now.  HI.

Dave (G0DJA)