Wiring alternative for 3-phase heating circuit to reduce current 7

[PaulKraemer]

Hi,

I have a tank that is used to melt glue. It has six heaters, each with a resistance of 10 ohms. It is powered with 208 VAC three phase.

Originally, it was wired as shown below with two heaters in parallel across each possible pair of lines (L1-L2, L2-L3, and L1-L3)...

I had asked a question yesterday about how to calculate the total current that this circuit would draw from L1, L2, and L3, which turned out to be about 72 amps. It turns out that this exceeds the rating of the breakers I have available to feed this circuit. With this being the case, I considered rewiring the heaters as shown below to reduce the current draw ...

... When I did the same current calculations (if I did them correctly), this decreased my expected current by a factor of four (down to 18 amps). This would make it easy for me to find a breaker to serve the circuit, but I am afraid that the corresponding factor of four decrease in heating power might make me unable to melt glue at my required rate.

I am wondering if anyone here can suggest another wiring alternative that would allow me to decrease the current draw by a factor that is less than four while still keeping my load balanced and having the same current flowing through every heater. I though about simply eliminating three heaters so that I would only hava a single heater across each possible pair of lines (L1-L2, L2-L3, and L1-L3), but I don't like this idea because it would mean I would then have less heated surface area in contact with the glue. I would really like to have current (albeit reduced) flowing through all six heaters. With six heaters and three lines, I just can't think of a way I can do this other than how I show in my second diagram (which results in a 4x reduction in power).

Any suggestions will be greatly appreciated.

Thanks and best regards,
Paul

 

[PaulKraemer]

Hi xnuke, Bill, crshears, jraef, and IRStuff, and che12345,

Thank you all for your advice and calculations. I understand that reducing the line current will reduce my wattage and therefore my heating capability. When I undertook this project, I did not know how much wattage would be required to melt the glue in the tank at the required rate to keep up with the process the tank is supporting. I believe the original wiring of the glue melter was what Che described as Delta with two resistors in parallel (line current 72.05A; total power 25,958.4W). Knowing that this would be too much current for any available circuit in my building, I rewired the heaters to what Che described as Delta with two resistors in series (Line current 18.01A, total power 6488.21W)

I was concerned that decreasing power by a factor of four might reduce my heating capability too much for me to be able to melt glue at the required rate. That is what led me to post my question, hoping there might be an alternate wiring that would allow me to decrease my power and current somewhat, but by less than a factor of four.

As Che pointed out, rewiring in Y formation with two resistors in parallel would give me a line current of 24A and total power of 8647.2 W. This is about 33% more power than the 6488.21 W I am getting with my current wiring. I tried out the glue melter this week and my melt rate seems promising even with the 6488.21W. I am going to try it out to see if it can keep up with our process. If not, I may rewire for Y with two resistors in parallel to get the extra 33%. If this is not enough to keep up with the process, we will likely have to upgrade the supply circuit.

I really appreciate all of your help.

Best regards,
Paul

 

[che12345]

Dear Mr. PaulKraemer
Just a suggestion for your consideration
1. When you eventually settled on a certain wattage (say W1) that will melt and maintain the glue to the desired viscosity; it may? take (too long a time) to [heat up from cold/solid state]. A possible solution is to have a higher wattage (say W2), where W2>W1 to heat up at a shorter time. Switch back to W1 when the solid starts to turn to liquid.
2. Attention: Do not run on W2. It is intended for start up only. It may? over-heat the glue.
3. This change over from W2 (short time starting only) to W1 (long time running) may be by a [timer or a thermostat]. Or manually by observation the state of the glue.
Che Kuan Yau (Singapore)

 

[PaulKraemer]

Thank you Che12345,

I didn't mention this in my previous posts, but power to the heaters is passed through an SCR that is controlled by a closed loop temperature controller. The temperature controller does timed proportioned control with (I believe) a one or two second cycle tim. With this control method, I believe that as long as I have enough wattage to achieve the required melt rate in the worst case scenario, the temperature controller should be able to reduce the average wattage (by decreasing the portion of each cycle when the SCR is ON)in situations when maximum power is not required.

I feel like I am just about ready to give it a try. I really appreciate your help.

Best regards,
Paul