AC SCRs

[TimWilborne 108]

Could someone explain to me how to control an SCR connected to 480 VAC? Not only how to make current flow through it but from what I understand you must send signals to it to keep current from flowing through it

[Big Country 3]

Someone can probably answer this better than me, but the best I can remember is that you have to give it .07 percent of the voltage this opens the gate and you control the the gate voltage to control the current flowing through it. Its been a while so I could wrong, if so please straighten us both out someone!

[Camel 1]

I go here for help alot of times. This specific section has information on semiconductor physics.

[TConnolly 13]

Once turned on an SCR conducts until either the current is removed from the SCR or until the SCR is reverse biased. Fortunately, AC current in the USA will reverse bias the SCR at 60Hz. Therefore to turn on the SCR you simply apply and maintain the necessary gate voltage, and when you want it to turn off you remove the gate voltage (on some high current SCRs a resistor network is installed at the gate to make sure gate potential does indeed drop to zero) and as soon as the AC current switches directions the SCR will be off. Often SCRs are used in power control scenarios where the SCR is gated at a particular phase angle of each 1/2 AC cycle. This requires the use of another controller board which synchronizes the AC power, the phase angle, and the command (usually an analog value from a PLC or other controller). http://www.ccipower.com/support/description.php (there is quite a bit of information under other links here as well)

[TimWilborne 108]

Thanks for the links. I'll probably back with more questions. Lots of info

[BobLfoot 301]

it was hinted at in an earlier post. Don't forget an SCR is a uni-directional device. An SCR which conducts during the 0 to 180 region of the sine wave will shut off and not conduct during the 180 to 360 phase. To work with 480 VAC three phase power you'll need six SCR's or two (2) per phase. You'll also have a slight hickup when you switch from the one set to the other. This is called the zero crossing problem. This is why TRIAC's are often used instead of SCR's.

[TimWilborne 108]

Actually it is 480VAC single phase with two of them. They go to the transformers of a flash welder. I'm mainly trying to understand what is going on between the firing board and the SCR. Nothings broke, I'm just trying to learn how they work so when something is wrong I can do more than just replace parts until they start working again.

[Gerry 13]

Actually, 3 SCR's and 3 diodes work just peachy! And control is simpler. But 1 SCR and 1 diode won't work for a single phase app.

[gravitar 3]

All the line-frequency AC resistance welders I've seen use two legs of a 460VAC bus, one of which is wired directly to one side of the weld transformer's primary and the other is switched through a pair of large power SCRs in inverse-parallel. Traditionally the SCR "hockey pucks" have been discrete components, but you see more and more of single-component triacs being used instead. Most people I've worked with don't know the difference and just call them SCRs anyway :) It really is "six of one or half a dozen of the other", whether you've got a pair of SCRs or a triac it is doing exactly the same thing.

[TimWilborne 108]

This is the way ours are. I'm in the group who wouldn't know the difference between a SCR and a Triac. White ceramic hockey pucks about 3" in diameter with metal rings on each side. Paint it black and I wouldn't know the difference if you used it at a hockey game.

[TimWilborne 108]

Can you explain how the resistor network on the gate would be setup?

[BobLfoot 301]

Since a minimal amount of voltage is required to keep the gate open placing a resistor divider network on the gate means that a higher level of voltage is needed to opne the gate sine the voltage is divided. This also means that it reaches 0 or below trigger voltage sooner than the main voltage.

[TimWilborne 108]

Ok wizard (or wiseman or resistor holder) I am a little confused. How to you figure out what voltage is required to open the gate and what resistors are required to assure it is closed?

[BobLfoot 301]

It has been some time since i did discrete component circuit design, but there is usually a data sheet which will tell you what voltage and current the gate need.

[OkiePC 14]

You can learn alot by monitoring the gate pulses with an o-scope. We normally do this and compare with a printed copy of "normal" to determine if a problem is in the driver card or one of the pucks. Ours are in Reliance DC drives from 50HP to 200HP 3ph, AC

[ilsk8 0]

Hello there :) I hope the attached doc is of any use explaining a bit. As far as firing SCR's, IGBT's, Triacks the most common way of sending gate pulses to the device is PWM. If you type PWM into google you will find a load of realy good information on the subject, or othwize just click the link below :) http://www.solorb.com/elect/solarcirc/pwm1/ http://www.cpemma.co.uk/pwm.html I have built the first circuit and works very well for messing about with. As you will read the PWM circuits adjust the duty cycle of the current/voltage being sypplyed to the end device. For medium frequency converter and resistance welders this is not how it is done, as there is another step to perform by the electronics first. From the PWM circuit a pulse of say 8 to 10 usS is crated and the the frequency of the waveform is modified so that the periodd petween the pulses are shortened or lengthend depending on the frequency required for a inverter for example. For your welder it is probably the phase firing or closed loop control mentioned in the doc. As sugested a current probe attached to a oscilliscope measuring the pulses across the device wile being referanced to the incomming supply of the controling electronics board is a good way of fault finding as that is one of the steps I perform when checking three phase rectifires. Phase_control.doc

[TimWilborne 108]

Thanks for the info. I will try to read deeper into it this weekend