Motor capacitor’s

Roger123 · Jan 12, 2020

Got a call today from the village I live in that one of their sewer pump motors was running high amps.
Went down to troubleshoot and I found a bad running capacitor.

Now I have questions about capacitors.

Can a running capacitor go slowly bad or is it just good or bad?

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CoolWill · Jan 12, 2020

Yes, they can go from bad to worse. Both directions. They can slowly decrease in capacitance over time and depending on temperature conditions. They can go completely open or short as well. Replacement capacitance isn't desperately important so long as the value is within a reasonable range. Voltage should be equal or greater.

Small fan motors might need 5 uF (mfd), multihorse motors a few hundred uF. Most often, if it fits, it ships. But if you have to, it's better to go up rather than down.

460 Delta · Jan 12, 2020

They can and do lose capacitance over time. I have replaced them on fans that started slow or ran noisy and all went well. I have replaced a few in motors that were hard to start with new or sometimes slightly higher capacity units in particularly difficult situations like air compressors.

drsparky · Jan 12, 2020

It's the capacitor plague!

micromind · Jan 13, 2020

As others have stated, capacitors can go partially bad.

If it's a run cap, make sure the replacement is also a run cap and not a start one. There's a difference. If a start one is in the circuit too long, it'll fail quickly (and sometimes catastrophically......lol).

MechanicalDVR · Jan 13, 2020

micromind said:
As others have stated, capacitors can go partially bad.

If it's a run cap, make sure the replacement is also a run cap and not a start one. There's a difference. If a start one is in the circuit too long, it'll fail quickly (and usually catastrophically......lol).

Fify

gpop · Jan 13, 2020

I hate caps as its one of the few things that can test good with a meter yet still be bad.

CoolWill · Jan 13, 2020

With a DMM, you have to leave the probes attached for a good while to get a real reading. The meter is charging the capacitor and a 1.5 V AA cell in the meter can only do so much. At any rate, most of the time, if I read a capacitor and it stabilizes very near it's rated capacitance, it is rarely bad. If it reaches a peak and starts to fall, it's probably bad.

frenchelectrican · Jan 13, 2020

The capaiators can go bad slow or quick .,

There is no fixed rqte when they fail and it can happened at the worst time .,,

even the capacitor can be partal bad and still run but you need to test the capacitor to make sure they are +- 10% range anything out of the range is replacement time.,,

HVAC outdoor units are worst I get the call from time to time to replace it and they do come in combo verison so be aware of it.

Roger123 · Jan 13, 2020

Thanks guys!

The motor was drawing close to 70 amps with no load. The maintenance guy pulled the motor from the well because most of the time high amps means some type of clog.

BTW, overload was set at 24 amps, that's getting changed out soon.

I check the voltage and it was in range, check for drop through the contactor and overload and found no significant voltage drop. Soon as I took the cover off the running cap I knew it was bad. Coincidentally this is the same motor that was reported to trip the breaker occasionally on start up. (another thread I started a little while ago)

New cap installed running amps when down to 12.5 under load.

Roger123 · Jan 13, 2020

OK, second capacitor question:

Most of the well pump motors are single phase and during change out I connect the capacitor wire in the pecker head which is red in color. Where does it get connected to inside the motor? Does it get connected to the armature?

frenchelectrican · Jan 13, 2020

The quick answer is if you have two wire pump the cap is at the motor head but once you get 3 wire verison then you will have a cap at the control panel which it usealy near the pressure switch loctation.

on three wire sub motors on single phase verison the red is start winding while the black is running winding and the two wire verison is combined at the motor head and it is sealed once they go to heck just replace the whole pump that is.

Just be aware of three wire pump some are three phase as well the sure fire way is ohm it out due start and run winding is different on ohm reading while three phase pump motors the ohms are pretty much the same phase to phase.

JRaef · Jan 13, 2020

Secondary issue: there is no “armature” on a Squirrel Cage AC induction motor. There is a rotor of course, but it has no direct connection to any electrical source. Current flow in the rotor bars is “induced” via the magnetic fields in the stator across the air gap.

Armature is a term for a rotor with windings on it connected by brushes or slip rings such as a DC motor, or on AC motors, a Wound Rotor motor, a Synchronous motor or a “Universal” motor (such as is used in small tools and appliances) which is actually a DC motor made to run on AC.

Motor capacitor’s

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