What would cause four 200 HP motors connected to a common MCC to overheat and fail?

[bdn2004]

There’s four 480 Volt, 200 Hp, motors that are started across the line. All are connected to a common Motor Control Center. They are protected by fuses and overloads. These motors are for pumps that are part of a larger automated system. When this failure occurred the system was in Manual Mode.

Sequence of Events:
1) The operator tried to start the first motor. Nothing happened.
2) Then he tried to start each of the other three motors. Nothing happened.
3) The fuses blew.
4) All four motors overheated and are permanently damaged

What could cause this? Why didn’t the overload relay take these circuits offline? Since this was an operator trying to start these manually – should he have waited a certain amount of time between starts?

 

[wroggent]

"Nothing happened" means the motor started successfully or the motor did not start?

Overloads weren't set correctly? Motors are in high ambient temperature? High voltage imbalance? Single phasing?

 

[waross]

How old is the system?
It may have been a missing phase and primitive O/L protection.
It may have been too many starts per hour.
It may have been a combination.
If the motors did start, a change in the piping resulting in too low back pressure will overload the motors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bdn2004]

Nothing happened = the first motor did not start. It could be the overloads were incorrect, we're checking.

This MCC is fed from a unit substation that also feeds other MCCs. And the other MCC's never experienced any problems.
It seems if there were a voltage issue, there would have been problems throughout.

The motors are outside and it's cold outside right now.

Any other ideas?

 

[wroggent]

So if the motor did not start the sequence of events was probably more like 4, 3, 1, 2, yes?

 

[itsmoked]

That one MCC was suffering a phase loss. Easy problem with intense cold and a loose connection.

Thermal overloads are marginal at detecting single-phasing (maybe 40% effective at best)
If you care about a motor or spend more than $2k on it it deserves phase-loss protection.

The already marginal overloads were freezing cold and therefore impaired in completing their dubious function.

It's cold as you say, perhaps the pumped fluid was frozen in the large surface area unprotected pump volume.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[bdn2004]

I've got more details, four motors A, B, C and D:

1) A-pump motor ran for a few minutes
2) Tried to start B-pump - it would not start, operator turned off the motor off
3) C-pump started and ran about 5 minutes (to this point, everything thought ok)
4) A weird noise
5) All Pumps stopped
6) Pump D motor was not attempted to be started during this time
7) Investigation began - Feeder breaker to the MCC tripped in the Unit Substation on Ground Fault
8) MCC Main Breaker Did Not Trip
9) Pump A motor - the fuses did not blow and it appears to be unharmed.
10) Pump B motor - two of the fuses were blown(B-phase and C-phase), motor meggers no good.
11) Pump C motor - one of the fuses were blown (C-phase), motor meggers no good.
12) Pump D motor - one of the fuses were blown (B-phase), motor meggers no good.

 

[itsmoked]

Single phasing still the most likely. 'Running' for a couple of minutes is classic single-phasing failure.

Not clear why the feeder GFI waited on three motors before tapping-out. Are all three motors really megging fried or are two megging questionable and one bad - the one that tripped the feeder?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

The incoming supply to the common bus lost a phase. Check the cables, cable terminations, fuses, main breaker etc. for a break in connection.

Muthu

http://www.edison.co.in

 

[jraef]

Keith is right, classic single phasing behavior. I’m thinking a high resistance arcing ground fault on the circuit feeding the MCC. The resistance was too high to allow enough ground current flow to trip the incoming OCPD (at first), but caused a severe voltage drop on one phase of the MCC which allowed motors that were already spinning to keep going for a while, but not start from a stop. Eventually the damage widened.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LionelHutz]

What make and model of overload relay is used on the motors?

 

[Arentsch]

These motors are all outside and are driving vertical pumps for a cooling tower. Each motor is fed with 3-350cu. in 2-1/2" Rigid Steel conduit. Each conduit is routed overhead on pipe racks 20+ feet above ground and into a building that houses the MCC. Is it possible that these motors were damaged by a lightning strike to the pipe racks that in turn sent transient voltage to the stator windings? The windings could have been shorted prior to the operator's attempt to start resulting in single phasing and other overtemp events. I checked and the motors have no surge protection. I also checked and found that several lightning strikes were recorded in the days prior to this event. This would explain the simultaneous failures of these motors. Is this a plausible deduction?

 

[jraef]

You have two different forum handles?

Once lightning is involved, all possibilities are on the table...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

I agree with jraef, with lightning lot of scenarios are possible including the one you suggest but often one no one even guesses.

The best protection may be lightning rod type protection as really nothing works perfectly on direct strikes.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Don't the cooling towers have their own lightning protection? I doubt it stuck all the motors at about the same time.

Muthu

http://www.edison.co.in

 

[CuriousElectron]

Does this mean the existing protection failed to protect against loss of phase ? With loss of 1 phase, motor draws more current to compensate for loss of 1/3 of input power, overloads take a a few seconds to trip on overload, but by the time they break the circuit the damage has already been done.

Thanks,
EE

 

[jraef]

With loss of 1 phase, motor draws more current to compensate for loss of 1/3 of input power, overloads take a a few seconds to trip on overload, but by the time they break the circuit the damage has already been done.

Only if the motor is fully (or near fully) loaded. If the load on the motor is low, the current on a motor ALREADY running will increase under a single phasing situation, but it may not increase to the point of tripping the motor. That's the validity of the question on what TYPE of overload relay they have. Older NEMA style overload relays would NOT trip just because the phase was lost. IEC overloads, despite being MARKETED as providing phase loss protection, still MIGHT not trip if the load is low. The IEC concept used on bi-metal OL relays just provides a biasing of the trip point to be lower than normal in order to compensate for the added heating effect in the motor from running on single phase. But again, it MIGHT run forever if the load is low enough (albeit without damaging the motor if that were the case).

That's why I have switched to using Solid State OL relays not that provide TRUE phase loss protection.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[racookpe1978]

Are the pumps centrifugal or positive displacement (near-certain they'd be centrifugal as water pumps to a cooling water (low pressure, high flow). Did you verify ALL pumps and ALL valves are correctly lined up and ALL water suction lines and screens are clear?

Are ALL pumps properly rotating (no jams, no ice, no vacuums or clogs). No impeller damage or casing/bearing problems?