Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>
Dr. Resonance
Hi DC,
I tried your design using a 0.08 inch gap, 280Vac, and 30amps on my program and it too came up with 110 turns. However, the flux density was 24600 gauss. I believe I physically have room for about 270 turns in one layer on the core, but will require a larger gap (0.45 inches). This also will reduce the flux density to 11000 gauss. I dont know where these cores saturate and if 24600 gauss is no problem, the fewer number of turns will mean less winding loss. Maybe you could shed some lite on this.
Thankyou for your response
Gerry R.
Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>
Epoxy both cores together and use them in parallel. Wrap 120 turns of #8 AWG around both cores. Tap at 100 and 110 turns.
Dr. Resonance
I just acquired two 120Vac 50A variacs that had a major diameter of 12 inches and an estimated cross section of 6 sq inches. The windings were burnt so I took the core out and removed the windings and found the actual cross section of core was 3 sq inches. I was originally planning on making two toroid ballasts and put them in series but now I'm thinking of combining the two cores and rewinding it as one core with 6 sq inches. I'm designing for 280Vac and using 8awg P180 insulation (180 degree C polyurythene) magnetic wire and using a 0.45 inch gap in the combined core.
The design program I wrote suggest the following performance (I assumed relative permeability of 2100):
268turns 29.7mh 25amps rms Bpeak=10114gauss Winding loss 107watts 245turns 24.8mh 30amps rms Bpeak=11080gauss Winding loss 141watts 227turns 21.2mh 35amps rms Bpeak=11968gauss Winding loss 178watts
The original 50amp variac had a tapered winding. 135 turns of single wound 11awg and 67 turns of double wound 11awg (2 strands of 11awg each in parallel). The two 11awg strands in parallel is an equivalent of one 8awg which seems OK for 50 amps.
Questions:
1. Since the variac was designed for 120Vac in and 0-120Vout, how did it get by with a 3 sq inch core at 50amps.
2. With my ballast design using the 6 sq inch core, the peak flux density is ~12000 gauss for the 35 amp case. What is the typical saturation level with these things realizing that the variac is a very old design using a silk insulation on the wire. The core appears to be made with spiral band of material.
Gerry R.
Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>
They will saturate. That's why when we used to use old variac cores we always gapped then at .080 inch. A fabricated steel clamp held the variac from spring apart and a bandsaw did the work. An 80 mil piece of plastic was epoxied in place to form the gap. They didn't saturate.
Now we use our own slug tuned cores with a small DC gearmotor to accomplish the same control and makes it a completely variable process. Current is adjusted completely independent of voltage.
Dr. Resonance