How to fire thyristors in a 3phase phase angle controller - Page 1
Forum section: electronics / projects

author: somlioy
author_rank: Regular Contributor
timestamp: July 20, 2019, 04:30:46 pm
content: Hello, working on a 3-phase phase angle controller and I have two questions- Is it required to control both gates at a thyristor individually or is it possible to tie both gates together?- How should the gate-signals be generated, with the possibility to change the firing-angle?Main circuit:

author: patrick1
author_rank: Contributor
timestamp: July 20, 2019, 06:06:32 pm
content: wow cool, il keep my eye on this thread, - sorry i dont mean too hijack it, - but imagine for 1 second you have neo magnets inside your motor, - and it was producing 1000v reverse emf's . - does anyone know of a gate driver that would handle that kind of abuse ?. -

author: duak
author_rank: Super Contributor
timestamp: July 20, 2019, 06:21:36 pm
content: Here's a link to a Master's thesis on three different methods to trigger the thyristors:https://pdfs.semanticscholar.org/aac2/ad713d9ffeeea06bda62cd87de7e791ba070.pdfThis is a few years old - now designers would probably use a microcontroller of some sort.

author: jbb
author_rank: Super Contributor
timestamp: July 21, 2019, 03:33:34 am
content: Ah, classic.Thyristor converters tend to be tough and long lived. Unfortunately they also tend to be big and heavy (magnetics for 50 or 60 Hz are large) and have terrible power factor. They are absolutely still in use for large industrial motor drives (>20MW) and some specialised power systems equipment.I'm not really well informed on thyristor design, but as a I recall there are some things to watch out for:Power thyristors devices are made up of many many little thyristor cells in parallel on a single dieExcessive dV/dt when the SCR is off could cause somebut not allof the thrysitor cells to trigger (current flows through stray capacitances). Current would flow through part of the die, causing overheating and burnout. Check your data sheet for the limits.I have also heard that excessive dI/dt could cause trouble too. Not sure why.In terms of gate drive:One driver per thyristorOnce on, they won't turn off unless you're using special devices such as a Gate Turn Off (GTO) thyristor. These require very specific driving provisions and snubbersA simple pulse transformer might be enough to do the trick (basically drive a continuous stream of pulses into the gate when you want it on). Refer to Figure 5 on page 12 of the thesis dual referencedA more advanced gate drive would deliver a brief pulse to turn it on, followed by a modest keep-on currentIn terms of control:The thesis is a bit old, and some of the comments made (e.g about temperature stability) are no longer trueYou may be able to find a suitable all-in-one thyristor control ICThis is quite possibly a job for a microcontroller. Using a crystal oscillator and timer peripherals enables high accuracy and high precisionFor 50 or 60Hz, it's very possible to sample the incoming supply with an ADC (5 kSamples/sec would probably do) and do a software PLLFYI, this scheme isn't great for induction motor drives because it changes the voltage but not the frequency. It has been, however, very popular for driving large DC motors using a 6 thyristor bridge.

author: somlioy
author_rank: Regular Contributor
timestamp: July 21, 2019, 12:10:47 pm
content: I'm planning to try to use this on a "dummy load bank", ie. electrodes in sea water, expecting around ~40kW. So this isnt something to be permanently installed somewhere.jbb: So a setup like this wont work, parallelling two thyristors to form a triac? (Imagine resistors instead of the motor.)

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 21, 2019, 12:36:52 pm
content: Quote from: somlioy on July 21, 2019, 12:10:47 pmA triac is not the same as two discrete SCRs wired in anti-parallel. You can trigger a triac in all 4 quadrants of operation (gate current polarity w/r/t MT1 polarity) but an SCR can only be turned on when its gate receives a pulse of current which is positive w/r/t its cathode, hence you have to use separate, isolated trigger signals for each SCR in an anti-parallel pair.Note that thyristors/SCRs (and triacs) have fairly specific limits on the dV/dt and dI/dt allowed during switching, and driving a gate on these devices is a bit more difficult (requires more energy) than, say, a MOSFET or IGBT. The usual approach for anti-parallel SCRs is a gate drive transformer with two separate secondary windings.

author: Circlotron
author_rank: Super Contributor
timestamp: July 21, 2019, 12:47:19 pm
content: Watching this thread.

author: jbb
author_rank: Super Contributor
timestamp: July 21, 2019, 11:16:53 pm
content: Putting SCRs back-to-back is a time honoured tradition. However, you’ll note that the gates are quite far apart in voltage (like the full peak line voltage!) so you’ll need to drive them separately to some extent. A dual-output transformer could be an option.If you want a TRIAC, then by all means use a TRIAC. They generally come in lower ratings, and are (according to rumour) a little less tough, but if you can find a reasonable device that’s not a problem.A salt water load is delightfully blunt-instrument. One thing to be aware of is that the load inductance will be small, so you should have a look at what dI/dt your device is rated for; a little series inductor may be required.For safety reasons you can’t just drop some electrodes in the ocean (what if some poor fish - or person! - swam through it?) and you should make sure your electrodes don’t contain any nasty metals (eg leaded solder, some brass alloys contain lead) that night leach out.What’s your power level? It may be feasible to use some space heaters... and you won’t end up with contaminated salt water to dispose of.

author: somlioy
author_rank: Regular Contributor
timestamp: July 22, 2019, 05:19:49 pm
content: I see, but is there any advantage using a transformer instead of a opto-coupler, like they've done in the attached circuit. Is this a possible solution?Regarding dI/dt. Say the thyristors are rated at dI/dt = 300A/µs. What value should this be compared to?I got a plastic barrel with sea water and electrodes, which are copper pipes. As per today we're doing load tests with the same setup but the copper electrodes lowered gradually into the barrel. This system is rated for couple houndred amps, also got a larger version of this probably capable of like 1000A or more. But for now I want to do this on the smaller setup since it's a bit "how u doin". Space heaters has also been used on smaller generators but they're problematic since the "load stepping" usually is a bit coarse. Might lead to getting some resistance wire creating a large heater that also then would be controlled by planned circuit.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 22, 2019, 07:01:08 pm
content: Quote from: somlioy on July 22, 2019, 05:19:49 pmNo, that circuit would be sketchy at 20A, much less 1kA, mainly because the energy delivered to the gate will depend on when in the cycle triggering occurs. This is fine for a static switch, and fine for phase control at low current (ie - under 20A), but not okay for large thyristors that might need as much as 1A at 3-9V for <10us to reliably trigger, but which can't tolerate that same voltage/current for the entire cycle without exceeding the gate power rating.If you've never worked with high power thyristors before, well... I'm not one to tell people what they can't do - who am I to judge? - but let's just say the learning curve is steep with these devices (that said, they are easier to use than GTOs!)Quote from: somlioy on July 22, 2019, 05:19:49 pmThis is pretty straightforward: use the inductor equation to figure out how much inductance is needed to limit dI/dt given the peak value of the mains voltage. Since a saltwater load is effectively a low-inductance resistance, you will almost certainly need additional inductance here; probably somewhere in the range of 2-4uH, depending on how fast the SCR turns on (generally 1-4us for phase control SCRs; <1us for inverter SCRs). You will need an RC snubber, with component values more likely in the range of 1uF and 1-10 Ohms, rather than the suspiciously generic-looking 0.1uF and 100 Ohms shown in the above schematic.Another note - electrolysis of saltwater is how bleach (NaOCl) is made industrially, though with DC, of course. Now, with AC the cathode and anode switch places 50 or 60 times per second, so ion migration can't really happen, but I do know that gases are produced in water resistors even when AC is applied, so beware. You might find it is altogether preferable to submerge coils of stainless steel wire rope into a barrel of deionized water to use as a water-cooled resistor, rather than use salt water itself as the resistor. Also, the coils of wire rope will definitely have enough minimum inductance.

author: Larry80
author_rank: Contributor
timestamp: July 22, 2019, 07:20:13 pm
content: Just want to chip in, which is not found in the literature. Be mindful about the energies which flow trough the RC snubber. All capacitors also act as resistor in AC circuit, which will flow trough even when triac is turned off. Use non-flammable resistors and X capacitors, at least in the end. You can estimate the amount of energy quite well by measuring the body temperature of your test resistor against the manufacturer charts. Factor in the ambient levels too, if you test at 20C flat and finished product is 50C vertical, how would it effect that equation? I would keep resistors nominally under 60C and it must not become dangerous if capacitor shorts.

author: duak
author_rank: Super Contributor
timestamp: July 23, 2019, 04:38:06 am
content: The optocoupler could drive an intermediate triac rated at 10 to 20 A that could then drive the main triac. Make sure the gate currents are within specification - some series resistance in each stage to limit current may be needed. Also, gate bypass resistors may be needed to handle any leakage current from the previous stage.Larry80 points out that the RC snubbers will also let a leakage current through even the triacs are not triggered. Also remember that the triacs cannot be considered fail-safe. ie., there must be definite opening of the line phases with a mechanical switch or connector.Small amounts of chlorine and hydrogen will be evolved even with AC, especially with hot or boiling water. The chlorine will tend to stay in solution and attack copper. Hydrogen is flammable and the combination with chlorine is explosive if allowed to collect in litre volumes. It can be triggered by light too.Take care,

author: jbb
author_rank: Super Contributor
timestamp: July 26, 2019, 11:01:01 pm
content: Quote from: MagicSmoker on July 22, 2019, 07:01:08 pmMmm, bleach. It may be a bad plan to tip that back into the ocean - do you have a plan for safe disposal?I just remembered a trick on the salt water resistors. If you don't need super-fast response, you can use a mechanical sliding stage to bring the bars into the barrel to varying depths; this makes a variable resistor and will have much better power factor than an SCR bridge.I also remembered that you can purchase resistors made from stainless steel sheet: see the HPR Grid Resistorsherefor an example. These can be immersed in deionised water as MagicSmoker suggests.