Resistance of a transformer windings.

[LeeH]

Does any of you clever chaps know what the resistance of a 5kVA transformer should be? It's a 240-110 CTE.

I have a resistance of 0.16 Ohms (1400A)

If I recall what little transformer theory I covered years ago the impedance will increase when energised but how much I have no idea.

Basically we did some remedial work on the main section board at our Manchester branch and the supply to the transformer won't hold in.

The OCPD is too small, it's a hager C32. Hager recommend a C63 or a D32 for a 5kVA so that's obviously a problem but it's been fine for years.

 

[PEG]

Hi bud,you might benefit from a bit of research on inrush problems.
There are modifications that can be applied to the transformer,to mitigate against such problems,but you may require approval to apply them...

 

[telectrix]

a 5 kVA transformer will give you a load current of approx 21A. while a 32A MCB will seem appropriate, the inrush current suggests a 32A type C or D, as Hager recommend.

 

[LeeH]

What I'm trying ask is wether the windings are at fault. I.e. A short circuit.

I have the hager data sheets and it's been fine as is for years

 

[telectrix]

slap a supply on it and clamp the cable. on no load it will draw far less than 20A.

 

[PEG]

Obviously,we do not know if the fault is at the transformer,if other work has been done,so satisfy yourself,that the OCPD is not seeing an actual fault elsewhere,if not,try energising the transformer,with a small load on the 110 side,sometimes,not always,it will hold and operate fine.

It could point to inrush current, as a problem.

This used to be a standard "instructed person" task,when determining by phone,if a site supply had a delicate MCB or a stroppy tranny.

 

[LeeH]

slap a supply on it and clamp the cable. on no load it will draw far less than 20A.

You're missing the point tel, I understand all the basics. The MCB will not hold even for a millisecond.

I have a tranny that's got a very low resistance across the windings. Them mcb IS to small buts it's been OK since it was installed.

I did some unrelated work upstream and now it won't hold in. The low resistance I measure is the question. Does this dramatically increase when ON. Or I guess to be technically correct at this point it will be impeadance.

Could be a tired C32, it should be a C63 or a D32. (Hagar data sheet)

I just don't want to waste £500 on a new one when it's perfectly fine. Only because I don't understand the theory.

VIR says it will never hold in (1400 plus amps). But with increased impedance it will.

 

[LeeH]

Obviously,we do not know if the fault is at the transformer,if other work has been done,so satisfy yourself,that the OCPD is not seeing an actual fault elsewhere,if not,try energising the transformer,with a small load on the 110 side,sometimes,not always,it will hold and operate fine.

It could point to inrush current, as a problem.

This used to be a standard "instructed person" task,when determining by phone,if a site supply had a delicate MCB or a stroppy tranny.

Thanks, unfortunately I locked it off when I left on Sunday!

 

[telectrix]

you can't go by a resistance reading as most of the impedance will be inductive. the ideal solution would be to supply ( no load on secondary) via a variac with the cable clamped. wind up from 0v and see what happens.

 

[bill01803]

FYE my 3.3 KVA site transformer primary winding measures 0.38 ohms, and that will take out a B32 if you catch the top of the cycle on switch on

 

[Marvo]

I'd expect a 5KVA 240-110 Tx to be way less than 2 ohms on the primary side. The impedance increases rapidly when it's energised. Go with a D curve MCB if you can meet disconnect times.

 

[Wilko]

Does any of you clever chaps know what the resistance of a 5kVA transformer should be? It's a 240-110 CTE. I have a resistance of 0.16 Ohms

Checking my portable 1.5/3kVA it has dc resistance of 0.5 Ohm, but that includes its inbuilt fuse and a bit of flex.

 

[telectrix]

FYE my 3.3 KVA site transformer primary winding measures 0.38 ohms, and that will take out a B32 if you catch the top of the cycle on switch on

my 3.3KVA tranny has a very high resistance ( to flying through the air when kicked. anyone got a cure for broken toes? ).

 

[Lucien Nunes]

Yes a few hundred milliohms sounds plausible, although I can't give a specific figure top of head for a 5kVA. The lower the resistance the better as this maximises the efficiency and regulation performance of the transformer; in a perfect world it would be zero ohms as with all wire and cable, but copper is the best conductor available at reasonable cost and we have to design around its small but not negligible resistivity. The actual winding resistances designed into a commercial transformer are a complex trade-off between cost, size, iron loss (off load) and copper loss (on load) and voltage regulation.

One of the main causes of inrush is magnetic saturation. In short, if you switch on at a disadvantageous point in the mains cycle (actually the voltage zero) the magnetic flux in the iron tries to swing from zero to twice its rated maximum, instead of negative maximum to positive maximum, which it cannot stand. So the inductance collapses momentarily and the low primary resistance passes a high peak current until the proper phase relationship between voltage and flux establishes itself.

 

[Rpa07]

Is there any kind of safety feature that is available for this anomaly Lucien or others? It can't be down to 'hope it holds' on power up! Very good explanations given in this thread - you should all get together and start a college to teach the next 'Gen' of sparks.
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