Help solve a debate between my twin brother and I (both electrical engineers) read message below for details

[jim dungar]

So you're saying if I can temporarily ground both the primary And secondary after power is removed I may have a better chance of measuring max inrush?

Laboratories have a hard time measuring the true inrush, I can't imagine you will be able to do it in the field.

 

[MD Automation]

So you're saying if I can temporarily ground both the primary And secondary after power is removed I may have a better chance of measuring max inrush?

Not, I don't believe so.

The residual magnetism "left over" after switch-off is in the core itself. It's not in the windings, you could remove all the winding coils from the transformer and the core would still have some magnetism in it, of a random amplitude and direction (polarity), all dependent on when you turned it off. You can't bleed it off, it's not electrical. Kind if like you can't simply make a magnet not magnetic (heating it up is cheating btw!!). Breaking it in half just makes 2 magnets

There are ways to minimize the inrush, one of which I think involves putting a (resistive) load of sorts in series with the windings. You energize the xmfr with those in the circuit, then after a bit the rotating magnetic field in the transformer has "synced" up with the mains, and the series loads work like current limiting resistors to cut down the inrush. Then you close a second set of contacts which shorts out the series loads.

But, for most typical cases, nobody bothers with complications like those.

 

[ptonsparky]

I wondered how residual magnetism could be affected by capacitance.

FWIU of this discussion, we as the typical electrician will most likely never be able to measure the max inrush of a transformer once it's been in operation.

 

[ggunn]

The residual magnetism "left over" after switch-off is in the core itself. It's not in the windings, you could remove all the winding coils from the transformer and the core would still have some magnetism in it, of a random amplitude and direction (polarity), all dependent on when you turned it off.

I didn't know that could happen; I learned something today. I suppose by "random amplitude" the randomness would be bounded by the maximum core magnetism the transformer core would experience when energized.

 

[MD Automation]

I suppose by "random amplitude" the randomness would be bounded by the maximum core magnetism the transformer core would experience when energized.

Yes, correct. And what I was blabbering on about in post #33 is that, if the cards don't fall your way at the next switch-on, the core magnetism can get pushed beyond that "normal" amplitude you are referring to.

For example, if the residual magnetism is leftover near a positive peak (you can call that a "North Pole Direction" if you like the N-S compass pointings) AND you switch on at a zero crossing with the sine wave ready to build up more of the same North Pole strength - you drive the core into saturation for a few cycles. The core was already "sleeping" near a peak and you went and woke it up by pushing it more that same way.

It's all a crap shoot about when you turned it off and when you turned it on again.

I just looked quick on the interwebs and this site has a nice little chart at the bottom that breaks down the switch-on scenarios into 6 basic categories...

PARAMETERS THAT DETERMINE TRANSFORMER INRUSH CURRENT – Voltage Disturbance

I circled the second one - which describes the worst case scenario. Again, the whole point of my original post - and what always interested me the most - is that the worst case inrush can happen when you fire the thing back up from the zero crossing. Very counter-intuitive.

 

[LarryFine]

Breaking it in half just makes 2 magnets

Kinda like a crumb.

 

[PE (always learning)]

Yes, correct. And what I was blabbering on about in post #33 is that, if the cards don't fall your way at the next switch-on, the core magnetism can get pushed beyond that "normal" amplitude you are referring to.

For example, if the residual magnetism is leftover near a positive peak (you can call that a "North Pole Direction" if you like the N-S compass pointings) AND you switch on at a zero crossing with the sine wave ready to build up more of the same North Pole strength - you drive the core into saturation for a few cycles. The core was already "sleeping" near a peak and you went and woke it up by pushing it more that same way.

It's all a crap shoot about when you turned it off and when you turned it on again.

I just looked quick on the interwebs and this site has a nice little chart at the bottom that breaks down the switch-on scenarios into 6 basic categories...

PARAMETERS THAT DETERMINE TRANSFORMER INRUSH CURRENT – Voltage Disturbance

View attachment 2570483

I circled the second one - which describes the worst case scenario. Again, the whole point of my original post - and what always interested me the most - is that the worst case inrush can happen when you fire the thing back up from the zero crossing. Very counter-intuitive.

hahahaha, this conversation has gone way further than I ever imagined.

If the inrush is so hard to determine, then why did Siemens produce all those pages of data documenting the absolute peak inrush and max practical inrush? Are you saying that data really isn't relevant?

 

[LarryFine]

hahahaha, this conversation has gone way further than I ever imagined.

In the Mike Holt forums?! No!!!

 

[topgone]

hahahaha, this conversation has gone way further than I ever imagined.

If the inrush is so hard to determine, then why did Siemens produce all those pages of data documenting the absolute peak inrush and max practical inrush? Are you saying that data really isn't relevant?

Any magnetic material when heated beyond its Curie temperature (approximately 700 degrees C) will lose its ability to magnetize (it becomes austenitic). Once cold, it behaves like a non-ferritic material. And, if one magnetizes it again by subjecting the material to a magnetic field, The iron then keeps some residual magnetism again. But that method is impractical.
Demagnetization of a core can also happen when the magnetism inside the core is reversed or diminished. The application of a DC on the winding of a transformer alternatively opposite at definite times, lowering the DC current at each injection phase reversal until the injected current has been lowered to zero will demagnetize the core. Please refer to IEEE Std 62-1995 on these.

 

[ptonsparky]

I wondered how residual magnetism could be affected by capacitance.

FWIU of this discussion, we as the typical electrician will most likely never be able to measure the max inrush of a transformer once it's been in operation.

Well, I proved that wrong with a .5 kva 1PH. Still don't know if it was the Max.
.680K amp at 120 volt through a x10 multiplier. 68 amps.