Part Winding Motor, Starter Timing

[joextech]

Called to replace a 230 volt 3 phase delta wound motor.
Motor had caught fire, no one could tell me when it had caught fire, (at start or while running) everyone knew nothing...
Sent the motor off for rewind.
While waiting for motor to be returned, tested the starter.
The start starter closed then 11 seconds later the run starter closed.
I have never seen a pert winding starter set that long.
Could this have started the fire?
I am used to 2 seconds delay, not 11 seconds with part winding motors.
What is your opinion on the timing.
Thanks, Joe

 

[itsmoked]

11 seconds seems awful long to me too for a coy er.. pert.. I mean - part winding start.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Eleven seconds seems long to me. I have seen new factory set part winding start blowers with less than 2 seconds timing. The second winding came in during acceleration to assist in the final acceleration to full speed.
respectfully

 

[Walden]

On refrigeration compressors with part wind motor windings, the time between the two windings is often 1 second: many times 0.5 second and I have seen the delay to be the time taken for the auxiliary contact on the first contactor to start the second. The killer of refrigeration compressor motors is the lack of or too low setting of the anti-cycle or limit start timer. Refrigeration compressors don't like to be started more than say, 12 times an hour. Joe, did your motor start and stop too frequently?

 

[itsmoked]

All it would need is a start or two with those settings to fry.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[LionelHutz]

I've typically seen 2 or 3 seconds max for a part-winding. Of course, you didn't say what the load was and that could make a huge difference on the motor failing or not.

 

[joextech]

Thanks all, you have confirmed my suspicions.

The motor is a 230/460 50 hp 3 phase 9 lead motor connected to a part winding starter wired for 230 volts. it is connected to a pump for a water slide.
I suspect two things, 11 seconds timing is way too much and attendants stopping and restarting the motor/pump while their friends were on the slide. (prank)

Thanks again for your opinions.

 

[jraef]

Hopefully, you used this for connection:

Here is the problem with Part Winding (PW) starting;

1) Low starting torque on the start winding (50% of FLT) which is sometimes not even enough to turn the shaft. The real purpose is to just establish the magnetic fields in the stator and rotor at a lower inrush (not necessarily starting current if you understand the difference) in preparation for the remaining windings to act to accelerate the motor. 1-2 seconds in Start is MAXIMUM. Longer settings will NOT reduce the starting current, a common misconception brought about by people not really understanding the concept yet using it to satisfy a utility requirement for Reduced Voltage starting. In actuality, utilities should NOT allow PW starting to be classified as such because it only reduces the voltage drop if the problem is a weak transformer, not a weak supply.

2) The motor heating during Start is extreme compared to any other method of RV starting. You are essentially at locked rotor current for a smaller motor, 50% of the main motor size, but the rotor may not actually be turning so that current stays that high without doing useful work. Again, remember that the intent was never to try to accelerate the motor, just to establish the magnetic fields. Therefore any more than a couple of seconds serves no useful purpose other than to begin to deteriorate the winding insulation.

In addition to someone cooking your winding by setting the transition time beyond 2 seconds, you have the other problem of short cycling. I strongly suggest you install a lock-out timer in your control circuit. This is usually highly recommended by motor manufacturers for PW started motors for the very reasons stated above. A good solid state Overload Relay that includes that feature would be worth the price of avoiding the next rewind.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
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[JERRYROBINSON]

More often than not, a part winding start motor is because of a mechanical issue, not electrical. It does very little to reduce inrush other than the break-away instantaneous. The time duration as stated earlier by several respondents at 2 seconds should be about the maximum setting, but checking with the motor manufacturer is best.

 

[waross]

Hi jraef;
When I was young and knew so much more than I do now, I knew those connections by heart. I connected two 40 HP blowers by the numbers. After the BANG BANG, (both motors started automatically in sequence) I discovered that those motors did not use NEMA numbering. The numbers were actually 6,7,8, not 7, 8, 9. It blew the internal star points open on both motors.
The good news was that warranty covered the repairs. My employer offered to pay for the damage that I had done, but the shop said that they felt that the motors should have withstood the abuse without those connections opening and so would support a warranty claim. Good people to deal with.
I always double check ther nameplate now.
respectfully

 

[jraef]

I too have been burned by non-NEMA numbering, I should have remembered. In my case the nameplate was correct, but what I didn't know is that the Weg 500HP dual-voltage motor was a 760/440V not a 440/220V design (as all their above NEMA sizes are by the way). So I connected to the High winding connection for my 480V supply. The voltage information was not on the inside of the pecker head with the wiring diagram and the motor was mounted so that the nameplate was not visible. Consequently I had no torque on the motor, even though it spun at the right speed unloaded. The problem wasn't discovered until the rock crusher was put into use. Because it was always started unloaded, nobody thought anything was wrong. But they kept tripping on overload, so much so that the intelligent operator assumed the OL relay was defective and disabled it. Smoked the motor of course. That was fun...