I'm renovating a Shizuoka ST-N CNC knee mill with a Bandit 3 controller from ~1980 in it. The spindle drive is a Todai Electric Co DC Motor Control Pack model number MOP-1500 which has absolutely no documentation online.

There are two Fuji Electric SRC3631-5-1 magnetic contactors in it, one for each spindle direction. However, one of them has this strange add-on component on the bottom of it.

The picture below is from an ebay listing with an identical add-on, except theirs has a knob with labels 4, 6, 8, ARC, while mine just has a large plastic phillips screw in the same position.

The small white rectangle next to the knob is a piece of loose plastic in a slot, and below it there's a button which it can press. In the configuration I received this part in, that button is held down by some sort of internal toggle mechanism, so pushing or pulling the white plastic does nothing. However, if I turn the knob a quarter turn clockwise, the toggle releases and the button becomes a normally open momentary switch which I can close by pushing on the white plastic. If I turn the knob back, the switch stays open until I press it again, and then the toggle engages to keep the circuit closed.

What is this add-on called? What's its point?

My best guess is that it's some sort of safety feature, where if something passes a threshold (adjustable with the knob) the switch will momentarily open. But, I need to know what it is because perhaps the toggle is the safety feature, and the operational state is un-toggled, with the circuit open.


Magnetic contactor with add-on picture


1 Answer 1


Looks like a settable thermal overload. A lot of industrial contactors for motors will have both a contactor and then a settable thermal overload system in series.

  • \$\begingroup\$ Ahhhh of course! I can't believe I didn't test this, but of course the toggle would also affect whether the three terminals on the left of the add-on connect with the main contactor's terminals. I thought those were just pass-throughs that were there for packaging reasons. Which is absurd in hindsight! There's even mention of thermal overload systems in a PDF I found on these contactors, but I glossed over it because I didn't see how this could serve that function based on the one contact I thought it was making/breaking. \$\endgroup\$ Apr 26, 2020 at 5:57
  • \$\begingroup\$ Inside there are three current sensing thermal elements, but all three are mechanically linkedto a single "trip contact" which is used to cut power to the coil of all contactors. \$\endgroup\$
    – JRaef
    Apr 27, 2020 at 17:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.