I have a very old table saw (runs off a pulley) that won't start unless I give the pulley wheel a gentle spin just as I turn on the switch to the saw. I'd like not to have to start the saw this way (for obvious reasons), but don't know enough about electrical motors, capacitors, centrifugal switches, etc. to know how to fix this problem. Can anyone out there help me?
Thanks.
Lynn Willis
Make sure your motor isn't underpowered or the saw arbor isn't binding preventing the motor from starting by itself. If it all free moving then it's probably the cap if it's capacitor start or the centrifugal switch if it's inductive starting or a loose associated wire. If it's a 3 phase motor there is a missing phase. You have to be handy to take it apart and fix it. You can get the parts at any motor shop or electrical supply wholesaler.
Those motors are pretty universal, and not that expensive. (Compared to the price of a new cast-table saw, at least.) Take the old one off, and go to local 'electric motor repair' place. Almost every town over a few thousand people has one. They can either rebuild yours, or sell you a drop-in replacement. If there is no data plate on the motor itself, they will want to know the make, model, and best guess as to the year of the saw. If you take it in with the pulley on, make sure you get YOUR pulley back, or one of the same diameter and cross section. And pick up a new belt while you are at it- as long as you have it apart, etc. Much like a furnace motor, these things should last 15-20 years, easy.
That is, unless you WANT to learn motor repair for a one-time job, of course. And yes, sticking your finger near spinning parts is something to be avoided. Pinch and crush injuries take a long annoying time to heal, and traumatic amputations can ruin your whole decade.
aem sends...
If the motor case has a lump on the outside it is a capacitor housing. Capacitors are fairly inexpensive and easy to change. You will need to open up the "lump", take the capacitor along with you. It will usually have 2 leads of the same color attached to the capacitor, it does not matter which one goes where - be very aware that capacitors can have a large dose of electricity stored up, don't be the path.
If the motor has two lumps it has both a start capacitor and a run capacitor.
If the motor doesn't have a lump, it is probably centrifugal switched to bypass the start windings. You would need to open the end bell with the weights. About all you could do would be to clean up any gummy residue that may be preventing the contacts coming back in to engage the start windings.
It would probably help to tell the brand name. Virtually all tablesaws have pulleys with the exception of the newer, cheap direct drives. Your motor will have a plate telling the Volts, Amps, Horsepower, This information would also help.
Other people have already given you a bunch of good advice about how to fix the motor. I thought I'd try to explain *why* it behaves this way:
A single-phase induction motor with only one energized winding will not start rotating on its own. If you give it a push in *either* direction by hand, it will then accelerate to speed and continue running in the direction you started it. But it won't start.
To avoid the inconvenience of manual starting, all single-phase motors have an additional winding of some sort. The second winding is mechanically rotated from the main winding, and it's fed with current that is somehow shifted in phase relative to the main winding. This creates an effectively rotating magnetic field, which makes the motor always start in one direction. Anything that prevents this second winding from getting power will also prevent the motor from starting on its own.
The simplest design mechanically is the split-phase motor. A centrifugal switch is supposed to apply power to the second winding when the motor is stopped, but disconnect before it reaches full speed. Switch contacts can get burned, or the switch just wears out.
A capacitor-start motor uses a capacitor in series with the second winding, again with a switch that opens when the motor reaches full speed. The switch can be bad, or the capacitor can be bad as well.
A capacitor-run motor keeps the capacitor and the second winding connected all the time. Here, the second winding both provides reliable starting and more running torque. Again, a bad capacitor means it won't start.
Dave
Wow! Thanks to you all for the advice, and special thanks to Dave Martindale for explaining the business with the two coils and the centrifugal switch. It was all very helpful. I'm hesitant to disassemble the motor to try to find the centrifugal switch -- I have a bad history with such things -- but I'm guessing that it's on one side or another of the motor, so I'll try blowing some high-pressure air in there to chase the sawdust away from the contacts. If that doesn't work, it's off to the motor shop we'll go.
Thanks again for all the help.
Lynn Willis Indianapolis/San Diego (winters)
Don't rule out that the motor start winding is open (motor bad). Don't forget to ohm out the start winding.
I also think it's the capacitor.
Thanks for all the help. I took the thing to the shop, they replaced the capacitor, blew out all the accumulated sawdust and cleaned the contacts of the stationary switch (whatever that is). The thing now runs like new. This motor has ventilation ports in the sides that suck in sawdust so I've put a deflector over the motor to keep that to a minimum. Maybe this thing's good now for another 50 years.
Thanks again, folks.
Lynn Willis
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