Selecting and Sizing Single Phase Motor Start Capacitor

[Stanfi]

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,

 

[davidbeach]

The specs for the original capacitor would be a really good place to start.

 

[waross]

Check the voltage rating of the capacitors, not the applied voltage. The voltage at the terminals of a start capacitor may be more than line voltage.
respectfully

 

[Stanfi]

This motor does not have a capacitor on it now. I am wanting to put one to it. That is why I am inquring as to how to properly size the capacitance of it.

 

[electricpete]

Is there a value on the nameplate?

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[electricpete]

my last post was referring to the motor nameplate.

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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[Stanfi]

Yes, this was not a cap start motor. It will start and run without a load on it. I have installed the motor on a small table saw. With the load of the blade (only a 8" blade), the starting amps are to high. My intentions were to put a capactitor on this motor so it would start easily.

However, it someone would have other ideas, I would be open to suggestions.

Thanks.

 

[electricpete]

Aha! We were thinking you were going to put it in series with an auxiliary winding. You want to put a cap in in parallel to keep the voltage up during start.

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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[Stanfi]

Ok, took a closer look at the nameplate this evening. The name plate does say "Capcaitor Motor". However, there are not any capacitors mouted to the case, that are typically seen on these motors. It is a Dayton Brand, the nameplate doesn't list any capactitor sizes though. Just your typical rpms, fla.. etc.

 

[electricpete]

Weird. I might be wrong about the cap hookup. Single phase motors are not my strength. I'll bow out and let someone knowledgeable pipe in.

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[Marke]

Hello Stanfi

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