3 phase transformer diagnosis

[Veketti]

Hi,

Hopefully someone in here is familiar with transformers and could help me diagnose issue with one 3 phase transformer. So I bought machine which runs on 200VAC and we have 400V electrics. With the machine came 3~ transformer which has been used in 400V electricity. However it will trip 25A C-curve circuit breakers immediately when powered even when there is no load on secondary side. I know these have higher startup current but the seller said it would work with 25A fuse. One thing is for sure, connections are correct.

I have taken some measurements. Primary coil resistances: L1 = 100 ohm, L2 = 143 ohm, L3 = 130 ohm. Secondary coil: L1 = 11 ohm, L2 = 21 ohm, L3 = 16 ohm. Measurements done when delta - star connections separated and by LCR meter. Resistance between primary - secondary coils: L1 = 675 kohm, L2 = 572 kohm, L3 = 575 kohm. Coil inductances: Primary L1 = 550 mH, L2 = 860 mH, L3 = 650 mH. Secondary L1 = 55 mH, L2 = 100 mH, L3 = 69 mH.

Also measured secondary coil voltages when primary coil had 60W resistive load (incandescent bulb) in series 230VAC. Secondary voltages: L1 = 62.8 V, L2 = 68.7 V, L3 = 63.2V. This measurement also done when coils separated from each others.

Is the transformer broken or the inrush current just so high that 25A is not sufficient?
Here is the ID plate of the transformer:


Here is the schematic drawing of the transformer
View attachment 13-00-Dyn5a.pdf

 

[FvM]

Probably just high inrush current. Use D MCB or fuses. Inrush current limiter would be another option, series resistors shorted by a time delayed contactor.

 

[Veketti]

Would star delta motor starter work to limit inrush current? Primary coil is in delta connection as standard. Or it only works for motors..

 

[FvM]

Star-delta starter doesn't help, it's too slow. Inrush problems of transformer and induction motor are quite different.

The effort for a series resistor starter is however less than for a star-delta starter.

 

[Veketti]

Ok, thank you. Might try that. Any idea what size resistors should be and maybe about the connection?

 

[Veketti]

Nevermind, I figured it out. Used 22 ohm 50W resistors and timed it for 0,6 seconds and it works.