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Induction heating. Low tq thyristors
2

Induction heating. Low tq thyristors

Induction heating. Low tq thyristors

(OP)
Self-commutated H bridge. Running at 400-600 Hz with DC up to 2 kV and output current up to 1 kA.

The original design uses three thyristors in each "leg" of the H. The reason is that fast thyristors were not available at 2 kV at the time of its conception and birth.

It seems to be time to refresh this granddad and give it better legs where a single thyristor handles each leg. That would make things a lot simpler (four thyristors instead of twelve) and make it less sensitive to snubber and voltage divider problems.

The Igate is standard, couple of volts and 1-2 A. Nothing special there. The tq is where the problem is. Tests have been made with long tq (like 200 us)and with no luck.

Been searching the Net for some time but don't find anything suitable. Any tips from EngTips?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction heating. Low tq thyristors

Hi Gunnar,

It's been a while since I was playing with inverter-grade thyristors, but Westcode used to be my go-to supplier for fast thyristors. They're part of IXYS now - did you see this range of thyristors? The tq's are low for their ratings: http://www.westcode.com/distthy.html Specifically the http://www.westcode.com/r1700mc18x-21x.pdf gets you to a tq of 40μs or less with 2100V and 1700A rating, depending on whether the other properties are acceptable.

I haven't used Powerex for inverter-grade stuff, only their very large power frequency types. A quick look on their website brought up http://www.pwrx.com/pwrx/docs/c712.pdf and http://www.pwrx.com/pwrx/docs/c770.pdf.

RE: Induction heating. Low tq thyristors

(OP)
Thanks Scotty!

That link gave a much better overview than I found during my futile attempts at finding the corresponding information. We will look deeper into those possibilities. Much deeper.

I am dealing with mechanical people and it has been a challenge to make them understand how the inverter works. I then made a "hydraulic" drawing (no standard symbols were used) to tell them what happens. It worked very well and they seem to understand it all. Looking forward to next meeting.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction heating. Low tq thyristors

(OP)
Sparky to Clanky:



H is main flow/current S is reverse flow/current

If flap doesn't close in time, it will stay open and the fastest valve (IRL, there are three in series in each leg) will take all the pressure/voltage. Which it isn't designed for.

The vane is the furnace oscillating like a pendelum between positive and negative voltage and triggering the release coils for A,D and B,C when limit switches are hit.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction heating. Low tq thyristors

That's very good Gunnar, a hydraulic oscillator.

I had actually envisaged the three thyristors in parallel to boost the current rating, not series for increased voltage rating. Paralleled devices are still fairly common today in very high current applications like static exciters for large generating plant as I'm sure you know. How did the designers handle the voltage division across the thyristor stack - or does it just rely on close matching of the three devices and hope for the best?

RE: Induction heating. Low tq thyristors

(OP)
This is what it looks like. Vertical axis in kV.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction heating. Low tq thyristors

It has the look of a current-source design with a near-resonant load. That strikes me as quite a brave circuit design - device matching looks fairly critical to operation. I can understand the desire to move to a single switch in each leg.

RE: Induction heating. Low tq thyristors

Normal thyristor can't turn off if current not go/pass to zero.
GTO may be turned off by gate control, but for standard thyristor need an auxiliary circuit that lead to reversing (decreasing to zero) current.
Some ideas may found here: https://www.quora.com/How-thyristor-get-on-turn-of...

RE: Induction heating. Low tq thyristors

(OP)
Thanks, iop. But we are a little bit beyond that stage. The discussion is about tq, withstand voltage and voltage distribution when employing several thyristors in series etcetera. I'm saying this so we don't get the thread cluttered with lots well meant but rather out-of-place "tips".

If you look at the wave-forms and read the comments, you may notice that the failure to turn of two of the three series connected thyristors is because they are much slower (around 200 us, than the "grey" one. The grey one turns off (in 20 us) the way it should and, doing so, gets exposed to a voltage that should be dealt with by all three thyristors in series. The current then drops to near zero so that there is no reverse current left to turn the still conducting (red and green) thyristors off. Semiconductors have improved over the years and we are now planning to use four 2 kV thyristors instead of 12 (three series connected/leg) thyristors with lower VDRM. That is what the thread is about and that is also indicated in the subject line.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction heating. Low tq thyristors

(OP)
Right! It is a Monte Carlo/Las Vegas situation.

That's why the snubbers have rather low series resistance and why there are parallel resistors for even voltage distribution. The design is old. Drawings are dated 1977, so it is about time to improve the whole thing.

I must say that it is a good design, for that time. It ran without problems till 2016.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction heating. Low tq thyristors

My mistake... in the first post you were talking about a 200us test that did not work and I thought the waveforms already refer to the modified version ... with a single thyristor on leg.
Such a big difference in tq, 1:10, is normal to lead to blocking the fastest.
Regarding this statement: "Tests have been made with long tq (like 200 us) and with no luck.", do you made tests with one 200us thyristor on a leg and fail to turn-off? If yes, maybe need to check dv/dt conditions/capability.

RE: Induction heating. Low tq thyristors

(OP)
OK. I see.

I wasn't there when the fail occurred, so there are no recordings from that.

The recording shown is when running at a reduced DC voltage. Low enough not to cause another fail. It shows that the fastest thyristor does what it is supposed to do and that the slower ones start to turn off. But no current "left" to do that properly.

New tests with other thyristors are planned within a few weeks. There is one furnace running. It has the 20 us thyristors in all positions. The problem is that the buying people had no idea what was actually needed and seem to go the easy way - buy the cheapest. Seen it too many times. I wish they would let the people that know what is needed do the procurement. But, no. MBA:s are slowly ruining the industry. There's more to life than Business Administration...

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction heating. Low tq thyristors

Do you have some details regarding firing control?
For such IH inverter with variable freq in 400-600Hz range, most probably use auxiliary thyristors with LC circuits to control turn-off.
With new 20us thyristors I think must modify timing for aux thyristors and LC circuit if any.

RE: Induction heating. Low tq thyristors

(OP)
Thanks for your interest. But if you care to look at wave-forms and the diagram below them, you will see that this is an H bridge where thyristors are commutated by turning on the other (diagonal) pair of legs.

Also, please note that this is a 40 years old plant that works OK as long as one doesn't try to save money by buying slower thyristors.

Re gating pulses: They are between 18 and 20 us wide with around 2.5 A amplitude and risetime in the order of 500 ns when measured with a 20 MHz DC clamp. No need to change anything there. And no need for auxiliary turn-off thyristors with the associated LC circuit or using GTO:s. The bridge is, as said, self-commutated.

The original question was mostly about finding thyristors that can be used. That question was answered to great satisfaction by ScottyUK and we really do not need any more inputs regarding this.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction heating. Low tq thyristors

If you've lost the turn-off sharing then the old devices likely had the Qrr or Ir matched and that requirement has been lost with the replacements. Properly matched devices don't require a huge amount of snubber for successful series operation.

I can't see 2000V SCR's being suitable for a circuit that operates at 2000V. You found some suitable 4000V or more rated devices to use?

RE: Induction heating. Low tq thyristors

(OP)
No, I don't think there are any such devices readily available. I live only around 20 miles from the ABB HVDC center in Ludvika and I am sure they have a few HV devices. But they are used in mains commutated 50/60 Hz applications and I don't think they can be used for a lot of other reasons as well. Dimensions, cost, current range (guessing) and ABB not being willing to sell are a few of them.

There is already a limit with the three devices/leg. The DC source has been limited to run lower than 2 kV and we are discussing if it is better to have a safety margin than to run at maximum capacity.

In my view, there are six series connected devices. Three in each leg. And the measurements we have made so far seem to show that the voltage distribution is such that each leg takes half the voltage. That would make 2 kV devices fit the bill. Time will show. No activity planned before we get the devices. Still have to decide what to use and then there is delivery time. It is not only I that decide. I am the "Messknecht", so to say.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction heating. Low tq thyristors

I think it's there an aux circuit made by a LC group and a SCR (near number 11); this one made turn-off.
As new Tq is around 10 times smaller, need to modify this LC circuit to increase 10 time resonant freq.
If no big Qrr difference between old/new SCRs, C must remain around and L decrease about 100 times.

RE: Induction heating. Low tq thyristors

(OP)
No, iop. That is the precharge/starting circuit. I know that you mean well. But your tips are of no use here. Really not.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction heating. Low tq thyristors

What's the voltage rating of the 3 devices in series? You probably should be above 90% of 3X that rating with the new single SCR to match the original design. Typical low-voltage puck devices are 1200-1800V each which would put you around a 4kV device.

You have the same issue with the 2 legs of the bridge as you do with the series devices. The devices won't share the voltage if they don't turn off at the exact same time.

There are technical reasons that lower voltage rated series devices are still a good choice. Lower Tq, lower gate drive requirements, devices switch faster, lower clamping force, distributing the heatsinking across multiple devices.

When you want big, then look at Dynex Power or CRRC Times Electric. Both specialize in big devices.

ABB does sell useful semiconductors. But I would agree that the HVDC center likely doesn't want to do walk-in sales.

RE: Induction heating. Low tq thyristors

Hi Gunnar,

during my time in induction heating business 20 years ago we did similar inverters with approx. 1000 V DC-link voltage using thyristors rated 2200 V with a tq of 100 us.

One of our classic competitors in this business used the series connection as you describe it here and this design has been copied by one supplier from India at least.

The market for fast thyristors is very small today, as the high power induction heating is the only relevant application today, and hence nearly all major suppliers have discontinued these parts. Westcode was aquired by IXYS as already mentioned. Recently IXYS became part of Littlefuse, this is the only company keeping still some focus on these devices.

Due to this I think that it is unlikely that you will find a device suitable to replace the series connection of three devices by a single one as there was no much further development, but you might be able to reduce the number of series devices to only two. But would this be worth the effort ?

My recommendation is to keep the series connection as it is. Major concern in replacement parts is that you most probably need parts with matched Qrr values.


RE: Induction heating. Low tq thyristors

(OP)
Thanks, Uwe.

It so happens that there's a workshop/conference on "Wide Bandgap" (SiC etcetera) power semiconductors in Stockholm this week-end and a couple of days after that. https://www.ri.se/en/scape-2019

I participated in last year's conference, but don't have the possibility this year. Anyhow, IXYS had a few guys there and I think they will be there this year as well. I talked to a friend that goes there and I hope that he can discuss the matter with the IXYS people. They seem to be the right guys. If they don't have it, or can't give any directions, I think that we can skip the plans on reducing the number of thyristors.

Hutzy told me. And you strengthened that.

My thinking was "sure, there must have been some development" but I think I was wrong.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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