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Definite Purpose Contactor for Powder Coat Oven

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  • Definite Purpose Contactor for Powder Coat Oven

    I recently finished building a powder coat oven. It's 3' H X 2' W X 6' Deep inside dimensions. It has four oven elements inside which add up to around 12,000 watts. They are wired in parallel and measure approximately 4.7 ohms. I've tested it and it does 400 degrees F. with no problem. Using a clamp meter separarately on the wires inside it's apparently drawing 51.4 amps. The wiring all the way from the subpanel where I'll be using the oven is four-wire 6 gauge 240 VAC. That's L1, L2, Neutral and ground.

    The design that I'm using here is on a website that shows construction and gave a materials list. I changed the framework design but basically I followed the instructions. Here is the link and I recomment it to anyone who is interested in doing their own powdercoating.

    http://www.powdercoatoven.4t.com/

    As I mentioned, I'm seeing slightly more than the rated resistive load. I've checked Amapcity charts on the internet and they show generally that enclosed 6 ga wiring is OK for 60 amps. My concern is the Cutler-Hammer C25 BNF240A Definite Purpose Contactor (DPC) that I'm using, It's single phase, two pole and rated at 50 amps resistive load. The control coil is powered by 120 VAC.

    I've been trying to locate a 60 amp DPC so that I am not over-extended. So far, the only ones I've seen on the internet are-three pole for 60 amps or greater resistive load.

    My question then is this: Would it be appropriate to use only two of the three poles that are offered on a 60 amp or larger three-pole DPC? I'm thinking it would but I'm not sure as I've not got a great deal of experience with this sort of equipment.

    I plan on updating the oven at some point to a P.I.D. (proportional, integrated, differential?) unit with a solid state relay to obtain more control over temperature but that's down the road.

    Any suggestions regarding three pole DPC usage on two pole power would be greatly appreciated. There are literally hundreds of three-pole DPC's on eBay at excellent prices so availability is no problem. The one I have is the largest I can find for two-pole.

    Thanks in advance.


    Oven.



    Inside View showing elements.



    Electrical Control Box (Pre-installation. The DPC is the black box in the center).

    Last edited by gnm109; 12-01-2009, 10:06 PM.

  • #2
    Better to use two of the available three than need three and only have two. I don't know if it is legal, but I've seen it a lot.

    Steve

    PS I have even done it......shhhhh

    Comment


    • #3
      Perfectly ok to use 2 of the 3 poles.

      Comment


      • #4
        No problem using only 2 poles of the contactor.
        Really nice job on the oven there.
        Great work!

        Comment


        • #5
          If the contactor is going to whacking on and off a lot, consider a mercury relay. They are often used in electric heating ovens. Use a contactor as a safety backup actuated by a high-limit thermostat to shut things down if the primary contactor welds itself shut. You might thank yourself if you leave the shop and the oven goes haywire!

          --Doozer
          DZER

          Comment


          • #6
            Thanks for the responses! I was hoping that two out of three poles would be OK. I notice that resistive loads come on gradually and apparently that's the reason that the DPC is rated higher for resistive loads than for Full Load Amps as with a motor. Once I install a DPC rated for 60 amps resistive load, I'll be good, although the one I have works OK.

            I'm going to be using my Mill and lathe to build special sidecar mounts out of steel. I've done some already and powdercoated them in a dedicated toaster oven with good results. The oven I have is large enough to take a motorcycle frame or four car wheels at the same time. It has an upper and lower trolley to roll the parts in once they're coated.

            I used metal studs for the base and 2X2X2" U channel for the top, sides and front and back. It's insulated with rock wool, sometimes called mineral wool.

            Thanks again.
            Last edited by gnm109; 12-02-2009, 04:29 AM.

            Comment


            • #7
              Cool. I have always wanted to try powder painting but it seemed very expensive and equipment intensive. Where do you spray the powder? Is there a booth required or does it just go all over? I think this place has good stuff (elements, themocouples, etc.,) for your oven and I believe they will sell to individuals via credit card.
              http://www.tempco.com/Default.htm

              Comment


              • #8
                Lemme run you over the hurdles:

                According to the NEC all conductors above ground potential serving an electrical load have to be opened simultaneously by a single device. Meaning your 230 single phase needs a pair of contacts; one pair on each leg. OK you got that but you're concerned with your preent DPC ampacity which from your description seems to be marginal to me. If you wish to divide your load your will need a second DPC. There's room for one in the enclosure.

                My suggetion is to control your heater bank with two DPC's wired to break both legs of the line current to the divided loads respectively. The DPC coils can be energized simultaniously by the control device (Thermostat? Temperature controller?) in series with an over-ride switch. You will, of coure, need a means of circuit protection and a disconnect serving the whole oven.

                The terminal at the lower left: are they ceramic binding posts? Are the small multple leads from the switched side of the DPC leading to the element? Are they supported and passed through openings with components rated for the oven's max temperature?

                Is the temp control via thermocouple? I see some termocouple sized wire in the lower photo. Is there a separate over-temp switch?

                I can't see it in the photos but is the wiring in the oven rated for the oven's skin temperature? Looks like 90degree C WEO cord and 75 Degree THHN conductor but no ratings are visible.

                Are the devices and wiring in the electrcal control enclosure attached to the oven exterior rated for the oven's skin temperature.

                Is there a bonded ground connected from the frame of the oven to the electrical ground of your electrical service? Oops! I see one with a fat gree wire hooked to it.

                My NEC dates from 1993 so I can't quote chapter and verse. It wouldn't hurt to find an up to date copy and work through the stuff on industrial ovens.
                Last edited by Forrest Addy; 12-02-2009, 01:53 AM.

                Comment


                • #9
                  gmn109,

                  Try Square D 8910DPA62V02. This is a 60A two pole definite purpose contactor with a 120V operating coil (other operating voltages available).

                  I use these quite a bit in my real job (Sports Lighting Contractor) and have found them to be very reliable.

                  If you have already purchased a 3 pole contactor, there is nothing wrong with using any 2 of the 3 poles for your purposes.

                  Tim
                  Last edited by tmc_31; 12-02-2009, 04:05 AM.

                  Comment


                  • #10
                    Originally posted by Forrest Addy
                    Lemme run you over the hurdles:

                    According to the NEC all conductors above ground potential serving an electrical load have to be opened simultaneously by a single device. Meaning your 230 single phase needs a pair of contacts; one pair on each leg. OK you got that but you're concerned with your preent DPC ampacity which from your description seems to be marginal to me. If you wish to divide your load your will need a second DPC. There's room for one in the enclosure.

                    My suggetion is to control your heater bank with two DPC's wired to break both legs of the line current to the divided loads respectively. The DPC coils can be energized simultaniously by the control device (Thermostat? Temperature controller?) in series with an over-ride switch. You will, of coure, need a means of circuit protection and a disconnect serving the whole oven.

                    I thought of using two DPC's but space is already tight in the box. I prefer to use one unit.

                    The terminal at the lower left: are they ceramic binding posts? Are the small multple leads from the switched side of the DPC leading to the element? Are they supported and passed through openings with components rated for the oven's max temperature?

                    Note that the picture of the control box is pre-installation. The binding posts are mounted into 1/8" phenolic dielectric sheet. No problem there. The output wires are not yet installed. Any small wires you see are control wires for the DPC coil on switches and neon lights for the panel to show power on/off, heating on/of and interior lights. All wires from the control box are in electrical conduit and the wires are oven-rated high temperature for the purpose, although they are isolated from the heat. The elements protrude into a cavity on the side where the wires connect. See the interior picture. The exterior of the oven where the control box mounts was at ambient temperature when I tested the oven recently thanks to the rock wool.

                    Is the temp control via thermocouple? I see some termocouple sized wire in the lower photo. Is there a separate over-temp switch? Yes, that's the thermocouple leading from the thermostat at the top. The thermosat controls the temperature. The oven will always be monitored during operation with an IR thermometer unit. The designer doesn't use one either.

                    I can't see it in the photos but is the wiring in the oven rated for the oven's skin temperature? Looks like 90degree C WEO cord and 75 Degree THHN conductor but no ratings are visible. Yes, the wiring leading to the elements is oven-rated from an appliance supply. Oven skin temperature outside is ambient. There are no wires exposed to temperature inside. All wires are outside of the inslulation media.

                    Are the devices and wiring in the electrcal control enclosure attached to the oven exterior rated for the oven's skin temperature. Yes, see above. The skin temp is ambient. The box is isolated from the side by an air gap anyway.

                    Is there a bonded ground connected from the frame of the oven to the electrical ground of your electrical service? Oops! I see one with a fat green wire hooked to it. Yes, it's four wires, 6 gauge all the way with a 70 amp breaker in a subpanel on a dedicated circuit.l There's also a large four prong connector to plug the oven in. Ground is tied ot the frame and goes all the way back. It's all per local code. Four wires are now required on everything except welders.

                    My NEC dates from 1993 so I can't quote chapter and verse. It wouldn't hurt to find an up to date copy and work through the stuff on industrial ovens I did. .
                    Interesting questions you raise Mr. Addy. Responses are in red above to the best of my knowledge. Remember, this is not my design but is based on a very succesful oven from the website link.

                    I just want to et a bit more capacity to keep things wihin limits.

                    Thanks for the response.
                    Last edited by gnm109; 12-02-2009, 05:06 AM.

                    Comment


                    • #11
                      Originally posted by tmc_31
                      gmn109,

                      Try Square D 8910DPA62V02. This is a 60A two pole definite purpose contactor with a 120V operating coil (other operating voltages available).

                      I use these quite a bit in my real job (Sports Lighting Contractor) and have found them to be very reliable.

                      If you have already purchased a 3 pole contactor, there is nothing wrong with using any 2 of the 3 poles for your purposes.

                      Tim
                      Thanks! I'll see what those cost.
                      Last edited by gnm109; 12-02-2009, 05:10 AM.

                      Comment


                      • #12
                        Doozer has a very valid point about the life of the contactor if it is cycling on/off a lot. I build a control panel for a high power heating application (25 to 100 KW 480 Volt 3 phase). What I do is very much like Doozer said. I use a contactor for high temp safety, over temp --> contactor OFF. For control I use either solid state relays (Crydom heavy duty series mounted on heat sinks or Continental relays W/heatsinks) or mercury relays for the very high power units. The control is programmed for time proportional control with a total cycle time between 10 and 30 seconds. This means that the relays might be cycling up to 6 times per minute at up to 120 amps per phase. To date I have had very few failures of the relays without something else failing first and taking the relay with it. This type of relay might be over your budget but it would be money well spent considering that it looks like you have invested lots of time and resources into a VERY good looking oven. Great Work . I hope this gives you some ideas.

                        Solid state relays are available from www.mouser.com

                        Robin
                        Robin

                        Happily working on my second million Gave up on the first

                        Comment


                        • #13
                          OKAY? why is it you are using a mechanical relay?

                          A SSR, solid state relay, using the 5volt pid oven controller.

                          A large contactor I had on the end of one oven when it'd bang in, the powder would be shook off hte part. took a week or so to realize that.

                          WIre, 600degree silicone jacketed wire around any heat, chases and going to the elements.
                          Excuse me, I farted.

                          Comment


                          • #14
                            "it's four wires, 6 gauge all the way with a 70 amp breaker in a subpanel on a dedicated circuit.l There's also a large four prong connector to plug the oven in. Ground is tied ot the frame and goes all the way back. It's all per local code"

                            Whoa! Someone correct me if I'm wrong, but you cannot connect conductors with less ampacity than the breaker supplying them. 70 amps would dictate larger than 6 ga., probably would have to go with 4. I am not trying to preach, and I am aware you have already run the 6, and are not likely to redo it. I would be inclined to swap out the 70 amp for a 50 in your panel, and just see what happens.

                            You should always think about what the fire / insurance inspectors will find in your shop if the worst were to happen. Big breaker + little wire = red stamp "denied" on your insurance claim!

                            The oven looks great. What will you use for the temp controller? Also, in regards to your contactor, do a search for SSR (solid state relay). They are silent, and can be fired on and off millions of time without problems.

                            Jon

                            Comment


                            • #15
                              Originally posted by Prototyper
                              "it's four wires, 6 gauge all the way with a 70 amp breaker in a subpanel on a dedicated circuit.l There's also a large four prong connector to plug the oven in. Ground is tied ot the frame and goes all the way back. It's all per local code"

                              Whoa! Someone correct me if I'm wrong, but you cannot connect conductors with less ampacity than the breaker supplying them. 70 amps would dictate larger than 6 ga., probably would have to go with 4. I am not trying to preach, and I am aware you have already run the 6, and are not likely to redo it. I would be inclined to swap out the 70 amp for a 50 in your panel, and just see what happens.

                              You should always think about what the fire / insurance inspectors will find in your shop if the worst were to happen. Big breaker + little wire = red stamp "denied" on your insurance claim!

                              The oven looks great. What will you use for the temp controller? Also, in regards to your contactor, do a search for SSR (solid state relay). They are silent, and can be fired on and off millions of time without problems.

                              Jon

                              I have all sorts of breakers available. 40, 50, 60, 70, 80, 100. I can easily change to 60 amps.

                              As mentioned in my original post, I'm intending to convert at some point to a PID controller with a matching SSR and thermocouple. Like other projects I do from time to time, I like to collect information beforehand and use the collective knowledge that I find here and elsewhere. Thus, I initiated this post. I also need to budget my funds so that I can hang on to my spouse. LOL.

                              I started out with an electro-mechanical relay because that was the design I worked from. The fellow that did the original oven on the website has been using one for years successfully and so have many others. Resistive loads are far easier to switch than motors and heavy machinery in any case.


                              Thanks for the comments.

                              Comment

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