Induction Heater Driver Problems

[ronv]

Here is my guess - and that's all it is.
In simulation the start up current is very high so i suspect your inductors were saturating ceating an "almost" short. The 2 inductors seem to have solved that problem. So now the bigger the part the higher the current. Since it is F2 you might get away with a slightly bigger fuse but you are probably getting close to what it can do. What frequency is it running at now? I think you meant 10 X 220nf, not Ufd.?

 

[Lighty]

Sorry, thats correct, C1 (tank Cap) is 2.64 uF (10x 220nF + 20x 22nF)

Think it was the problem.

Sorry, was late last night so didn't take note the the frequency.

 

[tcmtech]

So what exactly are you needing an induction heater for anyway?

A buddy of mine has a large scrap yard and salvage business and one of his many odds and ends is a used 77 KW industrial induction heater rig.

I want it but have yet to ever find a legitimate use for it! That and I only have a 240 VAC 200 Amp service in mys shop. Good excuse to step up to a 400 amp service though!

 

[Lighty]

Don't really have any specific use for it, LOL

at 77Kw that unit must be insane!

Just readly a project to build and an opertunity to learn some new aspects of electronics, but a really awesome tool to have. As i do a bit of mechanics it can come in handy for heating bearing etc.

Been thinking, as my transformer has 4 secondaries, which I have 2 parallelled at the moment, to increase the heaters power, would it be better to parallel the other 2 secondaries as well (25V 20A), or would it be better to rather have 2 in series, paralleled with the other 2 series (50V 10A), I know that it still works out at the same power (500W) but from what I've read its better to use higher voltage, lower current?

At the moment it heats up a thin knife in about 10 seconds, but from what I've seen on the net, units of similar design heat similar things up much quicker!

 

[tcmtech]

How you set it up rather depends on the characteristics of the primary induction coil and the circuitry you run it with.

For me I prefer higher voltages and lower currents being all the conductors and capacitors don't have to be as big but given a push pull circuit like yours they get fussy about over voltage spikes on the switching devices when the primary voltages go up.

That's why H bridge switching circuits work so much better. Any switching spikes can be redirected back into the power supply where their energy can be captured and controlled opposed to just shooting up and hoping that they don't go over the switching devices working limits.

The system my buddy has is about the size of a large refrigerator with a smaller unit that sits next to it for the cooling systems.

 

[throbscottle]

Someone else's take on a DIY induction heater: https://www.instructables.com/id/30-kVA-Induction-Heater/?ALLSTEPS

Fascinating thread, btw, very educational. Thanks for the good read

 

[Lighty]

I understand that the H-Bridge driver is probably a better.... Ok IS a better....driver, but I think it would come with its own problems. One that comes to mind is the resonant frequency, as a load is placed in the coil its inductance changes, therefore changing its resonant frequency, now it it being driving by a set frequency, it would become less efficient and draw quite a bit more current if I'm not mistake, so it would have to incorporate some sort of feedback as you originally stated regarding the SG3524.

If I could build a unit of about 1-2Kw it would be awesome, perhaps with power control.

 

[Lighty]

Just some further info, I placed 2x 0R27 (10w) in parallel (0.135ohm) in between the rectifiers and Fuse 2 (before the 15,000uF cap) and measured the voltage across it with my scope, with no load it peaked at about 1V, with a good load it shot upto around 5V, therefore in without a load the circuit is pulling a peak of 7.41 Amps, and 37.04 Amps which is a bit worrying, as the transfomer is only rated 25V 5A AC. These pulses wheren't very long, probably only about 20% duty cycle or so..... any comments? The same resisters placed after the cap was lower.

 

[tcmtech]

Transformers are generally not that fussy about being subjected to short high current loads on the secondaries during the peaks of the sine wave. They mostly care about averaged VA and wattage load levels.
If it doesn't run to hot to touch when loaded down for extended periods its probably just fine.

Now your rectifiers on the other hand may be sweating it out a bit if they are running at the upper end of there absolute maximum pulse current ratings.

 

[Lighty]

The rectifiers are rated for 6 or 8 amp, can recall, they are on a heatsink and just get a little warm, but not much. will upgrade them though.

 

[ChrisP58]

Even though the average current is the same through both fuses, the current through F2 has a much bigger impact on blowing the fuse due to it's high ripple content. Depending of the actual power factor, the I^2R heating in F2 could be two to five (or more) times greater than that in F1. Also, the peak current through the rectifiers is similarly much greater than the average load current.

The input voltage to the 7812 is 34V. (It's max in is 35V) For the 12 V output, it's dropping 22 volts. You can reduce the dissipation of IC1 by putting some resistance in front of it. But even better would be to find a lower voltage to feed it. Personally, I would just use a 12V wall wart.

 

[Lighty]

Thanks ChrisP58 for that info, explains why Fuse 2 was the one always popping and getting extreamely hot, even melting the pastic holder, I've now got a 15amp in there, which has been fine.

Thanks for pointing that out about the 7812, to be honest I didn't even think about that, although is was something I am aware of.... suppose I was too busy concentrating on the other problems and over looked it.

 

[Lighty]

Hi All,

So after quite a bit of reading and googling. I'm going to try build a higher power Series tank setup. At the moment on the unit that I've got running nicely is only about 120-150 watt (12V to 15V at 10amps) and probably a bit more on the 25V.

I'm aiming for about 1 - 2 KW. The problem I'm encountering is that all the "higher" power units run on rectified mains (220V) or so...... so I'm thinking of have a transformer wound for the application, but this won't be cheap.

From an articles or 2 that I've read, some have regulated the heaters power by using a "SCR dimmer" controller to control the power going into the step-down transformer, therefore reducing the output power, but this doesn't sound right to me? Others have used an Autotransformer, but this doesn't offer any mains isolation.....

Any comments are welcome.

 

[shortbus=]

Do you have access to an AC welder? In the US they are known as "buzz boxes" That would give you higher amperage at around 50V open circuit.

 

[tcmtech]

If I was you I would be looking at finding a 120/240:16/32 buck boost transformer.

Just go on eBay and type in "buck boost transformer 16 32" and pick what VA or KVA size you want!

 

[Lighty]

Thanks Gents,

I might be able to get my hands on an old AC welder for free, but it's probably a bit bulky and perhaps an over kill?

I had a look at the Buck boost transformers, but they don't seem to come cheap, perhaps in the USA it would be a viable option.

I have an old microwave transformer though, I was going to use it for my DIY spot welder, but perhaps I could wind it to supply about 40 -50 volts with about 25 - 20 amps? The microwave was rated for 1450W input and 850W output (both microwave power), I assume that the 850w rating is actually from the magnetron output after losses, so I'm thinking I should be able to get 1000W at least from the transformer?

Just one question about the series tank if I may, I'm battling to understand how the series tank behaves at resonance. From what I understand is that the impedance is extreamely low and that the circuit equates to a resistance component only? Surely this means it would act as a short circuit and cause an extreame load on the driving / inverter circuitry?