SCR control

Greetings,
I am still working on firing SCRs, The problem I am facing is that when I go hihger in frequency to about 150Hz I loose the out-put. meaning that the relay fully latches. I am using the PWM function file to control these SCRs. The max Frequency I want to use is 10Khz. Is this possible?

What in the world are you trying to do?
For one thing, you are going to need high speed SCR's to work at 10KHz, like the old chopper SCR's in older 6 step AC drives. It might be that your SCR's don't have enough time to turn off completely and are being re-triggerred.

How are you triggerring them? Relay outputs? Relays aren't going to run at 10KHz.

What are you using to commutate them off anyhow?

What are you trying to do?

The SCR will latch until supplied power drops. Are you using 60Hz AC to power the SCR's? If so, 120 Hertz will be the max.
You need DC and transistors to do PWM.

I have seen SCRs running about 33KHZ on an induction heater. It was also only 4 SCRs. They were fed with DC, highly filtered DC. The SCRs were INVERTER grade with a very specific turn on/off time.

I have seem many methods to fire SCRs, burst, comb, rapid fire, oscilliator to name a few but I have never seen PWM used to fire SCRs. When you fire an SCR is is on. Removing the gate signal does not turn them off. They have to be reverse biased or the current interupted.

SCRs usually use what is called a firing circuit to run them at the high frequencies you are talking about to do wave form re-generation. These are usually specific to inverters on large dc power supplies and for drives. If PLCs were capable of switching at that frequency, able to turn on and shut off in the time and have the electrical/mechanical ratings to turn on/off as many times so you would not be replacing the output card every 2 minutes... the manufacturers of VFDs and large dc power supples would install their PLCs for this purpose. Thus far I am not aware of a manufacturer doing this....

Sheldn said:

SCRs usually use what is called a firing circuit to run them at the high frequencies you are talking about to do wave form re-generation. These are usually specific to inverters on large dc power supplies and for drives. If PLCs were capable of switching at that frequency, able to turn on and shut off in the time and have the electrical/mechanical ratings to turn on/off as many times so you would not be replacing the output card every 2 minutes... the manufacturers of VFDs and large dc power supples would install their PLCs for this purpose. Thus far I am not aware of a manufacturer doing this....

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I do not see enough information in the original post to determine what should be done...ie;
What PLC
What type SCR and are there more than 1.
What is the application.

If PLCs were capable of switching at that frequency, able to turn on and shut off in the time and have the electrical/mechanical ratings to turn on/off as many times so you would not be replacing the output card every 2 minutes...

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PWM allows you to use a digital output to simulate an analog signal OR frequency modulation. Some PLCs are capable of 20khz or higher output.

I notice that any time PWM is mentioned pertaining to a PLC everyone states it should not be done, used, or will not work! Then why are some made capable of PWM?

Btw

the manufacturers of VFDs and large dc power supples would install their PLCs for this purpose. Thus far I am not aware of a manufacturer doing this....

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Omron's PLC+inverter

It wasn't so long ago that Omron introduced its first AC inverter for its motion control product line, the 3G3MV. Designed for small motor control (0.1 to 7.5 kW), the little inverter is nevertheless a powerhouse. It allows users, for example, to select sensorless vector control for applications requiring high torque at low speeds, and can produce up to 150% torque at 1 Hz. It is also loaded with I/O and communications options.

But last November, it acquired another 'option,' and this one is a bit more unusual that the rest. Omron added a plug-in PLC module—not just another attachment but a complete PLC equivalent to Omron's standard CPM2C-S PLC. Using dual-port RAM, the PLC connects directly with the inverter's I/O, key data, and parameters. The PLC has its own I/O and can provide these data directly to the inverter: encoder input, interrupt inputs, and digital, pulse, and PWM outputs.

The plug-in PLC (it is designated 3G3MV-P10CDT) has its own digital I/O. Of the six inputs, three can be configured as a high-speed up/down counters, which typically are used in conjunction with an encoder to allow the drive to track speed or position. Similarly, the first two outputs can work as pulse generators, making it easy to slave drives together in conveying and similar applications.

The combination, says Omron, makes an ideal control solution for networked or production systems requiring precise control of positioning and sequencing functions. It is also a solution for manufacturers of multi-step packaging lines, winders, and intelligent conveyor lines.

Omron shows some interesting possible applications. In one, a PLC+inverter is used as a pump sequencer to control water pressure in a three pump system. The PLC+inverter provides continuous closed loop control to the first pump and on/off control to two other pumps in parallel water pipes. An MMI screen gives local supervisory options, and a wireless transmitter sends SMS signals to maintenance personnel.

In another application example, a series of networked 3G3MV-P10CDTs provide distributed control over a small production line; the PLC in this situation would not only control the drive to adjust the speed of the conveyor, but also control machinery at a local station that is picking and placing components into the production operation.

I may have read too much into the question. I thought that Ernesto was asking about using a PLC with associated logic and I/O to do the firing of an SCR control circuit. Based on his comment about relays, thought he was using a PLC and without more information such as specialty modules and software being used offered some thoughts to keep the conversation going so we could get the details. If this was the question, then I suspect his observation is correct and my comments regarding switching frequencies would apply. If he is using a PLC with motion control and motor control and specialty modules hopefully we can get more details.

I notice that any time PWM is mentioned pertaining to a PLC everyone states it should not be done, used, or will not work! Then why are some made capable of PWM?

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I use this occassionally from a PLC to a CAT generator but more commonly in conjunction with either a Dawson Technologies Si-TEC TGC or Woodward EGCP2 generator controller. Each of these has PWM buit in. DC output not AC. It does work - very well - Caterpillar and the generator controller manufacturers use it all the time.

But last November, it acquired another 'option,' and this one is a bit more unusual that the rest. Omron added a plug-in PLC module—not just another attachment but a complete PLC equivalent to Omron's standard CPM2C-S PLC.

Click to expand...

Unusual choice of PLC I/O as the CPM2C has not been a success in Ozz, and perhaps elsewhere, due to it's price. However, the little dear really has a lot going for it as far as features go. The option would certainly be attractive if I was designing and implementing systems that could use the option.

There is absolutely nothing wrong with PWM but it is certainly normally used with DC devices to allow for high speed switching. Only very specific and expensive SCRs can switch fast enough for PWM, as mentioned previously in this thread.