Interesting power problem
Interesting power problem
(OP)
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.
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.






RE: Interesting power problem
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RE: Interesting power problem
RE: Interesting power problem
RE: Interesting power problem
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.
RE: Interesting power problem
RE: Interesting power problem
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RE: Interesting power problem
"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
RE: Interesting power problem
RE: Interesting power problem
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.
RE: Interesting power problem
If raghun is right then check that your overload is actually set-up correctly.
RE: Interesting power problem
That was my 1st thought too, seems odd that the 2 currents are sqrt3 apart.
RE: Interesting power problem
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.
RE: Interesting power problem
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
RE: Interesting power problem
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
RE: Interesting power problem
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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RE: Interesting power problem
I would like to see the actual schematics of the system(S).
RE: Interesting power problem
RE: Interesting power problem
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Interesting power problem
I am starting to wonder if the Delta-Wye transformer is actually 12470V to 1383, that would explain the sqrt(3) difference.
RE: Interesting power problem
RE: Interesting power problem
RE: Interesting power problem
RE: Interesting power problem