I have a lathe with a 1.5 hp 220VAC motor, built in the late 1980s in Taiwan. For lathes of this vintage, a "normal" motor control circuit consists of a couple switches (for forward/reverse), a couple contactors, and a 24VAC transformer to run the contactor coils. This lathe, however, is different:

Wiring cabinet

I have the normal 2 contactors, plus a circuit breaker (I believe these are referred to as "OL relays" or "overload relays"?), the 240VAC->24VAC transformer, and the bit that I don't understand: a small Omron MY4 24VDC relay. Buried under the terminals at the top of the relay is a small rectifier that takes the 24VAC from the transformer and produces DC(-ish) from it to run the relay. I can't find any other lathe wiring diagram anywhere that resembles this setup, and the manufacturer is long gone.

When I acquired this lathe, the fwd/rev switch (connected to the 4 wires labeled #3, #4, #5, #6 and circled in the above image in red) were disconnected, and presumably, the switch was faulty (the original owner had passed away, so I couldn't ask). I am operating under the assumption that if I wire up a new switch correctly, the lathe will operate.

I have worked out enough of the wiring diagram to conclude that the only way the Omron relay will be energized is by connecting #3-#6 at the fwd/rev switch. The thing I don't really get is why the relay is set up to "latch" itself; once it's on, it'd stay on until the e-stop was triggered (the e-stop switch is wired to #1 and #4, circled in blue above). This arrangement I do not understand at all.

Partial lathe control schematic

The fwd/rev switch is a "barrel" switch with the extremely imaginative part number "15A PN-1". There are 8 terminals on it, but because the switch was already disconnected when I bought the lathe, I have no way to know how it was connected. One interesting fact, however, is that there is a small jumper wire between two of the terminals (2 on one side to 4 on the other side). Other than that, I don't know why it's there or what two wires it would have connected.

Original barrel switch

For those interested in the motor, here is the nameplate:

Motor nameplate

I just worked up the courage to momentarily jump wires #3 and #6 (as labelled in the schematic), and I found that as long as I don't hit the e-stop or unplug the lathe, everything works as expected. This means that my schematic is probably at least mostly correct (as much of it is there, anyway), and the contactors and relays are working correctly.

Is this setup familiar enough to anyone that they could work out how the switch should be set up?

  • \$\begingroup\$ Hi, where does the coil's return wire go from the left contactor? \$\endgroup\$
    – rdtsc
    Commented Jul 16, 2021 at 17:13
  • \$\begingroup\$ Well, that's a good question. The contactors have 12 connections (3 in for the motor, 3 out for the motor, 2 more up top (1 NC, 1 NO?), and then what I assume are the coil connections, one low on the front and one low on the back. Not knowing which connections are really which, I can't really say (and I am not near the machine at the moment). \$\endgroup\$
    – Mark
    Commented Jul 16, 2021 at 17:22
  • \$\begingroup\$ It's probably worth saying that it has been suggested that the contactors are wired through N/O terminals on each other, in such a way that activating one contactor prevents activating the other. I haven't traced out the contactor connections to verify this (yet). \$\endgroup\$
    – Mark
    Commented Jul 16, 2021 at 17:24
  • \$\begingroup\$ I'm seeing U,V,W wiring (three-phase AC power out) due to the way the contactor is wired, but RS (two-phase power in) on the terminals. Could it be, that R,S,T was supposed to come into the box, and M5,V,M6 go to the motor? That way, M5 and M6 get swapped, resulting in reversal of direction. \$\endgroup\$
    – rdtsc
    Commented Jul 16, 2021 at 17:34
  • \$\begingroup\$ This was never set up and never intended to be set up for 3-phase. I believe it could be set up for 110, but I don't know that it ever was. One of the wires going into the motor is taped off there, and not connected to anything. I'm told that it would've been used if it were wired for 110V. \$\endgroup\$
    – Mark
    Commented Jul 16, 2021 at 18:12

2 Answers 2


It appears to be a split-phase motor, wired as shown, with its direction of

rotation chosen by energising either terminals 'M6' & 'U' or 'M6' & 'V'.

enter image description here

The DPDT reversing switch is to be be wired as shown below.

enter image description here

The purpose of the jumper is to interconnect the 2 poles (connected to terminal 6).


@rdtsc is correct. You can see from the photo that the right contactor terminals T1, T2, T3 are hardwired to T3, T2, T1 respectively on the left contactor, meaning that the order of the three phases are reversed when you switch from one contactor to the other. Therefore, in the end you want one of three cases: left contactor energized (one direction); right contactor energized (the other direction); or neither contactor energized (power off). You can never allow both contactors to be energized or you will short out two of the three phases. These are likely 24-volt contactor coils, so the emergency switch should disconnect the 24-volt power.

I'm not sure about your schematic. But in any case you want to design so that you won't change motor direction when under power, so make your switch/relay logic so that the power to both contactors is disconnected if you try to change directions when the motor is energized. Possibly your original switch had to pass through a de-energizing state when going from forward to reverse.

Since you have verified a single-phase motor with a capacitor, the motor will be wired as follows:


simulate this circuit – Schematic created using CircuitLab

What you must do is to reverse the auxiliary winding, which will cause the motor to run backwards.

Good luck!

  • \$\begingroup\$ I'm wondering if the original motor was 3-phase, and was replaced with a 2-phase model. I'm tempted to say "rip everything out and start over." \$\endgroup\$
    – rdtsc
    Commented Jul 16, 2021 at 19:20
  • \$\begingroup\$ I'm pretty much 100% positive it was never 3-phase. Lathes of this size (12x36) really never are. There's also a "220V" sticker on the outside of the wiring cabinet that is very factory-looking. \$\endgroup\$
    – Mark
    Commented Jul 16, 2021 at 20:01
  • \$\begingroup\$ An AC motor has to have at least a second winding in order to run in a known direction, and some switching of the windings is necessary to reverse direction. So you have three wires coming off the motor. It's possible that your motor has an integral capacitor; if it does, you can run with 2 wires but you can't change directions. Maybe this has been modified to eliminate the direction, which would explain the switch being disconnected. \$\endgroup\$ Commented Jul 16, 2021 at 20:32
  • \$\begingroup\$ The switch was disconnected because it was malfunctioning. I don't know what was wrong with it, but it's broken somehow. The guy who owned it bought new switches, but never got around to installing one. The motor is a "1.5HP 4-pole 220V induction motor". It definitely has a capacitor hanging off of it, I assume it's a starting capacitor. \$\endgroup\$
    – Mark
    Commented Jul 16, 2021 at 20:42
  • 1
    \$\begingroup\$ @KateMoon Yep, it's there now. This isn't really about the motor, though, this is about how the 24V control circuit is (supposed to be) set up. \$\endgroup\$
    – Mark
    Commented Jul 17, 2021 at 0:50

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