Blowing motor start capacitors

Thread Starter


Joined May 9, 2007
I have a 15 year old grizzly bandsaw with a 3/4hp 1720 RPM single phase 110/220v motor. The motor is being run on 110v. Several years ago it stopped starting without help (spinning by hand) but does seem to run fine once it is going.

I finally decided to fix it. I removed the old start capacitor (150MFD, 125v) and got a new one from Grainger (145-175MFD, 110-125v). I installed the new one and the saw started right up but within a few seconds I smelled the cap. I opened it up and there was oil in the box. I ran it with it opened and it was shooting smoke and some oil out the top edge of the cap. The cap also gets very hot - too hot to hold very long.

I pulled the motor and checked the capacitor cut out switch and it seems fine. With the motor off the contacts touch and make a good connection. When the motor gets to a few hundred rpm the weights spin out and open the contacts. I can hear the switch clicking into place as the motor slows after being shut off. I took it a part and cleaned the points just to make sure - they look good.

I did blow out all the dust (not too bad), and I pulled all wires out the back to check for breaks and they all look good.

Could I have gotten a bad capacitor or do I have another problem?

BTW: with the motor running at full speed (and no capacitor in place) one of the cap leads reads about 45v and the other reads 130v. Should these have any voltage on them at full speed?



Joined Apr 24, 2007
This is way out of my area. But, since no one else has responded, yet, maybe I can ask enough questions for you to be able to find the right question, and its answer, yourself.

If I understand correctly, you implied that the "capacitor cut-out switch" should be disconnecting the capacitor, above a certain RPM level. And yet, you measured a voltage between each capacitor connection and somewhere, with the capacitor removed and the motor at running speed.

That SEEMS to imply that the capacitor is not being disconnected. But that might depend on what, exactly, you were measuring.

Just to be clear, between what two points was each voltage measured? i.e. Where was the other voltmeter lead placed, for each of the two capacitor connections' voltage measurements?

I think I would also want to simply measure what voltage the capacitor was seeing across its terminals. i.e. Just measure the voltage between/across the two capacitor connections, with the capacitor removed and the motor running. Maybe you should also measure the voltage across/between the capacitor's two terminals, while it's installed, with the motor running.

Also, is it possible to manually open (and maybe also even unplug or disconnect) the capacitor cut-out switch, with the saw off (and unplugged)? If so, maybe it would make things more clear if you could measure RESISTANCES, with the switch both open and closed for each measurement, measuring a) across the capacitor connections (mainly WITHOUT the capacitor installed), and b) between where each lead should connect to the capacitor and where it connects to the switch, and c) between each capacitor connection and what it connects to on the other side of the switch (e.g. at motor wiring), and d) between each of the switch's connections (if possible) and where they connect to the motor, and, just for completeness, e,f,g,h...etc) from all of the capacitor and switch connections to the saw and motor frames/cases, and to any "ground" connection, and to each incoming AC power connection, and to each motor connection (if not already measured), and any other combinations of all of those (and any other identifiable) circuit points that I left out, to check not only for proper switch operation, but also for any possible unintended or short circuit paths. You might want to make a drawing or sketch, first, and then label it with all of the measured resistances. If necessary, you could also try to do something similar, but while measuring voltages, with the saw running and the capacitor both installed and not installed. (But be CAREFUL!)

Bear in mind that even if all of the above resistance measurements make the switch and related wiring look like it's all working OK, it's still quite possible that something very different is happening when the motor is running and things are spinning and moving. So, IF there's a way to disconnect just the switch's connections, as near to the switch as possible at first, and then run the saw WITH the capacitor installed (and observe the capacitor, and also measure the voltage across its terminals), that might be helpful in diagnosing the switch and wiring. IF that sort of thing can be done, then I'd also try it without the capacitor installed. And if there are any other places along the entire circuit, from motor to cut-out switch to capacitor, that can be disconnected, you might want to try disconnecting ONE at a time (with all others connected each time), to see if the problem will go away, which "should" help to narrow-down the location of the problem. Maybe start at the capacitor and work your way back toward the motor.

Or... maybe the new capacitor was defective. Or (probably just "grasping at straws", but) maybe its voltage rating was only marginally sufficient, and should have been higher. Or maybe some other capacitor spec didn't match the original's well-enough.

By the way (and the following might not matter at all, in this system, but, just for the sake of completeness, again) do you know if the cut-out switch is supposed to disconnect both sides of the capacitor, or just one side? (Do you, by chance, have the saw's wiring diagram?) And is your saw using a three-prong plug, or a two-prong polarized plug, or a two-prong non-polarized plug? And, while I'm at it, are you sure that the hot and neutral (and ground, if used) AC mains supply circuits are wired correctly?

- Tom Gootee


Joined Apr 2, 2007
i think there wud be a voltage across the switch after the capacitor is disconnected,
single phase induction motors can run more smoothly if capacitors are not disconnected however i don't know what ratings of capacitor or winding are req for it.

Thread Starter


Joined May 9, 2007
Thanks for the responses.

To answer some of the question you brought up...

- I was measuring the voltage between each cap lead and ground (motor body).
- The motor is wired through a switch and a 3 prong plug but I am bench testing it without the switch in the circuit (still using a 3 prong plug)
- The wiring (hot, neutral and ground) in the shop is correct.
- The motor has a start capacitor but does not have a run capacitor.
- The startup load on the motor is small - spinning 2 thin 14" alum wheels and a blade.
- I do not have a wiring diagram for the motor and it appears to be a noname (Chinese?) motor.
- I can disconnect the cut-out switch and see what happens.
- I will not be able to safely test the volage at the switch while it is running.

I was expecting the voltage to drop to 0 volts at the cap once the switch opens - I don't have a reason for expecting that - just that I thought that would cut the cap out and I don't know any better. Since it doesn't drop to 0 volts I thought there might be a short and that is why I checked for breaks in the wires.

From this info it appears to me that the cut-out switch is working but power is not being removed from the cap causing it to overheat and spit oil and smoke. I don't know what is suppose to happen at the cap leads or how to test it so I can't tell if it is working correctly or not.

This motor worked when new, has not be heavily used, has always been in conditioned space and appears to be pretty clean - it only had a little dust in it after all those years.

How likely is it that the start windings (does it have them?) have gone bad?



Joined Apr 2, 2007
i m not sure but maybe the voltage rating cud have something to do with it.
i once accidentally apply a higher voltage to the cap and it literally explode and there was oil everywhere.
since u havent got a fitting reply i'd also suggest you to post a query in the motor forums out there. u can always have an electrician look at it. its not always easy to assess things
without actually having a look at it.

Ron H

Joined Apr 14, 2005
Did you install the cap with the correct polarity?
Motor start caps are nonpolarized.
Jrappl: If you measured the voltage at each end of the cap when it was supposedly disconnected, the voltage at each end should have been the same (120V nominally), because the reactance of a 160uF capacitor at 60Hz is only about 17 ohms. On the other hand, if the cutout switch is open, or the capacitor is miswired, it will have considerable voltage across it and current through it, and the power dissipation of the internal resistance will cause it to overheat and go up in smoke.


Joined Nov 1, 2006
On a single-phase motor wired for 120V the cap, centrifugal [starting] switch & start winding are all connected in series across the AC terminals. The run winding are connected across the AC power termnals. In order for the cap to be smoking & boiling the oil out after the motor is up to speed there still has to be power on the cap from somewhere. It's possible there may be a carbon track across an insulator in the starting switch across the switch or to ground. Take a good look at the wire going from the starting switch to the cap & this part of the switch. If possible, megger across the switch terminals & also to the mounting holes.