Capacitor Issues

[mackos]

I have a Hayward pool pump where the start cap is 161-193 uf. How can you have a range of capacitance. Hayward is an american company. so maybe it can run on 110v or 240v. (just a guess). The other issue is ceiling fan caps, now these are start/run caps and I know from past experience that you can burn out the motor by fitting too high a cap eg was 2.5uf only had a 3.0uf in stock (goodbye fan). But yesterday I had a 1.8uf bad cap & fitted a 2.0uf . The customer is now saying that there is a smell when on high speed. Why are the ratings of fan caps so critical when on other induction motors ie pumps,washing m/c motors you can fit 30uf instead of 25uf with no ill effect. Thanks Mack.

 

[Lucien Nunes]

Start caps are normally electrolytic types because these pack a high capacitance into a small can. They have very wide manufacturing tolerances and drift over time, so one cannot rely on a precise capacitance value and the motor design must allow for that. A start capacitor that is some way off the ideal value will cause reduced starting torque due to incorrect winding current and phase shift, but as the start winding is only in circuit for a few seconds at a time, it doesn't matter if the current is a bit on the high side, so long as the motor starts. Neither the start winding nor the electrolytic capacitor are continuously rated anyway.

Run caps are usually film types as these are continuously rated, and accurate and stable in capacitance. The maximum capacitance is much lower for a given size of can than with electrolytic types, but that's not usually a problem as the required value for run duty is much lower anyway. Accurate capacitance is important because it affects the winding current and phase shift all the time the motor is running. If the capacitance is too high, the phase shift will be too low, torque will be reduced, current will be too high and the motor may eventually overheat.

Just how critical the run cap will be, depends on many details of the motor design and use. On a well- cooled, generously sized motor running light you might get away with a much greater error in capacitance than with a low cost, hot-running motor that is barely big enough for the job.

Re. The fan, it's possible that the winding was already damaged by the original cap failure, and has shorted turns that are now overheating, rather than just on account of the 3uF cap.

 

[mackos]

Start caps are normally electrolytic types because these pack a high capacitance into a small can. They have very wide manufacturing tolerances and drift over time, so one cannot rely on a precise capacitance value and the motor design must allow for that. A start capacitor that is some way off the ideal value will cause reduced starting torque due to incorrect winding current and phase shift, but as the start winding is only in circuit for a few seconds at a time, it doesn't matter if the current is a bit on the high side, so long as the motor starts. Neither the start winding nor the electrolytic capacitor are continuously rated anyway.

Run caps are usually film types as these are continuously rated, and accurate and stable in capacitance. The maximum capacitance is much lower for a given size of can than with electrolytic types, but that's not usually a problem as the required value for run duty is much lower anyway. Accurate capacitance is important because it affects the winding current and phase shift all the time the motor is running. If the capacitance is too high, the phase shift will be too low, torque will be reduced, current will be too high and the motor may eventually overheat.

Just how critical the run cap will be, depends on many details of the motor design and use. On a well- cooled, generously sized motor running light you might get away with a much greater error in capacitance than with a low cost, hot-running motor that is barely big enough for the job.

Re. The fan, it's possible that the winding was already damaged by the original cap failure, and has shorted turns that are now overheating, rather than just on account of the 3uF cap.

Hi Lucien
Thanks for your detailed reply. I understand a bit better now.as you say the tolerances are less on the start/run caps. I also overlooked that going from 1.8uf to 2.0uf is 10% greater and a lot of these caps have + or - 5% tolerance.Could I pick your brain on a similar vein. The Hayward pool pump I mentioned has a start cap which is disengaged by a centrifugal switch, but nearly all the other motors/pumps which just have start caps don`t. Do the start windings not have an effect on the run windings if not disengaged or are the motors wound differently. My theory lets me down on this side of things.
Thanks Mack.

 

[Lucien Nunes]

A start capacitor, by definition, is one that is disconnected once the motor is up to speed. That could be achieved by a centrifugal switch, current-sensing relay, thermistor or an external timer (on a large motor), but somehow the circuit has to be broken.

A run capacitor, by definition, stays in circuit all the time, creating a phase shift on the auxiliary winding that makes the motor run as a 2-phase motor, giving smoother torque and better power factor than a single winding. The run capacitor also enables the motor to start, although it is not referred to as a start capacitor.

Combining the two gives a capacitor start-and-run motor. The run capacitor is in circuit with the auxiliary winding all the time, but a larger start capacitor is connected in parallel during starting and then disconnected by one of the above methods. This gives the advantages of both: Increased starting torque plus improved running performance.

 

[mackos]

A start capacitor, by definition, is one that is disconnected once the motor is up to speed. That could be achieved by a centrifugal switch, current-sensing relay, thermistor or an external timer (on a large motor), but somehow the circuit has to be broken.

A run capacitor, by definition, stays in circuit all the time, creating a phase shift on the auxiliary winding that makes the motor run as a 2-phase motor, giving smoother torque and better power factor than a single winding. The run capacitor also enables the motor to start, although it is not referred to as a start capacitor.

Combining the two gives a capacitor start-and-run motor. The run capacitor is in circuit with the auxiliary winding all the time, but a larger start capacitor is connected in parallel during starting and then disconnected by one of the above methods. This gives the advantages of both: Increased starting torque plus improved running performance.

OK Thanks for that Lucien, much appreciated Mack.