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4hv.org :: Forums :: General Science and Electronics
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clean high voltage dc for induction heater

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IamSmooth
Tue Mar 09 2010, 12:11PM Print
IamSmooth Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
 		What are my options for a low-ripple dc power source for an induction heater? If I increase my capacitor bank I reduce my power factor.

Could I advance the voltage going into my rectifier/capacitor filter to compensate for the lagging voltage from the capacitor bank?
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Steve Conner
Tue Mar 09 2010, 12:13PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
 		The ripple doesn't matter unless you're trying to view clean waveforms on a scope.

For that purpose, I use a 600V regulated DC supply:
Link2

It doesn't have any PFC, so it has the same problem of poor power factor on the line. But luckily we use 240V over here.

Xantrex make a more modern version with PFC, that can sometimes be had used on Ebay if you're lucky.
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IamSmooth
Tue Mar 09 2010, 12:26PM
IamSmooth Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
 		My DC source seems to work well, but it looks fuzzy on my scope like you mentioned. Is the fuzzy waveform from the varying amplitude inconsequential?

You mentioned regulation. How much current can a regulator take?
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Steve Conner
Tue Mar 09 2010, 12:58PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
 		The fuzz doesn't affect operation. The only downside is that it makes the waveforms hard to see on a scope in the lab.

My power supply is rated 0-600V, 1.7 amps, 1kW output.
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Nicko
Tue Mar 09 2010, 12:59PM
Nicko Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
Steve McConner wrote ...

For that purpose, I use a 600V regulated DC supply:
Link2

Hey - I recognise that supply - is it doing the job for you ok?
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Steve Conner
Tue Mar 09 2010, 01:33PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
 		Yes, looks familiar, doesn't it? smile

I've converted the input to a Neutrik Powercon and the output to binding posts, but I haven't really used it in anger yet! Maybe 250V and a couple of mA for testing some nixie tubes.

Richie has the half-rack version with PFC, though, and he uses it in the way I described.

Solid-state power electronics will run off almost anything. In fact many SSTCs run off mains that hasn't been smoothed at all! The only problem is that if the ripple is excessive, it decreases the average output power, because less power is generated in the troughs of DC bus voltage, but the inverter still has to be rated to withstand the high power at the voltage peaks.

In SSTCs we only care about the peak power, because it's peak power that grows sparks, but in an induction heater, average power is what gets the metal hot. So there's a case for the power supply to be at least somewhat smoothed.
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IamSmooth
Tue Mar 09 2010, 06:37PM
IamSmooth Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
 		For a simple unregulated dc supply would putting an induction coil in series with one of the inputs be an acceptable means of correcting the power factor?
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Steve Conner
Tue Mar 09 2010, 06:59PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
 		Yes. I did this on my OLTC2 solid-state coil.
Link2

I got PF=0.8 with the choke, and 0.61 without. It's not worth trying to get it to 0.9+ with passive circuitry, you need an active PFC for that.
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IamSmooth
Tue Mar 09 2010, 09:23PM
IamSmooth Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
 		Would the reactance correction be as simple as 1/(2pi*f*C) = 2pi*f*L?
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Steve Conner
Wed Mar 10 2010, 09:39AM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
 		No. I have no idea what the equation for it is, or if one exists.

I just got a meaty-looking transformer core out of my junk box, got some wire that looked like it could carry about 20 amps intermittently, and wound on as much of it as would fit.

As I've said before, you'll never correct the power factor completely with a choke, because there are harmonics involved as well as phase displacement.
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