SCR Controllers
SCR Controllers
(OP)
I got a question about SCR controllers and I was wondering if anyone could help me out.
I'm trying to control a large resistance heater to high accuracy using an SCR controller. Issues have been raise whether to use the phase angle fired or zero voltage cross.
I'm more inclined to use the phase angle fired because i believe it can reach higher accuracies since zero cross is bound by each period of the 50 sine waves (50hz). However i'm also aware that phase angle fired is predominantly reserved for high response heaters such as wires due to the high Di/Dt. I've also read that phase angle fired tends to have high EMI (electro magnetic interference) due to the fact that it creates harmonics.
I'm very open to those, opinions and suggestions for using a Phase angle fired SCR for a high load (over a two dozen KW) resistance heater.
I'm trying to control a large resistance heater to high accuracy using an SCR controller. Issues have been raise whether to use the phase angle fired or zero voltage cross.
I'm more inclined to use the phase angle fired because i believe it can reach higher accuracies since zero cross is bound by each period of the 50 sine waves (50hz). However i'm also aware that phase angle fired is predominantly reserved for high response heaters such as wires due to the high Di/Dt. I've also read that phase angle fired tends to have high EMI (electro magnetic interference) due to the fact that it creates harmonics.
I'm very open to those, opinions and suggestions for using a Phase angle fired SCR for a high load (over a two dozen KW) resistance heater.






RE: SCR Controllers
If the heaters heat water, or some other large mass with high heat capacity, the zero crossing can give you good accuracy. But if you are producing heat in radiation form or heating thin/small/light things, then phase control is a must. The harmonics issue is not at all that bad. It is usually exaggerated and filters can reduce the problem if it gets real.
I would go for phase control.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: SCR Controllers
As skogs noted, normally phase angle is pretty much reserved for RADIANT heating. I do much work in the semiconductor equipment realm and they use some very large heaters in many applications that require insane temperature control. The only place we bother with phase angle(PA) control is with radiant heaters. Essentially lamps.
I somewhat disagree with skogs in that PA can and does cause far more EMI then zero crossing(ZC) - sometimes in mind numbing quantities too.
If you are not using radiant heating I cannot imagine you even being able to detect the difference. Except in your wallet. The key is to keep the cycle time reasonable.
There is an abyss between ZC and PA you cannot reasonably go much shorter than a second for ZC because the control become quantized due to the 1/2 cycle limit imposed on the power.
There are random SSRs and then there are PA controllers don't confuse them.
PA controllers are probably 3X the cost of ZC.
What are you heating?
What precisely are your heating elements?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: SCR Controllers
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: SCR Controllers
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: SCR Controllers
Speaking of flickering. In debugging a controller I built I needed a load. I grabbed a quartz lamp I'd ripped from a laserjet fuser (1kW) and hooked it to my SSR (ZC). I then proceeded to dink with the cycle times and set points and other parameters. At one point I was shocked to find the bulb, which was rather obvious, hanging from wires from my desk structure (cuz it was HOT!) shift from the expected stroboscopic flickering to long on and off periods of perhaps several seconds.
This was caused by the control signal beating with the turn on region of the ZC function. Since the ZC function limits the ON trigger period to about 20 degrees of the increasing line cycle.
I figured that there must be the occasional engineer out there driven nuts by a control system that was running thru these beating periods and not understanding why the system might just get into the setpoint region then suddenly have lousy swinging process temperatures and since normally you aren't using quartz heaters never 'see' the problem.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: SCR Controllers
In acutality, if your heater is small, you could get away with a burst cycle controller without the electrical utility noticing. Depends on how much the neighbors complain about lights flickering and electronics malfunctioning.
Phase displacement controller power quality problems on the other hand can be tamed with a band pass filter, harmonic traps, and a rapid response capacitor bank to correct lagging vars.
Mike Cole
RE: SCR Controllers
RE: SCR Controllers
Kidding aside thanks for the come-back.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: SCR Controllers
Nah, I've always intended to reply asap but slipped my mind. When I went through my email and saw the old scr controller threads I thought to myself it would be terribly bad manners of me not to thank you... despite being very late. So yeah i do feel bad it's been so long but truly, thank you everyone.
In the end, we purchased a phase angle fired SCR controller to control airflow using a high load resistance heater. I'm now responsible to test the minimum graduations of the controller. But since it's a very high voltage i'm hesitant as to where to begin. Perhaps there's a way to testing it using lower voltages input, control and output.
Why they have a Mech doing the work of an electrical or power engineer i haven't a clue...
'I wonder what happens when i connect these two wires toget-'
RE: SCR Controllers
Have a Happy and safe Holiday!
RE: SCR Controllers
RE: SCR Controllers
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: SCR Controllers
RE: SCR Controllers
Do you hang out with instrument techs a lot? Over here they're known as Tiffies and the majority jokingly (I think
British and probably European officialdom no longer recongnises anything other than LV (≤1000V AC / ≤1500V DC) and HV (>1000V AC, >1500V DC). The industry terms of MV and EHV are still heard but are no longer recognised by legislation.
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Sometimes I only open my mouth to swap feet...
RE: SCR Controllers
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: SCR Controllers
I once hired a woman (although this is not gender related IMHO) still in school at UC Berkeley for her EE degree, 3rd year. Her summer job was to just draft up control schematics for motor control panels that I had already designed. 99% of our panels used 24VDC controls. After a week of so she came to me asking "What are these circuits doing, because to me they don't make any sense. For instance I don't see what you could possibly do with these all series capacitors and varistors!" Being that this was all relay logic, I was puzzled by her question, then came to find out that she was interpreting N.O. contacts as capacitor symbols and N.C. contacts as varistors. I said "I thought you were an EE student?" She replied "Well, EE / Computer Science. We never work with this "High Voltage" stuff in class." To her, High Voltage was anything above TTL!
This was in 1983 and represented my first exposure to the fact that University EE programs were tuning out specialists who were no longer being exposed to the full gamut of EE disciplines.
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: SCR Controllers
RE: SCR Controllers
It is OK for me - I make some money out of other peoples ignorance. But it is bad for society.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...