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star/delta problem
3

star/delta problem

star/delta problem

(OP)
I have a three phase motor that is tripping the overload soon after start-up. L1 and L3 are pulling 40 amps, whereas L2 is pulling nothing. Motor rated for 20A. There is full voltage on each phase going to the motor on one set. There are two sets of cables going to the motor windings and the second set has L2 and L3 at half voltage, while L1 has full voltage. Can anyone explain what might cause this?

RE: star/delta problem

(OP)
full line voltage

RE: star/delta problem

There is an open circuit some where. Could be in the motor winding or cabling or the contactor.


 

Rafiq Bulsara
http://www.srengineersct.com

RE: star/delta problem

What Rafiq said. Concise and accurate. lps

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

RE: star/delta problem

(OP)
okay will check that. Thanks a mill.
 

RE: star/delta problem

When you said "full line voltage" in response to the fuse check question, a star-delta setup can sometimes fool you on using voltage as a way to check fuse integrity because you might be seeing voltage through the windings of the motor. Fuses should be checked by removing them and checking continuity.

From your description I would venture to say you have an Elevator controller set up for star-delta starting. These applications are notorious because they only use 2 contactors (strictly to save money) and one side of the motor windings is energized at all times. So if you have a welded contact, which happens a lot on star-delta starters, and you are checking for a blown fuse by measuring voltage, it will falsely read as "full voltage" even though a fuse is bad because the motor windings do not present enough resistance to show a significant voltage drop on a 9VDC battery (what's in your VOM).

So even if you do have a blown fuse, check the contacts of your contactor(s) to make sure the cause wasn't a welded contact. And by the way, that most likely damaged the motor windings as well. Such is the down side of that cheap 2-contactor configuration.

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— Kilgore Trout (via Kurt Vonnegut)
  
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RE: star/delta problem

(OP)
Thanks for the reply jraef. It is a three contactor setup. When I say full line voltage, I mean full voltage from the fuses when the motor itself is isolated. I won't be on site again until tomorrow so I'll let you know what I find. Thanks again.

RE: star/delta problem

A note on welded contacts. I have seen contacts welded in the OPEN position also.  

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

RE: star/delta problem

(OP)
Found the problem. It was the link in the contactor that links L1, L2, L3. It was loose- very loose. Looked like it hadn't been tightened when it was installed. Thanks for pointing me in the right direction everyone.

RE: star/delta problem

hvaac:
Glad that you found the issue and was able to fix it. Also thank you for the update, not everyone does that.

Rafiq Bulsara
http://www.srengineersct.com

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