Hi,
I had a motor start capacitor (450V, 7uF) die in a tumble drier.
I had a 6uF, 400V one handy, so I used that while I waited for the replacement part.
It seemed to be fine, except the drier seemed to be stopping early.
After switching back to the spec'd part it was fine again.
For my edification, does anyone know why this would be the case?
I can only guess that it's causing the motor to run hot and trigger the motor thermal cutout, but I don't know why.

 

That size capacitor is typically a film-type run capacitor, not start.
If so its capacity could affect the motor run current and thus its operating temperature.

 

If the capacitor was actually a START capacitor, switched out of the circuit by a start relay after just a few seconds, that would not have much effect on the motor run speed or temperature. But if it was a RUN capacitor, then the effect would last for the entire cycle, which could affect both speed and temperature.
Why does the TS think it was a start capacitor, and not a run capacitor??

 

When you used the 6 µF, 400V capacitor, the lower capacitance may have caused insufficient starting torque.

 

Also You should be looking at specifically Motor run-rated types, keep away from Chinese origin versions!!
Specifically rated for start are generally bi-polar electrolytic motor start rated, momentary operation, motor run can be oil filled paper.

 

I assumed it was a start capacitor because you can hand start the drum in the dryer and it will keep running, but that is with the 'dead' capacitor (2uF, vs spec 7uF) in place. The new one is sold as "universal start/run". Perhaps the dead one has enough C to keep the motor turning over, but not enough to start. It sounds like it's both start and run. The temporary 6uF vs spec 7uF should be enough, especially for an unloaded motor and the 400V vs 450V I would have though would just risk blowing the lower V-rated capacitor, not affected performance.

 

I upgraded the old "generic" to a Ducati! That should make the drier go faster!

 

I also found this Blog that sheds some light on what's happening. I think that 6uF might just make the difference. The motor runs hotter, so the thermal protection cuts in. I think it's just a start capacitor. Other motors have both a start and a separate run capacitor.
https://www.aces.edu/blog/topics/farming/start-and-run-capacitors-for-electric-motors/#:~:text=The start capacitor may be,load (amps) and overheats.

 

Did you check the device that switches out the start capacitor?? That part can certainly fail as well.

 

If it's a start capacitor that is only engaged during starting, then it would have no effect on the motor run temperature.

 

If it's a start capacitor that is only engaged during starting, then it would have no effect on the motor run temperature.

That's what I figured. That's why I'm confused.

 

That's what I figured. That's why I'm confused.

Thus it seems logical that it's a run capacitor.

 

Not all of the schemes for disconnecting the start capacitor are so very obvious, which was discussed in a different thread a few months back. There was a mention, I think, of some mechanism included with the start capacitor, that reduced its function during the run mode. So a better examination of the original capacitor label is in order, to see if perhaps it included such a feature.
Of course, if the replacement is performing as desired, then perhaps it also includes such a feature, which should be revealed on the label.

 

A tumble dryer is usually a small motor so only a combined start/run cap is all that is required.

 

A tumble dryer is usually a small motor so only a combined start/run cap is all that is required.

True! The small motor on our tumble dryer is ONLY 1/4 HP, and it only uses one capacitor, which I have never had to replace. The only part that needs replacing is the fan disc on the air blower. The shaft coupling connection keeps wearing out. I have replaced that same part on several other friends tumble dryers, all by the same one manufacturer: ADMIRAL.

 

Maybe I should have looked a little harder and asked ChatGPT before hitting this forum.
"The capacitor is an important part of the tumble dryer’s electrical system, as it helps to ensure that the motor runs smoothly and at the correct speed. Without the capacitor, the motor may run too slowly, causing the tumble dryer to take longer to dry your clothes, or the motor may struggle to start up altogether....... the capacitor also helps to reduce the amount of heat generated by the motor. "
Same, 1/4HP, one single start/run capacitor. Other motors I'd used had dual capacitors, one for start and another for run.

 

I don't think ChatGPT woul give any usefull help. You need to establish if the capacitor is in circuit all the time or just when it is starting. Without taking the motor apart to see if it has a centrifugal switch I think the easiest way to find out is to connect an AC ammeter (Set to the 10 amp range.) in series with the capacitor. If it is only a start capacitor then the meter will only show current for a few seconds at most when the motor is starting. If it shows a continious current then is is connected all of the time. (Or the centrifugal switch is shorted.)

Les.

 

It will probably be simpler to meter the voltage across the capacitor, with a 10K resistor across it. If it is only a start cap, the voltage will start to drop toward zero within a few seconds of starting. If it is a RUN capacitor the voltage will not drop much after a start.