Wild leg on 230v 3 phase star - delta motors

[Terrodd]

I'm not an electrical motor expert, but I hope someone can help me. I have a situation where I have an injection molding machine that has a 230v, 3 phase, star-delta start motor. I have gone through 4 motor contactors in 6 months. Some of the time my 100 amp ckt breaker on the wall will trip during the transition from star to delta. Once the machine is running it will remain running with no problems.

When I measure between the legs I have 230v phase to phase and 230v, 230v, and 0v, when measured phase to ground. I was told this was the famous "wild leg" situation. However, reading through the forums here, it sounds like if it was a "wild leg" I should read 120v, 230v, 120v, respectively. I'm confused.

The machine manufacturer tells me that the fact that this is a 3-wire configuration with a wild leg insted of the 4-wire system, this is my problem. The Electric company says it is OK. I have 2 machines doing this and I have replaced all contactors, had motors tested, wiring triple checked, circuit breaker on wall replaced, etc.

I tend to believe the machine technician that the real problem is with the incoming power but I dont know how to prove it or argue the fact.

From reading the forums here I see there are some very talented and knowledgeable people and I'm hoping one of you can help. Thanks.

 

[busbar]

“…230v phase to phase and 230v, 230v, and 0v, when measured phase to ground.” This is describing a corner-grounded-delta configuration—in the simplified sense, as you imply, the motor only cares about seeing three {balanced} phase-to-phase readings. Unbalanced voltages-to-ground are not likely causing your problem.

“…if it was a "wild leg" I should read 120v, 230v, 120v, respectively.” A 3ø 4-wire delta configuration should measure around 120, 208 and 120 volts to ground. The middle reading is the “wild leg.” Again, this is not likely causing your problem. You do not mention if the corner-grounded system is furnished by the utility or from a local transformer owned by the plant.

Voltage-to-ground readings probably aren’t the direct cause of the limited contactor life, and normally in their own right don’t cause the failures your are experiencing. Of concern is the need for balanced phase-to-phase voltage readings.

What are the motor-nameplate horsepower and full-load current ratings as compared to the tripping 100A breaker? Does the breaker serve more than one motor? Are you using NEMA- rated magnetic starters? What is the duty; id est, how often are the motors being started? Is the motor-overload protection properly sized? What are the current readings on the three motor phases? It’s more important that they are balanced, and at or below the nameplate values. Specifically, what in the starters is failing and causing the need for replacement?

 

[jbartos]

Suggestion: The busbar posting is certainly heading in the right direction. I just would like to mention:
1. The nominal voltage systems is actually 240V/120V. What busbar means by 230V is the motor terminal voltage rating.
Also, sqrt(240**2 - 120**2)=208V wild leg
sqrt(230**2 - 120**2)=196V wild leg
sqrt(230**2 - 115**2)=199V wild leg
2. The trip between wye - delta needs more carefull analysis. Sometimes, it is only needed to increase the circuit breaker rating.

 

[Marke]

Hello Terrodd
You do not mention a) which contactor is failing, and what the motor rating is.
Star - delta starters are prone to causing problems, often unidentified, but essentially they typically place more stress on the supply than direct on line starters due to the open transition switching between star and delta. In order to minimise the current at change over from star to delta, you need to set the timer so that the switching occurs when the motor has reached full speed. If the switching occurs at part speed, the high start current could result in the circuit breaker tripping.
Contactor failure would tend to indicate an inappropriately rated contactor. If you advise the motor rating and which contactor is failing and its AC3 rating, perhaps someone can comment.
Best regards, Mark Empson

http://www.lmphotonics.com