Hi DC,
Here is the program output for your design if you find it useful:
******************** BALLAST DATA ********************
Ballast Voltage = 280 Volts rms
Ballast Current = 30 Amps rms
Line Frequency = 60 Hz
Outside Toroid Diameter = 12.0 inches
Inside Toroid Diameter = 6.8 inches
Toroid Core Area = 6.0 square inches
Relative Permeability = 2100
GAP Size = 0.08 inches
********************** RESULTS **********************
Required Inductance = 24.8 mh
Required Number of Turns = 110
Volts per Turn = 2.5 volts
Peak Flux Density = 24613 gauss
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