3 phase to 1 phase motor swap, contactor wiring

[Eric Myket]

I recently picked up my first sliding table saw, a Grizzly G0588, and I'm swapping out the 7.5hp 3-phase motor for a single phase motor. I'm finding some curious things via multimeter when I look at rewiring the contactor though, following this guide (https://springercontrols.com/news/single-phase-motor-3-phase-contactor/). With the saw plugged into single-phase 240V, probing the contactor shows 240V between L1 and L2/120 each to ground (as expected since that's how I wired the input plug), but for some reason there's still 38V between L3 and ground even though the input on that wire isn't connected to anything at all. This is true even if I completely disconnect the wire going to L3, the L3 wire still shows 38V to ground, and likewise T3 overload shows about 35V to ground when the contactor is closed and the L3 wire is unconnected. The wiring on this saw isn't complicated either, there's just power in, and out to the motor, with the main on/off switch, 1 E-stop, and a blade change interlock all connected to the contactor in between it all.

I'm afraid to follow the guide I linked and jumper between T2 overload and L3 until there's no voltage on L3 on its own, in case of creating a short.

Should I be interpreting this as a failure/short inside the contactor itself somehow? I wasn't able to observe the saw running when I bought it, so that's possible. Is there some other explanation anybody who's done this can suggest?

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[Bill Dufour]

Put one meter lead into ground of the outlet. Hold the other probe metal in your fingers and check for voltage. I am guessing is is acting as a transformer winding. Probably very little amps there. Clean dry floor good shoes, etc
Check with a actual needle meter not a digital VOM. That may be enough load to drop volts down to zero. Run meter as amp meter from L3 to ground and see if any amps flow.
Try a 12 volt bulb from a car, protect from it exploding. My guess is the voltage is too low for a wiggie to register.
Bill D

[Alex Zeller]

I would verify that the contactor is rated to handle the current draw of your new motor. How big of a motor are you switching to?

[Dan Friedrichs]

As Bill said, it's almost certain that you're just measuring some high-impedance circuit that can't actually source any amps. The overload, for instance, has some current sense coils that are probably coupling to the L3 contact.

You could verify this by putting some load (light bulb, etc) across the 35V and observing it "disappear".

[Eric Myket]

Thanks everyone! You were right, once I hooked up a load to L1 and L2 the voltage on L3 dropped to .5V.

The contactor is well within spec, I'm hooking up a 3HP/16A motor, vs. ~20A per phase for the original motor.