Synchronous motor field measurements.
Synchronous motor field measurements.
(OP)
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.
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.





RE: Synchronous motor field measurements.
15A sounds really low for the resistor contactor - it may be rated 80A for a short time but I doubt it's happy breaking 80A.
There are other manufacturers of contactors suitable for that applicaton. I know AB makes some and there was a thread there not to long ago with a bunch of others listed.
RE: Synchronous motor field measurements.
RE: Synchronous motor field measurements.
ABB definately has a 130A contactor with N/C and N/O poles. Either get one with both and take a chance without the overlap or much better just use 2 of them and trigger the N/C contactor with an aux contact from the N/O contactor.
Search for the recent thread here since there were a number of good manufacturers posted. It was called something like "sources for DC contactors".
RE: Synchronous motor field measurements.
RE: Synchronous motor field measurements.
Bill
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"Why not the best?"
Jimmy Carter
RE: Synchronous motor field measurements.
If you start the motors without discharge resistors connected you may get away with it a few times, but for sure a ground fault will occur in the field winding...an expensive repair job will follow.
regards, rasevskii
RE: Synchronous motor field measurements.
I'm checking the current with a clampmeter set for AC on the resistor circuit.
RE: Synchronous motor field measurements.
RE: Synchronous motor field measurements.
rasevskii
RE: Synchronous motor field measurements.
Field discharge resistor values determine how fast your field energy is dissipated. The problem with sparking on change-over from exciting to field discharging is solved by the overlap in the making-breaking of the NO-NC contacts.
And BTW, rasevski is correct, "don't fix it if it ain't broke", but having rough contacts means they need attention.
RE: Synchronous motor field measurements.
I tried to locate a suitable replacement that was well suited to the job, and was surprised to find that they don't really exist. Everyone i've spoke to from ABB didn't know anything about them even though I have a catalog showing the contactors. GE broke the molds years ago, but will still make them for about 5 grand and a 4 month lead time.
I found a few new old stock GE in the surplus section of the storeroom, so now I'm installing them. All I'm trying to do is understand the problem, as well as fix it properly.
RE: Synchronous motor field measurements.
If the field discharge resistor was erroneously replaced by a 3-ohm resistor; the induced field current could be something like 600A, assuming the rotor resistance is very small(I arbitrarily used 0.2 ohms). Then that error caused the NC contact of the field breaker to fail!
Even if the contact overlap allowed the NO contact to make before the NC contact of the field discharge part, that does not change the picture as the SCRS only fire when gated and no current can flow through the SCR circuitry relieving the NC contact current. The old setup could not experience this as the DC generator can absorb some of the current when the NO contacts of the field breaker mate.
IMHO, you can use a normal field breaker provided you don't miss the field discharge resistor value.
BTW, the ammortisseur(damper) winding of the synchronous motor determines how long the motor will pull-up to synch speed(assuming you use DOL). The field winding (with the field discharge resistor in-circuit) has little effect during starting.
RE: Synchronous motor field measurements.
In the other thread on this topic I eventually found the Telemecanique contactors I'd mentioned previously, if the info is of use at this late stage. 440V DC per pole at DC-5 rating.
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If we learn from our mistakes I'm getting a great education!
RE: Synchronous motor field measurements.
On the field discharge resistor, there is the possibility of leaving that permanently connected across the field, but the ohm value has to be about 10 times the hot field ohm value. The power dissipation of this resistor has to be considered as it will be a continuous loss. Consult the motor OEM for advice, if they still exist, that is.
regards, rasevskii
RE: Synchronous motor field measurements.
Others invited to comment here...
rasavskii
RE: Synchronous motor field measurements.
Scotty, I'm looking into those contactors now. I asked a Schneider rep about a month ago, but he had no reccomendation.
RE: Synchronous motor field measurements.
We all agree that you have zeroed-in on the cause of your troubles: field discharge resistor value was wrong at 3 ohms instead of 33.4 ohms! Other than that, I think you don't have other problems. I'd stick with the adage posted earlier by rasevskii "don't fix it if it ain't broke". But if you believe Murphy's law is so real on your side of the fence, you can secure bigger and better field breakers, it's your call.
RE: Synchronous motor field measurements.
aged. The field breaker that exploded and took out the exciter was not the one that had the miswired resistor. I've replaced 2 field breakers with the GE originals, but I'm going to need 3 more at some time.
Scotty, I looked at the telSYS, and I hope that they can configuration them for me, because those are some nice looking contactors.
RE: Synchronous motor field measurements.
RE: Synchronous motor field measurements.
RE: Synchronous motor field measurements.
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.
RE: Synchronous motor field measurements.
RE: Synchronous motor field measurements.
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.
RE: Synchronous motor field measurements.