How close tolerance does a run capacitor have to be? 1

[OperaHouse]

There is a discussion with a guy that says his air conditioner fan motor over heated and died because the manufacturer used a 7.5uF instead of the 5uF as stated on the motor. This motor has a run and a start capacitor. I think the cap just opened up or the bearings started to freeze. A friend lets me scavenge old HVAC units for parts and I've seen higher value caps on motors before. Someone told him that the cap has to be within 10% or the motor greatly overheats. I've always thought a little higher was better than being under. Certainly that much higher wouldn't damage the motor. The manufacturer must do it for some reason. Just curious.

 

[ScottyUK]

Generally the manufacturer will use the smallest capacitor he can get away with to minimise costs. In my experience larger caps just cost more, never heard of a motor dying as a result of using one.

 

[waross]

Generally a run capacitor does not affect the motor at all. It affects the KVARs drawn from or delivered to the grid,and affects the power factor but it does not have a direct affect on the motor.

Bill
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"Why not the best?"
Jimmy Carter

 

[Compositepro]

Thought a run capacitor gives a motor greater torque capability since it keep the start winding partly energized while running.

 

[waross]

Hi Compositepro. I stand corrected. I was thinking three phase motors.
A capacitor in series with the start winding should ideally shift the phase angle of the start winding current to coincide with the physical electrical angle that the start winding is offset from the run winding.
However this shift is not 90 degrees from the applied voltage but 90 degrees from the normally lagging current of the run winding.
In addition, the phase angle of the run current will shift with the degree of loading.
I suspect that the motor manufacturer spec'ed the minimum value of capacitor to run the motor and the A/C manufacturer used the optimum size for their application and fan load.
The wrong value run cap may cause overheating of the motor, but the more likely cause of burn-out may be bearing failure, start capacitor failure, or starting relay failure.
If the oversized cap caused the burn-out, I would expect an early failure. Warranty claims would be expensive.
How old is this unit?
Is it possible that the capacitor has been replaced in the past with the wrong value?
Can the wrong value capacitor cause motor heating? Yes.
Is a jump from 5 uF to 7.5 uF enough to cause over heating? Probably not.
Bill
--------------------
"Why not the best?"
Jimmy Carter