Electric Motor Starting Problems

When the motor sits and hums instead of starting, the problem could be in a switch or in the starting capacitor. March 3, 2009

When I try to start my Delta X5 230v 3hp Unisaw (GPE switch), I get a strong hum and the blade turns slightly. Is there a starter that can be tested and /or replaced? Is the motor shot? The saw is only 5 years old. It had been working fine the previous day (very light use) and the following morning would not start. Has anyone had a similar problem?

Forum Responses
(Cabinetmaking Forum)
From contributor A:
The magnetic switches eventually get tired. You can open it and raise the amperage setting so you get a little more juice at startup. That will get you through a few more days while you wait for the new switch to show up in the mail.

If you open up the cover of the switch there is a white adjustment screw that has numbers on it. I think it is set at about 18- 20 amps at the factory (mine is a 5hp so I don't know exactly your setting is), but you can turn it up a couple of amps without doing any damage. It is only the starting amperage.

From contributor P:
Single phase? Check (or just replace) the starting capacitor. Cheap fix from your local Grainger. I've been running machinery for a long time, and have never had a mag starter "get tired," but maybe I'm just lucky that way.

From contributor S:
I agree, check the start capacitor (if your machine is single phase). The start capacitor provides a quick shot of extra current to help get the motor moving, and (in some motors) tells the motor which direction to run.

If the capacitor is defective, often the motor will just sit there and hum because it is trying to run in two directions at the same time. If you give the motor a little spin, it will take off and run in the direction you spin it.

If your motor is 3 phase, there is often a small centrifugally-activated switch inside the motor that briefly routes current to the start windings. If this switch is bad, your motor will have a difficult time starting. This switch can be replaced fairly easily.

When messing with capacitors, always short out the capacitor leads with a screwdriver. The function of a capacitor is to store electricity, and they can/will bite you if you don't discharge it.

Back in high school, we would take an ignition condenser (which is nothing more than a capacitor), charge it off a spark plug wire, then leave it sitting on someone's desk. Invariably, the poor slob would pick it up, hold the capacitor in one hand, and touch the wire lead with the other. It was no different than touching a spark plug - it was great fun!