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The no-volt-release switch stopped latching on my single-phase planer thicknesser. The motor is under 1.1kw (1.5hp) and I understood that I would be able to drop in a generic switch.

Picture of Motor Information Plate

However now that I have come to wire in the switch I find my motor has three wires (plus earth) and not the simple live and neutral I expected.

In case it's useful, here are some pictures of the old switch and how it was connected:

Picture of old switch label

Unistop seem to be out of business and a like for like replacement not available.

Picture of old switch connnections

Note the yellow jumper. I have no idea what these letters mean.

Picture of old switch internals

The motor casing tells of a circuit diagram to be found inside the terminal block but alas there is nothing there, here's a not particularly helpful picture of inside the terminal block:

Picture of inside the termianl block

The canister on the right contains a capacitor which is part of how the motor copes with a single phase supply.

I've done a bit of work with this detaching everything and doing continuity testing to establish the following circuit diagram for the motor (apologies for the dodgy art skills):

Picture of terminal block circuit diagram

What I think we have here is a Wye arrangement very similar to this arrangement, the diagram on the left below..

enter image description here

The difference on my motor is that the bottom of the three coils (brown-brown) is connected by thinner wire and has a far higher resistance. I understand this may indicate a starter coil which is possibly connected to an internal centrifugal switch leaving the upper two coils to run the motor slightly out of phase due to the capacitor.

So far so good I hope, but I'm not 100% sure how to connect the Brown, Black & Blue cables that come out of the terminal block to the Brown and Blue ones coming from my single phase 230v mains power.

I can tell you that ignoring the black wire and connecting live and neutral to brown and blue doesn't work, the motor judders back and forth and doesn't get started.

I can also tell you that connecting black to mains neutral and the other two wires to mains live instantly trips my electrics.

So... how do I wire this thing up?

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  • $\begingroup$ Bit of an update this evening... I tested for continuity again and found the brown-brown coil no longer has continuity. Maybe I burned it out :( To investigate further I dismantled the whole motor and the coils look really clean and good and interestingly there appear to be only two of them. The brown wires disappear inside the coils but I'm not clear what they are for. $\endgroup$
    – James
    May 23 at 23:06
  • $\begingroup$ Just a note for how to use the site: I doubt anyone could answer your question about this specific device without a schematic of the device internals, unless they were very familiar with this precise make and model, and even then we have no idea if the starting configuration matches the schematic. Also no one with any professional ethical sensibility will want to give you advice on how to tinker with something that may kill you if you make a mistake. Always use extreme caution working with electricity, especially an unfamiliar device. $\endgroup$
    – RC_23
    Jun 28 at 2:48
  • $\begingroup$ @RC_23 Thanks - I'm keeping things unplugged when working on it and discharging the capacitor each time but I agree - it's not without risk. I guess I was hoping somebody would show up and say "ah yes, the Kity 1.5HP, I know those.." or maybe "careful, those capacitors are not to be fooled with" etc.. $\endgroup$
    – James
    Jun 30 at 10:57

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Just for posterities sake and the next person trying to change a switch on one of these Kity motors... this is what I did:

enter image description here

That's right - if I remove the mystery brown wire from the equation and don't connect it at all, the motor runs. I connected mains live (brown) to motor brown, and mains neutral (blue) to motor black and the motor runs.

Essentially, it's like this:

enter image description here

This does raise a question about what the thin brown coil is for (perhaps something to do with the old style NVR switch, or a heat sensor or something..) but that's a question for another day.

I'm not an electrical engineer - use this solution at your own risk!

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