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I have a bench grinder that doesn't want to start. I figured out the problem: bad centrifugal switch. It's a two-contact, normally open affair (it starts closed, then as the grinder spins up the weight is taken off and it opens). One of the switching contacts is great. The other looks like the part that was actually making contact got ripped off, or fused to the bottom and then when it sprung back ripped off, or something like that. I verified it's the switch causing the problem because the broken contact arm actually stuck a bit when I closed it, so I pushed it closed until stuck, reassembled the grinder, and it started normally.

This grinder is ancient and the switch would be a complete pain to replace, both mechanically and simply finding a replacement. I'm wondering if I really need both contacts to actually open/close for longevity in the future. Could I just solder the ripped contact arm down to the contacting section? I'm assuming that both have to be closed in order for current to flow, so is it enough to just have the centrifugal part of the switch only open/close that single contact arm?

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  • bypassing one of the switches removes a failsafe component from the motor design ... if the remaining contact should get stuck closed then the motor may experience a runaway condition, which could cause the grinding wheel to fly apart and cause damage to persons and property
    – jsotola
    Oct 2 '20 at 3:02
  • I suppose I can just repair it by soldering a small copper shim onto the broken arm, then? That should allow it to make contact.
    – Surgo
    Oct 2 '20 at 3:08
  • The motor brand and model number would be helpful. The motor may start fine but will overheat if it can’t switch to the run winding in a few minutes.
    – Ed Beal
    Oct 2 '20 at 10:48
  • @EdBeal the idea is that since the switch has two contacts, only one of them can do the switching. It's an ancient Dayton 1Z07 (likely from before the company was purchased by Grainger).
    – Surgo
    Oct 2 '20 at 13:58
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Surgo I understand the goal I repaired a similar motor not long ago 1-1/2 Hp on a 56 motor frame Dayton brand. The contact set in my unit was 120/240v in the low voltage connection only 1 contact was needed (to tell the truth I did not look to see if it was different for 240v) These centrifugal switches are used across many sizes and brands. I got the machine running again by changing over to the unused contact. I needed a backup or better repair and then found an aftermarket centrifugal switch assembly for under 15$ (I Could not find the correct one on grangers website).

Google or whatever search engine Search for centrifugal motor switch they are sized in mm so you will need your motor shaft size in mm I believe they had 1/2 inch through 1.5”. for example I needed a 16.5 I think it was a L16 it screwed right in. This was cheaper than $235.00 for a new motor, 35$ Later I had a new motor 2 bearings and the switch. You may want to check your bearings because end play is what damaged my switch.

I did look for your wiring configuration and came up empty so you may have to trace the connections on the board to the windings they may not be connected as mine were not. They could have a combination of changing the coil configuration + a capacitor and at the time of manufacturing it just used dual contacts where only 1 is needed to bring the cap in and start winding.

If the start winding is held in the motor will overheat because of the extra field coil that provides the torque to start. so look at the configuration it may be fine but you need to know or you will let the magic smoke out if left on for long.

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  • Thanks. I temporarily repaired it today by soldering a piece of copper onto the broken end of the switch's arm so it could make contact, but I'm under no illusions of a permanent repair. I'll look for a new stationary switch using the method you described. The wiring is pretty simple luckily -- only two of them, one attached to one end of the switch the other attached to the other :-)
    – Surgo
    Oct 3 '20 at 1:44

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