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So recently I got my hands on an old pump motor, unfortunately the connection diagram is missing. I have already determined this is a 2 phase motor, and that in order to make it work I need a motor-run capacitor.

However I was wondering can I use any motor-run capacitor or do I need one with a specific capacitance to get the 90 degrees of phase difference? If it would be important I live in a area with 230V AC out of the wall.

Thank you, for any help/tips

ps: i have included photos of the stator; Top of the stator, where the diagram used to be and the capacitor The stator, with the wires going directly to the coils

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  • \$\begingroup\$ The capacitor is determined by the motor power rating and the use of the motor, the mech. load. It is differnig if you need a hard start, load is always present or you start the motor unlaoded, also if the motor will freely spin at max. revs or it will be loaded. \$\endgroup\$ – Marko Buršič Jan 21 '16 at 13:27
  • \$\begingroup\$ It's a single phase, capacitor start motor. See this post. The phase shift doesn't have to be 90. A lesser value will do as long as it's enough to give a rotation direction. \$\endgroup\$ – Transistor Jan 21 '16 at 18:17
  • \$\begingroup\$ You better expand your question with motor data and the application for it, to get an approximative cap value. \$\endgroup\$ – Marko Buršič Jan 22 '16 at 12:03
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Yes, you need a cap. The cap must be rated for the job. A 250VAC or better metal film cap will be needed.

The phase shift is never exactly 90 degrees but it does not have to be. The capacitance needed is roughly proportional to the size of the motor. If your cap is small you will still get starting torque but not very much. If your cap is too big, you get more starting torque but you increase no load current consumption and risk burning out the motor. Running torque does not change much with cap value.

If your load is "easy" like a fan you have a wide range of capacitance to work with. If your load is "hard" like a piston compressor you will have to get the cap right be ensuring that the motor starts under load. Under no load on the bench you could start with 10uF and take things from there.

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  • \$\begingroup\$ My local electronics store sells motor-run capacitors rated upto 480V. I would be using the motor as a blower for a furnace, but as i mentioned the motor has no markings, labels or anything, it is just the stator, rotor and housing. Could it be harmfull if use a capacitor that has a value of 60 microfarads if i use it without load? Or it doesn't hurt to have "too much" microfarads? Thank you for the reply, \$\endgroup\$ – Fenix Computers Jan 22 '16 at 14:41
  • \$\begingroup\$ Good your load is easy so you can start on the small side without stalling the motor .Be conservative and place 2 of your caps in series to make 30 microfarad .If you run 60 the cap current and hence the winding current will be about 3 to 4 Amp .Measure your wire diameter to make a sensible final cap decision . \$\endgroup\$ – Autistic Jan 22 '16 at 20:55
  • \$\begingroup\$ Thank you a lot for your comment, the shop sells different values of capacitors, and I was wondering about what the harm could be of having one that was to big, but I will buy one of 30 microfarads. \$\endgroup\$ – Fenix Computers Jan 23 '16 at 14:32

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