Hello all,

Just thought it would be worthwhile posting this question here in case others could benefit from the discussion that I expect will arise.

My question relates to an application issue involving the following;

Supply:
Primary 22kV, secondary 415V 50Hz, rating 315kVA

Motor:
Squirrell cage induction motor, 160kW 3-phase 415V 50Hz, 1448 RPM, FLC = 265 Amp, LRC = 577% FLC, LRT = 212% FLT, BDT = 273% FLT

Load:
Centrifugal pump

Starter:
3-phase Auto-transformer starter (Korndorfer connected) currently starting on the 50% tapping.

Line amps:
Chart recording reveals line amps increase to appox 3000 on mommnet of switch on. This gradually reduces to 1600 amps over the first second and is maintained at that level until the starter transitions to full voltage (6 seconds). At that stage the current drops immediately to 245 amps ie below motor nameplate FLC.

Line voltage:
Chart recording reveals this drops from nominal 250VAC phase-neutral to approx 117V at the momment of switch on. This gradually increases to 186V and is maintained at that level until the starter transitions to full voltage. At that stage the voltage rises immediately to 242V.

On occassions the start will cause the breaker on the 22kV supply to trip. Engineers involved have used that information together with other instrumentation to confirm the above-mentioned start current and start voltage recordings to be correct.

Theory suggests a start current (measured on the line side of the transformer) should not exceed 420 Amps asuming transformer losses of around 10%. In my opinion the supply transformer bordering on being too small for the connected load. Having said that I struggle to accept the high current & low voltage levels being recorded. The numbers don't appear to stack up!

Any assistance would be greatly appreciated.

Regards,
GGOSS

Hello GGOSS

That is an interesting situation. How much smoke has come out of the auto transformer??

It sounds very much to me, as though there is an error in the construction or connection of the starter. I suspect that the supply is being applied to the tap rather than the top of the transformer. This would oversaturate the iron in the transformer and cause it to draw a high current, resulting in the drop in the supply. It is probably just as well that the impedance of the supply is limited by the supply transformer.

If the installation is too far for you to visit site, ask them to disconnect the motor and run the starter through it's paces, checking the current while doing so.

What is the chance that the supply is connected to the motor terminals and the motor is connected to the supply terminals, ie the starter is connected in reverse. This would certainly explain the results and it would appear that the motor is getting to full speed, and I can not see that happening on the 50% tap, especially with a low supply voltage.

Just my first thoughts.

Best regards,

Hello marke,

No smoke out of the transformer......yet!

It would appear we are on the same wavelength with regards to the possible misconnection of the transformer by the contractor. Unfortunately it's not an easy place to get to and I am not as yet convinced that there is anyone there with sound understanding of the the 'technology'.

Having said that, I expect they will be able to perform current measurements with the motor disconnected, I might also get them to measure the voltage on the motor terminals during a start.

Thanks again and regards,
GGOSS

Hello GGOSS

I expect that with the motor disconnected, there will still be a significant current on the supply, and that the output voltage will be higher than line voltage.
I would prime them to drop the unit off line pretty smartly if the voltage on the motor terminals is high, or if the line voltage is low.
Without the motor connected, I expect that the voltage will be much higher and if the supply is connected to the tap, then the higher supply voltage will drive the smoke out faster!!

Good luck,

Hello Marke,

Confirmed - supply connected to tapping, transformer primary connected to motor. In other words voltage was increased (x 2) instead of being decreased (x 0.5).

Made the necessary changes to the power wiring, all tested OK.

I think the fact that this starter has run continuosly over several months limited possibility of failure. If it had been started repeatedly damage would have been highly likely.

Regards,
GGOSS

Hello GGOSS

Thank you for the update. Great to get a problem solved and an answer/solution.

It is the first time that I have heard of a reverse connectd AT starter, but I have found several reverse connected soft starters in the past.

I would say that it is lucky that the supply impedance was high enough to limit the start ciurrent. If this had been done on a solid bus, the transformer and the motor would probably have been damaged, unless the supply protection operated first.

Just goes to prove, in problem solving, never assume the obvious!!

Best regards,

Wow, good catch guys. I'm impressed.