We have two 600 HP , 4160 V, motors (condensate pumps) that have had internal shorts in the past six months. The pumps are four years old and tested and maintained each year. The pumps are 100% capacity and are tested and changed each week. Is a weekly starts adversly shortining the life of the motor? If testing and changing is needed for reliability is a montly run better? The motors are outside in but under a shelter. The data shows no exessive vibration, normal RTD temps, and no other signs that a failure was coming.

Chris welcome, I want to introduce you to our mega discussion:

http://maintenanceforums.com/eve/forums/a/tpc/f/209103451/m/3621013522/p/1

called Best strategy
where the effects of duty x standby is discussed

We like to know to which camp you adere based on your personal experience

I never run 50/50 as I don't want to reach 0 at the same time. Generally 60 lead 40% lag kind of a deal for 2 pumps. If you're doing it by hand then weekly is OK unless remote but computer is better or PLC, etc... 50/30/20 is OK for 3 pmps;;;;;;;etc...you get the idea

50-50, switching each week, means 26 starts/stop per pump, every year. If in that week you are also testing, aside other things, then this number of starts/stops will climb rapidly.

Do you have problems with mechanical seals?

Steven,

There have been to date no problems with the pumps. It is the motors only that have failed. The pumps operate under a vacuum of .143 bar and the water quality is demin grade. This is a a CCGT project in Mexico.

Steven,

Thanks for the welcome, I need a little time to see to which camps are avaliable based on the choices, looks very good.

Cheers,
Chris

quote:

Originally posted by Sam Pickens:
I never run 50/50 as I don't want to reach 0 at the same time. Generally 60 lead 40% lag kind of a deal for 2 pumps. If you're doing it by hand then weekly is OK unless remote but computer is better or PLC, etc... 50/30/20 is OK for 3 pmps;;;;;;;etc...you get the idea

Thanks

Could you elaborate more on the topic failed?
What, how, when than we can come to a possible why

Steven,

The pumps were commissioned in late 2001 and went into weekly maintenance swapping, auto start standby pump as is normal in the power industry. Each year the pump is visually checked (lift checks), the coupling is checked and motor maintenance is performed. Motor works are meager checks,electrical connections inspected and tightened as needed. Routine oil testing is done, and a on-line monitoring with a multi-line and Bentley vibration monitoring. Data is archived in OSIsoft PI. All readings have been flat with no adverse tends have been identifed. At the four year mark the "A" pump motor is pulled and set to a local shop for cleaning and inspection. The pump motor was brought back and placed into service in Jan 06. In March 06 the "B" pump motor shorted out, cause was a scratch on stator bar causing short. Motor "B" sent to shop for re-wind and came back to site last week. On July 4 "B" pump was tested and was placed back into service. Two hours later on RTD opened and caused the pump motor to trip. The "A" pump auto started and shorted out 1 min into service. The "B" pump and motor was checked the spare RTD put in service and the pump was put back into service. The "A" motor just went to shop for disassembly and repair. We do not know where the fault is yet. This is to random of a failure and there must be a process to prevent future like kind faults. Sorry for the ramble but to get to why.


Cheers,

Chris

Chris:

would need to see photos of the failure and information on the actual duty cycle, operating environment and power conditions (ie: from the multilin). The pictures might suffice. Do you have photos of the actual failures and the operating area that can be posted?

Howard

The mega discussion mentioned one of the advantages of the duty/standby policy over the periodical pump swapping is "to avoid Fail to start". If your motor experiences internal short frequently, doesn't it mean fail to start after many start/stop cycles?

Would you conclude that the motors failed after 4 years of operation, or did you had other (minor) failures in this period?

Could it be that another event outside the electric motors caused the failures?

Sorry, I am a mechanical guy, but with MotorDoc around you are in good hands

Chris,

Every start causes a large current surge, as you and other readers are aware. These can cause rotor bar or insulation faults eventually, so the fewer the number of starts over the life of the motor, the better. However, 4 years life with weekly starts seems very low indeed.

Have you been able to rule out external influences? Let us see - poor(read cheap) design, spikes in power supply waveform, high humidity, non-uniform air gap, wrong voltage, rain water ingress etc.

The findings of the repair shop on the B motor will give us more information. Experts like Howard will be able to point you the right way if you provide the as-found report, photos etc.