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-   -   Trying to figure out SCRs (http://talk.craftweb.com/showthread.php?t=12656)

Antti Torstensson 08-25-2020 06:11 AM

Trying to figure out SCRs
 
Iíve been controlling my SiC elements with just an SSR with mains voltage. Just this weekend I found an amazing deal for an SCR and bought it. While waiting for it to arrive Iím trying to figure out how and if I can actually use it. Iíve been reading a lot the past few days but I have absolutely no experience with SCRs.

Iíve read that with phase angle fired SCRs it is not recommended to go below 50% of the mains voltage. How does that work for controlling SiC elements? Seems way too much for minimum available power output initially. Especially with the power reserve you must have to compensate for element aging. Or is it even possible to use phase angle firing mode without a transformer to control SiC elements (in a glass blowing furnace application)?

I could of course use the SCR in burst firing mode but would there be any advantages to my current setup that does the same with an SSR. I also had a stupid idea of just limiting the voltage with the SCR and use the SSR for burst firing.

Thanks in advance for any help!

Antti Torstensson 08-27-2020 06:13 AM

I did some re-reading and re-thinking. The Kanthal handbook says this:

"As there is no current flow for part of the supply
cycle, phase-angle controlled loads will give rise
to an apparently poor power factor, even with a
purely resistive load, and this can lead to problems
on large installations, especially if the firing angle
is small. Generally, the starting voltage with new
elements should not be less than 60% of the supply
voltage, to minimize this effect."

I now think this must mean that the elements should be sized in a way that you initially get full required power with 60% or more of the supply voltage. Although thatís not what it says, itís the only way I can make sense of it.

Still seems that the power factor would be really low especially while the furnace is idle. Why exactly is a poor power factor an issue? Does it make the electricity company unhappy?

Randy Kaltenbach 08-27-2020 11:42 AM

Well, it's been over 40 years since I've thought about power factor so take this with a grain of salt.

Purely resistive loads (good power factor) are the easiest type of power for the power companies to deliver. Non- resistive (reactive) (poor power factor) loads are more problematic for them. Therefore, they hit the users of poor power factor equipment with surcharges.

Antti Torstensson 08-28-2020 06:05 AM

So lets say I size the elements so that the designed maximum power (12 kW) is achieved at 140 V with new elements. So 12 kW is the real power and apparent power is 32,4 kW (calculated with 230 V line voltage). That would give a power factor of 0,37 when the furnace is operated at full power and most of the time the power factor would be even poorer. Am I getting it right and is this really how SiC furnaces are operated with a phase-angle firing SCR without a transformer?

Randy Kaltenbach 08-28-2020 11:28 AM

I honestly don't know how those things would be configured or operated. I think you need to talk to an electrical machinery expert. (Unless someone on the board has this knowledge)


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