SCRs

This topic contains 12 replies, has 3 voices, and was last updated by John C John C 7 months, 1 week ago.

  • Author
    Posts
  • #8001

    Ben Wallace
    Participant

    Hello,
    I have some older machines with DC drives running them with power devices called SCRs. So I was wondering how SCR’s work and how do they compare to modern devices?
    Is it worth changing over to more modern type drives?
    Appreciate any help here
    Ben


  • #8002
    John C
    John C
    Galco Representative

    Hi Ben, and thanks for asking Galco this afternoon.

    SCR means Silicon-Controlled Rectifier. They have been around a long time now. I started in this business 34 years ago and they were here long before I was.
    They work a lot like Diodes in that they only conduct electricity in 1 direction, kind of like an electric check valve if you will. The key difference between a Diode and an SCR is the Diode will conduct by itself without any type of control connected to it. The SCR on the other hand will require a Gate signal to tell it when to turn on. Diodes only have 2 terminals, the Anode and Cathode, when the Anode is simply more positive than the Cathode (by 0.5 volts or so) it turns on and conducts. An SCR also has to be more positive on its Anode than the Cathode but also requires a positive pulse on the Gate with respect to the Cathode in order to conduct. Then, like a Diode once the polarity reverses across the device it shuts back off again. Actually the SCR shuts off when the current through it goes below the “Minimum Holding Current” level, is technically how it works.
    The latest and greatest device for power control is the IGBT, Insulated Gate Bipolar Transistor. However it is only used in AC drives not DC, they still use SCR’s to this day. As far as changing over to more modern drives, that depends on a lot of different factors and would be better left to our engineering department. You can contact us by scrolling down to bottom of this page, click Contact Us. Or you can check out our engineering page from the top of this page by clicking on Engineered Systems.
    Hope this helps you out.
    Regards
    John


  • #8003

    Ben Wallace
    Participant

    Ok John, thanks for the information, I’ll take a look.


  • #8454

    Clayton Eldridge
    Participant

    You mention DC drive still uses the SCR, but I would like to know what else they are used in?


  • #8455
    John C
    John C
    Galco Representative

    Well this is a good question. Good morning Clayton. The SCR is still in use today in some AC drives and regeneration modules besides the DC drive. Plating Rectifier is another type unit uses SCRs to name a few.


  • #8456

    Clayton Eldridge
    Participant

    How is an SCR used in an AC drive?


  • #8457
    John C
    John C
    Galco Representative

    This is a great question, thanks Clayton.
    In an AC drive the SCR can have 1 of 3 different jobs, 2 involving the Precharge function an AC drive must do at power up, unless it is an AC current source drive. Some larger HP AC drives will use a phase controlled front end to precharge the bus using SCR’s instead of a resistor. The second function an SCR can have is the Bypass device for a precharge resistor in a drive. Most common device for Bypass was the Contactor, but an SCR has no moving parts and doesn’t make much noise when it is fired. The 3rd job an SCR can have in an AC drive is the Inverter output for a Current Source type VFD. The first 2 jobs mentioned are for Voltage source type drives.


  • #8458

    Clayton Eldridge
    Participant

    Inverter output with SCRs, how is that even possible and what is difference between Voltage source and a Current source?


  • #8459
    John C
    John C
    Galco Representative

    Well Clayton I’m so glad you asked.
    First I should probably discuss the primary difference between Voltage and Current source AC drives. Most all drives today (Spindle dirves, Vector drives, Servo drives, etc.) are a voltage source type, in other words they use a bank of Electrolytic capacitors to filter the DC voltage. In a Current source type drive, you have to use a Choke (Inductor) to filter the DC current that feeds the Inverter section of the drive. All AC drives rectify AC to DC and then invert it back into AC. The SCR in the Inverter has to be Commutated off after each time it is fired in order for the drive to work at all. This is accomplished by using AC capacitors wired between the output phases to the motor in a Delta fashion, both in the positive side of the Inverter and the Negative side.


  • #8460

    Clayton Eldridge
    Participant

    Very interesting, but can you describe how the AC capacitors in the Inverter work to communicate the SCRs?


  • #8461
    John C
    John C
    Galco Representative

    I think you ment to say “Commutate” right?


  • #8462

    Clayton Eldridge
    Participant

    Yes


  • #8463
    John C
    John C
    Galco Representative

    Well it is difficult without a diagram to reference but I will try to explain better.

    Picture a DC bus, horizontal line across top is positive across the bottom is negative, ok. From the top to the bottom we are going to series up 4 devices, a diode then a SCR then another SCR then another diode, so Diode SCR SCR Diode. Some diagrams may show SCR Diode Diode SCR, but it works either way. This string of devices makes up 1 output phase, so now put 3 of these strings side by side for a 3 phase output. All the Anodes across the top to plus bus, and all the Cathodes across the bottom to the minus bus. OK. Now connect 3 of the Commutation Capacitors in a Delta fashion (triangle if don’t know delta) and connect each common point between the SCR and the Diode. What happens is, as each SCR is fired it forces a polarity change on these capacitors and the discharge of a polarity will force the previously conducting SCR off.
    OK?


You must be logged in to reply to this topic.