Breaker Charging Motor, Major Voltage Dip

[MD88]

Hey guys, I'm having a problem that I just can't figure out. We have a fairly new 5kV switchgear lineup from Powell. We had the whole thing site tested to NETA standards back in April and everything was fine. One feeder wasn't ready so we didn't energize it until last week... when we closed the breaker, all the relays and lights shut off, only to come back on a few seconds later! Through doing it a few times, we figured out that once the breaker closes and the charging motor starts up, the voltage on the 125VDC control circuit (fed from a charger and battery stack) is dipping severely up until the point that the spring is charged, then it goes back up to 125VDC. The next time, we watched the screen on the charger... the voltage went from 125VDC to 10, then 7, then 3, then as soon as the motor was wound up, went right back to 125VDC. We figured it was something with the motor (either a resistive short or a partially seized shaft), so we took the breaker out and put in a spare we had... exact same thing!! Does anyone have any ideas??

Thanks

 

[templdl]

Personally I would load test the 125dc powersupply first. It may not be able to provide the current necessary to power the charging motor and the voltage drops severely as a result.
The charging motor as I recall is a basic universal motor similar to a drill motor.

 

[zog]

Hey guys, I'm having a problem that I just can't figure out. We have a fairly new 5kV switchgear lineup from Powell. We had the whole thing site tested to NETA standards back in April and everything was fine. One feeder wasn't ready so we didn't energize it until last week... when we closed the breaker, all the relays and lights shut off, only to come back on a few seconds later! Through doing it a few times, we figured out that once the breaker closes and the charging motor starts up, the voltage on the 125VDC control circuit (fed from a charger and battery stack) is dipping severely up until the point that the spring is charged, then it goes back up to 125VDC. The next time, we watched the screen on the charger... the voltage went from 125VDC to 10, then 7, then 3, then as soon as the motor was wound up, went right back to 125VDC. We figured it was something with the motor (either a resistive short or a partially seized shaft), so we took the breaker out and put in a spare we had... exact same thing!! Does anyone have any ideas??

Thanks

The motors are not universial, but I assume you verified the ratings on your motor are correct. Is it just this breaker that is causing the dip or is it all of them?

NETA standards do not require electrically charging the breaker (They don't require a lot of things that should be checked but that is a whole different topic). NETA only requires an insulation test on the motor.

Typically for NETA testing they will manually charge the breaker on the floor, there is no trip testing required per NETA for a MV breaker, just VCB integrity, contact resistance, bus connections, and insulation tests. It sounds like a charging gear lubrication issue, have you tried manually charging the breaker and comparing the effort it takes compared to other breakers? I would also ask if any spray lubricants were used, they will make everything work great at the time of testing but actually displace the real grease and cause failures in the mechanism after a few weeks, never let anyone use "Technition in a can" on your breakers.

 

[zog]

MD88, you ever figure this out?

 

[MD88]

Not yet... we haven't had a chance to shut down any areas of the plant to test any breakers yet. I also thought it was a motor lubrication issue, but the chances of 2 breakers which were stored in separate rooms having the exact same issue seems pretty slim.

 

[kingpb]

Are you saying this happens when any of the breakers in the line-up are charging, or is it only this breaker cell?

Do you have a copy of the DC battery/charger duty cycle to make sure the motor charging was accounted for in the charger design, especially with everything else energized.

It will be interesting to see what Powell comes up with, I assume you have engaged them in the fix.

 

[zog]

Not yet... we haven't had a chance to shut down any areas of the plant to test any breakers yet. I also thought it was a motor lubrication issue, but the chances of 2 breakers which were stored in separate rooms having the exact same issue seems pretty slim.

Not a motor lubrication issue, a charging mechanism lubrication issue. And I don't see the chances as slim at all, stored in the same place, likely had the same "PM's" done to them at some point. If spray lubes were used by the testing company and then they sat idle for a period of time I would say the odds are pretty good.

 

[brian john]

Check the battery
Check the charger.
Measure voltage at the battery with the motor operating
If the battery voltage is dipping that low, you have a bad battery pr too small a battery.

If battery voltage does not dip measure current while operating the motor. Is the current proper for the charging motor? If not motor issue. If so check conductor connections.

 

[SG-1]

I am suprised that the volt dip as measured at the charger did not stall out the motor. I do not think the voltage at the motor terminals could have been as low as reported by the charger.

 

[zog]

I am suprised that the volt dip as measured at the charger did not stall out the motor. I do not think the voltage at the motor terminals could have been as low as reported by the charger.

You are right, we recently did a series of charging motor tests for this exact thing for Exelon Nuclear, burned up a few charging motors during the testing too.

 

[templdl]

The Charging motors that I'm Familiar with are basicallyu a drill motor chasis that has brushes. The Darned things actually look like an unpainted drill motor. They are used because of their high torque and can be run on AC or DC which makes them a universal motor. In This case they run on a DC supply.

 

[jayelvington]

The inrush at 125VDC is just shy of 9 amps and the current draw during spring charging looks to be just shy of 5 amps on that particular motor. That is based on a sample of one that I did a while back. I have curves for 100VDC, 105, 125 and 140VDC.

Jay Yelvington

 

[Open Neutral]

a) Measure the voltage at the motor.
b) Also measure the voltage at the battery bank.

Compare and contrast.

 

[wirenut3]

Are these motors fed from a capacitor charging box? Does the box have a way to discharge the capacitor for service means? One type I worked on you would remove a screw on the cover and that would discharge the capacitor. If you left the cover opened the device wouldn't charge up.

 

[zog]

Are these motors fed from a capacitor charging box? Does the box have a way to discharge the capacitor for service means? One type I worked on you would remove a screw on the cover and that would discharge the capacitor. If you left the cover opened the device wouldn't charge up.

I have never seen a cap feed a motor, trip coils yes, motor no.

 

[templdl]

Cap trip devices do not have anywhere near enough VA to run a motor. I agree with Zog that the CTD is used to trip a shunt trip when power is no longer present to activate the ST. The CTD often is not capable of storing energy for a great deal of time and as such the ST muct be actvated within a relatively short time after a power failure.

 

[ATSman]

Westinghouse

Westinghouse

The Charging motors that I'm Familiar with are basicallyu a drill motor chasis that has brushes. The Darned things actually look like an unpainted drill motor. They are used because of their high torque and can be run on AC or DC which makes them a universal motor. In This case they run on a DC supply.

Westinghouse, Cutler-Hammer, now Eaton use a drill motor (yes they are universal) made by B&D with a modified shaft on their MV VCP-W vacuum breakers. They were also used in the LV DS breaker line. Not sure what the newer Eaton Magnum LV breakers use.

 

[mull982]

I am suprised that the volt dip as measured at the charger did not stall out the motor. I do not think the voltage at the motor terminals could have been as low as reported by the charger.

I agree the only thing that could cause the voltage to drop this low at the charger would be a high impedance in the charger ( loose connection) or too much current draw on the charger that cause too much of a voltage drop across the internal impedance of the charger.

You are right, we recently did a series of charging motor tests for this exact thing for Exelon Nuclear, burned up a few charging motors during the testing too.

I'm guessing the motors burned up due to the fact that tye were stalling. If the motors were drawing too much current this current could cause an excessive voltage drop across the internal impedance of the charger and thus a voltage drop at the motors themselves. Do you think the motors were burning up because they had too much current in their windings, or not enough torque to move their load?

 

[zog]

I'm guessing the motors burned up due to the fact that tye were stalling. If the motors were drawing too much current this current could cause an excessive voltage drop across the internal impedance of the charger and thus a voltage drop at the motors themselves. Do you think the motors were burning up because they had too much current in their windings, or not enough torque to move their load?

Actually it was amazing how low of voltages charged the breaker, charging time increaced when voltage decreased and current rose as you would expect but these were pretty tough little motors. It was an interesting week of testing.

 

[mbeatty]

The motors are not universial, but I assume you verified the ratings on your motor are correct. Is it just this breaker that is causing the dip or is it all of them?

NETA standards do not require electrically charging the breaker (They don't require a lot of things that should be checked but that is a whole different topic). NETA only requires an insulation test on the motor.

Typically for NETA testing they will manually charge the breaker on the floor, there is no trip testing required per NETA for a MV breaker, just VCB integrity, contact resistance, bus connections, and insulation tests. It sounds like a charging gear lubrication issue, have you tried manually charging the breaker and comparing the effort it takes compared to other breakers? I would also ask if any spray lubricants were used, they will make everything work great at the time of testing but actually displace the real grease and cause failures in the mechanism after a few weeks, never let anyone use "Technition in a can" on your breakers.

Excellent point. I've seen this too many times to count. :happyno: