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Synchronous motor field measurements. 5

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Thedroid

Electrical
May 18, 2008
196
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.

 
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Well, you don't really want to leave the field open circuit with power applied to the stator. Otherwise, the rotor will produce a very high voltage.

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.
 
I'm pretty sure that AB doesn't make one, and there are very few that have the overlap of the contacts. I found one in an obscure ABB catalog, and had trouble getting any information other than made in France. The one that was just installed, is actually a new in box contactor made by GE specifically for the task. It was sitting on the shelf as surplus, because the originals were removed from service years ago. The main contacts are rated for 100A continuous and just the NC is rated at 15A continuous and 80A for one minute. All three poled have nice magnetic blowout coils, and healthy size contacts. Heres a copy of the instructions for the contactor.

 
 http://files.engineering.com/getfile.aspx?folder=ae8b4cd0-b7ae-4d19-b19f-e4933b859881&file=GE_Field_Contactor_Instructions.pdf
Sorry, I meant ABB has DC contactors.

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".
 
Finished installing the new contactor today and still have some interesting readings. I can't figure out what the value of AC induced voltage from the field is during startup. I measure 3A on this motor, and on an identical setup 55A. We checked the field with a megger, and thought that the values looked suspect. 500V test showed 500 megohms, while the field wouldn't take a 1000V test. The pedestal bearing had leaked oil on the brush gear, and there was signs of possible flashover. We thought that this was the issue, and the AC induced voltage was going to ground. We cleaned up everything and still have the same result. The motor starts just fine, but I'd like a better understanding of whats going on. I was told that I shouldn't megger the field at anything over 250V, but this doesn't make sense to me. I would think that the builder would insulate the field in anticipation of higher AC induced voltage.

 
Use ohms law. The resistance of the discharge circuit multiplied by the current when the circuit is interrupted will give you the maximum induced "Kick" voltage. If you have megohms and no discharge path you can expect very high voltages. The arcing as the contactor opens will drop the voltage quite a bit below the calculated values, but not always enough to prevent serious damage. The inductive "Kick" voltage on a field without proper discharge components is probably a much higher voltage than induced AC voltages.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
You are really asking for trouble if you megger the field at 1000V. 500 V is the norm for DC fields, be they rotating field windings or stationary such as on exciters and DC machines, generally. Why not just put in the tried and true GE field breakers and be done with it. Are you sure GE does not supply these anymore?

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
 
I have installed the original breakers on both of these motors, and the discharge resistor is wired in properly. I'm just trying to make sense of the different readings. The motor that reads 3A had the discharge resistor hooked up wrong for the last 6 yrs 3 ohms instead of 33.

I'm checking the current with a clampmeter set for AC on the resistor circuit.

 
I took more measurements on both motors Friday. Both field discharge resistor circuits measured about 57A through 33.4 ohms. This gives me 1903V. If this is the case, why can't I test the insulation resistance over 500V?

 
If everything is now working properly, why keep messing around with it? Go right ahead and megger at 1000V. You might be looking for another job afterward. Don't fix it if it ain't broke. Just some advice...

rasevskii
 
Perhaps you just had switching sequence errors that's why your field breaker got damaged. On start-up (whatever your method of starting is, softstarter was mentioned), you will only close your field exciter at around 98% of speed (or your will have problems when you close excitation earlier). Then you can load up. You just reverse the sequence when stopping the synch motor, i.e. unload first before you put the equipment off. Contact wear is very common with DC contacts, all you need is to maintain or replace the contacts as well as clean the arc chutes to prevent flash overs. Rough contact surfaces hasten pitting of the surface, and our experience is to just to burnish them contacts with very fine grain paper!
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.
 
Well, it was broke. We have 5 of these motors that were updated about 6 yrs ago with new contactors that had no overlap. All of these contactors are in very bad shape, and one has exploded and taken an expensive exciter with it. Contact maintenance is not the concern, because the contacts themselves are in great shape, it is the barriers and permanent magnets seperating the poles that are being destroyed by the excess current flowing because of the lack of overlap.

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.

 
From what I understood, you took measurements at starting. You got 57A through the 33.4-ohm field discharge resistor(momentary). Since the field discharge resistor is in-circuit during start up, the NC contact of the field breaker did carry 57A for a portion of the ramp-up time(10 seconds).

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.
 
theDroid,

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!
 
Try Impulse NC of North Carolina, now part of Reuel, Inc. They are using a lot of DC switchgear for Traction substations. Also Cableform makes a line of DC contactors, mainly for mining locomotives and streetcars. However specific designs incorporating a field discharge NC contact may not be part of the available catalog items.

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
 
If you were to go for the permanently connected resistor, that would have to be a bank of resistors located in a dedicated ventilated enclosure, for which you may or may not have space. Based on 6.6 A /one tenth of the field current/ that would dissipate about 1650W continuously. Size it for 4 or 5 KW. Located at the motor, not the substation, would be better.

Others invited to comment here...

rasavskii
 
I didn't think about the SCR's having any affect on the current. We don't change the gating on the SCR's at startup or shutdown, we just allow field voltage to float while the motor is down. Even with the SCR's, wouldn't the L/R cause troubles with the contactor, if there was no overlap of the contacts?

Scotty, I'm looking into those contactors now. I asked a Schneider rep about a month ago, but he had no reccomendation.

 
Theroid,
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.
 
The confusing thing is that all 5 field breakers that have no overlap were on their last leg. The one with the 3 ohm resistor had the NC pole damaged, and the other 4 had the NO poles dam
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.

 
The GE field breakers are looking real good. I'm waiting for a price for the Telemecanique option so we can compare them with the old GE's. They look like really nice contactors.

 
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