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Re: Toroid ballast design



Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>


Well, both variacs had burnt winding and all I wanted was the cores. Don't know what caused the damage but stacking the cores and keeping the number of turns high seems like the way to go. I will try to vary the gap to get the right range of inductances. This may be a good test to see if my program works :o))) I'll report the results.


Gerry R.

Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>

We spacewind the layers with some small 1/8 x 1/8 phenolic strips so we can blast 200 CFM of air thru the core as well.

Sounds like they were pushing it a bit.

Dr. Resonance

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