OT: GE Multilin 469 / CT sizing for 800hp motor

I have a 800hp 4160V induction motor (FLA 104) that is tripping on current overload and I have a question about the CT sizing.
How can the motor protection unit (GE multilin 469) that uses a 0-5A CT input measure 500Amps when the CTs used are labelled 100:5 Amp. The 469 would have to be able to see 25A? This unit has been in service for many years and the 1,000 HP motor (FLA 127 A) uses 150:5 CTs without problem.

Shouldn't the primary of the CT be greater than the FLA?

thanks,
Brian.

Shouldn't the primary of the CT be greater than the FLA?

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I think you are right. Personally I use the circuit breaker rating to size the CT ratio. ie if the supply is protected with a 100A then the CT would be 100/5A or as I understand it 100A = 5A this is however per phase or single CT.
I also don't understand

How can the motor protection unit (GE multilin 469) that uses a 0-5A CT input measure 500Amps when the CTs used are labelled 100:5 Amp.

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If your 800 HP is drawing 500 FLA then I think your CTs should be 500/5.
Assuming your GE unit is setup for 5 amp input representing 100% FLA.

Dan Bentler

The GE 469 unit is displaying up to 500 amps pretrip.
The FLA is 104 Amps.
I'm suspecting that I am not getting accurate readings and getting false tripping.
The GE 469 parameters seem to be OK where the 100:5 CT is concerned. I just can't explain how the 100:5 CT into a 5A CT input can read 500 Amps.
We just had the motor sent out to a repair shop because we suspected a broken rotor bar but they have just informed us that the motor is fine.

Brian.

Bryan

Cranked the numbers at
104 FLA 4160 volt 3 phase and 0.8 PF I get 802.6 HP.

So 100 to 5 CTs sound about right. About the only thing you need to check is if they will take the starting current.

What does this motor drive? Are you sure load is not overloading motor?
By the numbers you are running motor right at 100%. I would double check motor current.

When does motor trip? Is it during start or while running?


Dan Bentler

Hi guys,

Thanks for the replies,
I had to go home, had worked 12 hour days since the beginning of Sept and needed a weekend off.

The equipment is the primary rotor of a rock crusher, the secondary being the 1000 hp. High inertial load coupled with a fluid clutch.

This is what they found.

The new 469 multilin was the wrong unit. Instead of having a 5Amp CT input it was a 1Amp unit. (We don't have any X:1amp CTs here so not sure why this was ordered, apparently it is common in Europe.)

So our displayed currents were actually 5 times greater than actual.

I am guessing our plant spent between $150K and $250K sending the good motor out and contracting two seperate portable crushing operations to make up the production.

I think I did everything I could in light of the fact I had no safe way to verify the current, but I really suspected it was bogus, should have checked the PN, we had a P1 and we needed a P5,

live and learn,

thanks all,

Brian.