What's new
What's new

single phase motor not starting- capacitor?

stoneaxe

Stainless
Joined
Mar 2, 2010
Location
pacific northwest
Help out a electrical idiot here, please.
The bandsaw motor went bad- was running fine, then started real weak, shut it down and tried to restart and it just hums.
2hp, 120/240v, single phase. Has one start capacitor.
Motor is wired 240, but the start capacitor is marked 110V. It has been running like this for 25 years, at least. Visually capacitor is OK.
The on off switch is 2 pole manual, seems OK, but the motor is pulling 50 amps per side attempting to start....
Motor spins freely.

does this sound like a capacitor problem?
I do not know how to test the capacitor, putting an ohm meter on it shows OL for a few seconds, then it starts counting down resistance from about 40k, 35k, 29k,20k etc- I think this reading might be the bleed resistor between the poles?
When It is replaced, should it be a 240 volt replacement?
And what about the MFD rating- this one says 704- most seem to have a range of ratings.
Any assistance would be much appreciated. ThanksDSCF1222.JPG, stoneaxe.

DSCF1219.JPGDSCF1221.JPG
 
Capacitors normally start low with an ohm meter and then go up. Starter caps are cheap. Starter caps don't last forever. Go 240 Volt.
CarlBoyd
 
If its humming its usually an indicator of an open start circuit, meaning start switch or bad capacitor. Get it spinning by hand then try to start it. If it starts, its most likely the capacitor or centrifugal switch.
 
While you have the motor out, remove the rear end bell and inspect the start switch contacts and operation of the centrifugal weights and actuator for free movement. The start winding is 120V operated so a 110V cap is fine. If you try to get a higher voltage rated one it might be too large to fit under the cover. 704MFD or slightly higher should work just fine. But get a decent quality replacement form Grainger or your local motor shop if you want another 20 years out of it, and relocate the bleed resistor so your start switch don’t get burned up.
 
Academic issue:
cap = 704uF
resistor = 15K
time constant for discharge = R*C = 11 sec
At 5*RC the it's going to be discharged around 98%.

It's nice of those Dayton boys to assume that you can't take the capacitor cover off in under 55 sec. :drink:

Don't worry about the capacitor. After your first one it won't bother you much.
 
While you have the motor out, remove the rear end bell and inspect the start switch contacts and operation of the centrifugal weights and actuator for free movement. The start winding is 120V operated so a 110V cap is fine. If you try to get a higher voltage rated one it might be too large to fit under the cover. 704MFD or slightly higher should work just fine. But get a decent quality replacement form Grainger or your local motor shop if you want another 20 years out of it, and relocate the bleed resistor so your start switch don’t get burned up.
I was hoping to not have to pull it out...
thanks for the cap advice- that is what I found looking- higher voltage was too big.

Then I found some manufacturer listings chart that simply listed the cap case size as a 1, or 2, or 4, etc-is this diameter? circumference? length? or what?
 
A good way to check a capacitor is with a 12v transformer and an ammeter. Something like a landscape transformer or control transformer or buck boost transformer can be had cheap. You put the capacitor across the supply and measure the current and voltage. An online capacitor calculator will tell you what the current should be.
 
I went the easy way- sent the cap specs to my local motor shop and had them match it up- the new cap and a cleaning of the start switch contacts fixed the problem for now. After I get another saw, this one will be pulled from service and be refurbished with new start switch ,bearings, etc.
 








 
Back
Top