Electric motor problem

[Robert in Md.]

I have a table saw with a 2 hp Baldor,
220 volt, 1 phase motor on it. When the start button is pushed the motor hums and acts like it trys to turn.
I,m thinking the start capasitor is shot. Does this seem reasonable.? I can get the capasitor off without removing the motor.
I don,t want to remove the motor under this saw if possible, I remember how much trouble I had getting this motor bolted up and under the saw table 30 years ago when I was 40 years old.

Robert in Md.

 

[Bob M]

Robert - You can try replacing the capacitor and see if that fixes it.

More likely however the problem is the centrifugal starting switch. Could be the switch mechanism has become loaded with sawdust and is not closing contacts when it should. Or the contacts are welded or burned out. Could also be a burned-out start winding inside the stator.

Unfortunately the motor must be opened to check/correct a bad centrifugal switch.

 

[Bob Huntress]

Take a 10,000 ohm resistor, when you take the capacitor off. Without touching the two legs, touch the resistor to both terminals on the capacitor to discharge the stored electricity, which has the potaintial of preventing a 41th birthday. It could, however be a start capacitor. With the unit unplugged, you can spin the blade freely?

 

[Bob Huntress]

Not to argue, yet, most of the time that I see the contacts burnt or welded, the indicator is the when we attempt to stop the unit, it must be either unplugged or the breaker flipped, as they often weld in the closed position. I know that it is possible, yet, do you think it is as likely as contaminates distroying the motor bearings or such, as he stated that the motor does hum, like it's trying to start. From what he states, it is getting power. it just isn't starting.

 

[redtom]

I have an old sears saw with a 1 hp motor did the same thing. The flyweights in the starter were full of saw dust. Took it apart and blew it out-ok now.

 

[Bob M]

Bob - I think you are confusing a magnetic STARTER switch with a centrifugal STARTING switch.

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A magnetic starter is a large power handling relay. It's contacts close/open thus starting /stopping the motor as commanded by start and stop push buttons. (Incidentally also magnetic starter generally also incorporates overload sensing circuitry to open the relay and stop the motor under overload conditions)

Now if a magnetic starter's contacts become welded shut a motor indeed can only be stopped by shutting off the power (unplugging the cord or opening the breaker) exactly as you describe.

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On the other hand the centrifugal starting switch is a switch whose contacts supply power to an auxiliary starting winding in the motor. At startup when the motor shaft is stationary, the contacts are closed and power is supplied to the starting winding. This causes the shaft to start to spin. Then as the motor shaft comes up to speed the contacts open, shutting off current to the starting winding.

Now if the starting switch contacts become welded shut they continue to feed current to the starting winding after the motor comes up to speed. The heavy current draw causes either the breaker to trip or overload button to pop.

Or if the starter switch contacts fail to close when the motor is stopped, or if the contacts are are burned out, the starting winding will not be energized when the power is turned on. The motor now will hum but the shaft will not turn. (The motor will also overheat if left very long.)

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Robert's problem description (motor hums but fails to start) points to issues with the starting winding circuit - the centrifugal switch, the starting capacitor or the starting winding itself.

 

[Bob Huntress]

Oh I fully agree that the start windings could very well be the problem.

 

[36 Coupe]

A motor can develope excessive end play and fail to close the starting switch.Common problem fixed with fiber washers.Start capacitor is next suspect.

 

[Stephen Newell]

More than likely the motor is shot. Your easiest and cheapest fix would be to take to a shop that rebuilds electric motors to evaluate. Since you don't want to take the motor off you might inspect the brushes. If the saw is that old they may be bad. I also had a sears table saw that had brakes built into the motor that would stop the blade when the saw was turned off. When the saw got old the brakes stayed on and the motor wouldn't start until I took the brakes off.

 

[Anonymous-0]

If the motor is an open frame try blowing it out with compressed air. I worked in a sail boat shop a couple of summers and we had to periodically blow everything out with compressed air to get the saw dust out. The barn fan in the wall would get slower & slower and finally stop until it was blown out. Cheap fix if it works. Good luck! joe-

 

[John T]

GREAT EXPLANATION BOB, I could have done that with a lot more words however lol lol

My uncle had a bench grinder with a split phase motor in which he purposely disabled the start winding, that way he could turn it on and give it a good hand spin in the direction he wanted it to turn and off it went. I was a kid then n considerd him a genius.

Youre combination of mechanical as well as electrical knowledge is showing !!!!!!!!!!!!!!

John T

 

[PYCHUNG]

DIRT DOBBER

 

[Bob M]

Thanks for the compliment John!

Bob