Motor start capacitors rated 110V being used in 240V motors and failing - Page 1
Forum section: electronics / repair

author: calzap
author_rank: Frequent Contributor
timestamp: October 28, 2024, 07:38:56 pm
content: 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

author: Gyro
author_rank: Super Contributor
timestamp: October 28, 2024, 09:15:26 pm
content: 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.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: October 29, 2024, 08:11:45 am
content: 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 abunch of videosabout 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 bootsand alsohere-Capacitor housings-Capacitor mounting brackets-'Universal' capacitorfor quick repair or motor testing (contains 6 different and combinable capacitance values in one part).Quote from: Gyro on October 28, 2024, 09:15:26 pmYou are probably right : usually the current in both windings of a PSC motor should be about the same.

author: calzap
author_rank: Frequent Contributor
timestamp: October 29, 2024, 11:58:45 pm
content: 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

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: October 30, 2024, 07:03:07 am
content: Hello calzap,The motor might rotate firmly without load even if it suffers from asingle 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.

author: CaptDon
author_rank: Super Contributor
timestamp: October 30, 2024, 12:43:46 pm
content: 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.

author: calzap
author_rank: Frequent Contributor
timestamp: October 30, 2024, 04:36:17 pm
content: 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

author: CaptDon
author_rank: Super Contributor
timestamp: October 30, 2024, 08:08:18 pm
content: 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.

author: johansen
author_rank: Super Contributor
timestamp: October 30, 2024, 09:04:11 pm
content: Quote from: calzap on October 28, 2024, 07:38:56 pmthe 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.

author: calzap
author_rank: Frequent Contributor
timestamp: October 31, 2024, 12:54:24 am
content: Quote from: johansen on October 30, 2024, 09:04:11 pmYeah, 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

author: johansen
author_rank: Super Contributor
timestamp: October 31, 2024, 10:09:29 pm
content: Quote from: calzap on October 31, 2024, 12:54:24 amSeveral starts per day will do it

author: calzap
author_rank: Frequent Contributor
timestamp: November 05, 2024, 02:15:06 am
content: 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

author: johansen
author_rank: Super Contributor
timestamp: November 05, 2024, 03:20:30 am
content: Quote from: calzap on November 05, 2024, 02:15:06 amits 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.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: November 05, 2024, 08:02:58 am
content: Hello calzap,Quote from: calzap on November 05, 2024, 02:15:06 amSuch 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 amFor 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 amHm, the defective underrated capacitor seems to have spawn additionnal damageIt 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

author: calzap
author_rank: Frequent Contributor
timestamp: March 15, 2025, 01:02:58 am
content: I finally got around to opening the motor. Found a broken spring for the centrifugal switch. See the pic. So, even though the underrated start capacitor was leaking and on its way to total failure, it was the inoperative centrifugal switch that finally led to stoppage.Of course, neither the pump maker (AMT) or motor maker (WEG) could supply a spring. After a lot of searching, found a pair of springs from McMaster that were close. Installed them and a properly rated capacitor, and the very expensive pump now works.Mike

author: Gyro
author_rank: Super Contributor
timestamp: March 15, 2025, 01:25:42 pm
content: Yep, that would do itGood catch and outcome!

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 15, 2025, 08:11:33 pm
content: Hello calzap,That's good news, thanks for the update.If the centifugal switch is defective, it will kill the start capacitor for sure. Those are not designed for a permanent connexion to mains voltage.You can reel your own springs by using a lathe.Quinnshows how to do it the right and safe way.Of course, you may also find some 'warrior mode' tutorialswhere hurting your hands or even your face is included.As your springs fit in a pump motor, usingstainless steel spring wireavoids corrosion and therefore failure.I sometimes like to email my thoughtsto a manufacturer (in your case AMT and/or WEG) that sells expensive but crappy gear.It may be satisfying to explain that you are not fooled by the fact that they are not considering improving their products just to sell more.Well, obtaining a response is not guaranteedBecause designing things that are 'defective by design' is an art form that is not understood by everyone^^.

author: Analog Kid
author_rank: Super Contributor
timestamp: March 15, 2025, 10:08:04 pm
content: Quote from: timeandfrequency on March 15, 2025, 08:11:33 pmAny decent hardware store in the US should have a collection of springs which will yield one close enough for gov't. work.

author: calzap
author_rank: Frequent Contributor
timestamp: March 15, 2025, 10:45:42 pm
content: The springs I got from McMaster were 302 stainless and same dimension as the old springs. Corrosion shouldn’t be an issue like it was for the OEM springs. Doubt I could have gotten stainless steel springs from the local hardware store. I’m in the countryside with about 1 hour round trip to get something from local towns, so it almost always pays to have small stuff delivered.Problem with upgrading start capacitor to 250 VAC is it won’t fit in the detachable steel compartment on the side of the motor. I’ve found these compartments online but would have had to buy in quantity at a high price. So, a custom “can” has to be fabricated. In the past, I’ve used a section of 2 3/8 inch (60 mm) round steel fence post with post-caps on the ends. It was heavy duty and took a lot of work. This time, I’m using a pair of skinny tomato paste cans. Easy to cut with tin snips and create overlapping tabs, and the “caps” are already built-in.Mike

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 15, 2025, 11:23:34 pm
content: Hello calzap,Indeed, you already told us that the 250 VAC capacitor is somehow oversized and does not fit in the genuine compartement.And I was also suprised that capacitor housings can be really expensive (up to 100 bucks).Tomato paste cans is for sure a cheap but clever alternative to protect the capacitor.Good to know that you could procure stainless steeel springs, rather than the rusty genuine parts. McMaster-Carr is a true lifesaver.If you think that the defective centrifugal switch is the main failure cause of your pump, you might monitor its nominal speed with such adedicated sensor.Output is 4-20 mA so a vanilla threshold relay with current loop input will convert the analog signal to a binary condition.With a downstream time relay, you can then blank out the startup period.The goal could be to switch off the pump and trigger an alarm condition if nominal speed is not reached after, let's say, 5 seconds.

author: calzap
author_rank: Frequent Contributor
timestamp: March 16, 2025, 08:49:00 pm
content: Thanks for the input. I’ll look into a rotation detector. The motor and pump share the same shaft, but there is about 2 cm of exposed stainless steel shaft between them. I’m presuming, a small magnet would need to be cemented to the shaft in order to use the Millispeed. There are other types of rotation detectors (optical, for example) that could be used.However, there are a number of protective devices for the pump already. In this case, the overload protector that’s part of the contactor assembly did a fine job. Other protectors are under/over voltage, output pressure, input flow, and loss of prime float-switch for input. I’ve thought about adding thermal protection, but that would need to compensate for ambient temperatures which can range from 5-40 C. Pump is in a building where it won’t freeze.Biggest control problem is the float switch on the input. It’s in a side arm near the pump inlet. The problem is that it’s pond water that’s pumped, and pond water is full of bits of algae and other plant materials. These bits are very sticky and sometimes cause the float arm to stick. The switch is magnetic, and the arm always sticks in the open position indicating loss of prime. This is far better than the opposite because it’s failing safe, but still annoying. In the past, we kicked the sidearm or would occasionally have to take it off for cleaning. Recently, I tried moving a permanent magnet around the outside of the side arm (it’s PVC) … worked great. My helper considers that to be “very spooky”.So, I’m going to install a DC electromagnet on the sidearm with a momentary-DPDT-center-off switch so the field can be reversed. I tried a supposed 50N magnet, which was too weak. Next will be a 250N magnet.Mike

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 16, 2025, 10:32:34 pm
content: Hello calzap,The Millispeed Internet page says :An inductive sensing device located in the nose of the Milli-Speed enclosure will detect a metal target. This target can be an existing bolt head or device attached to a shaft, such as 4B’s Whirligig® sensor mount.Maybe a call with the manufacturer's application engineer would provide additional information on how to setup such a sensor on your pump.Indeed, an optical sensor can also be a good choice. For maximum contrast, you can paint the shaft with black liquid rubber(*) and then glue a small reflective target.You need a sensor that converts the frequency (the target passes in front of the sensor) to an analog signal (e.g 4-20 mA current loop). The downstream low speed detection system will then be identical to what I already described earlier.Quote from: calzap on March 16, 2025, 08:49:00 pmA temperature difference transmitter with current loop output should do the work. The first sensor catches the ambient temperature. The second sensor can be clipped to the motor housing, put in the wind of the motor fan or at best put inside the stator coil.Unfortunately, I could not spot such a device :'(Is is an interesting technique you suggest to cleanup a float-switch, which is generally speaking a very reliable level sensor.Maybe you can surround the float-switch with a mesh screen to limit its obstruction :-10 x 10 mm mesh-5 x 5 mm mesh(probably needs support)Water level can also be measured withultrasonic non-contact sensors. Same fromE+Hwho also sellsmicroware radar level sensors(80 GHz).If you prefer buy locally, brands like Honeywell probably sell similar gear.(*) insensive to water, will never peel off