SCR Switching for Butt Welder

This might be of interest to you:

http://www.completepowerelectronics.com/scr-parallel-operation-tutorial/

SCRs turn off when the current stops (reverses). With inductive loads, this does not occur at zero voltage.

If your load is inductive you should look at this:

http://www.electronicspoint.com/threads/scr-triggering-for-high-current-inductive-loads.37461/

There is definitely something missing from your first sentence. An SCR can only turn on, and stay on until the voltage across it reaches zero (at or near the zero-crossing of the supply voltage). As such, it can effectively lower the output voltage, but it can never increase the voltage above the input voltage.

Now it's possible that your 18-36V "input" might refer to the trigger voltage, which is NOT the input voltage to the SCR. A very small trigger current pulse is required at the appropriate time during the power half-cycle to turn on each SCR.

If you have a three-phase device, it must have three separate SCRs (or more likely six SCRs). If you attempt to jumper these inputs and/or outputs, you will see the magic smoke!

A circuit diagram of what you want to do would be most helpful!

I take it you are wanting to use a standard three phase 48 - 530 VAC 50 amp zero crossing turn on Solid State Relay as a single phase 150 amp unit by paralleling all three phases?

In most three phase solid state relays they are built as three independent single phase units in one package so yes it should work just fine although for a 4 KVA transformer on a 480 VAC line its average current draw is ~8.33 amps so the need for having the SSR set up to work as a 150 amp unit is way overkill and just using one phase would be more than sufficient.

You may well be right, that he's talking about a SSRelay, not just SCRs! I was assuming he wanted to connect the 3 outputs of 3 SCRs connected to 3Ø 480V supply together to power a single transformer.

To the OP: There is a BIG difference between input (supply) voltage and current, and the control (trigger, for SCRs) voltage and current.

The input voltage must always be at least slightly higher than the output voltage (due to losses in the SCRs), and the input must supply all the current that reaches the output.

The control (trigger) voltage is nearly always much lower than the supply voltage, and has a vastly smaller current than the supply current. In the case of SCRs, that trigger can be a very short pulse once each power line half-cycle.

It seemed pretty obvious to me that he is talking about a SSR not a SCR and that he, like many people who are not that familiar with solid state switching device terminology, have the two names switched around.

With a SSR the specs of 48 - 530 VAC @ 50 amps Three Phase is a very common rating for the AC power side. So is Zero crossing Vs random turn on. Lastly 18 - 36 VAC is also a common input side rating for SSR's designed to work with systems that use common 24 VAC control circuitry.

As can be read about here.

Crydom Solid State Relay Catalog

SCR's are not rated in that fashion or values and they do not come in three phase either.

After re-reading the original post numerous times, I have to agree. I have to admit that I didn't pay attention to the zero-crossing turn on that he did mention.

I still have a problem with calling the control voltage an input voltage, although thinking of it as an amplifier pretty much removes that objection.

Now the question becomes one of adequate control. That depends greatly on the size of whatever he is butt-welding. I just finished building a welder that has a 6kVA weld transformer, because that was what was supplied, but a 1kVA or even smaller transformer would do our job of edge welding thin foils to thicker substrates, at least with adequate transformer cooling. We sometimes use a fraction of a single half-cycle to do a weld, and a zero-crossing turn-on device could not do that. If he is welding stuff large enough to require several or many full cycles, then the SSR should work.

You nailed it!! It was my headline that confused everyone, sorry. it is a SSR

That's okay. I have come to terms with the problem of knowing everything is getting others to accept it.

Thanks to all

Yes...my error, I was in a rush to get something on line.

Yes, the control voltage is 24 vac. Yes, it is a SSR. There would be more than one sine wave for the operation as the steel wire (approx. .223") is butt welded. There is an automatic shut off (limit switch )when a certain distance has been reached on the weld being pressed together.

I knew the SSR was heavy enough for the application, but some engineer in France with a certain company couldn't answer my question on the load being shared across the three poles if I wired it that way. Even after sending him a diagram and his company's part number he kept rambling on about one pole would switch first followed by another. I kept telling him this relay was being used to start motors according to the catalog with locked rotor current so how could one switch followed by another on a zero crossing SSR. Maybe it had to be translated into English and something got lost in the translation.

Just one more thing, we have some large heavy resistance welders here at out facility. One is a 330 KVA 480 3 phase. The output is only about 8 - 10 vac. Yes, the SCRs are water cooled, and the operation is regulated with the amount of current, parts of sine cycles, slope, and impulse, repeat cycle. In tending to the program boards and the SCRs , I get easily mixed up when writing something down real quick in a rush. Sorry for the confusion.

Thanks again, I going to try this application like I theoretically envisioned.

You are forgiven! Please come back an let us know the results!