Interesting power problem 2

[EE7860]

I have an interesting question if any one can help.
2 identical compressor motors with the following data.
600HP, 2300V, 390RPM, 152FLA
Both were run uncoupled to check the amps.

One being fed from Delta-Wye transformer with the following data
12470V-2400Y/1383
2000/2240(OA) KVA, %Imp 5.24, transformer tap is set at 12160V. The motor running amps are approximately 104A on all three phases.

Second Transformer is Delta-Delta 13200-2400V, 1500KVA, %Imp 5.43, Transformer tap is set at 12540V. The running amps recorded are approximately 65A on all three phases.

Both transformers are ungrounded, The system voltage is 12470V. In the past motors were exchanged but the amps stayed with the transformer, in other words on Delta-Wye configuration motor pulls more amps.
Any suggestion? I thought power requirement will be the same in any case and the motors will run identical on both systems.

 

[electricpete]

Strange. Was the 2400V bus voltage measured?

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[EE7860]

Yes it is measured and is 2400V

 

[EE7860]

Although paperwork and nameplate proving to be identical motor. Can they be wound differently causing this issue?

 

[TestBeforeTouch]

Delta-Delta or Delta wye connected transformer makes no difference to the motor if the voltage and installation are correct.
But,how many leads does the motor have? If the motor has more than three leads than each motor may have been wired incorrectly when it was put on the first transformer.
The no load amps on the first transformer seem too high even for a low speed motor. The amps on the second seem about correct.

 

[EE7860]

Thanks alot, I am trying to get the original rewound reports (15 years old issue). There seems to be only three leads at the packerd head coming out from the motor casing. Quite possible one of them is wound wrong. What bugs me is the fact I was told that motors are switched around in the past and high amp issue seems to stay with the Delta-Wye transformer. If motor is wired wrong then high ampere issue will move as the motor is moved too?

 

[electricpete]

If motor is wired wrong then high ampere issue will move as the motor is moved too?

Yes. Miswound motor is not consistent with your original problem description (behavior stayed with the transformer)

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[waross]

With a system voltage of 12470 V;
"transformer tap is set at 12160V." This should supply 2461 V.
"Transformer tap is set at 12540V." This should supply 2387 V.
This would be the cause of your current discrepancies.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[REM76]

Although unrelated, I would consider high resistance grounding the transformers versus ungrounded.

 

[RRaghunath]

Typically, the no-load current of motors varies from 20% for 2-pole machines to 60% for 20-pole ones. Thus, 104A no-load current, which comes to 68% of 152A (FLC) seems possible for a 390RPM motor.
Could it be that the 65A is being measured is not the line current but phase curren (inside the delta), in which a case the current measured would approximately be (104/Sqrt3)A.
I can't think of any other explanation, the location of CTs may be holding the key.

 

[LionelHutz]

Waross or raghun have the most plausible answers. Either one could be the problem.

If raghun is right then check that your overload is actually set-up correctly.

 

[Zogzog]

"Could it be that the 65A is being measured is not the line current but phase curren (inside the delta), in which a case the current measured would approximately be (104/Sqrt3)A"

That was my 1st thought too, seems odd that the 2 currents are sqrt3 apart.

 

[TestBeforeTouch]

If there are only three leads coming out of the motor than how the motor is wound makes no difference since you can't hook it up wrong.
Have you looked at the CT ratio of the first transformer and compared it to the meter scale. I would try primary current injection on the CT's to verify the installation.

 

[NAZ55]

I would like to respectfully disagree with waross's answer on this one.

The ratio of Voltages and currents should be the same since power is constant, where as according to his explaination 2461/2387 = 1.03 and 104/65 = 1.6

Sincerely

Zazmat

 

[Kiribanda]

EE77860,

I too agree what Waross has pointed out.One of the motors is operating at 7% higher (2461 V)than its rated 2300 V and the other is 3% higher (2386 V) at nominal sytsem voltage of 12470 V.

So definitely the first motor might be out of its performance range and drawing higher current. My advice is

1) Check the motor test data sheets
2) Contact manufacuterer for the motor operating voltage range.

Hope this helps.

Kiri

 

[electricpete]

I would like to respectfully disagree with waross's answer on this one.

The ratio of Voltages and currents should be the same since power is constant, where as according to his explaination 2461/2387 = 1.03 and 104/65 = 1.6

I agree that the large difference in currents would not be expecteed to be explained by the small difference in current. For uncoupled motor, I would say the current is primarily inductive and it acts more like a constant impedance (inductance) than a constant power load. But still, the change in current should correspond to the change in voltage which is much smaller, so again I agree with the conclusion the transformer ratio doesn't explain the change in current.

But when things are unexplained, it is productive to check what you ACTUALLY have instead of what you think you have. That's why I asked about the voltages in my very first post.

Another thought - as several pointed out, the difference is a square root of 3. I'm pretty sure that is not just a coincidence. Somewhere among verifying the connections at the motor, connections at the transformer, and at the monitoring location, the square root of 3 will be explained.

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[BJC]

Where are the CTs in the system? Did you measure with a portable clamp-on ammeter or take a reading from the CTs in the system? Are the CTs the same ratio?
I would like to see the actual schematics of the system(S).

 

[EE7860]

Thanks for all the thoughts, the currents were measured directly at the cables through flex CT's. This eliminates the CT error or location issue. My initial thoughts were about transformer tap differences, but calculations didnt compensate for the approximate 40A difference. Since amps were measured at the packerd head via flex CT's, I dont believe there is an issue of line or phase currents there.

 

[electricpete]

any voltage monitoring on these buses?

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[NAZ55]

Could you also tell us if there were any voltage measurements taken on the low side of the transformers?
I am starting to wonder if the Delta-Wye transformer is actually 12470V to 1383, that would explain the sqrt(3) difference.