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  • Low voltage control for a magnetic starter.


    I have looked for information specific to my questions but have not been able to find it, if all of my questions have been covered off already please direct me to the appropriate post.

    My scenario:
    What I have is a 3hp single phase 240v motor for an older Torit dust collector that is new to me, posted in the DC branch of the forums here. I would like to buy a magnetic starter for said unit, to allow the use of multiple remote stop/start stations, a total of three for now, and that could eventually change to four…
    To date my work with magnetic starters has all been line (typically 240v) voltage and with no additional control stations, just the control that is on the starter. For this scenario though I would like to use a control transformer to reduce the control voltage to 24V and I would like to run the stop/start stations with 18/3 LVT wire. The motor starter would also need a 24v coil in this scenario.


    My questions are as follows:

    1. Sizing the control transformer, would 50 VA be enough for three or possibly four remote stop/start stations?

    2. I am thinking it would be most convenient to take the power for the 24V control transformer from a nearby 20A 120v receptacle line, are there any problems doing this?

    3. Does the control transformer need to be mounted in an enclosure, or does only the splice to 120V need to be enclosed?

    4. Were I to install the control transformer in an enclosure would that make any changes to the sizing (thinking latent heat…)?

    5. I generally prefer to buy from a local electrical wholesaler but have noticed that many of you seem to do your purchasing for motor control on-line, on Amazon etc. Buying locally I know that everything meets CSA and that I can get similar stuff or replacement/repair coils, parts etc. Siemens is available locally for instance. Any comments specific to sourcing?

    6. Any other tips or suggestions?

    Thanks in advance,
    Mark

  • Thread Continues Below...

  • #2

    Re: Low voltage control for a magnetic starter.

    if Im reading this right no dont do that. You really dont want a situation where the same machine is fed by two sources. you can avoid that. It's kind of an electrical no no

    if you want a low control voltage then you may be able to use a bell transformer and get around 17VAC or so. or it could be 12 or 24VDC the reason to keep the control voltage low is so the wires to your stop button dont need to have as high insulation properties.
    usually with a 3 phase motor there is a contactor and that includes a motor overload. the contactor is wired so that if the stop button is pressed it unlatches the power to the coil of the relay. then you need a start button because you dont want a situation where releasing the stop button can start the machine.

    usually any stop switches and door switches can be run in a loop called a safety loop. this power can latch or unlatch the solenoid when the safety circuit is closed. If anyone opens a door then the power to the coil is interrupted. one of the contacts on the contactor is used to maintain this circuit in such a way that if the power is interrupted it is impossible to restart until the safety loop is closed because that same power is what is pulling in the solenoid coil. the start button can momentarily close the solenoid and as long as it is latched then the power is maintained. once it unlatches then the start button needs to be pressed to latch it again.
    you dont want a situation where you can plug a machine in and see it start without pressing the start button. you cant have a situation where releasing a stop button can start the machine either.
    I think your contactor will need 4 contacts 3 for three phase and one for the holding circuit.
    usually there is also an overload which connects, its a separate part and often they are adjustable within a range so you can size it to the motor.
    the circuit that is used for starting probably also wants to be matched to the coil of the contactor. you can use 110 or one of the phases but if you want that wire to be 24 volts so it doesn't need heavy insulation and protection than you could use a transformer. that transformer isn't running much so it doesn't need many amps. 24V is a common control voltage so you could use a 24V transformer.
    doorbell transformers are funny because you aren't supposed to install a transformer inside an electrical box they are made to attach to the outside of an electrical box. the connections are made in the box but the transformer in that case goes on the utside of the box.

    if it's 110 then it is possible to wire a machine without the latching circuit, for example you can use a starter that has a built in stop button and a start button that work by mechanical means and it may also have a spot for your lockout lock . and an overload. in other words they dont use a holding circuit.

    If you want remote stop buttons then you need a safety loop, a transformer and a coil for the contactor that matches the power supplied by the transformer.

    personally I think id say look for a electrical supplier that deals with schneider, call with your info motor specs etc. let them help you size a configuration that works. some are quite a large box and if you dont mind the size then there is lots of room for your transformer. If you don't need a remote stop button then I dont think you will need the transformer because you wont need external wiring.

    you can probably spend 500 to 1000 or so on all this stuff. Your machine may already have a contactor? does it have 4 contacts and one not used? if so then you may already have that part? If you can reuse existing parts that could help save cost.

    Last edited by stickman; 05-04-2021, 05:17 PM.

    Comment


    • #3

      Re: Low voltage control for a magnetic starter.

      1 the trans former just powers the coil I'm not sure of amps but it is not a high draw, adding a bunch of buttons, they dont use power, voltage loss in the wire is too minute to worry about. the contacts of each switch could have slight resistance but the only thing using power in this circuit is the coil and it's not drawing lots. the transformer is only supplying enough amps to keep the contactor's coil energized. It's not high on amps and it wont be high on heat.. warm to the touch maybe but nothing should ever be paint burning hot..

      2 - no get the power from inside the control box, if you can put the transformer there too.

      3 it depends on the transformer, some have insulation some are quite bare and made to install inside a control box and that's common. you may have a 24 V LED adapter for your kitchen undercounter lamps , that could have a cord and be electrically enclosed in its own plastic casing for example similar with a laptop charger..

      4 dont worry much about heat, but I'd use a metal box not one of the little plastic boxes as they wont have room. some are specific and dont have a lot of extra space.
      5 I'd try a supplier that sells schneider. You do have other choices of course.

      don;t run out and buy this without confirming it's right but this is the sort of thing I'd expect may be appropriate. The supplier should be able to guide you and always best to consult with a real electrician which I am not. the particular parts may be purchased to your specs, but just to give you some idea.. the start and stop button will go where the blue thing is. Id ask the supplier about configuring it with a power supply for your safety loop. I think the box will be large enough to make it work and house the transformer.

      https://www.se.com/ca/en/product/LE1...fused-type-12/

      you seemed to size the wire heavy for the start/ stop buttons. If they are just 24V then you dont need heavy wire.
      this type of panel will have some mollex connectors, If you give the dealer the specs he should be able to make sure you have the control circuit worked out. They will configure it for you and help you get the right parts.

      you will also need some start stop button panels. they could be any manufacturer.

      There are lots of other ways you can do this by cobbling parts together but I think this would be nice and neat and tidy and safe. the panel is made for various configurations.

      you can beat the price by buying offshore stuff. there is some real junk out there too. many makers seem to come and go, I'd stick with some common manufacturer so you can get individual parts locally.
      Last edited by stickman; 05-04-2021, 05:26 PM.

      Comment


      • #4

        Re: Low voltage control for a magnetic starter.

        one thing a bit weird is having a dust collector with all these start stop/ buttons.. You know Its possible to have sensors that can sense the power draw from your machines and then send a signal to turn it on or off, or to open gates for the machines you are using while the D/C runs continuously. that way the dust collector just reacts when a machine is used rather than to need to turn the D/C on and off manually. the sensors are normally near a power wire and they sense the induction. you can get ones that plug in and will run 110 items.. but for 3 phase you might then need some sort of special contactor to do that. you would need a part that can read the state of the sensor and use that signal to turn things on and off. the sensor will be low voltage. This can be done and it might work better but i can't really recommend parts for it. I'm sure Siemens will make the parts for all that .. Im not sure if that would be considered a PLC , or just something similar..

        my thoughts are that you may consider such a system in comparison before spending. it might be nice not to have to worry about turning on and off the dust collection manually and the price might be close?

        Comment


        • #5

          Re: Low voltage control for a magnetic starter.

          Just a thought but something to think about. With a 3 hp motor you don’t want to be turning it on and off too often like for every cut as the heat builds up. You can also get the remote 240v switch at BusyBee so you just carry the fob around in the shop to cycle the blower and this would be a lot cheaper and simpler than running a low voltage coil all about the shop.
          Erik

          Canada's Island Paradise - Prince Edward Island

          Proud member of the Wadkin Blockhead Club

          Comment


          • #6

            Re: Low voltage control for a magnetic starter.

            I have something similar to what you are looking to do. My pieces were collected over some time from various sources.

            I have 5 stations. I can start and stop from any one at any time.

            I have a 24v transformer in a junction box above my panel. That 24 volts goes out to the buttons and actuates a rather large relay that energizes and de-energizes the receptacle my DC is plugged in to.

            If you have questions feel free to PM me. This can be done. Do your research. You can do it.

            Comment

            • Thread Continues Below...

            • #7

              Re: Low voltage control for a magnetic starter.

              Thanks for the response guys.

              Stickman:

              I just want to clarify a couple things that perhaps I could have described better in my original post.
              -currently my dust collector does not have any motor control or switchgear, nothing more than a peckerhead with some wires dangling…
              -the motor for this dust collector is SINGLE PHASE, 3HP, 240V.
              I want to purchase a new magnetic starter and would like to get it with a 24V coil. (the coil would control the contactor, the contactor would be sized to run my 240V 3 HP single phase motor)
              The main reason that I would like a magnetic starter is so that I can use low voltage “3 wire control” to run my start/stop stations.

              Specific comments to your reply, Stickman

              -I understood that a doorbell transformer was “to small” as in not enough VA (typically 20 or so?) to properly engage the coil in a magnetic starter.
              -your comments on installing the doorbell transformer “not inside the box” was more or less what I understood as well. Hopefully someone else who is using or has used low voltage controls for a magnetic starter can comment.
              - I thought 18/3 LVT wire was about as small as could be used for this, what size were you thinking code would allow for this?


              Erikm

              You are right, generally I do not want to be turning the dust collector on and off with every cut for the reasons you stated. I also typically let the collector run for a few minutes after I have shut down the tool I am collecting from. I was really interested in the remote start/stop stations to allow starting from the area I am working in, the TS for instance will be at the opposite end of the shop from the DC. For what I am doing I could actually get away with three remote starts only (no remote stops) and then walk back to the DC when I want to shut it off…
              I will have to see how the remote start/stops price out for this and then make a decision. Thanks for the suggestion.

              cheers,
              Mark

              Comment


              • #8

                Re: Low voltage control for a magnetic starter.

                Remote control works great for DCs and obviates the need for a control transformer. I have installed a number of themcu

                Comment


                • #9

                  Re: Low voltage control for a magnetic starter.

                  General Band Saw Starter.pdf General Band Saw Starter.pdf
                  Originally posted by cmzinbc View Post
                  I have looked for information specific to my questions but have not been able to find it, if all of my questions have been covered off already please direct me to the appropriate post.

                  My scenario:
                  What I have is a 3hp single phase 240v motor for an older Torit dust collector that is new to me, posted in the DC branch of the forums here. I would like to buy a magnetic starter for said unit, to allow the use of multiple remote stop/start stations, a total of three for now, and that could eventually change to four…
                  To date my work with magnetic starters has all been line (typically 240v) voltage and with no additional control stations, just the control that is on the starter. For this scenario though I would like to use a control transformer to reduce the control voltage to 24V and I would like to run the stop/start stations with 18/3 LVT wire. The motor starter would also need a 24v coil in this scenario.


                  My questions are as follows:

                  1. Sizing the control transformer, would 50 VA be enough for three or possibly four remote stop/start stations?

                  2. I am thinking it would be most convenient to take the power for the 24V control transformer from a nearby 20A 120v receptacle line, are there any problems doing this?

                  3. Does the control transformer need to be mounted in an enclosure, or does only the splice to 120V need to be enclosed?

                  4. Were I to install the control transformer in an enclosure would that make any changes to the sizing (thinking latent heat…)?

                  5. I generally prefer to buy from a local electrical wholesaler but have noticed that many of you seem to do your purchasing for motor control on-line, on Amazon etc. Buying locally I know that everything meets CSA and that I can get similar stuff or replacement/repair coils, parts etc. Siemens is available locally for instance. Any comments specific to sourcing?

                  6. Any other tips or suggestions?

                  Thanks in advance,
                  Mark
                  1) yes 50 VA is fine

                  2) mount the control transformer inside the box with the contactor

                  3) ground one side of the 24 volt secondary

                  4) install a 2 ampere time delay fuse on the other secondary lead of the transformer.

                  5) provide primary protection for the control transformer, 1 ampere time delay fuse should be fine.

                  I've included a drawing of a starter I made for my bandsaw...........Rod. General Band Saw Starter.pdf

                  P.S. If you want a contactor PM me your mailing address.
                  Attached Files
                  Last edited by Rod Sheridan; 05-05-2021, 08:07 AM.
                  Work is the curse of the riding class.

                  Comment


                  • #10

                    Re: Low voltage control for a magnetic starter.

                    you can ignore my comments about wire size. I think you are on the right track and I visualized it as being larger conductors , It's not carrying a lot of amps.

                    the example of the doorbell transformer was pretty much just a way to describe it's mounting. I'd put the transformer in the control box.

                    the voltage needs to match the col and you are right in using 24VDC, your choices of coils will be limited and that's normal. the could be 110 if the start and stop button didnt' have to run all over the place.

                    Id prefer to start and stop to be working off the sensor on the machines cord , or if it were to open gates that might be better because you will be sharing the suction. manual gates are an option.
                    there is something to not getting carried away with complexity. you are on the right track.

                    Comment


                    • #11

                      Re: Low voltage control for a magnetic starter.

                      Rod,
                      thanks for the reply and generous offer. I will take you up on that, PM to follow. If I am mounting the control transformer in the box with the motor starter I will see if I can get a 240/24 transformer, rather than bring additional wiring into that enclosure.
                      One question on your diagram though, as I am not planning on any pilot lights for either the stop or start circuits am I correct in using 18/3 LVT for the control wiring?

                      iamtooler,
                      in this case I would prefer to hardwire the circuits but if I get too much sticker shock I will reserve the right to change my mind...


                      stickman,
                      thanks for the follow-up

                      appreciate the comments and ideas from all of you,
                      cheers,
                      Mark

                      Comment

                      • Thread Continues Below...

                      • #12

                        Re: Low voltage control for a magnetic starter.

                        I normally use 16AWG for low voltage control systems
                        Work is the curse of the riding class.

                        Comment


                        • #13

                          Re: Low voltage control for a magnetic starter.

                          Originally posted by Rod Sheridan View Post
                          I normally use 16AWG for low voltage control systems
                          Hi Rod,
                          Interested in your thoughts re grounding the stop start stations when using 24V control wiring. All other electrical work seems to require it but I wanted to check in on this before ordering the material.

                          The way I read your diagram the secondary, 24V side, of the 240/24 transformer is grounded/bonded with the white wire, should this be done at each of the remote pushbutton stations with the white wire as well, or should a separate dedicated ground be run from the position the white wire is bonded to at the transformer?
                          I have deleted the pilot lights from your drawing and added one additional start stop - apologies in advance for the poor artwork...

                          thanks again,
                          Mark

                          Attached Files
                          Last edited by cmzinbc; 05-10-2021, 09:26 PM.

                          Comment


                          • #14

                            Re: Low voltage control for a magnetic starter.

                            Originally posted by cmzinbc View Post

                            Hi Rod,
                            Interested in your thoughts re grounding the stop start stations when using 24V control wiring. All other electrical work seems to require it but I wanted to check in on this before ordering the material.

                            The way I read your diagram the secondary, 24V side, of the 240/24 transformer is grounded/bonded with the white wire, should this be done at each of the remote pushbutton stations with the white wire as well, or should a separate dedicated ground be run from the position the white wire is bonded to at the transformer?
                            I have deleted the pilot lights from your drawing and added one additional start stop - apologies in advance for the poor artwork...

                            thanks again,
                            Mark
                            Hi Mark, one terminal of the secondary of the transformer is to be grounded, doesn't matter which terminal you pick. That is the only bond to ground location.

                            There are 2 safety reasons for that;

                            1) if a primary to secondary fault were to occur in the transformer and the secondary was ungrounded, that primary voltage would be coupled to all of the secondary circuit.

                            2) an ungrounded secondary can result in extremely high voltages to ground due to capacitive coupling through the transformer.

                            Regards, Rod.
                            Work is the curse of the riding class.

                            Comment


                            • #15

                              Re: Low voltage control for a magnetic starter.

                              Originally posted by cmzinbc View Post

                              Hi Rod,
                              Interested in your thoughts re grounding the stop start stations when using 24V control wiring. All other electrical work seems to require it but I wanted to check in on this before ordering the material.

                              The way I read your diagram the secondary, 24V side, of the 240/24 transformer is grounded/bonded with the white wire, should this be done at each of the remote pushbutton stations with the white wire as well, or should a separate dedicated ground be run from the position the white wire is bonded to at the transformer?
                              I have deleted the pilot lights from your drawing and added one additional start stop - apologies in advance for the poor artwork...

                              thanks again,
                              Mark
                              Hi Mark, just looked at the drawing, you cannot remove the coil for M1 otherwise it won't work. Also put the other stop button outside of terminal 3........Rod.
                              Work is the curse of the riding class.

                              Comment

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