I picked up a table saw(very large cast iron top) 5 hp single ph motor, has
a magnetic switch on it . It will run only when you hold in the start
button, shuts down as soon as you let go, tough to use that way:)... I
cleaned out the switch and box, checked for loose wires, found no arced or
burnt contacts , no difference. I have my trusty multi-meter and am ready to
troubleshoot as soon as one of the RCM electrical guru's rains a bit of
knowledge on me. What do I do next?
I don't know about saws specifically but usually if there is a magnetic
switch it is so that a break in the magnetic hold circuit will stop it for
safety purposes. Any open in the safety circuit will stop it. That would
include a stop switch, a safety switch that opens if the guard is removed
from the blade, a cover is open that shouldn't be such as one over the
belts, etc. I'd look for something like that.
"I'm not grown up enough to be so old!"
You have to check the wiring. See the page from Ugley's Manual:
Basically, the start button briefly energizes the coil, which generates a
magnetic field which closes the contacts so the motor runs. It also closes a
little relay bolted onto the side that's normally open. That relay has to be
wired from power to the coil, so if the relay's engaged and the power is on, the
contactor stays engaged. If the power goes off, then the coil is de-energized
and the contacts and the secondary relay open (by passive spring action) so when
the power comes back on it won't restart by itself. The Brits call this a
"AndrewV" wrote:(clip) It will run only when you hold in the start button,
shuts down as soon as you let go (clip)
So we know the start button and the relay coil work. There should be a pair
of contacts on the relay, in parallel with the start switch, which start to
supply holding current to the relay, so when you let go the button, the
relay doesn't open. Find this part of the circuit, and look for burned
contacts or a broken wire. That HAS to be it.
"AndrewV" wrote in
Control Power comes in, goes through the 'start' button (normally open),
possibly through some safety switches and on to the coil on the starter
contactor. From a secondary set of contacts *(normally open) on the start
contactor, Control Power goes through the 'stop' button (normally closed)
and any safety switches to the coil on the start contactor.
Upon you pushing the start button and any 'start condition safeties'
satisfied, the coil is energized and pulls in the motor contacts along with
the secondary Control power contact. This secondary contact, creates a
closed loop running through any safety switches and the 'off' button,
effectively holding the coil in the 'on' or 'energized' state.
When you push the 'off' button, or any safety in this closed loop is
opened, power is removed from the coil on the start contactor, thereby
disconnecting power from the motor.
Trace that circuit. It may also go through some thermally
tripped switches on the contactor, to drop power when it draws too much
current -- such as when the saw stalls, or when one of a three-phase
feed is open. (Is this powered from three phase?)
It depends on how things are wired. I agree that they *should*
be wired so the start button is only in parallel with the hold contacts
on the contactor, but I've seen things mis (or stupidly) wired many
Maybe I'm mis-understanding - heaters are closed circuit. The
contacts close, current flows through the contact tips, then through
the heaters to the motor windings. If the motor draws too much
current, then the heaters (bimetal springs) trip open and shut down
the current. Heaters can't heat up unless current is flowing through
them. Sometimes a coil wire is fed through a heater so when it trips
out the coil drops out and opens the contactor.
You lost the attribution -- the part which you are questioning
was written by: Grant Erwin
not by me.
I *think* that what he meant was that if the contacts controlled
by the heaters are open (tripped), then the start button should not move
the motor. I agree with this -- as long as it is correctly wired, with
the button paralleling *only* the hold-in contacts on the contactor
(relay). But, as I have already said in another followup to him, I have
seen mis-wired things in too many places to want to bet that it is
correctly wired. :-)
I think what Grant meant by "Heaters are open circuit it doesn't start
when you push the start button, can't be the heaters" is, "If the
heaters are open circuit it won't start" and "since it does start, it
can't be the heaters."
Since the motor starts when you push in the start switch the start
switch is wired directly to the coil. There is a hold circuit that
keeps the coil energized when you release the start switch. This
circuit consists of usually two devices, an aux set of contacts on the
contactor and a stop switch. The stop switch is a normally closed set
of contacts. These are wired in series. Once the aux contacts are
engaged the curent travels through the stop switch and the aux contacts
which are closed when the coil is pulling them in. If you hit the stop
switch it opens the circuit and the contacts drop out, turning off the
Sometimes they wire the stop switch in series with the start switch but
not in your case. The aux contacts are always across the start switch.
Okay, things are getting a little confusing - the original poster said
that the table saw RUNS when you HOLD IN the push button.... and
my original comment to that post was that he should check the
continuity across the stop button contacts as they may have sawdust in
them or something to keep them from making their normally closed