115v 2 Phase furnace rewiring, help needed

Hello all,

I have an older US made furnace, "Hevi Duty" brand, with the most recent patent date being in the 1930's. Data plate says its 115v 2phase 18kw rated to 2000 F. The current wiring ends at the hookups for the heat elements. I imagine the relay and temperature controls would have been installed elsewhere and where left when the furnace was moved.

Anyway, I have single phase 240v in my shop, along with a 3 phase rotary converter for my other machines. I can't find much info on 2 phase or how to go about hooking one up in either 1 or 3 phase arrangements. I was hoping someone here might be familiar with this. It seems like its just 2 115v legs each operating half the elements. There are also the remnants of two separate thermocouples inside, presumably one for each set of elements.

Also, I plan to use a modern SSR and PID to control temperature.

If it really is two phase, heaters don't care about phase angle, so it should work fine on single phase of correct voltage.

Can you measure resistances and draw up a sketch of what you find?

18kw is a lot for 115v so 230 would be the target voltage.

JST said:

If it really is two phase, heaters don't care about phase angle, so it should work fine on single phase of correct voltage.

Can you measure resistances and draw up a sketch of what you find?

18kw is a lot for 115v so 230 would be the target voltage.

Click to expand...

Can't post images but essentially its 2 circuits with 5 sets of elements each. Each circuit is 1.5 ohms.

Doing a little calculating, 115v with 1.5 ohms ends up approximately 9kw. So two 115v 1.5 ohms circuits would make the 18kw listed on the data plate.

So powering both circuits on 220v, at 3 ohms = approximately 18kw, and 80 amps, would work? I would think it would take forever to bring it up to temperature at 220v since its about 15x15x20", but maybe these old furnaces were just built that way or possibly its 480v (2 phase with four 115v wires?)

This may just be a matter of semantics. 115 volts looked at with an oscilloscope measuring the voltage to ground shows a nice SINGLE sine wave. If the inputs now are across each leg of 230 volt supply you still have a SINGLE sine wave of twice the amplitude. Each leg is out of phase by exactly 1/2 cycle, so the two live wires MIGHT be considered as two phases, relative to ground. Wikipedia has a nice explanation of single, double, and three phase power that is worth reading.

18kw is 18kw. It will heat as fast on any voltage, so long as there is 18kw going in.

As for phases, standard 230V is just 230V single phase, center tapped.