Electronics > Repair
Motor start capacitors rated 110V being used in 240V motors and failing
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johansen:

--- Quote from: calzap on October 31, 2024, 12:54:24 am ---
--- Quote from: johansen on October 30, 2024, 09:04:11 pm ---
--- Quote from: calzap on October 28, 2024, 07:38:56 pm ---Not the first time, I’ve found failed start capacitors rated 110V being used in 240V motors. 

--- End quote ---

the reason they can do this is because the capacitor doesn't connect to the 240v line, it is permanently connected to the midpoint of the two 120vac windings, which are in series for 240 and parallel for 120.

such 5 wire motors are not reversible on 120vac but when connected for 240 you can reverse the motor by connecting the start switch from line 1 to line 2.

--- End quote ---
Yeah, I know.  But somehow it doesn’t work.  They last a couple of years before failing in pump motors that have many starts per day.   Maybe there’s a start or stop surge that exceeds their rating.  These motors have voltage monitors to protect them from too high or low from the utility company, so that’s not the problem.

Mike

--- End quote ---

Several starts per day will do it
calzap:
Replacement capacitor arrived 2-Nov, 3 days late.  Installed today.  Motor started to turn over slowly, then overload protector in controller tripped.  Although there is no burnt smell, my guess is that the insulation on some of the start winding wires had melted, and the conductors are touching.  Will do a teardown to check.

Installed new motor and pump.  Looked at the start capacitor.  And yep, just like before, rated 125 VAC for a motor operating at 240 VAC.  Unusual capacitance though, 475-650 uF, made by NGM.  When I decode the model number using NGM’s catalog, it decodes to a minimum of 500 uF.  Would like to replace it with one rated 250 VAC.  Haven’t been able to find one of that capacitance at any voltage.  Any opinion on whether a 540-648 uF capacitor, which is common, would be OK to use?

Mike

johansen:

--- Quote from: calzap on November 05, 2024, 02:15:06 am ---Replacement capacitor arrived 2-Nov, 3 days late.  Installed today.  Motor started to turn over slowly, then overload protector in controller tripped.  Although there is no burnt smell, my guess is that the insulation on some of the start winding wires had melted, and the conductors are touching.  Will do a teardown to check.

Installed new motor and pump.  Looked at the start capacitor.  And yep, just like before, rated 125 VAC for a motor operating at 240 VAC.  Unusual capacitance though, 475-650 uF, made by NGM.  When I decode the model number using NGM’s catalog, it decodes to a minimum of 500 uF.  Would like to replace it with one rated 250 VAC.  Haven’t been able to find one of that capacitance at any voltage.  Any opinion on whether a 540-648 uF capacitor, which is common, would be OK to use?

Mike

--- End quote ---

its not critical at all.

and you're right no one make s 250vac caps that large because its not really physically possible to fit them into the standard frame size, so they just parallel lots of smaller ones. i've seen RPC panels which look like they have 10 soda can sized start caps in parallel.

if the old bad motor is bad.. once you get it up and running yeah it will smoke those shorted turns. but at startup, those shorted turns don't have much current flowing in them and they severely reduce the starting torque. 

you could probably double the stock starting capacitance before you run into trouble where the increased thermal heat into the start winding;
 exceeds the reduction in heat in the start winding because the capacitor is energized for a shorter time.
timeandfrequency:
Hello calzap,


--- Quote from: calzap on November 05, 2024, 02:15:06 am ---[...] it decodes to a minimum of 500 uF.  Would like to replace it with one rated 250 VAC.  Haven’t been able to find one of that capacitance at any voltage.  Any opinion on whether a 540-648 uF capacitor, which is common, would be OK to use?
--- End quote ---
Such a huge value is usually used as starting capacitor, which is disconnected a few seconds after motor start.  Do you actually need a start or a run capacitor ?
Here is a cheapy : 500 µF 250 VAC start capacitor. Do not use as run capacitor.


--- Quote from: johansen on November 05, 2024, 03:20:30 am ---its not critical at all.
--- End quote ---
For a start capacitor, you're right. But if calzap's issue is about the run capacitor, I would not bet on that : main and aux coils currents should be about the same value and the actual run capacitor value is therefore important.


--- Quote from: calzap on November 05, 2024, 02:15:06 am ---[...] Motor started to turn over slowly, then overload protector in controller tripped. Although there is no burnt smell, my guess is that the insulation on some of the start winding wires had melted, and the conductors are touching.  Will do a teardown to check.
--- End quote ---
Hm, the defective underrated capacitor seems to have spawn additionnal damage :( It would be interesting to know if a non-destructive teardown is sufficient to locate the failure.
In some cases, you don't see or smell anything, but newertheless you have a single shorted turn or maybe as you guessed, an insulation issue.
As suggested above, making a ringing test or measuring the motor's current can sometimes be more reliable than visual/smell inspection. If you have an identical replacement motor, you may also measure/compare the coil's inductance.
I imagine that a megohmmeter(*) used for insulation testing has already its' place in your toolbox, so this is an easy test to do.


(*) Top notch megohmmeter for industrial environment :
- Fluke 1507
- Fluke 1587 FC
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