Motor capacitors

In both start and run application the capacitor is being used to limit current to the start windings. The capacitance is determined in the manufactures design. Run motors having lower capacitance values .5 to 50 MFDs to limit continuous current. Start having higher as that the start windings are switch off.

Check WWW Graingers they list the many different valves they stock for both types of motors.

Hello Guest you said:

what if anything is the difference between a motor start and motor run capacitor?

It has already been mentioned that run capacitors tend to be of lower capacitance than start capacitor's. However the major difference is that a run capacitor can remain in the circuit indefinitely. A start capacitor is intermittent duty only. A start capacitor will overheat and fail if left energized too long.

The primary purpose of either capacitor is to promote motor starting and or direction of rotation. All single phase motors must employ some sort of starting device.

In the case of a permanent split phase capacitor motor, there are two sets of windings in the motor. By placing a capacitor in series with one of windings the phase angle is changed, which in turn determines the rotation, and allows motor to start. As already mentioned an overly large capacitor can allow increased current in the windings and result in premature motor failure.

In the case of a capacitor start induction run motor the start capacitor is only in the circuit for a short period of time. In either case a motor that employs a capacitor has higher starting torque than one that does not. (Three phase motors excluded)

Bottom line start capacitor's cannot be used for continuous duty. Run capacitors generally have metal cases and are liquid filled, at least in the larger industrial machines.

Multiple run capacitors can be used. Capacitance is additive , when capacitors are linked in parallel.

I'm not sure why you're correcting your power factor, however the following link may be of interest to you. The link contains some good information on power factor correction.

http://cr4.globalspec.com/thread/18626/Energy-Saving-to-25

In simple terms, start capacitors are used to give the motor (single phase induction motors) a "boost" when starting since their inherent starting torque is low. If the start capacitor on a small single phase motor is removed from the circuit, one can apply power to the motor and manually give the shaft a spin and the motor will run. This is what the start capacitors do. The run capacitors are there to "season" the motor operation by maintaining a constant current to the motor run windings. There are many motors that do not have run capacitors, only start capacitors. As for power factor correction capacitors, a discussion on that subject would take more space than is available here. You can try the link below for more information on that subject.

http://www.lmphotonics.com/pwrfact.htm

Start capacitors have high starting torque and are solely used for providing high startting torque for induction-run motors. Once the motor reaches about 70% of its total rpm, a centrifugal switch or potential relay removes the start capacitor from the circuit. It is used only for starting purposes.

Run capacitors provide lower starting torque for motors. Run caps are permanently wired between the start winding and the run winding to keep both windings out of phase with each other by 90 degrees electrically. Run caps remain in the circuit 100 percent of the time during operation. Start caps and Run caps are not inter-changeable.

Have a split system air conditioning system that has developed a hesitant sounding loud clatter upon start up. Can this be caused by a faulty run capacitor?