# Synchronous Motor Capacitor Sizing

I am troubleshooting a 1967 GE synchronous motor used to run a reciprocating compressor. It is a 1000 HP machine. I am trying to figure out the size and purpose of the capacitor connected to the DC field. Is it there to absorb voltage fluctuations?

The capacitor has burnt up and can't be tested (arcing destroyed it)

Nameplate data:
KVA 800 Horsepower 1000 Volts 2300 Amp 200 RPM 225 cycles 60 Phase 3 PF 1.0
Excitation Volts 250 Amp 79

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jim hardy
Gold Member
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Capacitor across the field? I'd have expected either a "field discharge resistor" or a surge arrestor.

https://www.gedigitalenergy.com/products/manuals/spm/spmman-a9.pdf
Shunting a resistor around the motor field during starting is accomplished with a field contactor. Optimum application of excitation (that is, closing the field contactor) requires accurate sensing of motor speed and rotor angle. This SPM provides this function. Optimum speed for pull-in varies with motor design and with the field discharge resistor value

Can you post a photo of the capacitor ? Blowing up such parts suggests the amortisseur windings could need attention.

http://www.wmea.net/Technical Papers/May-2009-Sync-Motor-Excitation-a.pdf
Open circuit fields during start creates high voltages [10,000 volts or more] – damage to fields, slip rings!
Either a short circuit or a resistor should be used during start.
Using an optimal resistor can give 30-50% more start torque

“Thyrite” voltage surge protectors act as backup to resistors and contactors across the fields

good luck !

old jim

The field current is probably rectified from an AC source. The capacitor is probably for smoothing the DC field current after it is rectified. These are just guesses based on other applications I've seen. More information would help.

What excites the field? Are there any other big Caps in that circuit?

jim hardy
Gold Member
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The capacitor is probably for smoothing the DC field current after it is rectified.

I've wracked my brain and cannot figure out why anybody would put a capacitor of any size across the inductive field of a synchronous machine. It opens the door to subsynchronous resonance troubles.

I hope we see a picture of the gizmo. I'd expect a selenium surge suppressor.
http://www.deantechnology.com/pg_common/DisplPDF.aspx?ItemIndex=13&DbTable=techinfo [Broken]

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I've wracked my brain and cannot figure out why anybody would put a capacitor of any size across the inductive field of a synchronous machine.

I think you are correct and now I can't remember what I had in mind...

More Investigation

I may have been leading you in the wrong direction(as i was), it appears to be a shaft grounding system. The brush being directly on the shaft before the compressor, connected to the resistor?, then to ground through the motor casing.

I think I'm looking for resistor sizing, thanks for any help.

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jim hardy
Gold Member
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Here's an educational article on shaft grounding.
http://www.marubun.co.jp/product/measurement/electric/qgc18e000002i83w-att/nip_understanding_803.pdf [Broken]

Shaft voltage is normally modest, the grounding brush is basically just a static drain.

Why'd it blow?
Was there some sort of event on this machine?
Centrifugal shorts are really hard to find because they go away when the machine slows down.

I'd replace it temporarily with a 240 volt light bulb , around 200 watts . If it illuminates there's trouble enough that it should be easy to find. There should be not more than a very few volts on the shaft when it's connected through such a low impedance. But the lamp won't explode if it sees full exciter voltage.(unless the field breaker trips).

Meantime maybe somebody will find the instruction book for that device. Could it be a capacitor to suppress RF from brush arcing?

Are both bearing housings insulated or just one?

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The brush assembly was not being maintained properly and there was some major arcing. This is the only brush directly on the shaft. I like the light bulb idea!

jim hardy
Gold Member
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' major arcing' ?

Shaft voltage arises from magnetic imbalance in the machine. Flux that's asymmetric around the shaft couples it so current tries to circulate along shaft , through bearings and back via motor frame.
A couple volts is typical open circuit . If neither bearing is insulated a very few amps might flow depending on imbalance.

Above around 500 horsepower machines are big enough it's necessary to insulate one bearing to break the circuit. Shaft current will pit journal bearings. It makes ball bearings acquire stripes so regular that your first impression is "What in the world put those raccoon tail stripes on that bearing race ? " Of course when the bearing fails catastrophically it wipes out the evidence. The natural reaction then is to mutter "Galling" and try a different grease.

When VFD's became commonplace , shaft current troubles moved into the world of small motors. So there's a LOT of information out there now much of it scholarly. Thirty five years ago it was mysterious.. i wracked my [edit:[STRIKE]brain[/STRIKE] ]alleged brain over a pair of 450KVA machines that intermittently ate bearings. They weren't insulated..

If you have significant arcing it suggests to me you might have trouble beyond a dirty brush. Maybe the insulated bearing is no longer insulated ?
Is something elevating that shaft voltage well above ground ?
It's worth looking at some simple things . Light bulb tells you something even if it stays dark.

How's the vibration on that machine? Does it vary with excitation? That'd be a symptom of shorted rotor turns which cause magnetic imbalance - you have less mmf hence less flux at slots with a shorted turn. Those can be centrifugal and if bad enough can ground the field....
These guys' Dad literally 'wrote the book' on diagnosing shorted rotor turns in-situ.
Red trace is slot by slot mmf.

http://www.generatortech.com/

Good luck - keep us posted !

old jim

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Hmmm, peel it open and let's see what's inside!

It is a capacitor, we were finally able to shut down a similar machine and it tested and 6.8 uf. We cleaned up the leads on the "bad" one and it tested the same.

Question is, why a capacitor to ground on a shaft grounding system?

jim hardy
Gold Member
Dearly Missed
It is a capacitor, we were finally able to shut down a similar machine and it tested and 6.8 uf. We cleaned up the leads on the "bad" one and it tested the same.

Question is, why a capacitor to ground on a shaft grounding system?

my GUESS is it's a high frequency bypass to keep RF caused by sparking at the grounding brush from propagating on down the wires to whatever (if any) shaft voltage monitoring system is there.

You see a similar application on old brush style car generators. It keeps brush static out of the radio.

An old fashioned pocket transistor radio is useful for keeping an eye on big motors. Set it to AM and tune between stations. Walk around the motor listening to the radio. You'll get used to how radio-noisy it is . A change is an early warning that you are developing arcing or corona . Try it on rainy vs dry days... search on "Partial Discharge Monitoring"

old jim