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I am creating a schematic for my machine which is supposed to do the following with the push of a button. All motors are three-phase AC motors.

  1. Run Motor 1 CW until it hits its right limit switch.

  2. Then run Motor 2 CW for a given time with using a timer relay.

  3. Then run Motor 1 in CCW until it hits its left limit switch.

Should I be using two contactors for Motor 1 or can I create a schematic in which both motors only have 1 contactor and the rotation is switched by something else?

I am not very knowledgeable of PLCs.

I would like to keep my costs low while still building a reliable system.

I would like to learn more about how to address changing rotation in terms of relay logic and necessary components.

I would also appreciate any further resources that could assist me when building such a schematic for 3-phase AC motors using relay logic.

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3 Answers 3

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Two contactors are a must to change the phase sequence for reversing a three-phase motor.

The power and control circuits are as shown.

enter image description here

When a cycle is initiated, on resumption of power after an interruption, the machine will complete the interrupted cycle.

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  • \$\begingroup\$ Is there a reason why you numbered motors differently? Aren't they both supposed to be M1? \$\endgroup\$ Commented Jul 23, 2023 at 9:37
  • \$\begingroup\$ Hi Burak, M1 is the reversing motor that runs CW and CCW and M2 the motor that runs CW only. \$\endgroup\$
    – vu2nan
    Commented Jul 23, 2023 at 10:59
  • \$\begingroup\$ Oh, alright I see. Sorry, I confused contactors and MPCBs. Then, are MPCBs thermic relays? \$\endgroup\$ Commented Jul 23, 2023 at 12:05
  • \$\begingroup\$ Hi Burak, MPCB stands for Motor Protection Circuit Breaker. An MPCB is a specially designed circuit breaker for protecting electric motors against overload, short circuit, under voltage and phase loss. \$\endgroup\$
    – vu2nan
    Commented Jul 23, 2023 at 17:55
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Changing rotation on a 3-phase motor is rather simple. Just pick any two phases and swap them.

I would think you would want a 3rd mode where the motor is not turning, simply disconnect those same 2 phases (the 3rd phase can be left connected). I would dwell in this position for a short time rather than do an immediate reversal. So 3 modes:

  • Fwd - L1 to A, L2 to B, L3 to C
  • OFF - L1 to A, L2 to nothing, L3 to nothing
  • Rev - L1 to A, L2 to C, L3 to B

You will want some sort of interlock so it is impossible for both directions to be selected at once: if you connect L2 to B and also L3 to B, that's a dead short.

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I am not familiar with 3 phase AC motors only BLDC motors. You can easily reverse the direction of a bldc motor by switching one phase leg. It can be easily done especially if there are hall sensors being used. I have successfully converted an automobile alternator into a 3 phase DC brushless motor. The bridge rectifier is removed and identify the 3 plase wires. youll either be in a WYE or Delta config. The brush connectors then get powered with DC voltage and you have a powerful brushless 3 phase DC motor. configuration

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