Synchronous motor field measurements. 5

[Thedroid]

I'm working with 2 different motors, and I'm trying to interpret some basic test results. I have 2 1250hp 4160V 40 pole sync motors, Field is 250V 66A and the field discharge resistor is 33.4 ohms. The Field is supplied by an SCR rectifier thats mounted in the substatation by the starter. The motor is soft started with a ramp time of 10 seconds. I'm trying to measure the ac induced current coming from the motor as it accelerating and decelerating, and also at the moment the field is applied and removed.

The field circuit breaker on one of these motors was destroyed awhile back, and didn't have any overlap between the NO and NC contacts that apply the field, and insert the resitor. I wanted to measure the ac induced current, so hooked up an analyzer, and set my clampmeter to capture inrush, since I didn't want to stand in front and watch. I was able to meausure 1800V 55A going through the resistor and NC contact for several seconds before the NC contact opened. Motor started without any problems, but don't want the field or contact damaged. The contact is rated for 15 A dc continuous, and 80 A for up to 1 minute.

I measured a different motor which still has the old contactor with no overlap, and it was around 3 A at start, and 15A at stop. This made me scratch my head, because I was expecting higher readings. The second motor took longer to reach sync speed, so maybe this would affect my reading.

The exciters were originally MG sets, but were replaced with the SCR rectifiers. Originally the field circuit breakers were vintage GE bar contactors, and were replaced with with Joslyn Clark contactors with permanent magnet blowouts. These contactors didn't have any overlap, and have either let smoke, or are going to. A better option couldn't really be found, but I did stumble across a few GE's. They're old, but still new in the box. I watched from a distance after installing one, and those old school arc chutes sure put on a show.

 

[LionelHutz]

Is the motor loaded during a start? Do the controls energize the field before the motor reaches synchronous speed?

 

[burnt2x]

Is the motor loaded during a start? Do the controls energize the field before the motor reaches synchronous speed?

This item should have been the first to be examined,IMO! As previously posted, the mills should have been properly grounded-out before shutting down.
Another thing, experience tells me it's better if you have a voltage relay across the rotor field which initiates/blocks field breaker preventing closing other than the safe window (98%-100%) of synch speed.

 

[Thedroid]

The field is applied at 95.5% of sync speed. The mills are started up loaded, and the motor is brought up to speed using a softstart. Historically the mills were ground out before shutdown, but now they are taken down loaded.

 

[LionelHutz]

OK. If the mill was clutched then it would have reluctance synchronized either in phase or 180 degrees out. As it is, the contactor is likely closing into a fairly high AC field voltage. This could be part of the reason you are seeing the contactor damage. But then, poor quality contactors could be the reason too...

Can't you get closer to 100% speed, say at least 97.5%?

Do you synchronize on the rising AC current or just randomly? This can make a big difference on how nicely the motor synchronizes.

 

[Thedroid]

We use a SPM to determine the proper sync speed. I can try to get it closer to 100%.