Selecting and Sizing Single Phase Motor Start Capacitor
Selecting and Sizing Single Phase Motor Start Capacitor
(OP)
I am wanting to implenent a start capacitor into an existing 120V 1hp motor. Can someone provide me with the proper forumlas, or reference material for selecting and sizing the correct start capacitor?
Thanks,
Thanks,





RE: Selecting and Sizing Single Phase Motor Start Capacitor
RE: Selecting and Sizing Single Phase Motor Start Capacitor
respectfully
RE: Selecting and Sizing Single Phase Motor Start Capacitor
RE: Selecting and Sizing Single Phase Motor Start Capacitor
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RE: Selecting and Sizing Single Phase Motor Start Capacitor
By the way, is it possible this was not a cap start motor to begin with? (perhaps a split phase motor). Have you tried starting it without cap?
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RE: Selecting and Sizing Single Phase Motor Start Capacitor
However, it someone would have other ideas, I would be open to suggestions.
Thanks.
RE: Selecting and Sizing Single Phase Motor Start Capacitor
That should be easier for the group to figure out. My first guess would be to size it like a power factor correction capacitor by ensuring it delivers less vars than the motor absorbs at no load... to avoid resonance. But I'm not sure.
I think there are vendors that sells power boost units like this as an add-on for existing motors.. they should have application info depending on motor speed and horsepower
Now that the question is clarified, I'm guessing there will be other comments.
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RE: Selecting and Sizing Single Phase Motor Start Capacitor
RE: Selecting and Sizing Single Phase Motor Start Capacitor
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Selecting and Sizing Single Phase Motor Start Capacitor
I was under the same impression as ElecricPete.
If the motor starts OK as it is, then you should not tinker with the start winding arrangement.
When the motor starts, it does draw a severe overload current which can be in the order of 600% of the motor rating.
You can add series impedance to reduce the voltage on the motor terminals and thereby reduce the start current. This will also reduce the start torque by the current reduction squared.
At start, the power factor of the motor is very low, typically as low as 0.15, and rises as the motor accelerates. You could reduce the start current by connecting shunt capacitance, but the problem is that the capacitance needs to change with speed which is very difficult to achieve. You could have a number of steps and have the appropriate steps controlled by speed sensing circuitry. - all getting too complicated in my book. I expect that it would be cheaper to change the motor to a three phase motor and use a single phase in, three phase out inverter. That will keep the start current to less than 150% of the rated current of the motor.
Perhaps someone out there has acutally achieved dynamic pf correction during start as a means of reducing start current.
Good luck,
Best regards,
Mark Empson
http://www.lmphotonics.com