# AC motor run capacitor

Discussion in 'General Electronics Chat' started by walley10291, Apr 15, 2009.

1. ### walley10291 Thread Starter New Member

Apr 15, 2009
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I understand that a motor run capacitor allows the motor to run smoother and more efficiently; my question is, can a run cap be added to a motor that does not already have one, and if so, how to calculate the correct size ( mfd ) that would be needed? Thanks

2. ### RAH1379 Well-Known Member

Dec 13, 2005
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i believe the purpose of the capacitor is to get the motor started because of the inrush current demand, once the motor gets up to about 80% of speed a centrical switch usually cuts it out of the circuit.

3. ### KMoffett AAC Fanatic!

Dec 19, 2007
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That's a "start" capacitor, not a "run" capacitor.

Check pages 19+ on
http://www.scribd.com/doc/6883830/TutorialMotorBasicsLecture

Ken

Dec 13, 2005
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5. ### zisuo88 New Member

Oct 30, 2009
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wow,I have a same question,thank you

6. ### electr Active Member

May 23, 2009
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look for power factor correction capacitors.
There are great links out there (also on this website) that guide you throught the cap's value calculation.

7. ### Duane P Wetick Active Member

Apr 23, 2009
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Motor run capacitors correct the power factor, thus reducing the line demand current. The motor's current remains unchanged. A general rule that I use says that 50 mfd. capacitance corrects 10 amperes @ 480 VAC / 60 Hz.

8. ### electr Active Member

May 23, 2009
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You cant set the capacitance without knowing the motor's inductance.
By doing that, you can even increase the current's intake.

9. ### GetDeviceInfo Senior Member

Jun 7, 2009
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a permanent split capacitor motor will have it's oil filled cap in series with an auxiliary winding. This provides an out of phase current for the appropriately positioned winding, giving smoother operation. This is a bit different than a capacitor start where the start winding is only engaged until the speed hits about 75% of rated speed, and is used for starting torque.

Power correction is a different beast all together, and you need to ask if it's worth the effort.

Jun 22, 2012
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11. ### strantor AAC Fanatic!

Oct 3, 2010
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No, I can't see it either. :?

Oct 3, 2010
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13. ### htroberts New Member

Jun 22, 2012
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I don't know if that's the same as the original link, but it's certainly informative.

Thanks,
Heath

14. ### Brownout Well-Known Member

Jan 10, 2012
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I can't see it either, and I'm not a new member.

Correction: I can see it after refreshing the page.

15. ### gerty AAC Fanatic!

Aug 30, 2007
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Still cannot see it even after refreshing page, must be on the banned list.

16. ### ohmster New Member

Jul 12, 2016
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The URL is quite valid. Perhaps this TinyURL will work to get someone there.
http://tinyurl.com/h65secy

Jul 18, 2013
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Max.

18. ### crutschow Expert

Mar 14, 2008
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The purpose of the start capacitor is to create a rotating magnetic field in the auxiliary winding to start/run a single-phase induction motor, since such a motor won't start without help (single-phase induction motors will run without any help once started, but the explanation for that is somewhat complicated).
Thus if the capacitor fails in such a motor it won't start, it will just sit there and hum.
In effect the capacitor and auxiliary winding create a rotating field, similar to one created by a two-phase AC supply into a 2-phase motor (or 3-phase supply into a 3-phase motor).

Louis Wilen likes this.

Jul 18, 2013
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A tad more ..1ph induction motor, without the start cap the field oscillates across 0-180°, so no revolutions, the start winding and cap are calculated to produce an almost 90° phase shift to this main current.
The run winding current lags the applied voltage by a few degrees and the start voltage leads by almost the 90° producing the "2 phase' supply with optimum phase shift.
Max.

Last edited: Jul 14, 2016
Louis Wilen likes this.
20. ### ohmster New Member

Jul 12, 2016
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Yes this is a very old thread but will have new life because people like myself who are homeowners with limited funds, have to fix our own home A/C units. Mine stopped cooling the home and when I went outside to look at the condenser unit, the main exhaust fan was not turning and I could hear a loud hum from the unit. Obviously the unit has power but the fan, for whatever reason is not turning. I have worked as an electronic technician on home electronics for my entire life, but not with large A/C motors. This thread and the offered links for education got my home A/C repaired, for essentially \$6 to pay for a new run capacitor for the fan instead of calling an A/C repair service and paying hundreds of dollars that I do not have right now. Without a thread like this and the education I got from it, I would never have been able to fix the outdoor condenser unit.

The capacitors and circuits used in a large A/C motor are *nothing* like one would deal with fixing TV sets and stereos. The capacitors are unique and have a totally different purpose and function as used in a home A/C unit. I could not even measure them as the Fluke 85 DMM, a very handy and expensive meter, does not measure "microfarads". I took the caps into an A/C parts store and had them checked. The 10uF fan run capacitor was down to 3uF, clearly a bad part that must be replaced. Even if it did not repair the unit, it had to be replaced if ever this was to work. Fortunately, it did fix the unit and for the first time in days, I got a good night sleep without waking to a 90° degree home, soaked in sweat. So 4 years old or not, this thread has an ongoing, useful purpose and made the difference for me by allowing me to fix my own home A/C system. Thanks guys!

Last edited: Jul 13, 2016