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?
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
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.
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
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.
Keep the whole world singing . . . .
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
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
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.
Indianapolis/San Diego (winters)
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.
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