Soft Start Problem

[JES2727]

A new customer of mine is having a problem with a 480V , 25hp motor. It began tripping the breaker at apparently random times, sometimes after 10 hours, sometimes after 24 hours, etc. But now it will run only for about 60 seconds before tripping. It is supplied through a Cutler Hammer soft starter. The starter is giving an error message code 25, phase imbalance. I'll be looking at this first thing in the morning. I could use some guidance as far as what to look for and what to check out. What would cause a phase imbalance? The customer seems to feel the soft starter is the problem, but C/H tech support does not agree, of course. I'm no expert on this sort of thing. Other than taking some amp and volt readings, I don't know what more to do. I don't own any analyzing equipment. Any advice would be appreciated.
Thanks.

John

 

[RayS]

My first guess would be a loose connection somewhere on 1 leg- that would explain the intermittent faults, and the new phase imbalance.

A little phase voltage imbalance can be a big problem- something like 25% more heating of windings with 3% imbalance. I think 3% would be the max. imbalance you would want to allow.

 

[kingpb]

Use this link, particularly pay attention to the current portion near the bottom.

http://www.usmotors.com/default_products.htm

 

[Wes G]

Check current

Check current

The "phase imbalance" message usually indicates a current problem. I would check your current drawn by the different phases for balance. Do you have a motor that has a winding that has a phase winding that has an internal short?
___________________
Wes Gerrans
Instructor
Northwest Kansas technical College
Goodland, KS

 

[ed_man]

I would bet that you will find that all three phase currents are reasonably balanced. You need to determine exactly when the problem is occurring. When the time-span was longer, did the breaker trip in the middle of a run or does the motor start and stop a lot? Is it possible that the breaker is only tripping during startup? If so, you may need to simply increase the initial starting voltage. This is a common problem that causes the motor to draw LR amps for an extended period of time, until something gives. Also, remove the T-Box cover from the motor. Inspect the connections and look for arc marks. As far as the phase imbalance alarm, I would not get stuck on that. When a breaker trips, sometimes misleading alarms will appear. Unless its a large current imbalance (> 10% - 15%) its probably not going to affect the breaker. Breakers should be rated at least 125% of rated motor current. Finally, start the motor and listen carefully. Does the motor ramp up smoothly or does it growl or bog down in the missle of acceleration. Either scenario could indicate misfiring SCR's, also very common.

 

[templdl]

"tripping breaker"
Is it a thermal magnetic (TM) or magnetic onlt MCP?
If TM is it tripping thermally or magnetically? If thermally the breaker commonly can not be reset and closed right after a trip. If it can be reset and closed it most likely tripped magnetically. If the breaker has an adjustable magnetic pickup setting where is it set?
If it's an MCP it tripped magnetically. Since it does have an adjustable magnetic pickup setting where has that been set? Also, a tripping MCP commonly indicates that the motor is failing. A TM breaker that's tripping instantaneously often responds the same.

Also, is the breaker feeding the softstarter which then feeds the motor or is it he soft starter feeding the motor through the breaker?

 

[JES2727]

Thanks for all the advice. I visited the site yesterday. It was not the breaker, but the starter itself that would open the circuit. I first checked the lugging, and everything seemed ok, except that the motor (125 hp) was fed with 3/0 welding cable and the wiring compartment was stuffed with cardboard. But no signs of arcing or loose connections. When we turned on the power, we had an error message " phase imbalance". I checked the incoming voltage and discovered that we had only about 250 volts on the line side of the starter between phases B & C. As it turns out, they had swapped the starter with one from another unit and thats when the problem got worse. The mechanic forgot to tighten two of the lugs on the line side of the starter! I tightened everything up, and it started right up. It's still running right now. However, when we attempted to start the other unit the starter would not function. All the LED's flashing in a crazy fashion, and not resetting. He's ordering an new starter this morning and having it shipped overnight for Saturday delivery. I'll be back on Monday to replace it, and back again during the shutdown in early November to replace the welding cable.

John

 

[don_resqcapt19]

and back again during the shutdown in early November to replace the welding cable.

While most welding cable is not permitted to be use for this type of application, check the info on the cable. It is available with dual welding/RHH listing, and if so and if used with the correct terminations, it would be ok for this use. This type of cable is rare, but it does exist.
Don

 

[templdl]

Right on Don.
If welders cable is used all bets are off should it be terminated with standard lugs. Welders cable should be terminated using crip on terminals and the terminal bolted.
I keep an example of a lug failure on a 250a breaker that escalated to a L-G fault which then included the adjacent phase
Dave