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crackle 17th Oct 2020 3:01 pm

Transformer voltages?
Can someone please help me out here I think my brain has ceased to function.

Consider a transformer with connections 0v, 240v and 400v on the primary and a 24v secondary.
If 400v was connected between the 240 and 400v terminals what would the OP volts be?

Would it be 60v


barrymagrec 17th Oct 2020 3:02 pm

Re: transformer

crackle 17th Oct 2020 3:04 pm

Re: transformer
Sorry when you say briefly do you mean the OP would be 60v before the primary fused itself which I presume would be fairly rapidly .

crackle 17th Oct 2020 3:26 pm

Re: transformer
I just hasten to add this is not something I am proposing to do. I am just confirming the likely outcome and cause of a fault with a friends piece of industrial equipment.


Radio Wrangler 17th Oct 2020 3:52 pm

Re: transformer
I wouldn't expect the secondary to get up to the calculated voltage because the core is likely to saturate long before. You're applying 400v to a 160v winding, 2.5 times the intended voltage.

When the core saturates the primary inductance plunges and the primary current goes through the roof. It's a race between a fuse and the wire going first.


crackle 17th Oct 2020 3:57 pm

Re: transformer
I have suggested to my mate he should check the continuity of all the relay windings that the transformer was supplying.

Skywave 17th Oct 2020 6:26 pm

Re: Transformer voltages?
I've just done the arithmetic: result = 60v. A theoretical result, of course.


broadgage 18th Oct 2020 12:07 am

Re: Transformer voltages?
Yes about 60 volts IN THEORY. In practice the transformer would draw a grossly excessive current due to magnetic saturation, and would either be destroyed, or might be saved by prompt operation of the fuse or circuit breaker protecting the primary winding.

If this is an accident that has actually occurred, rather than a theoretical discussion, then relay windings would PROBABLY have survived the very brief over voltage.
If the transformer looks undamaged and still works, it was probably saved by prompt operation of the fuse or circuit breaker.

If future failure of the transformer or the relay coils/other loads powered by the transformer would put human life at risk, or be exceedingly costly then I would advise replacement.
For less critical applications that is probably extravagant.

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