# Motor Capacitor Needs Replacement - Need Help



## jgedde (Nov 27, 2011)

I was up to my in-laws for Thanksgiving.  My father in law has a vintage Delta Milwaukee drill press that we just pulled out of storage in his barn and put into his newly built shop.  When put into storage, the motor worked.

Cutting to the chase, I replaced mouse chewed fabric covered wiring in the motor and the line cord.  The motor runs if you give it a healthy spin by hand and the bearings are shot.  Clearly the cap is no good - especially since it was crushed!  Unfortunately, the cap's markings are not readable.  So, I have no idea what capacitance we need to get this motor working again.

I took the motor home with me for new bearings and a cap.

It's a 1/2 hp 115/230V motor wired for 115V.   At work, I have a box full of new and used motor caps, so chances are I'll have something that'll work.  My best guess based on what markings are left is 16 microfarads, but my level of certainty is only about 30% (could be 160 uF for all I know).  Can anyone give me an idea what cap value to start with to get this thing running? 

Thanks!
John


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## geotek (Nov 27, 2011)

It's not very critical.  A good rule of thumb is about 70mF/hp.  Or in your case something around 35mF.  If the motor spins up too slow, use a larger cap.


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## Tony Wells (Nov 27, 2011)

General purpose of  starter cap is to throw the starting winding about 90 deg out of phase with the runing coil in order to get things moving, then as the cp charges, it ceases to conduct, taking the start coils out of the picture. Too small and it won't have time to come to speed, and hence torque, so will probably coast back down and stop. Too large, and the stat coils will stay energized too long and get warmer than intended. Unless you get really oversized, too large is better than too small. It won't hurt to experiment a little. You might be able to get the proper cap size from the motor mfg webite.


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## jgedde (Nov 27, 2011)

Tony Wells said:


> General purpose of  starter cap is to throw the starting winding about 90 deg out of phase with the runing coil in order to get things moving, then as the cp charges, it ceases to conduct, taking the start coils out of the picture. Too small and it won't have time to come to speed, and hence torque, so will probably coast back down and stop. Too large, and the stat coils will stay energized too long and get warmer than intended. Unless you get really oversized, too large is better than too small. It won't hurt to experiment a little. You might be able to get the proper cap size from the motor mfg webite.



Now that were delving into sparky talk, perhaps the best way to size this cap would be to measure the L of the start winding at some nominal current (likely zero) and calculate a C to yield 90 deg phase shift at 60 Hz.  Easy cheesy...  If I assume that maximum torque is required at startup, and since there is no back EMF present at zero rotational velocity, I would think the DC test current for L measurement should be the RMS of whatever current the start winding takes with 115 VAC applied to it.  Our trusty old General Radio RLCQ test set is just the ticket for this!  

Clearly this is overthinking the problem, after all it's just an in-law's drill press which will likely see little use.  On the other hand, he did more or less ask me to get it going for him and any job worth doing is worth doing right.  Right?  Oh heck, I'll just measure L at zero current.  This'll yield a cap that's 10-20% too big (SWAG).  Since the tolerances on those electrolytics are so crummy, perhaps I should even add additional margin.  Then again, 70 uF / hp sounds like an easy rule of thumb to apply.   If the motor runs, makes good torque, and doesn't overheat, everyone ends up happy.

What's weird (scary?) is that I'm an EE for a living and actually design motor controllers (radiation hardened brushless DC motion controllers for space) and it never occurred to me to simply apply basic theory to this problem.  Strange how the obvious theoretical approach gets lost when it comes to hobby work....

Thanks!
John


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## Tony Wells (Nov 28, 2011)

That's the way it works, John. Too close to it to see the shortcut. 

Good to see someone use the old GR test equipment beside to gather dust too!


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