Murphy A574 Mains transformer getting hot

[MotorBikeLes]

If I needed to monitor transformer current, I have two different TEK current probes, but I also have a Waugh isolation amplifier, so probes can be connected directly to HV points. The unit has optocouplers inside separating the HV from the scope.
I have frequently used the Ch 1 add to Ch 2 but with Ch 2 inverted on occasions.
I also have a little TEK 212 scope, completely isolated and plastic cased. Limited by its 200KHz bandwidth though for some purposes.
Les.

[Argus25]

Originally Posted by Ed_Dinning

Hi check that the lamination stack is well butted together with minimal airgaps between lams.
The clamp bolts can be loosened and the lams tapped together to reduce these gaps.
This is often a common cause of overheating on old transformers.

Ed, that is interesting, I have never tried this.

Some transformers that were vacuum varnished have the lams very well stuck in the place from new. Recently I was able to un-stack a faulty power transformer in a Tek TM501 power unit, and unwind and push out the faulty primary winding (as it was layer wound with paper and cardboard and not a plastic bobbin) and glue in a new primary winding. It was a very difficult un-stacking due to the excellent varnishing there.

[Graham G3ZVT]

Originally Posted by Ed_Dinning

Hi check that the lamination stack is well butted together with minimal airgaps between lams.
The clamp bolts can be loosened and the lams tapped together to reduce these gaps.
This is often a common cause of overheating on old transformers.

This is what I was thinking. ie gaps formed between the "Is" and the "Es". I seem to recall being taught that frame output transformers deliberately have a gap, but I can't recall why, I think they can get hot in operation, perhaps that's a clue?

[Argus25]

I seem to recall being taught that frame output transformers deliberately have a gap, but I can't recall why, I think they can get hot in operation, perhaps that's a clue?

Gaps are often put in either chokes or class A audio output transformers, or Frame output transformers as there is a DC current in the winding. Without the gap to increase the reluctance of the magnetic circuit, the Iron core can be pushed too far up its B-H curve and the magnetic saturation causes a profound drop on inductance.

Class B output transformers don't need the gap because the currents in each half of the primary cancel out the magnetizing effect on the core.

(A frame output stage is not dissimilar in overall design to a class A audio output stage)

[Ed_Dinning]

Hi Gents, ref gaps, Argus beat me to it.
Generally speaking there should be an absolute minimum gap in the type of mains transformers we use, difficult if it is a varnished core.

Ed