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I intend to replace the capacitors on my house air conditioner condenser. This video claims that discharging the capacitors is not necessary, because the circuitry will discharge through the fan motor if you unplug while it is running.

https://www.youtube.com/watch?v=Jz3DyCSKGB4

  1. Will using this technique really discharge the capacitor?
  2. Is this a best practice, and is it safe?
  3. What is the best way to handle capacitor discharge for an AC condenser?
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    I agree with Ben's answer. But if you want to make sure with the power off short across the terminals, some use a power resistor , i have used a screwdriver for the last 35 years with only 1 time getting a good arc (the power was still there oops). + Ben
    – Ed Beal
    Jun 24, 2016 at 16:46

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Most of the motors associated with fans and such, utilize an induced magnetic field, and don't have a permanent magnet. So as the capacitor releases its charge (quickly), a magnetic field would be induced in the motor and cause the motor to turn maybe once or twice, and that should spend most, if not all, of the the stored charge. So the capacitor will release the charge to turn the motor... or more technically, the motor will draw the stored current out of the capacitor (quickly).

With respect to the kind of motor that is turning, for most any AC fan motor, it is highly doubtful that a dynamo-style (using a permanent magnet) motor is being used (except on wind mills). I mention this because the efficiency of the discharging effect could be different compared with a dynamo-style motor... but I also think it should be kind of the same (roughly). The difference being that inertia of the spinning dynamo, will continue to generate electricity, until it nearly stops- at which point it would draw the stored current from the capacitor.

But something else to be aware of is that, typically, large capacitors do not perfectly insulate or isolate or hold charges... they naturally discharge within themselves after a while. Furthermore, large capacitors usually have a bleeder resistor attached to the capacitors terminals. Discharging through motors and other electrical items where a circuit exists is (probably) the primary way, though.

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  • So this statement is consistent with your answer: "you can expect the run cap or start cap to already be discharged when you remove the power from the unit." Is that right?
    – steampowered
    Jun 25, 2016 at 16:47
  • @steampowered Kind of... Ed Beal has the right idea though. Always make double sure. If you count on the capacitor (or any other electrical or mechanical thing) to work perfectly, then you may not be able to count your fingers later.
    – Ben Welborn
    Jun 27, 2016 at 12:11
  • So expect them to be discharged when the power is disconnected. But prior to removing the capacitor, test the capacitor to be sure it is discharged using a short or a resistor across the terminals just to be safe.
    – steampowered
    Jun 27, 2016 at 22:23

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