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Pump motor postmortem

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calzap:
The pump is a two-stage centrifugal pump and was powered by a 3 HP, 240 VAC, 60 Hz, 1 Ph motor running at 3450 PRM.  It had been in use for 10 years, used to keep water pressurized in tanks, and controlled by a pressure switch.  It had over 6000 hours on it, but I still consider it a premature failure.  Just before failure, it made a bit of noise, which increased suddenly with a small amount smoke seen before it was shut down.

The pictures below show a roasted motor with giblets.  Main course is burnt stator windings, followed by a melted fan blade, and finish with a thoroughly baked run capacitor.

Run capacitor is same brand as motor ... WEG.  Start capacitor is a Lorenzetti and tests OK.  So, what failed first?  Probably not the stator coils.  If the insulation on them had failed first, I would have expected to find a small area that was severely burned, maybe with some melted metal, with progressively less scorching further out.  Instead, it appears that there was relatively uniform heating with little or no arcing between conductors.  My vote is for the fan blade.  If the fan blade fails, everything overheats including the coils and capacitors, which toast and die.  The fan blade was nylon and force-fitted to the motor shaft.  My guess is that it started losing its grip on the shaft which started things heating, including the shaft, which caused more slippage, and so on.

For a motor/pump combo that costs US$1000-2000, would it be asking too much for a decent run capacitor and a metal fan blade that is solidly attached to the shaft?  Maybe so.  And a thermal fuse  ... how much would that have cost?  The motor bearings were in good shape, and the pump is fine.

One other gripe is the motor shaft is extended out so that it is also the pump shaft.  This saved some money because the manufacturer didn't have to buy a coupler or a frame capable of holding a separate motor and pump.  However, it makes the motor unique to the pump.  Unless I want to saw the shaft from the motor, buy a coupler and build a frame, I'm stuck buying the same motor to run the pump.

Mike in California

Gregg:
Sometimes you get what you pay for, i.e. cheaper product = cut corners (often times because bean counters can’t find the funds until things fall apart and then funding suddenly appears out of nowhere).   |O
The motor doesn’t look like it has been in ideal operating conditions or it wasn’t designed for the conditions where it was placed.  Note the rust on the laminations and dirt inside.  A totally enclosed fan cooled motor would be much better.  How are the bearings?  I’d bet they are shot like the rest of the motor and may have been the main cause of death.  Bad bearings may have helped the cheap plastic fan to spin on the shaft and no longer provided proper cooling.  But it doesn’t look like the rotor crashed into the stator.
Was the pump run continuously or were there a lot of start / stop cycles?

calzap:
Pump was use for irrigation and operated to keep pressure tanks charged, so operated by a pressure switch.  Lots of on/off cycles 7-8 mo/yr; not used during rainy season.  Actually had excellent environmental conditions.  Was completely indoors in a pump house and run only in the dry season.  There were no pipes or valves running above the motor.  But 10 years is a long time, and I would expect any non-stainless, non-protected steel to rust in that time.  Huge amount of air would have been blown through it in 6000 hours of operation accounting for dust and dirt inside.

Mike in California

andy3055:
You forgot an important part. The centrifugal switch on the motor shaft that disconnects the start capacitor from the circuit after speed is up. Most often those fail due to continuous on/off operation. If the switch was stuck in the off position or pitted so bad that it does not make contact, the next time around when the motor has to start, the start winding is off circuit and the run winding will be getting hot as the motor does not spin.

If you say the run cap has failed, it could also be that the run cap just failed due to age. In the 10 years of operation, its life span would have been already finished.

The pump may also need replacement eventually. A better bet would be to buy a motor and pump that are externally coupled so that only one part needs replacing anytime. Ultimately, it is about the $$.

calzap:
Good point on the centrifugal switch.  I'll inspect it before tossing the motor.  The pump metal parts are in great shape.  Case is cast iron and vanes are stainless.  I'm impressed with the pump, but think they could have chosen a better motor to attach to it.

There is a rebuild kit for the seals and gaskets on the pump.  I may go ahead and cut the shaft from the motor, buy a better motor and coupler, build a frame, and get the pump going again.  There is already a replacement pump/motor in operation, but we like to keep a spare ready to go.

Mike in California

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