Motor start capacitors rated 110V being used in 240V motors and failing

[calzap]

Not the first time, I’ve found failed start capacitors rated 110V being used in 240V motors.  See pic of most recent below.  Strange thing is I get different readings on 3 different meters.  The Klein refused to give a value; just indicated it’s bad.  The Agilent gave a value at the low end of the rated range, which is 540-648 uF.  The DER EE gives a value below the rated range.  But with the leaking crud and motor refusing to start, there’s no doubt it must be replaced.

It's irritating that motor manufacturers (at least two of these have been WEGs) don’t use a 250V capacitor in a motor that can be wired for 120V or 240V.  Also irritating that the metal compartment for the 110V-rated is too small for a 250V-rated capacitor.  So, I must fabricate a new compartment too.   End of rant.

Mike

[Gyro]

That's pretty crappy underrating of a bipolar electrolytic. I suppose it's possible that the cap might only see 110V during start (the rest being dropped across the start winding) but I certainly wouldn't bank on it. That drop might be factored into the rating anyway.

Even if the value is still reading somewhere near, the ESR will have gone through the roof.


EDIT: If the motor can be wired for 240V or 110V, I wonder if it is possible to use a lower value cap in the 240V configuration.

[timeandfrequency]

Hello calzap,

I agree with Gyro : what you describe is really of bad choice due to such a blatant underrating. The capacitor failure that will result may even fry the motors' windings.

AC Service Tech LLC produces a bunch of videos about motor capacitor. Check out at least this one



... where the influence of the incorrect capacitance value is measured, and a basic formula in run conditions (60 Hz) is provided. In AC systems, many of the small to medium sized motors or compressors are PSC (Permanent Split Capacitor) motors that require an external capacitor.

As you have to replace the capacitor for 220 - 240 VAC mains, you may find these these sites/products interesting :
- Capacitor boots and also here
- Capacitor housings
- Capacitor mounting brackets
- 'Universal' capacitor for quick repair or motor testing (contains 6 different and combinable capacitance values in one part).

EDIT: If the motor can be wired for 240V or 110V, I wonder if it is possible to use a lower value cap in the 240V configuration.

You are probably right : usually the current in both windings of a PSC motor should be about the same.

[calzap]

Thanks for the info.  New capacitor should be installed 31-Oct.  As expected, the 250V capacitor will need a bigger housing.  Hopefully, motor windings are OK; the overload protector did trip and there was no burning smell.  I’ll do a post to let you know the outcome.

Mike

[timeandfrequency]

Hello calzap,

The motor might rotate firmly without load even if it suffers from a single shorted turn.
Only a ringing test...



... of each motor coil will dispel any doubts.

With a shorted turn, the motor has actually lost its' torque : you can truly stop it by hand, just with a piece of wood pushed againt the shaft.

If the motor draws an abnormal amount of current by reference of what's written on the motor plate, this means also bad news.

But of course, I hope the best for your motor.

[CaptDon]

The current is across an impedance so the E=I X Z may apply and the voltage measured across the capacitor could be only 60vac. Surely there will be transients and so forth that would seem to make a capacitor rated to at least the line voltage mandatory. I am sure the manufacturer had 'planned in' service calls and eventual replacement. Look at G.E. locomotives, they will sell you one dirt cheap and even provide the financing but they will kill you on service and parts!!! Service after the sale is where the real money is. EMD's were both easy to maintain and very cost effective overall. Now the government agencies are basically trying to do a 'cash for clunkers' program to eliminate and destroy some of the hardest working power units ever made, the GP40 and SD40 series. They are also destroying the 'head end power' gensets during the MTBA rebuilds. Some of those HEP units had about 2500 hours on the meters and looked like new. Hard working long lasting products no longer seems to fit the profit model. 'Designed to Fail' should be printed right on the equipment catalogs.

[calzap]

GE Locomotives has been owned by Wabtec, not GE, for 5 years.   Don’t know if that has made a difference in quality or pricing.  EMD has been shuffled around for 20 years, now part of Caterpillar.

Mike

[CaptDon]

Yes, I was hired by G.E. and retired from Wabtec but also ran the 140 passenger full service tour boat for the past 11 years. That equates to 70 to 80 hour weeks during the boating season! I always thought no one could screw up a business better than G.E., until Wabtec came to town! Wabtec wanted 120 day net billing and DigiKey and Mouser both said "Shop somewhere else". Pretty sad working for an engineering lab that can't buy parts. Wabtec wanted a 120 day payout for private contractors but if you would accept a 5% decrease in the pay amount they would pay quicker. B.T.W., they just got 50 million to create hydrogen powered locomotives!! There is a damn fool idea they would never try if it weren't for sucking on the green garden gnome government money pool. The LNG units never reached maturity, just bandaid after bandaid. Did you ever see how much methane is released into the atmosphere when filling an LNG tanker? Or how much 'pop vents' while the tanker is sitting in the sun? Clean real clean...not. At least methane goes up as opposed to hugging the ground until an ignition source is found. LNG tends to hug the ground until it warms up.

[johansen]

Not the first time, I’ve found failed start capacitors rated 110V being used in 240V motors. 

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. 

[calzap]

Not the first time, I’ve found failed start capacitors rated 110V being used in 240V motors. 

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.

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

[johansen]

Not the first time, I’ve found failed start capacitors rated 110V being used in 240V motors. 

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.

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

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]

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

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,

[...] 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?

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.

its not critical at all.

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.

[...] 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.

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