Re: Motor starting jolt

Gary Johnson

The Leeson 110088.00 is a capacitor start, capacitor run motor.  The two caps are in parallel when starting, then the centrifugal switch disconnects the much larger starting cap. Such motors can produce more starting toque than a plain capacitor start motor. They also offer higher efficiency and lower power factor since the run capacitor roughly cancels the inductive reactance i.e., it is approximately resonant. 

It is NOT resonant during startup. The start capcitor provides nearly all the required phase shift, thus a very large value is needed, somewhere around 10X the run cap.

Starting torque will decrease with smaller starting capacitor values, but of course you must avoid a stall condition under any expected starting load. You could experiment by substituting other values, or placing them in SERIES with the start capacitor to create an effectivley lower value  Ctotal = C1*C2/(C1+C2). Never alter the value of the RUN capacitor. Another factoid: Without the start capacitor, the system becomes a Permanent Split Capacitor motor, which still has storting torque, but not very much. So you could give it a try with no start capacitor at all (with no load, pelase).

Limiting the inrush current will also reduce starting torque. So the suggested experiment with a long skinny extension cord is actually a nice simple test to see how it behaves. 

The soft starter like Jim recommended is a good solution. (If I was starting from scratch, I'd go with a DC permag motor and variable speed, like I did for my mill.)



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