amperage loss on a 3phase motor with y-delta start up.

[Iaconomechanic]

I am working on a motor in an air compressor. The motor has equal amperage on the y starter leads. but when the delta kicks in the amperage on L1 goes from 32amps to 14amps. And the L2 32 to 40amps, L3 32 to 5oamps. I have checked and double checked my wiring for the y-delta start wiring for this motor. My motor guy says its not the motor but it could be the delta starter. But my voltage is the same on the top of the starter as it is on the bottom. Give or take 3 volts. I just cant figure it out. To give you more info. the motor hp is 125 voltage is 480v. I personally think the motor as a problem. Can anyone give me some help.

 

[roydm]

Are you sure you dont have one coil backwards in Delta?
Does the comressor stall?
Regards
Roy

 

[rbulsara]

I would check out what roydm suggested first.

 

[Iaconomechanic]

Thank you Roydom for your info. It is so strange i have all the coils the right way. the motor comes up to full speed. When the wye is in i have even amps across all three legs but as soon as the delta kicks in i get this really funky amperage imbalance. But it only happens when the delta starter pulls in. My motor guy says that there is nothing wrong with the motor. But i cant help but feel there is something wrong. if im following the coils right i have everything wired right and i am not droping in voltage when the starters pull in. I tested this by droping the motor wires and starting the machine and testing the voltage at the bottom of all three starters and i have full voltage. The only strange thing is that when i take amperage on the motor i dont get even amps across the coils. like if its a 12 wire motor i have four wires that make up a coil and if i test the amperage on these four wires i should get the same amperage on all for wires. But i dont say one coil is wires 3,6,9,12. If i go to each wire on this coil i get two lines with 14amps and two lines with 42amps. Am i wrong in my thinking that they should all have 14amps or all have 42amps?

 

[itsmoked]

Sounds like one of your contacts or connections are bad. They can measure perfectly unloaded, as you've done, then under load not carry any useful current.

As your system transitions the motor gets single phased. (very bad)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Measure the voltage from the top of the starter to the bottom of the starter. L1 to T1, L2 to T2, L3 to T3.
The voltages should not be more than a couple of volts. The lines with the heaviest current may have slightly more voltage drop.
If the line with the least current has the greatest voltage drop you may have found the problem. If not, recheck all your connections. Don't forget or neglect to include the overload relay heaters in the voltage drop checks.

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

 

[Marke]

This sounds like a common problem with the winding ends going to the wrong phases.
If the starter performs correctly in star, I would say that the motor is fine.

I would recommend that you use a multimeter to identify and mark both ends of each winding group. Assume that the ends going to the star or wye contactor are the finishes and the other ends are the starts.
Positively label each with winding number and start or finish.

Number the incoming phases, 1,2 3 and ensure that the order across the input of the main contactor and the delta contactor are the same. I usually check by measuring L1 on K1 (Main contactor) to L1 on K2 (Delta contactor) then L2 on K1 to L2 on K2 and L3 on K1 to L3 on K2.

Next connect the finish of winding 2 (start connected to phase 2 of the main contactor) to the output T1 (phase 1) of the delta contactor. Connect the finish of winding 3 to T2 of the delta contactor and the finish of winding 1 to T3 of the delta contactor.

To prove that you have it right, isolate the coil of the delta contactor and initiate a start. When the main contactor is closed, and the delta contactor is open, measure input to output on each phase of the delta contactor. You should get phase to phase voltage (i.e. 480 V) on each phase. If you do not get equal voltages across each phase of the contactor, you have a wiring problem.

Best regards,
Mark.


Mark Empson
L M Photonics Ltd

 

[zlatkodo]

Cause may be:
- Voltage unbalance, because, 1 % of voltage unbalance causes current unbalance 5-10 %.
- Bad contakt in delta contactor,
- In motor – windings ( which is probably with several parallel circuits), one of coils is with short-connection between turns, or one of coils is wrong coiled ( if that happens from begining).
- I recomend:
Check or replace delta contactor.
If still is same situation, dont worry, because current is still less then the nominal.
Also you can check the motor, if you can somehow accurately measure the resistance of each phase-winding ( UI- method because of small resistance).
Zlatkodo

 

[rbulsara]

The motor may be ok but the "delta" wiring is completed in the starter. I suspect delta wiring in the starter.

I would disconeect the load the start the motor directly in delta (without load). That should check out if motor's delta connections are OK or not OK. Carefully note down which terminal was connected where in completing the delta.

If this works, then have a good electrician check out starter wiring.