So I have a control panel which controls the speed and direction of a motor in a sliding mechanism. the direction can be inverted using a tri-state selector on the panel. the selector input is a COMMON signal coming from the drive and the output is P1 "direction 1" and P2 "direction 2" which are going back to the drive. what I want to do is to add 2 limit switches on the mechanism and allow the drive to revert direction either by the selector or the limit switches. knowing that in order to the drive to be able to reverse the direction, the signal (P1 or P2) has to be continuously active high, So I will add a relay to achieve that with the limit switches "Latch Circuit". but the problem is how to link the selector and the limit switches without the need to add more components rather than the relay.

These are the available components that can be used:

enter image description here

  • \$\begingroup\$ I see you not thought about surge current or a proper Servo design. Imax = 2Vdd/DCR \$\endgroup\$ Aug 17, 2020 at 21:36
  • \$\begingroup\$ @tonyStewartSunnyskyguyEE75 The design aleary exists, I would like to add the feature of controlling the motor direction by the 2 limit switches. By the way its an AC geared motor connected to the VFD \$\endgroup\$
    – ahm_zahran
    Aug 17, 2020 at 21:54
  • 1
    \$\begingroup\$ Still not a great plan. It is better to compute stored mechanical then electrical energy and slow down early with extra sensing before end stop than reverse full power. End stop should stop before change power in the other direction, then have near end stop and full crash prevention end stop. \$\endgroup\$ Aug 17, 2020 at 21:57
  • \$\begingroup\$ For sure Sir, the VFD has a decelration function activated when limit switch is energized. Also the speed is too much low, in the range if 2 RPM or less. So there is still enough time for the mechanism to stop before something crashes. \$\endgroup\$
    – ahm_zahran
    Aug 17, 2020 at 22:08
  • \$\begingroup\$ ok then with control V/f ramps ok. But it must be one in logic not power. For power brakes only. \$\endgroup\$ Aug 17, 2020 at 22:24

1 Answer 1


Here's a way to meet the requirement.

enter image description here

  • \$\begingroup\$ Seems like you have not considered one of the main requirements, which is the signal must be continously active high in oreder to the VFD to rotate the motor in either direction. And that's why I'm using a latching relay. \$\endgroup\$
    – ahm_zahran
    Aug 18, 2020 at 10:24
  • \$\begingroup\$ Hello @vu2nan, Yes it's a latching type circuit. No worries 👍 \$\endgroup\$
    – ahm_zahran
    Aug 18, 2020 at 10:56
  • \$\begingroup\$ Hi Ahmed, 'Motor On' is a motor start/stop holding relay. I am sorry I have missed mentioning that. Will amend the schematic accordingly. P.S. I have reposted my earlier comment, with changes, as I could not edit it. I have also amended the schematic. \$\endgroup\$
    – vu2nan
    Aug 18, 2020 at 11:12
  • \$\begingroup\$ Then add back back inverters to make it latch. \$\endgroup\$ Aug 19, 2020 at 12:54
  • \$\begingroup\$ Hi Tony, That's just an electromagnetic relay, with its own holding contact, not shown in the schematic. \$\endgroup\$
    – vu2nan
    Aug 19, 2020 at 13:26

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