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Registered Member #123
Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162
Extreme Electronics wrote ...
I have some 600v 26A triacs on the way along with MOC3020 which maplin had in stock
The MOC3020 is not a zero crossing opto, so you will need some ZC triggering system to give you zero crossing turn on of your variac. MOC3020 PDF
But if you are building a phase controller they are ideal. Just remember that you need to control the turn on delay (from zero cross), not the on time for phase control to work with a triac.
Derek
Thanks Derek during the blurr that was last night (work unfortunately not drink lol) I hadn't looked at the chip properly.
Registered Member #123
Joined: Fri Feb 10 2006, 12:58PM
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Posts: 162
Anders M. wrote ...
Zero crossing is actually the worst time to turn on a transformer, contrary to common belief.
Anders M.
Thanks Anders, that made for interesting reading and made sense of it all. OK so the plan changes again ha ha!
Ok I think by the time I have finished I had just as well put one of my 600A 600V thyristor bricks on the end and put in PWM and ditch the variac idea altogther.
The next thing I have to consider is I run my smoothing caps as a doubler to push upto over 600vdc and was the basis of my design. I need to look if that arrangement can be done without too much hassle. I cant see why not bit then I usually don't!
OK homework time. I will go and hunt out a circuit for this thing and see if I can build a solid state variac:-D I have some controlled bridges in brick form also so that may work better.
I assume the controlled bridges use thyristor to regulate pwm and the diodes after them to rectify, correct?
Off out to the shop now for a head scratching session!
OK just looked what I have in the shop. I am running a doubler as I said so I take the live mains 240v then rectify one way neg through one dieode and then other way through other diode for pos this then gets fed to to 2 series caps with neutral tied to the middle of them. Meaning I get 600 v dc floating across half bridge.
Now I have thyristor bricks which contain just that one pos thyristor one neg. So thinking just just feed it the same except controlling the pulse width of the thryristors to reduce power. Sound good? These will be fed from a choice of 2 pulse width chips either tl494 or uc2846. this will drive the chips I have ordered for some isolation which will in turn drive the IXYS MCC250 14IO1 thyristor brick which is 1400v and 250A From memory. SO not really a full solid state variac but should work fine for my requirements
Registered Member #74
Joined: Thu Feb 09 2006, 09:17AM
Location: Nottingham UK
Posts: 99
Anders, Interesting reading, but I don't think it applys to the case of a variac which is turned down to zero when the power is applied. There is no residual flux and the transformer is not running near saturation. Once the power is applied then the zero cross is no different from nornal mains.
If Colin is building a replacement for the variac, then the problem geas away again as there is no large inductance to switch in the first palce. Infact the problem is driving a large capacitance of the DRSSTC bus caps.
Registered Member #123
Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162
Extreme Electronics wrote ...
Anders, Interesting reading, but I don't think it applys to the case of a variac which is turned down to zero when the power is applied. There is no residual flux and the transformer is not running near saturation. Once the power is applied then the zero cross is no different from nornal mains.
If Colin is building a replacement for the variac, then the problem geas away again as there is no large inductance to switch in the first palce. Infact the problem is driving a large capacitance of the DRSSTC bus caps.
Derek
Yes that will be the new problem Derek. I am hoping starting up with reduced pulse width will keep surge current low enough to stop breaker tripping. If not then will have nifty power control and will STILL need to change the breaker:-D I will get a drawing put together and post up for a sense check on here later. if ok will build it the weekend and see how it goes. I will also have to hope the caps are large enough to not tune too close and cause ringing as I have experienced with power factor caps on large induction heater
Registered Member #123
Joined: Fri Feb 10 2006, 12:58PM
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Sorry to double post but this is seperate to above almost.
Just been thinking about my proposed plan and I was thing that a single fire on gate would be enough but it won't. Reason being output will be dc at first which works fine as pulsed so thryistor will turn off at zero crossing on it's own. Now when the smoothing caps so there job then thats not so and it will never return to zero until shutdown. This means I cannot rely on the thyristors turning themselves off I think???
Am I following this correct or will they turn off when input ac goes to zero there by losing forward bias???
Also just trying to figure out how to sync the tl494 to the mains phase angle. I think there is an option on the chip for this though so will do some reading tonight, see if i can make my head hurt some more!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hey Colin,
This always worked fine for me. The UC3526 is synced to the mains and does the phase angle controlling, while the TL494 produces a constant train of pulses during the whole time the SCRs are supposed to be on, for hard firing. It'll do phase angle control of practically any load, things I've tested it on include NSTs, hoovers, and of course DRSSTCs
The only problem with it used to be that you had to remember to turn the DC bus voltage knob down to zero before plugging it into the mains, or it would come on at full power and blow the fuse. I was happy with that for a while, but lately I've wanted to operate it with a footswitch that broke the mains for safety reasons, so I needed the coil to just start up at a preset power when the mains was connected. So I've modified it to include a soft-start that gets round this issue.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Colin, Steve -
Unless I missed something in this discussion, Colin is still using a variac to control input power. His problem is that the inrush current (magnetizing current) initially drawn by the variac trips his breaker.
This may sound heretical, but isn't the simplest, most reliable, lowest parts-count solution just to use a low-value power resistor, electromechanical relay, and R-C "timer" to reduce inrush current during the first few milliseconds after power is applied? The resistor is in series with the variac (input side), and the relay's normally open contacts are in parallel with the resistor. A simple R-C delays the relay closure for a 1/2 second or so, allowing the maximum inrush current spike to be limited by the power resistor.
Obviously, the parts count of the attached schematic could be reduced even further by using a single power resistor, a single 1000uF relay cap, etc, etc. The power resistor really doesn't require a huge power rating because the duty cycle is so short. The relay contacts should be rated for the full load current.
(I realize that an "electromechanical relay" attached to a DRSSTC may seem archaic, but it may also be a rational solution for the immediate problem, and really is no more primitive than an "electromechanical" variac.)
Registered Member #123
Joined: Fri Feb 10 2006, 12:58PM
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Posts: 162
Steve McConner wrote ...
Hey Colin,
This always worked fine for me. The UC3526 is synced to the mains and does the phase angle controlling, while the TL494 produces a constant train of pulses during the whole time the SCRs are supposed to be on, for hard firing. It'll do phase angle control of practically any load, things I've tested it on include NSTs, hoovers, and of course DRSSTCs
The only problem with it used to be that you had to remember to turn the DC bus voltage knob down to zero before plugging it into the mains, or it would come on at full power and blow the fuse. I was happy with that for a while, but lately I've wanted to operate it with a footswitch that broke the mains for safety reasons, so I needed the coil to just start up at a preset power when the mains was connected. So I've modified it to include a soft-start that gets round this issue.
Thanks Steve. I was slowly coming round to the conclusion while browsing the TL494 data sheet that there was no way to sync the chip directly to 50hz. You have saved me more head scratching :)
What was your reason for hard firing the thyristors Steve? I was just going to fire them and let things take course when the input mains passed zero. I am assuming they would then change bias and turn off. Then again it woul dbe good practice to ensure they stay conducting in such a noisy environment yes? If they turn off early I would think the device would see large spikes and possibly damage thyristor.
I will investigate the uc2856 as that chip has PWM and sync all onboard and better yet it's in my parts drawer already :)
So without Hard firing it's very simple indeed as chip has soft start facility to. use this to drive the opto's and in turn drive the thyristors.
Registered Member #123
Joined: Fri Feb 10 2006, 12:58PM
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Posts: 162
Herr Zapp wrote ...
Colin, Steve -
Unless I missed something in this discussion, Colin is still using a variac to control input power. His problem is that the inrush current (magnetizing current) initially drawn by the variac trips his breaker.
This may sound heretical, but isn't the simplest, most reliable, lowest parts-count solution just to use a low-value power resistor, electromechanical relay, and R-C "timer" to reduce inrush current during the first few milliseconds after power is applied? The resistor is in series with the variac (input side), and the relay's normally open contacts are in parallel with the resistor. A simple R-C delays the relay closure for a 1/2 second or so, allowing the maximum inrush current spike to limited by the power resistor.
Obviously, the parts count could be reduced even further with a single power resistor, a single 1000uF relay cap, etc, etc. The power resistor really doesn't require a huge power rating because the duty cycle is so short. The relay contacts should be rated for the full load current.
Regards, Herr Zapp
Thanks Herr Zapp, Thats was the route I was going and have all the parts to do so. I did however decide to make things interesting for myself and go with the solid state version. This will be a great learning session for me :)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
So what are you actually wanting here? A soft-start for a variac, or a soft-start for a DRSSTC? It wasn't clear from the thread.
If the former, just go to RS and buy one of those time delay relays and a bigass resistor. Job done in half an hour.
If the latter, then it starts to make sense to use phase angle control of SCRs, because then you can use them as a "solid-state variac" to control your DC bus voltage. So if you were using a variac before, the SCRs can replace it, as well as providing a soft-start to slowly charge your bus caps.
I must admit that I prototyped my DRSSTC with an ordinary variac, and then installed the solid-state circuit once the coil itself was working reliably.
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