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.)
Best Answer
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.