Motor Starting Capacitor Question

[Joe A Faulkner]

I have a used Grizzly Dust Collector- 2HP 125v motor. I've only had it for a year, and used it very sparingly in that time, but I think it is over 20 years old.

Today I had some shop time and it was put to use. I am making three wall cabinets, so I had the DC running while ripping sheet goods for the sides, ripping some hickory for the face frames, some popular for hanging rails, and using the SMC to cut rails to length. I probably started the DC 3 to 4 times - no issues. Towards the end of the afternoon, I had it off for 20 minutes while sorting through the lumber rack to find more popular for the hanging rails. When I went to rip the popular, the DC hummed, but did not spin up. My guess is that the motor starting capacitor needs replaced.

I know very little about capacitors or electric motors for that matter, so I am hoping a Creeker who is knowledgeable in this area will help me out. A few questions come to mind.

1) Based on the description of my DC's behavior, does it sound reasonable that the problem is the Motor Starting Capacitor?
2) Is it common for capacitors to fail?
3) Is there any significance in the poles/leads? In other words, does it matter which wire goes to which lead? I don't note any markings on the leads on the cpacitor.
4) One web site had a category of "Motor Start" capacitors and another category of "Motor Starting" capacitors. Are these different concepts or is one man's Motor Start capacitor another man's Motor Starting capacitor?

I apprecitate any help on this topic. I'd really like to get the DC operational again.

[Lee Schierer]

Capacitors do fail, but usually not in just a couple of years of use. Check to insure your impeller is free to rotate and that it isn't jammed by a piece of scrap or shavings. Most motor shops can check the capacitor for you for a nominal if any fee. Also check to see if there is a thermal reset button on the side of the motor. Try it again after the motor has had a chance to cool and see if it starts. Lack of ventilation may have over heated the motor.

[Rod Sheridan]

Hi, at 20 years of age the capacitor could certainly be bad.

As Lee indicated a motor shop can test the capacitor for you, and check the other items Lee mentioned.

The capacitor doesn't care which wire is connected to which terminal, and and a motor starting and motor start capacitor are the same thing.

Just buy a capacitor with the correct voltage and capacitance (expressed as micro-farads or uF).............Regards, Rod.

[Randy Henry]

Mine just went out on my Woodsucker. I called Grainger, gave them all the numbers on the old one, and for $11.00 each, I ordered 2, to have a spare if the new one fails. My d/c is 8 years old, so I guess it could go out again.

[scott spencer]

If the caps are easily accessible, it's usually pretty obvious from a glance when a cap goes bad....it's worth a look, but I'd also be sure the impeller is clear as Lee suggests.

[Joe A Faulkner]

Thanks All.

Lee's questions prompted me to return to the shop. Ventalation is not really an issue. The DC sits near the middle of the shop. The motor is about 5' off the ground with a good 5' of clearance to the ceiling. It also is a fan cooled motor. I reconfigured the unit into a Thein-based separator. I checked to make sure that the motor was spinning, removed the housing around the fan, blew out the area and was pleased to not see too much dust flyout. Hit the on switch and right before the motor spun up, I noticed arching on the wires connected to the motor.

I let it run for a few minutes, then turned it off, unplugged it, hit the on switch, thinking this might drain any residual voltage in the capacitor, and then tighted the wire terminals on the motor. Now it spins up without any arching, so I think I'm back in business. Of course all of this was after ordering a replacement capacitor on-line. Oh well, for $13 including shipping and handling, I'll have a replacement on-hand, in the event it is needed.

I'm glad I have it working again, as next up is ripping out more stock for the face frames and doors. While I've only had the DC in operation for about 9 months, I've come to appreciate working without a dust mask.

[ray hampton]

I would try a different approach to discharging the capacitor, maybe a water metal pipe or short across the plug blades with a screwdriver and then flip the switch

[Dan Hintz]

I would try a different approach to discharging the capacitor, maybe a water metal pipe or short across the plug blades with a screwdriver and then flip the switch
I would suggest against such a thing... fast discharges on a cap not designed for such rough abuse can destroy it in short order. Once in a blue moon might be okay if you really need to get your hands in there fast, but every time you do you're causing damage.

[Jim McDonald]

Simple capacitor check if you have an analog ohm meter;
Short capacitor out to discharge. Touch leads with probes. Ohm meter will respond in one of three ways.
1. If good, needle will swing over to the right, pause and swing back to the left.
2. Needle will not move. Cap is open, replace.
3.Needle will swing to right, will not return to the left. Cap is shorted out, replace.
Note, you will have to reverse the probes each time that you want to repeat the test.

[Charles Lent]

All of this is good information if the capacitor is bad, but more likely the cause is a malfunctioning centrifugal switch in the end of the motor. Dirty contacts or the slide collar sticking can cause the capacitor not to be re-connected as the motor slows down. The next time that the motor is started it tries to do so without the capacitor in the circuit, and the motor will fail to start and just hum. Before you replace the capacitor, take the time to be certain that the centrifugal start switch is functioning correctly. It's inside the motor end cover where the wires go from the capacitor. There is a pair of weights on levers that surround the motor shaft and fly out away from the motor shaft as the motor comes up to speed. They move a plastic collar along the motor shaft. This collar can stick to the shaft and not move easily from rust or dirt. All it usually needs is cleaning and a little lubrication. The switch itself is a spring contact that is moved by the collar, causing it's contacts to open when the collar slides on the motor shaft. These contacts also need to be clean and in good condition. If all of these parts are working correctly, then the capacitor is likely the problem and needs replacing. I know, the capacitor is the easiest part to get to since it's outside the motor, but in 50+ years of working on motors I've found that the mechanical parts of the centrifugal switch are more frequently the cause of the problem.

Charley

[Paul Saffold]

Not to hijack the thread cause I think my problem is related. We'll one of my problems any how :eek:.

I have a Penn State DC-3 portable 1 1/2 hp with a shop-made separator. About 10% of the time I go to start it the motor hums. I give it a shake or kick and it starts OK. It is about 20 years old but has done this for a long time. Since I don't have a remote start on it I'm right next to it and shake it right away. I know there aren't chips jammed in it. Am I doing any damage to it operating it the way it is? Sounds like I need to follow Charles Lent's advice and check the centrifugal switch.

2nd question. Dan, or anyone else, what is the better way to discharge a capacitor before checking it?

Thanks, Paul

[Dan Hintz]

what is the better way to discharge a capacitor before checking it?
Through a resistor of appropriate wattage. A metal bar has essentially zero resistance, so a very high current flows. With a good resistor, say 100 ohms, current is severely reduced and is more gentle on the cap's plates.

[Paul Saffold]

Thanks Dan.

[Paul Saffold]

Update. Thanks Charles for your advice. The contacts on the centrifugal switch were out of alignment and barely making contact and looked burnt. Slightly bending the spring and a little sandpaper smoothed the contacts and all is well.