Possible to use relays to get 1960's switch gear to control VFD?

Discussion in 'The Projects Forum' started by andrsn, Jun 12, 2014.

  1. andrsn

    Thread Starter New Member

    Jun 12, 2014
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    I've got a circuit I'm trying to implement and I'm having some trouble determining exactly what type of switches/relays to use to accomplish my goal.

    Background:
    I have an old milling machine that originally ran on 3-phase power, but because I don't have 3-phase power at my house, I am using a Teco-FM50 Variable Frequency Drive to power/control the motor with standard 240 2-phase power. Everything is wired up and the VFD is controlling the motor great.

    Now I want to take advantage of the VFD's ability to have external switches control the VFD/Motor functions because i'm sick of having to walk all the way around the machine to get to the VFD's control panel and its not super safe to have your stop button conveniently out of direct reach. Ideally, I would like to use the machines original switch gear to control the starting and stopping of the motor and that is where I'm having trouble. The VFD has a common 12V terminal and a Start/Stop terminal, when this simple circuit is open the motor is off when the circuit is completed the motor starts.

    The machine has 2 switches that were used to control the motor start stop function and ideally I would like to continue using them as they originally functioned. There is a momentary NO switch used to start the motor, and that is the only thing that should be able to start the motor. There is also a large lever that activates a switch that can be wired NO or NC. When the lever/switch is in one position the momentary button is allowed to start the motor, when the lever/switch is in the other position the motor is not allowed to start (in this position there is a mechanical brake that should never be engaged when the motor is started).

    The concept I have come up with to accomplish what I want is for the momentary switch to act like a circuit reset switch, when it is pushed it closes the circuit and when released the circuit stays closed until the other switch/lever is engaged (the push button does not need to be able to stop the machine). The switch/lever can only open the circuit/stop the motor or close the circuit/allow the momentary push button to then start the motor. The switch/lever should never be able to start the motor because it's heavy and sometimes will fall into the open position on its own.

    I cannot figure out what type of relay I need to get these buttons to function how I want or if any relay will do this at all. Sorry if my description of what I want is hard to understand, I'm trying to make it as easy to read as possible. I researched latching relays, dpdt relays and went down some other internet wormholes, but I still cannot figure out how to get this circuit to work as i want.

    I also read this forum post and it seemed like what I needed but I cannot get it to function correctly:
    http://cocoontech.com/forums/topic/...ule-requiring-a-reset-following-power-outage/

    skip down to the post by WayneW, I did go down the street and buy the Radio Shack #: 275-217 relay.

    Here is a picture of the machine, just in case anyone is interested. You can see the lever and push button in the photo (the first lever on the side of the machine you would come to if you started at the top and worked your way down, there a few levers on there....):
    [​IMG]
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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  4. andrsn

    Thread Starter New Member

    Jun 12, 2014
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    that looks like it should do what i want it to, i swear i tried that configuration last night when i was playing around with it. i now think i may have bought the wrong relay radioshack #275-217 (125vac 10amp) when i probably need radioshack #275-218 (12vdc 10amp)....
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
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    What vfd are you using?

    Most will operate from momentary switch.

    You could also leave the original wiring and connect vfd control to motor starter contact.
     
  6. andrsn

    Thread Starter New Member

    Jun 12, 2014
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    TECO FM50 as stated above. I wired up the momentary switch on its own, turn on when button is engaged, turns off when you let go.

    original wiring has been removed, all 40lbs of it... and original on off control was a 3 wire system so i'm pretty sure it wouldn't have worked anyway.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    See the three wire P.B. control on the Schneider PDF.
    Which is most likely what was on the original contactor circuit.
    Instead of the contactor, a small D.P. relay should work.
    Max.
     
  8. andrsn

    Thread Starter New Member

    Jun 12, 2014
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    I see the one on page 6. That is an awesome pdf, definitely gonna print it out for a reference. i think the relay will work for the start/stop with the combo of a switch/push button i just need to make sure i've got the right relay and play around with it. it does need to be maintained to keep the motor on. the fwd rev and speed control are on separate circuits, those i have figured out.
     
  9. strantor

    AAC Fanatic!

    Oct 3, 2010
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    That sucks, since this would have all been super easy - just run your VFD run signal through the existing (not existing) spindle run 3 phase contactor, then all the original logic is preserved.

    That bit of trivia out of the way, now lets on move on to your actual solution.

    all you need is a DPDT relay with a 220V coil. try this:


    [​IMG]
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You also could use low voltage control if present in the enclosure or 120vac if you already have that relay.
    I normally retain the contactor on the input side for E-stop condition.
    In the event it does have a brake, you can often assign the multi function output to de-energize the brake when 'At-Zero-Speed' is assigned to the output via the parameters.
    You don't want to energize the brake to stop the spindle normally, especially with a VFD.
    Max.
     
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