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Living and working in West Africa. We cannot purchase capacitors here, we can only beg (and pay) used parts from the workshops which rewind electrical motors.

Our gray-water-pump is broken, the always-connected run capacitor is dead. It is a simple 400W single phase induction motor. I can start it by hand in either direction. But it will not start any longer by itself. The dead capacitor is marked 6 uF.

I found no 6 uF capacitor locally, have already asked in three different towns at several workshops. I found a used 5 uF (cylindrical shape, same size as the dead one) and a 1,25 uF (small box-shaped unit).

Because of the keeping-the-water-out-seals, I cannot open the pump many more times, because the rubber rings suffer with each opening and closing of the case.

Question: Should I install the combination of 5 uF + 1,25 uF in parallel or just install the 5 uF? What are advantages and disadvantages to consider please?

Are there formulas which I could apply, having very little documentation? (It is just a dirt-water-pump from a garden-center labelled 400W.) If I could calculate the "nominally needed capacitance" then I could better decide whether to go 1 uF too low or rather 0,25 uF too high with the added complcation of installing two different capacitors in parallel.

What do the users think please, who got real-life repair experience?

I can tell you that the motor without any load (dry on my work bench) does start on the 5 uF alone. But I wonder whether it will be "enough" with the added load of having the blades submerged in the water (physical friction). The local repair-veteran (without any formal education) has recommended to just install the 5 uF. But I would be 17% off the original capacitance.

Why do I not just order a new 6 uF capacitor from Germany? I did, but mail-order and shipment to West Africa takes several months. And I do love my family and want to repair our toilet-flushing system. So your help is appreciated please. This is not a theoretical question but meant to help me fix a real problem. Thank you. (This is my first question here. If I got it wrong, please do not hate me, rather please help me edit to get it right. I love Stackexchange.)

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    \$\begingroup\$ I'm definitely not going to answer this (I know nothing about motors), but knowing that most capacitors like that have a larger tolerance than 17%, I would just use the 5 µF. Of course, maybe the original one had more than 6 µF, and maybe the new one have less than 5 µF, but it's probably still within the accepted range. How do you know the capacitor is dead? \$\endgroup\$
    – pipe
    Mar 18, 2017 at 11:37
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    \$\begingroup\$ I know the old capa (local nickname) is dead, because: a) all the symptoms fit (I did a lot of reading online about this): the motor can be started by hand in either direction and is running fine, once started b) as I also said in my question, with a "new" used 5 uF capacitor the motor will start but not with the old dead one c) the local repair-guys have some rather spectacular methods of testing (involving bare contacts, bare hands, live mains voltage and sparks and sound-effects) - so my local workman has also tested and confirmed my old capa as dead and tested the new capa as working. \$\endgroup\$
    – Martin Zaske
    Mar 18, 2017 at 11:48
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    \$\begingroup\$ Capacitor values for a motor are not that critical. It needs to be there to establish an initial rotation direction, and 5uF will give slightly less starting torque. But I agree, I'd go with just the 5 uF. \$\endgroup\$
    – user16324
    Mar 18, 2017 at 12:53
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    \$\begingroup\$ Capacitors designed for continuous connection are designed differently from those designed only for brief connection during starting. If a capacitor that is designed for brief connection is connected continuously it will likely overheat and fail rather quickly. Only comments are given for questions expected to be closed because they don't fit the overall objectives of this site. \$\endgroup\$
    – user80875
    Mar 18, 2017 at 14:41
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    \$\begingroup\$ Capacitors that are only connected during starting are usually called motor-start capacitors or just start capacitors in American English. Capacitors that are connected whenever the motor is energized are called motor-run or run capacitors. I don't know if British English is different, but I believe it is very similar in this case. Unfortunately for people who do not have a lot of experience with English, there are a lot of alternative words and expressions that can cause confusion. \$\endgroup\$
    – user80875
    Mar 18, 2017 at 15:52

3 Answers 3

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First I must repeat that I know nothing about motors! However, knowing that most capacitors like that have a tolerance larger than 17%, I would just use the 5 µF. When they originally selected a capacitor with 6 µF of capacitance they knew that it could have a large manufacturing spread.

Of course, maybe the original one had more than 6 µF, and maybe the new one have less than 5 µF, but it's likely still within the accepted range.

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  • \$\begingroup\$ See, I gave you the tick, as promissed. But just to have the last word, I want to say that I know a bit about motors and even had formal training during my studies (mostly forgotten because long ago). And I want to repeat that all the motor-run capacitors which I found available for mail order had a tolerance of 5% given. You could have an even laster word and post a link to some sloppy capacitors for sale with more than 17% tolerance and I would grovel nicely. If or when I find the missing original dead capacitor, I will post what tolerance is marked on it, if any. Over and out. \$\endgroup\$
    – Martin Zaske
    Mar 18, 2017 at 21:02
  • \$\begingroup\$ @MartinZaske I'm talking about a bog standard electrolytic, for example this from a reputable manufacturer. ±20%. The motor starter caps I found where these, with a tolerance of about -0%/+20%. \$\endgroup\$
    – pipe
    Mar 18, 2017 at 23:46
  • \$\begingroup\$ @MartinZaske Here is a 5 µF capacitor with a tolerance of -10%/+150%! \$\endgroup\$
    – pipe
    Mar 18, 2017 at 23:48
  • \$\begingroup\$ thank you again for having searched more examples of real-world capas; you will have noticed that for motors, electrolytic are not used. And I learnt valuable information from your examples which are properly meant for motors. Your example about extreme tolerances is just the honesty of the manufacturer: at nominal voltage the tolerance is -10%/+50% and only if you decide to operate it at 10% of the nominally permissible voltage, then you get such extreme tolerances (and you should probably find another model, unless you need to allow for surges or whatever). greetings. \$\endgroup\$
    – Martin Zaske
    Mar 21, 2017 at 13:01
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I needed a 6 uF replacement and only found a used 5 uF and a used 1.25 uF. With the help of several answers, I decided to only install the 5 uF capacitor.

The motor started fine on its own. Having assembled the pump with its pumping "blades" (extra mass) and the outer case it still started fine in my workshop.

Then I re-installed the 1-inch hose and the supporting rope and tested inside our gray-water tank. I learned that I now have less starting torque. But in the wild, the pump is starting just fine, even suspended almost 1 meter down in gray water, i.e. having an actual "load" in the outgoing hose. It is not mud, just water from our washing machine and shower.

So, nominally, I am 17% low on capacitance, and the used capa might even be somewhat lower than what its label says. But my pump is repaired and we no longer need to run with buckets to flush our toilet.Thank you all. Maybe this will help another user...

Update2: It is now May 18th and the pump is still starting every single time with load (in the water) at first flip of the switch. It never hesitated with my replacment capacitor with 5 uF. In the meantime a brand-new 6 uF capacitor has arrived in the mail; but I do not even want to open the pump again. I will keep it as a spare part for when my second-hand fix will die of the heat some day.

Update3: Today Oct 11, 2017; pump still starting fine every time. And keeping the replacement capacitor on standby.

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  • \$\begingroup\$ Question about manners in this forum: Is is better to mark my own answer as "it solved my problem" (with hindsight) so that others can see at a glance that this option worked. Or is it better manners to leave it unmarked, as it is my own answer? \$\endgroup\$
    – Martin Zaske
    Mar 18, 2017 at 18:43
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    \$\begingroup\$ Its fine to acceppt your own answer if you think that nobody is likely to contribute a better one. In this case I would consider the matter closed with your answer. \$\endgroup\$
    – Rev
    Mar 18, 2017 at 18:51
  • \$\begingroup\$ Yes, thank you. It might help marking the question as closed. I will do it in a few days unless some person can give an answer which will help more users like: "On such motors, you can go off the original capacitance by 25% higher or 5% lower and still start, because the creators normally design for 12% surplus torque for xxx reason." \$\endgroup\$
    – Martin Zaske
    Mar 18, 2017 at 19:00
  • \$\begingroup\$ Mark it. Any one of us who wrote comments instead of actual answers have ourselves to blame for missing out on the bounty. We should know better. \$\endgroup\$
    – pipe
    Mar 18, 2017 at 19:00
  • \$\begingroup\$ @pipe I really appreciate the help I am receiving from Stackexchange, so I would gladly pass you the bounty, if you like: Please write an answer about going for the 5 uF only option (and keep the funny remark about not knowing anything about motors) - and I will mark it as the "helpful answer". That way you get something and the information is online for others to find and I feel less like a parasite, having taken all yours times for nothing. \$\endgroup\$
    – Martin Zaske
    Mar 18, 2017 at 19:05
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Do not use an electrolytic capacitor for this AC application. The previous capacitor was both start and run. It may have lost its capacity through overheating. So, choose a non-electrolytic capacitor with a high temperature rating, such as 105 C. If the two capacitors you have are non-electrolytic, are rated for high temperature, are rated for motor applications, and rated for the absolute maximum AC Voltage that may get applied to the motor, then connect them in parallel. Using two capacitors increases the outside, heat dissipating area, which may increase the time to the next failure. You may need to ask your local electricity supply authority what maximum AC Voltage is provided - not the standard, but the actual. If you run your motor from a local generator, then use a multi-meter to check the maximum Voltage - you may need to measure at many times during the day; if your multi-meter uses a moving needle to indicate Voltage, then you can leave it connected all the time, and just note the Voltage each time you walk past.

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