Unisaw magnetic switch quit working

[Jack Budak]

I have a Rockwell unisaw from either 1969-70. The motor is a 3 hp baldor single phase 220 V and the motor says 32 amps. The magnetic switch stopped working and the saw will only run if I physically hold/push the switch button in. I disassembled the switch, blew it out with air. The switch does not have the overload protection or a reset button like the newer ones. I was going to buy a new switch but there are 4 wires running to the switch. All the new switches I have seen only have room for 3 wires. I did notice that the new 3 phase switch have 4 wires running to the switch. I have attached a couple of pict of the box on the back of the machine. If you look close next the red wire there is a red plastic tab with grey overspray on the tip. I thought it was a reset switch but it does not move. Does anyone have a solution to this? Thanks in advance! Jack
d.2.2.jpgphoto2.jpg

[Steve Rozmiarek]

Good chance it's the coil gave up the ghost. It's that little gadget that has the spade connectors in each side, near the center of the whole thing. If you have a ohm meter, check across the poles. I don't know what the correct number is right off, but it should read something. I have trouble finding Furnas components here for the stuff I work on.

[Rollie Kelly]

Check the seal in contact, bottom contact marked with numbers 2&3. Check for bad contact surface and or loose connection on either end of wire #2. Do this after removing all power to the machine (gotta make the lawyers and ins. cos. happy)!
Rollie

[David L Morse]

If after checking what Steve and Rollie recommended you've not found the problem it would be worthwhile to make sure you have 24V coming out of the transformer as it is another possible point of failure.

Also, it should be noted that your switch DOES have overload protection. It's marked in red on the attached photos.
OVP Schematic.jpgOVP Photo.jpg

[Steve Rozmiarek]

I figured the pros would be all over this by now, so I'll toss out another potential problem. The way that starter works is by using a 24 volt control circuit to energize the magnetic contact that switches the 220v. The on/off switch will be using 24 v. There are several possibilities, one is the coil that energizes the magnetic field that I mentioned earlier, another is the transformer itself that makes the 24 volts isn't working. You need a meter to see it, but that would certainly shut down the contactors too. As Rollie said, loose connections are a possibility too. There is often a reset button, fuse or breaker on the control circuit transformer power supply line, your's is the red button at the top.

I use these a lot in 480 volt power situations, and those are the most common issues. If you hear the contact "thump" but no machine power, you have a dead leg on the main power, or bad contact on the mains. I don't think that is the issue here. I don't know why they chose to step down the voltage to 24v for the control circuit. They could have used a 120v coil and skip the transformer to do the same thing.

[Steve Rozmiarek]

If after checking what Steve and Rollie recommended you've not found the problem it would be worthwhile to make sure you have 24V coming out of the transformer as it is another possible point of failure.

Also, it should be noted that your switch DOES have overload protection. It's marked in red on the attached photos.
OVP Schematic.jpgOVP Photo.jpg

Sorry David, guess I was typing mine when you posted.

[David L Morse]

I don't know why they chose to step down the voltage to 24v for the control circuit. They could have used a 120v coil and skip the transformer to do the same thing.

The transformer with it's multiple voltage primary allows this switch to be used from 120V single phase to 480V three phase with only minor wiring changes, no need for different coil voltages. See the primary tap information on the schematic and the maximum horsepower chart in the lower left corner. It's a "universal" switch.

[David L Morse]

Jack, do you want to try to repair this switch or do you prefer just replacing it?

If you want to try the repair route you'll need at least a continuity tester for diagnosis. A multimeter gets you the continuity test and also the useful ability to test voltage, assuming you're comfortable working on live circuits.

[David L Morse]

... the saw will only run if I physically hold/push the switch button in. I disassembled the switch, blew it out with air.

When you say "switch button" do you mean the start button that you normally use to turn the saw on? If so then Rollie's post is the answer: there's a problem with those contacts on the contactor.

[Steve Rozmiarek]

The transformer with it's multiple voltage primary allows this switch to be used from 120V single phase to 480V three phase with only minor wiring changes, no need for different coil voltages. See the primary tap information on the schematic and the maximum horsepower chart in the lower left corner. It's a "universal" switch.

Makes perfect sense.

[Mike Heidrick]

Contacts on the contactor look pretty rusty/corroded. I'd clean those up PDQ (unplug your saw). Definately get out a meter and test for continuity. If you dont feel comfortable doing this take it to an AC motor shop.

[Alan Schaffter]

That appears to be a very old, rebuilt, or home-brew Rockwell/Delta LVC (low voltage control). Essentially it uses 24V (low voltage) through a latching circuit to to operate the main contactor. It uses a spare set of contacts on the contactor for the latching circuit. When you depress start which has N.O. (normally open) contacts 24V energizes the contactor coil. When the contactor closes 24V runs through the stop button and one or more over-current heaters and possible an electrical box interlock to the coil and a spare set of contacts to keep the contactor coil self-energized (latching circuit). When you hit stop, which has N.C. (normally closed) contacts, you open the contacts, breaking the latching circuit which de-energizes the contactor coil and all contacts (24V and 220V) open.

What that means is that if the motor runs when you mechanically operate the relay as you say, the 220V wiring and contracts are good, but won't operate when you hit the start button, the contactor isn't closing and you have one or more problems in the 24V control circuit. (If, when you press and hold the start button, the contactor closes and saw runs but stops when you stop pressing, your problem is most likely 1, 2, 4 or 6)

Possible causes:

1. There is loose, broken, or improperly connected wire in the 24V circuit.
2. One of the heaters is bad (the heaters break the start/latching circuit when there is an over-current condition.) You can jumper across the 24V LV terminals on the heater to test each heater.
3. The 24V transformer is bad or wired incorrectly (wrong input or output voltage. if multi-voltage- yours doesn't look OEM)
4. The 24V (latching circuit) contacts on the main contactor are bad.
5. The 24V coil on the main contactor is bad.
6. The stop button does not close properly (high on my list of possible causes). You can test this easily by putting a jumper across the stop button terminals (N.C.). CAUTION: If that is the problem, you'll need to remove the jumper or pull the plug to turn the saw off after testing. An OEM replacement button assembly (Furnas) is likely not available and if it is it could cost > $100 unless you find one on Ebay. You can sometimes find a complete LVC box for that price. If it is the standard Rockwell-supplied momentary start/stop button assembly it will be sealed so can't use compressed air to blow it out.
7. If equipped with a electrical box interlock switch (some Rockwell Delta LCV boxes had them), you won't be able to start the saw normally if it is mis-adjusted, broken, or the cover is off the box.

OEM LVC start/stop assembly (what is behind the big square red and green buttons):

[Jack Budak]

Alan and David, here is a picture of the actual switch. It appears to be very simple from the ones I have looked at from other modern saws. Remember this saw is from 1969. I wanted to mention that I inspected the contacts of which there were 6 on what I am calling the magnetic switch in the box on the back of the saw. 2 looked new and 4 were very pitted. I tried to clean the but without totally dismantiling it was very tight and difficult. After cleaning them the saw is reacting the same. It would turn on when I held the start button down but would shut off when I released the start button. Could worn out contacts cause my problem? Should I just replace this part? What is this part called. I am not good with the nomenclature of anything electrical because I build wooden boats for a living. I thank y'all for your help!
switch.jpg

[David L Morse]

Ok, let's step through this. When you push the start button the motor starts but it shuts off when you release the start button. Clearly the transformer and contactor coil are good or the contactor would never operate. Let's follow the current path starting at the transformer lead marked "1 SPLICE". From there it goes to the stop switch which is normally closed. If it were not closed then there is no current path and the saw doesn't start. Therefore the stop switch is good. Through the start switch (which obviously is good) we get to the known good contactor coil and then through the overload contacts which must also be good since otherwise the saw would not start. The only thing left is the control contacts on the contactor whcih are in parallel with the start switch and should maintain the current path after the start switch is released. As Rollie pointed out earlier this is your point of failure since logically nothing else is a problem. Your contactor (magnetic switch) has a total of eight contacts. It's the ones you didn't find that are causing the problem.
To be clear, the momentary switches show in your latest photo are NOT the problem; no need to replace that assembly.

[Jack Budak]

David, I just pulled everything out of the box so I could get full access to everything. I have attached a pict of the contacts I told you about. There are 3 pair ( upper and lower) on each side. These are the ones I said were pitted. When they are making contact there is 2 pair (upper and lower ) that you can see in pict 2. Is this the 2 contacts that I didn't find? Should I just replace this part or do a cleaning of the contacts?contacts.jpgcontacts2.jpg Thanks so much!