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From wikipedia:

If a wrong capacitance value is installed, it will cause an uneven magnetic field around the rotor. This causes the rotor to hesitate at the uneven spots, resulting in irregular rotation, especially under load. This hesitation can cause the motor to become noisy, increase energy consumption, cause performance to drop and the motor to overheat.

What is the tolerance for the run capacitor's capacitance?

  • I have a compressor with 50 µF run capacitor. The capacitor went bad.
  • Manual reads that the run capacitor should be 60 µF.

Which one would you go with?

What is the tolerance for this capacitance?

EDIT: there is also a separate Start condenser in this motor setup. The question is only about the Run capacitor.

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  • \$\begingroup\$ Most capacitors are +/- 5% tolerance. 50uF or 60uF are really not very different. But use what the manual say. These capacitors are generally specified with Temperature range (-25 ->75 °C) and lifetime (3000 h) \$\endgroup\$
    – Antonio51
    Nov 7, 2021 at 8:38
  • \$\begingroup\$ @Antonio51 difference between 50 and 60 is 17% which I think is very significant, and above 5% you gave. I was not talking about manufacturing tolerance, but only acceptable tolerance for this particular problem I described above. Generally, permanent-split-capacitance motors have two phases that are wound 90 degrees apart. The motors perform best when the main-phase current and auxiliary-phase current waveforms have this 90 degree phase relationship. The capacitor is used to provide this phase shift \$\endgroup\$
    – Tagar
    Nov 7, 2021 at 23:21
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    \$\begingroup\$ Is the capacitor an original or did someone already replace it? Can you contact the manufacturer and ask for their recommendation? I would consider it more likely the manual didn't keep up with a motor change than the wrong cap was originally installed. But if it's a replacement, it could easily have been different. \$\endgroup\$ Nov 7, 2021 at 23:44
  • \$\begingroup\$ @SpehroPefhany That's original. Yes we will try to contact the manufacturer. If they can't make documentation match, I am worried what first like of support can tell over phone. So I was asking from theoretical standpoint which way I should go. \$\endgroup\$
    – Tagar
    Nov 8, 2021 at 0:01
  • \$\begingroup\$ @Tagar Ok for the difference of 17%. What I meant is that if you try 50uF or 60uF (as as stated in the answer), no "real" problem can occur, except "perhaps" a loss/gain of power. You will perhaps also "hear" what is the "true value". \$\endgroup\$
    – Antonio51
    Nov 8, 2021 at 7:02

1 Answer 1

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If the Wikipedia information is valid at all, it is about large a large deviation in capacitance value, not the a value that is the nearest available value above or below the recommended value. In general, increasing the capacitor value increases the motor starting torque at a slight loss of motor efficiency. However motors used on loads that have a high starting torque usually have a start capacitor that is disconnected as the motor approaches full speed. A motor that has a permanently connected run capacitor is usually connected to a load like a fan or centrifugal pump that has a low starting torque requirement. If the motor has two capacitors, the start capacitor is primarily responsible for the starting torque.

If you suspect that the run capacitor has failed prematurely, make sure the replacement capacitor voltage rating is equal to or higher than the recommended voltage rating. The recommended voltage rating is usually about 125 to 150 percent of the supply voltage. Buy a brand that you know to be good quality or buy from a supplier that you believe sells good quality products.

See also: PSC motor behavior when changing run capacitor value

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  • \$\begingroup\$ Thank you for the great answer! Why do you think that the wikipedia article may be incorrect? The Run capacitor determines a phase difference for an electric motor and it has to be as close as possible to 90 degrees. Thanks for the the link - it confirms that the Run capacitor capacitance is important for efficiency of this kind of motor. Btw, I do have a separate Start condenser, with a much larger capacitance of 250 µF. My question was only in regards to the Run capacitor. Would you go with 50 or 60 µF? What is accepted difference. This is an electric motor for a bigger air compressor. \$\endgroup\$
    – Tagar
    Nov 7, 2021 at 23:27
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    \$\begingroup\$ If the original capacitor lasted several years, I would be inclined to think it was the correct value and replace with the same value. In Wikipedia. the part about uneven magnetic field and irregular rotation doesn't make much sense and is not supported by an academic source. If you use 50 uF or 60 or something in between you may not be able to detect any symptom of degraded motor performance. The best you can do is to compare the running current with the nameplate rating. \$\endgroup\$
    – user80875
    Nov 8, 2021 at 3:28

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