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Relay question for the non-electrically challenged amongst us.

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  • Relay question for the non-electrically challenged amongst us.

    I want to add a motor reversing relay to my CNC lathe equipped with a 90v 3/4 hp Baldor DC motor. Would this relay handle the power? If memory serves the motor is rated @ 7.5A.

    http://www.ebay.com/itm/NTE-R55-11D2...item43b099a931
    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

  • #2
    I believe you'll need a relay with DC-rated contacts. The one referenced is rated for AC only. However if the relay only operates when DC is not flowing, it should work.
    Last edited by Alan Douglas; 02-12-2013, 02:39 PM.

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    • #3
      If you intend reversing when running then you need a relay with magnetic arc-blow out, If you stop or inhibit a drive first you can use an ordinary relay.
      P&B PRD model is one with Arc Blow out.
      The one you show is on the light side for DC motor switching.
      Max.

      Comment


      • #4
        I think it would probably work fine .. the starting current for the motor is pretty close to
        relay limits .. but .. I wouldn't be afraid to use it ..

        are ya sure you wouldn't want to spend just a little more and be sure

        http://www.ebay.com/itm/NTE-R55-11D2...item3cce2ba49e

        edit: Max was replying the same time as me .. I would give his advice more consideration.
        Last edited by Mike Amick; 02-12-2013, 02:49 PM.
        John Titor, when are you.

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        • #5
          Be careful when setting up a relay system to reverse a DC motor as many controllers do not like the load connections to be rapidly reversed without going thru a dedicated OFF time. Check the wiring specs for the controller for reversing procedure. No problem with the motor.

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          • #6
            I have used the typical cube relay before for DC switching, the contacts just do not stand up.
            Max.

            Comment


            • #7
              Thanks, definitely not my area of expertise. Further review of the manuf. spec. shows [email protected] so it's too small.

              Probably work for my use 'cuz I usually cut small stuff at way less than full power but I need to look around some more.
              Milton

              "Accuracy is the sum total of your compensating mistakes."

              "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

              Comment


              • #8
                Oops, I missed ya'lls other learned responses, thanks!

                Definitely will be making sure Mach is programmed to make the motor come to full whoa before reversing. My B.O.B. has 2 relays, 1 already switches the Inhibit terminals on the KB controller and the other one will switch the reverse relay...when I get a good one.
                Milton

                "Accuracy is the sum total of your compensating mistakes."

                "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                Comment


                • #9
                  OK, how 'bout this one? http://www.ebay.com/itm/Siemens-Furn...item3f2141af97
                  Milton

                  "Accuracy is the sum total of your compensating mistakes."

                  "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                  Comment


                  • #10
                    That looks like the P&B I referred to, it has the Arc blow out magnet between the contacts.
                    Max.

                    Comment


                    • #11
                      OK thanks Max. That puppy looks manly enough for the job and then some.
                      Milton

                      "Accuracy is the sum total of your compensating mistakes."

                      "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                      Comment


                      • #12
                        HI Milton,
                        Only thing I'd worry about is that it's rated 20A for *non-inductive* loads - and a DC motor's very much inductive... No problem if Mach does it's job and removes motor power and waits for it to stop before switching, but if anything glitches it could produce some pretty arcs! In which case:

                        It may do the job, as you'll be running at around 1/3rd the non-inductive rating - If I were using it (and overly worried that anything that can go wrong, will go wrong) I'd either get one rated for inductive loads, a higher non-inductive rating or put a MOV surge arrester across the motor terminals to deal with any inductive spikes if/when the relay opened with the motor spinning.

                        Just my ha'pennorth,
                        Dave H. (the other one)
                        Rules are for the obedience of fools, and the guidance of wise men.

                        Holbrook Model C Number 13 lathe, Testa 2U universal mill, bikes and tools

                        Comment


                        • #13
                          Originally posted by Hopefuldave View Post
                          HI Milton,
                          Only thing I'd worry about is that it's rated 20A for *non-inductive* loads - and a DC motor's very much inductive... No problem if Mach does it's job and removes motor power and waits for it to stop before switching, but if anything glitches it could produce some pretty arcs! In which case:
                          The arc blow out does a pretty good job, I have used these on large inductive loads OK.
                          Max.

                          Comment


                          • #14
                            Thanks gentlemen; as always your knowledge and input is greatly appreciated.

                            The relay just arrived and it's definitely a manly bit of kit and bigger than I thought it'd be. I hope I can find room for it!

                            Ebay sellers (and FedEx) amaze me. I ordered it Tuesday around 3:30 pm and it was on my desk when I got back from lunch today at 12:00 PM. It was $24.99 with free shipping and arrived from Roanoke, VA via FedEx ground. Can't beat that! Now I've got something else to play with this weekend.

                            As to the arcing, hopefully my memory (and coding skills) will allow me to program in a 1 or 2 sec. delay after every M5 and before each reversal. M5's instantly open up the I1/I2 inhibit circuit on the KB controller too so that should help, no?
                            Milton

                            "Accuracy is the sum total of your compensating mistakes."

                            "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                            Comment


                            • #15
                              You mean close the I1 & I2 to inhibit.
                              This resets the drive.
                              Also, you should have a couple of spare N.C. contacts on each of the PRD's, if you are so inclined, you could put a dynamic braking resistor across these, when both are open the resistor is placed across the motor.
                              This helps if you have high rpm and/or a high overhauling load.
                              Max.

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