Transformer rewind the only cure?

[Uncle Bulgaria]

One of the watch timing machines I'm working through has a mains transformer that is getting very hot to the touch.

I've removed it from the chassis and measured the resistances of each winding. Hooking the 240V tap up off-load to my variac results in a reported draw of 130mA, though the secondaries report reasonable voltages. My investigations are shown in my notes attached. The only thing I haven't done is get the 1kV Megger out from storage and test the insulation, but since the transformer's overheating off-load I don't think it would add anything.

The laminations and securing screws are all painted and undisturbed.

Is the only hope a rebuild of the transformer?

I have no idea of the outlay required for one of the Good Members here to 'do a rewind', and fear I haven't the room or confidence at the moment for making the jigs and items required for a first rewinding myself. What are my options apart from shelving the project for the nonce?

[Diabolical Artificer]

You could just fit a fan, if there's room, that's the simplest fix.

Andy

[Ed_Dinning]

Hi Andy, not a good idea, the fan will only make the resultant fire worse!

Ed

[high_vacuum_house]

Without taking into account the inductance and power factor (I.e if it was a pure resistance) the resulting dissipation is just over 31 watts which is a lot of heat to dissipate in the transformer. I presume you are using the highest voltage tapping on the primary winding. I presume it was designed for 50 Hz mains supply not 60 Hz.

How does it look if you reduce the mains voltage to say 220 or 200V. If the secondary voltages are still ok and within specifications then add some load and see if they drop. If all is still okay you may get away with putting a large wirewound resistor in series with the primary to drop the voltage (10-20V) though this would be a last resort .

Christopher Capener

[Lucien Nunes]

The only thorough test for shorted turns is a ring test, however with the present symptoms I don't think there's much doubt that it has shorted turns. Because the excess dissipation takes place in a concentrated zone within the winding, the maximum temperature within can be very high, so the fault tends to progress as nearby insulation degrades and eventually it goes up in a puff of smoke.

It does sound like you need a rewind. As a temporary solution you could make an external box with whatever odd assortment of spare transformers you might have lying around that offer the necessary voltages amongst them.

[dougietamson]

I've just checked a known good large-ish PT from an old solid state 150W PA.

The core area is 1660mm2 and weighs 2.8Kg

It draws 101mA at the primary with no load.

[Uncle Bulgaria]

I think it's beyond a fan, Andy! Before I removed it from the chassis to make these tests, and check it wasn't coupling between the frame or problems with the machine's circuitry, I realised it was having problems when it was making bubbling noises!

Thank you for the ideas.

Lucien - I've had an offer from our resident transformer guru if I want a kosher rewind. I haven't got a stock of transformers (except for some overlarge Vickers ones) but Mr Dinning says the values are not unusual so could possibly find another with the same taps. The unit is under repair so I don't need to have it in use immediately, but I guess it would be handy to have it not in pieces on the floor... It also interferes with experiments to investigate the workings of the spark marking method used for the paper roll (my thread on AEG rectifier diodes is for the same machine).

high_vacuum_house (always liked that username) - yes, it's 50Hz, on the highest resistance winding, which was properly connected to the 240V selector tap. It has an interesting five possible voltage selections with appropriate fusing requirements!

dougietamson - this weighs about 1.2kg, so comparatively with yours the draw seems excessive!

[Uncle Bulgaria]

I've been having a look for suitable transformers to test this out. I don't want to lay out for a good rewind until I know the rest of the unit can be made to work! Before I post a 'wanted' I'd like to check what I'm looking for.

My measured values for the secondaries shown in my notes attached in the first post are 250V, 300V and the 6.3V heater winding. Assuming these values haven't been affected by the shorted turns, why are there twin secondaries?

The watch-timing machine this is from has one secondary rectified for valve HT, and the other goes to the spark generating rectifier and coil that creates marks on a paper roll when fired. Are there two secondaries for increased current capacity, rather than simply taking both in parallel from a single secondary? Perhaps the extra 50V is crucial.

Could I take both these supplies for testing purposes from a 250-0-250 secondary, for example?

[roadster541]

Looking at the circuit diagram for the Greiner chronografic-record. the on load ac voltage for the amplifier HT is 290v, which matches, quite nicely, your 300v off load and presumably, a similar amp. I would have expected the heater voltage to be around 6.5v off load, so maybe this is the damaged winding. I can't help with the transformer voltage for the spark generator as I don't have any paperwork on that model. I guess that the spark generator has a separate winding to reduce the effects of that circuit on the amplifier/timing circuits.
I have various ex eqpt xfmrs here if you wish to rig up a temporary supply, I am "at home" for the next week.
Rod