Failed electrical cable. Is there a physicist in the house?

[bungeejumper]

After a bit of research, it seems that pond pumps usually use permanent magnet motors. The same principles apply, however. If the stator is just a mains frequency coil (with no fancy electronics), the speed will be fairly constant

Thanks for that, jfgw. I'd always supposed that the speed of the pump was effectively set by the 50 hertz AC cycle, but maybe that was why I got thrown out of physics classes?

BJ

[csearle]

Suppose there were an intermittent fault in the primary windings of the transformer which shorted out a few turns - the voltage in the secondary windings would go up. Say it went to 30v instead of 24v - the water spray would be higher, but the additional current would fry the bellwire. The intermittent fault has been disturbed as you've rewired, and all is ok for now. If the spray height goes up again, you could confirm this theory with a voltmeter.

A shorted winding in a transformer effectively shorts out the whole transformer. It is like shorting out the secondary - the primary will take a lot more current until something (such as a fuse) fails. There would have to be a simultaneous short and open circuit for the secondary voltage to go up.

I am intrigued by this. Any chance of explaining why that should be? I was imagining too that if, say, a significant number of windings of the primary were to have fused together, presumably through a breakdown of the insulation, then as well as the current obviously increasing there would also be fewer turns around the core. Wouldn't that then alter the turns ratio from say 10:1 to e.g. 9:1 and push the output voltage up on the load?

I suspect somehow that I'm overlooking something as you are clearly quite well up on things of this sort.

Regards,
Chris

[jfgw]

Thinking about it, I would not wish to say unequivocally that the secondary voltage could not increase but I would normally expect something to fail within a very short time if it did due to the high currents involved.

To illustrate, I have created this diagram of an ideal transformer with some shorted turns,



While it is true that there are now, effectively, only 900 turns in the primary (which would give 26.67 volts on the secondary were it not for the short), there are also, in effect, two tappings on the primary which are 100 turns apart and which will have a voltage induced across them. Since they are shorted, this part of the winding will carry a lot of current. Consequently, the remaining 900 turns of the primary will also carry a lot of current.

In an ideal transformer, a short such as this will present itself as a dead short across the whole primary. A real transformer could, I suspect, continue to operate for the short time that it took the shorted section of winding to cook completely or until something such as a thermal fuse blew.

Of course, it could be that the shorted section fails open, in which case there would be, in effect, a primary of 900 turns and no shorts. This would provide a higher secondary voltage although I would not expect a transformer in this condition to continue to do so for very long. In the OP's case, as the transformer has not been replaced and the pump is now working normally, I consider this a most unlikely scenario.

Julian F. G. W.