Isolation transformer220VAC-220VAC 250 Volts on secondary?

[nightfire]

Recently acquired an old isolation transformer from fleabay. Manufactured according to VDE standards, and to be able to be used with 220 VAC- so that thing is quite old, probably build in the 80s. Solid stuff, real metal case, solid switch.
After putting a new fuseholder and lamp (some more modern LED with integrated resistors) for the secondary side in it the big moment came:

After power-on, I measured at the power strip at my bench around 233 VAC from mains, and at secondary output my Agilent U1272A measured 251 VAC- after puttting some load on it via the LowZ-Position, it still was at 250 VAC.

Question here: Is it normal for these transformers to have an open terminal voltage that is 5% higher than the primary? Or do I have to suspect some shorted primary side, just enough to work but be effectively some windings shorter and therefore raising the voltage on secondary?

[IanB]

What is the rated load for the transformer on the nameplate? Does it give VA, or an output current?

If so, you should probably put the rated load on the transformer before measuring the voltage.

[nightfire]

Rated load is 160VA. With 220 VAC back then this would translate to 0,72A with resisitive load- the fuse on the primary is a 0,8 A slow fuse...

[bdunham7]

That's fairly typical.  Add a small (50W or so) load and that voltage will start coming down.

[madires]

10% more isn't unusal for larger transformers. For small ones you can easily have 50%. The LowZ mode doesn't help in this case because the load is still way too small.

[nightfire]

OK, some test:
Input approx 233 VAC, 100 Watts lightbulb on secondary, and 244 VAC on secondary.

Not that bad, but I'd rather like the secondary a bit more in the direction of the input voltage at small loads, especially because I will use some devices on it, that will use less than 20 Watts...

[Ian.M]

IIRC the EU mains voltage limits are 230V RMS +/-10%, i.e. 207V to 253V.  (Was harmonized as 230V +10%,-6% from 1995 to circa 2010, which was a lower limit of 216V)

Load it up with a 160W resistive load (or as close as you can to that) and check its output is still within those limits.  If so, and if it doesn't overheat in 15 minutes running I'd bet its OK.

If you need to test equipment at a particular voltage, *WITH* *ISOLATION*, put a Variac in front of it.

If you don't have a Variac, use a 20VA 12V-0-12V transformer, or similar, to buck the voltage fed to it in switch selectable 12V steps.   

[nightfire]

OK, a Variac will be one of the next projects, but in this case I only want to look that the devices I intend to hook up onto it will not get damaged...
And those probably will include some old stuff that I will repair and comes from the 220V world. But, usually 5% additional tolerance should in most cases not be the factor that decides about yay or nay...

[Zero999]

OK, some test:
Input approx 233 VAC, 100 Watts lightbulb on secondary, and 244 VAC on secondary.

Not that bad, but I'd rather like the secondary a bit more in the direction of the input voltage at small loads, especially because I will use some devices on it, that will use less than 20 Watts...

Take a 24V transformer, with the same or greater current rating, as the isolating transformer, connect the primary to the mains and the secondary in series anti-phase with the secondary of the isolating transformer. You might need to experiment swap the connections to the primary, or secondary of either transformer, to get it to reduce the voltage by 24V, rather than increase it. This should also improve the regulation slightly, as the 24V transformer's secondary voltage will be higher, with a light load, thus drop the voltage more, compared to when fully loaded.

[tunk]

Ignorant question, would it be possible to run it in reverse?

[nightfire]

Hmm... at least it would involve a complete re-wiring of the connection board of the transformer, which is fixed, as I think. Also some 4mm banana outlet is put on the outside wit a connection to a "shield" for the secondary winding- which probably is not moveable.

[madires]

Ignorant question, would it be possible to run it in reverse?

Yes! However, the number of windings of the primary and secondary side are usually the same. So you'll have the same no-load voltage.

[Zero999]

Ignorant question, would it be possible to run it in reverse?

Yes! However, the number of windings of the primary and secondary side are usually the same. So you'll have the same no-load voltage.

That's not true, there are normally slightly more turns on the secondary, hence the higher voltage, when unloaded. Running it in reverse will reduce the voltage by the same factor. If it gives 251V out, with 233V in, then it'll give 215V out when unloaded, run in reverse.

[Doctorandus_P]

I agree with the above.
A higher output voltage is normal.

Normally the specifications are given for the full load conditions. With big transformers (2kVA)  the output is usually a few percent higher, and with small transformers (5VA) something like 30% higher.

Primary and secondary probably do not have the same number of turns. Usually there is a small difference to compensate for the voltage losses in the transformer under full load.

[madires]

Yes! However, the number of windings of the primary and secondary side are usually the same. So you'll have the same no-load voltage.

That's not true, there are normally slightly more turns on the secondary, hence the higher voltage, when unloaded. Running it in reverse will reduce the voltage by the same factor. If it gives 251V out, with 233V in, then it'll give 215V out when unloaded, run in reverse.

And there are also variants with additional taps for a few % +/-, like +5% and -5%. Your example calculation ignores that the no-load voltage is larger than the load voltage (or: no-load voltage factor >= 1). It doesn't work the same way as the turns or voltage ratio. Let's assume the secondary's load voltage is 242V (I took simply half of the difference 18V/2). For a no-load voltage of 251V the no-load voltage factor is about 251V/242V = 1.0372.  The transformer's voltage ratio is 242V/233V = 1.0386. When we swap primary and secondary windings we get a new output voltage of 233V/1.0386 = 224V. With the no-load voltage factor we have a no-load voltage of 224V * 1.0372 = 232V.

[Zero999]

Yes! However, the number of windings of the primary and secondary side are usually the same. So you'll have the same no-load voltage.

That's not true, there are normally slightly more turns on the secondary, hence the higher voltage, when unloaded. Running it in reverse will reduce the voltage by the same factor. If it gives 251V out, with 233V in, then it'll give 215V out when unloaded, run in reverse.

And there are also variants with additional taps for a few % +/-, like +5% and -5%. Your example calculation ignores that the no-load voltage is larger than the load voltage (or: no-load voltage factor >= 1). It doesn't work the same way as the turns or voltage ratio. Let's assume the secondary's load voltage is 242V (I took simply half of the difference 18V/2). For a no-load voltage of 251V the no-load voltage factor is about 251V/242V = 1.0372.  The transformer's voltage ratio is 242V/233V = 1.0386. When we swap primary and secondary windings we get a new output voltage of 233V/1.0386 = 224V. With the no-load voltage factor we have a no-load voltage of 224V * 1.0372 = 232V.

I don't follow you. My example did take into account that the no-load voltage is larger than the loaded voltage. The no load secondary voltage is proportional to the turns ratio. Extra turns are added, to compensate for the voltage drop, so the transformer is not really 1:1. The transformer will be specified for the secondary voltage, when fully loaded, to equal the primary. If the unloaded secondary voltage is 251V, when the primary is 233V, then the turns ratio is really 251/233 1:1.077. This means when it's run in reverse, with 233V in, the unloaded voltage will be 233/1.077 =  216V.

[madires]

I stand corrected, the turn or voltage ratio is measured without load. I've put the cart before the horse.

[BravoV]

I own 3 isolation transformers, all of them, their secondary windings are always tad higher.

Although I don't know their windings count, but I always measured their inductance (with LCR) when I got them as part of the inspection routine, and for example my biggest one, the primary winding inductance is 301mH vs secondary = 304mH.