Magnetic switch

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?
Thanks
Andrew
Reply to
AndrewV
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Duct tape the start button?
Just kidding. :P
Reply to
DeepDiver
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.
Best Regards, Keith Marshall snipped-for-privacy@progressivelogic.com
"I'm not grown up enough to be so old!"
Reply to
Keith Marshall
It has separate stop and start buttons?
There should be an auxilliary contact across (in parallel with) the start button contacts that holds the contactor on (self-hold).
See figure 5-13 here:
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Maybe contact "M" is bad, or one of the wires has come off.
Best regards, Spehro Pefhany
Reply to
Spehro Pefhany
You have to check the wiring. See the page from Ugley's Manual:
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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 "no-volt release".
GWE
Reply to
Grant Erwin
I would check to see if the contactor coil is fried. This is what holds the "switch" on after you press the start button.
Shawn
Reply to
Shawn
"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.
Reply to
Leo Lichtman
"AndrewV" wrote in news:429fb8c6$1 snipped-for-privacy@alt.athenanews.com:
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.
Reply to
Anthony
Check the continuity across the stop button contacts. Ken.
Reply to
Ken Sterling
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?)
Good Luck, DoN.
Reply to
DoN. Nichols
Heaters are open circuit it doesn't start when you push the start button, can't be the heaters. - GWE
Reply to
Grant Erwin
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 times before.
Good Luck, DoN.
Reply to
DoN. Nichols
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. Ken.
Reply to
Ken Sterling
Thanks for the replies, I'm hoping to dig into it later today ....I'll post an update.
Andrew
Reply to
AndrewV
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. :-)
Enjoy, DoN.
Reply to
DoN. Nichols
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." -jiw
Reply to
James Waldby
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 saw.
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.
John
Reply to
john
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 circuit status. Ken.
Reply to
Ken Sterling

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