Hi All,

I have finally got around to getting myself a Isolation transformer for my bench setup as I'm doing more and more diagnostics on power supplies and mains powered domestic electronics.

The transformer is a 3kVA 1:1 ratio that is second hand and came out of a UK Ministry of Defense facility.

Now the questions:

Plugging it into the 244V AC mains the transformer draws 335mA with nothing connected to the secondary side.
Is this normal?

When first powering it up there is an audible jolt type noise which I guess is to be expected and I've tried to measure the inrush current with a clamp meter. Using the max hold facility on the meter (UNI-T UT201) I measure 483mA (this is an average of 8 measurements)
I plan to put the transformer on it's own 6A MCB breaker.
Bearing in mind the inrush current, should the MCB be a type B or C or something else?

Many thanks.

Dan

 

Plugging it into the 244V AC mains the transformer draws 335mA with nothing connected to the secondary side.
Is this normal?

Yes that is normal. If you feel the unloaded transformer you will feel some heat and likely some vibration. That heat is energy being dissipated even when there is no load on the transformer. Ideally for bench use it would be mice if you used a GFCI (Ground Fault Circuit Interrupt) outlet or breaker.

Ron

 

Thanks for your advise.
My work room mains supply has got a GFCI (we call them RCD (Residual Current Devise) here in the UK) and the isolation transformer is down stream of that. I've just designed and installed a mains distribution board just for the room and can patch in various MCB's breakers to the bench depending what i'm working on.

Because of the current load of the transformer (and therefore the cost involved) I won't have it routinely powered unless I need it.

Many thanks again.

 

Because of the current load of the transformer (and therefore the cost involved) I won't have it routinely powered unless I need it.

Note that it's likely most of that unloaded input current is the inductive magnetizing current which is reactive power and is not charged as real power by the mains meter.
A Kill-A-Watt meter can tell you both the real and reactive power it's drawing.
It's also handy for checking the power draw of appliances as well as the mains voltage and frequency.

 

There is some power dissipation due to I^2·R loss of the magnetizing current and the resistance of the primary winding. There will also be some hysteresis loss in the core. 335 mA of magnetizing current is by no means excessive for that size of transformer.

A clunk at turn-on is not unusual. Normally, with a simple inductive load (and for practical purposes, that is what a transformer with no load on the secondary would be expected to look like, since the losses previously mentioned are quite small) the current would start at zero and ramp up. BUT ... the iron core of a transformer can remain quite strongly magnetized after the primary is disconnected - it just depends on timing. When the input power is re-applied, if the polarity is such that magnetic field from the winding is in the same direction as the residual magnetism, you can get a transient saturation of the magnetic core. A current spike results because the winding looks like a resistor instead of an inductor. The following half-cycle will largely remove the residual magnetism. It won't harm the transformer but it is something that may need to be considered in selecting a switch for the primary of the xfmr. Most circuit breakers are way too slow to even begin to notice the transient.

Most ordinary isolation transformers aren't very isolating at high frequencies due to interwinding capacitance. This can be an issue for certain things (e.g. trying to look at waveforms in an active power factor correction circuit using an oscilloscope).

Some isolation transformers aren't actually 1:1 turns ratio, but have a few extra turns on the secondary to compensate for voltage drop due to winding losses. This can be helpful, of no consequence or annoying, depending on what you are doing.

 

Hi All,

I have finally got around to getting myself a Isolation transformer for my bench setup as I'm doing more and more diagnostics on power supplies and mains powered domestic electronics.

The transformer is a 3kVA 1:1 ratio that is second hand and came out of a UK Ministry of Defense facility.

Now the questions:

Plugging it into the 244V AC mains the transformer draws 335mA with nothing connected to the secondary side.
Is this normal?

When first powering it up there is an audible jolt type noise which I guess is to be expected and I've tried to measure the inrush current with a clamp meter. Using the max hold facility on the meter (UNI-T UT201) I measure 483mA (this is an average of 8 measurements)
I plan to put the transformer on it's own 6A MCB breaker.
Bearing in mind the inrush current, should the MCB be a type B or C or something else?

Many thanks.

Dan

Use a 6A type C mcb , for added protection you can use a RCD too.

 

Thanks all for the information. Inside the box containing the IT there was no markings regarding connections of primary/secondary and I assumed it was 1:1 and therefore made no difference either way.

The point about it not being a true 1:1 makes a lot of sense...

What I will now do is measure the voltage output from both directions (so to speak) and see if there is in fact a difference..

Thanks again.

 

What I will now do is measure the voltage output from both directions (so to speak) and see if there is in fact a difference..

You may want to give it a light load like an incandescent lamp bulb when you try that and please do post back with any findings.

Ron

 

Thanks all for the information. Inside the box containing the IT there was no markings regarding connections of primary/secondary and I assumed it was 1:1 and therefore made no difference either way.

Some isolation transformers have an electrostatic shield windinding betwen primary and secondary that gets connected to PE, so with those the PE connection would probably located along with one set of the main connections, so I would treat that side as the input side.

 

Hi All,

So to test if the isolation transformer is truly wound 1:1 I connected it in both directions to see if there was a difference in output:

The mains supply voltage today is 241V and with it wired A>B the output was 248V

And conversely when it was connected B>A the output was 230V

So it appears that the transformer is not true 1:1 but a different ratio that compensates for loss.

In both cases the output was driving an incandescent lamp: (241V÷144Ω=1.67A)

 

The "primary" works best when it's the winding closest to the core.