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How to fire thyristors in a 3phase phase angle controller
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somlioy:
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:
patrick1:
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 ?.   -
duak:
Here's a link to a Master's thesis on three different methods to trigger the thyristors: https://pdfs.semanticscholar.org/aac2/ad713d9ffeeea06bda62cd87de7e791ba070.pdf

This is a few years old - now designers would probably use a microcontroller of some sort.
jbb:
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 die
* Excessive dV/dt when the SCR is off could cause some but not all of 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 thyristor
* Once 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 snubbers
* A 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 referenced
* A more advanced gate drive would deliver a brief pulse to turn it on, followed by a modest keep-on current
In terms of control:

* The thesis is a bit old, and some of the comments made (e.g about temperature stability) are no longer true
* You may be able to find a suitable all-in-one thyristor control IC
* This is quite possibly a job for a microcontroller.  Using a crystal oscillator and timer peripherals enables high accuracy and high precision
* For 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 PLL
FYI, 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.
somlioy:
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. :))
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