Single phase motor capacitor value: which is better?

[bklauba]

We are trying to determine which value to chose as a running cap to use with a small single phase motor. This 120 VAC motor is a rotary vibrator, used for material flow aid (packing tables), and runs roughly 30% of the time. The motor mfr. supplies a 2.5 uF cap; our customer (an OEM) specs a 4.0 uF cap on their drawings.

When running with the 2.5 uF cap, the current thru neutral is 342 mA, and 177 mA thru the cap. Voltage across the cap is 180 VAC.

When running with the 4.0 uF cap, the current thru neutral is 313 mA, and 280 mA thru the cap. Voltage across the cap is 198 VAC.

We have run both versions of these continously (one hour plus, stable temperatures), and see little difference in the temperature rise. On a wooden table covered with some rubber matting, so as to create the worst case heat dissipation conditions, in a 70 degree F room, temperatures on the outside of the housing at the top (warmest spot) did not exceed 128 degrees F. Winding temperatures are likely to be not much more, which is not all that hot.

There is no detectable difference in start up or other performance behavior that we can see.

Is there any inherent advantage of the 2.5 or the 4.0 valued caps, other than lifetime of the caps (both the caps we are using are rated 400 VAC at least).

BK

 

[aolalde]

In my opinion the 4uF will be better since compensates more reactive power, reducing the current demanded from the power supply.
The capacitor life probably will be the same as far as both are designed to operate up to 400VAC. Probably the cost makes a difference for a large amount of motors.