MikeAlmogy Posted March 17, 2020 Share Posted March 17, 2020 Hi all. I hope that i'm in the right forum. If not please transfer this post to the right one. So i need to built a heat treatment oven for a client. I got the refractory insulation bricks, i notched/curved them to the coil diameter using a router and now i'm working on the steel box in which they all be placed. The design in for 4 heating coils of 4000W, 240V each. one on each side of the box. The opening will be from the top and the steel will be hanging from the top. My question is how to connect the coils. In parallel or serial. My goal is to use as lower amperage as possible. It is a single phase oven. I have a small background in electricity and electronic and i do understand the principales between parallel and serial connection but can not make up which will use less amperage. Since the coils are 240V i'm not sure if that means that i can even connect them in serial. since the voltage on each coil is 220/4. how will it effect the coils? In parallel, the voltage is 220V on each element but the current is the sum of all currents. So it might be too much then i can provide. I feel that i'm missing something or do not understand something. Thanks, Mike Quote Link to comment Share on other sites More sharing options...
Latticino Posted March 17, 2020 Share Posted March 17, 2020 You need to go back to your basic electronics books and talk with your coil supplier. Coils have a characteristic resistance and will only provide the power output (wattage) if correctly connected. For each coil the power equals the volts multiplied by the current (P=V x I) and the volts equals the current multiplied by the resistance (V=IxR). So if you connect only one coil to power at the rated voltage you can see that the coil will pass 4000/240=16.6 amps of current. 240/16.6= a characteristic resistance of 14.4 ohms. You can directly measure this with the meter you should have if you are doing this kind of construction. Any way, for a serial circuit the resistance is simply additive (Rtotal= R1+R2+R3+R4). That means the resistance the 220 volt supply sees is 14.4 x 4=57.6 ohms. Therefore the current passed by the circuit is 220/57.6=3.8 amps, and the power output is 220 x 3.8 or only 840 watts. Connected in parallel the total resistance can be found by the following formula (I'm not going into the derivation tonight, look it up yourself ☺), 1/Rtotal=1/R1+1/R2+1/R3+1/R4. At 14.4 ohms each, that gives a circuit resistance of only 3.6 ohms. Many paths for the current lowers the resistance and allows more current to pass 220/3.6=61.1 amps. This is a good thing for coil heat output because then the power output will be 220 x 61.1=13,444 watts. However you need a breaker, contactor and supply wire capable of drawing that current, and the coils need to be able to withstand same without overheating and burning out. Last option would be to connect a pair of coils in series, and each of those two in parallel. Needless to say that will be a compromise between a full series circuit with insufficient power and a full parallel with potentially too much current draw. The math for that is easy from the equations I've given you and I'll leave that as an exercise for you. 15.3 amps and therefore 3,361 watts output. I strongly suggest that you get a real electrician involved in this build. You have a basic lack of understanding of circuits that could be dangerous, even fatal. You still need to know how to correctly size feeders, make terminal connections that are electrically isolated and can withstand the heat, chose a contactor or SSR that can take the current, interface with a PID controller and power same without having stray inductive current mess up your signal, wire the thermocouple, properly ground the system, wire a safety door switch... Don't know what the laws are in Israel, but if you build it for a client here and they get hurt or burn down their shop you would be liable. Note that I've not had much sleep tonight and I'm on a tablet, so not checking my calcs. No stamp tonight, get a local EE to certify, or just buy a commercial unit! Quote Link to comment Share on other sites More sharing options...
MikeAlmogy Posted March 17, 2020 Author Share Posted March 17, 2020 Hi and thanks :-) I will see what i can do. In anyway i think i'm screwed. Thanks Mike Quote Link to comment Share on other sites More sharing options...
MikeAlmogy Posted March 17, 2020 Author Share Posted March 17, 2020 Ok, just to make sure i got it right. I can use 2 coils in serial then connect them to another 2 serial coils in parallel. My numbers are 12.1 ohm resistance, 18.1 amepre and 4000 Watt. Am i wrong? this is exactly what i want. Mike Quote Link to comment Share on other sites More sharing options...
Latticino Posted March 17, 2020 Share Posted March 17, 2020 Not enough info. Delta or wye connection. Sketch your work. Three phase is a little different than single if I recall. Something with square roots. Look it up. Resistance of individual coils doesn't change, just overall resistance of the circuit. Need to know how much poweris required for the box you designed and select coils and circuiting to match, then controls to suit. Joppa Glassworks used to have an excellent design manual for that. I'm just a Mech E, you need an Electrical engineer. Maybe Steve can help you. Quote Link to comment Share on other sites More sharing options...
MikeAlmogy Posted March 17, 2020 Author Share Posted March 17, 2020 2 minutes ago, Latticino said: Not enough info. Series or parallel. Sketch your work. Three phase is a little different than single if I recall. Something with square roots. Look it up. Resistance of individual coils doesn't change, just overall resistance of the circuit. Need to know how much poweris required for the box you designed and select coils and circuiting to match, then controls to suit. Joppa Glassworks used to have an excellent design manual for that. I'm just a Mech E, you need an Electrical engineer. Maybe Steve can help you. I edit my original message after running some more numbers. No 3 phase. Quote Link to comment Share on other sites More sharing options...
Latticino Posted March 17, 2020 Share Posted March 17, 2020 Close, depends on actual coil resistance. I got 15.3 A and 3.36 kW for that configuration given a coil resistance of 14.4 ohms. Measure to be sure. Quote Link to comment Share on other sites More sharing options...
MikeAlmogy Posted March 17, 2020 Author Share Posted March 17, 2020 I'm sure that the actual numbers are different. My wife father is an electrical engineer. I will run the circuit design and numbers with him. The next obstacle is to figure out how to use a 2 phase setup in order to lower the amperage from each phase, but this is more complicated for me at the moment so I need to run it with him. My original thought was to put each serial coil on a different phase but it is not how this works so a professional help is needed. Thanks for helping me figure the basics out. Thanks! Quote Link to comment Share on other sites More sharing options...
Steve Sells Posted March 17, 2020 Share Posted March 17, 2020 Electrically speaking, actual 2 phase exists but not in Israel. You really need to get an electrician involved before you hurt someone. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.