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Thread: SCR filter?

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  1. #1
    Join Date
    Jan 2005
    Posts
    238

    SCR filter?

    Ok, where to start.

    I seem to have run into a problem that I think can only be solved by filtering the output from a DC Regen drive that I bought new, to replace the original Westamp analog Spindle drive that my mill originally came with.
    The old drive was circa 1979 and finally became too unstable and unreliable, so I decided to replace it.
    Things didn't go quite as planned though.
    The DC drive I bought is a KBRG-255. 5hp 180Vdc. Seemed like a perfect match for the 5hp,180VDC Peerless motor and in fact it does work quite well. But, I'm not too happy with the resulting harmonics and potential for rapid or increased brush wear on the motor. When the motor spools up, it starts with a low growl and at full motor speed, 3600 rpm, it sounds like a jet engine at half throttle. A very high pitched, whinny sound.
    Some may find this cool, but it's not for me. If I wanted the squeal, I would of kept the old drive, as it was quite loud.

    So in any case, I came to the conclusion after reading many posts and doing some research, that SCR drives although reliable and affordable, have some draw backs. Their outputs ( form factor ), is not quite pure DC and they draw more on the incoming AC line then fully filtered drives. They wear brushes out quicker and create more heat in the motor when not running at full rpm.

    My question to all this is, can I use the big capacitors that came with the analog drive and it's inductor coils, and make my own filter.
    If I take the output from the SCR drive and pass it through two Capacitors in parallel with an inductor in series between them, would it smooth out any left over ripples?(thus reducing noise)
    How would this affect reversing the motor?
    Has anybody tried this or am I the only one with this issue?
    Any constructive input is welcome.
    Thanks.

  2. #2
    Join Date
    Dec 2003
    Posts
    24216

    Re: SCR filter?

    Not it is not a good idea to use capacitors on the output side of the SCR bridge.
    They operate by phase angle control of the 120Hz AC into the bridge.
    You would be much better off obtaining one of the KB/Baldor PWM versions.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  3. #3
    Join Date
    Jan 2005
    Posts
    238

    Re: SCR filter?

    Hmmm. well that is not good news. I looked around for days for a PWM drive that can handle 5hp DC motor, and couldn't find any. Baldor and KB have up to 3hp, but that's the maximum.
    So what your saying is that nothing can be done? To me, this is an industry shortfall.
    I guess that is the one drawback SCR drives have. They are noisy. I wonder how the mill will deal with all this added electrical noise.
    The KB drive seemed like a good answer to the replacement of my analog drive, but now knowing this, I regret it.
    Would you know what is the form factor from the output of the KBRG255? KB Electronics doesn't have it published in their spec sheet. I'm hoping 1.1 to 1.3. But that seems unrealistic being that they go through motor brushes quickly.(from what I've read).
    Thanks for your help AL.

  4. #4
    Join Date
    Dec 2003
    Posts
    24216

    Re: SCR filter?

    Not sure how the F.F. would be rated but the best results for SCR drives is the 3phase variety, they were even used in servo power in the early days, the % ripple is less than 30% compared to 100% on 1ph and the ripple freq is 360
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  5. #5
    Join Date
    Jan 2005
    Posts
    238

    Re: SCR filter?

    Ok. I will be keeping an eye out for a used servo drive like the AMC. For the time being, I will have to run what I've got.
    I wish I would of known the draw backs of the SCR drive beforehand, but no one was complaining much, so I figured it was a solid choice, as they've been around for a long time.
    I hope the motor won't be too affected by the KB drive and that the brushes can last at least a couple months.
    Again, thanks for the wisdom.
    Good day.

  6. #6
    Join Date
    Jan 2005
    Posts
    238

    Re: SCR filter?

    Al, one more question. If I remember correctly in a DC motor, Amps = Torque and Volts = RPM. In other word, amps = amount of work potential and Voltage is how fast it can be done. I know this is simplifying things but just the basics will do. It's been a while and I don't have time to dust off the old books.
    I remember back in university that a good analogy they taught us, was that voltage can be thought of as pressure and amps as volume.

    For anybody who cares, this rule is also simplified for the combustion engine. Torque is the amount of work it can do and horsepower is how fast it can be done.
    Thanks Al.

  7. #7
    Join Date
    Dec 2003
    Posts
    24216

    Re: SCR filter?

    Yes Torque is equal to current and voltage RPM.
    The old water analogy used to be used, that voltage is = pressure and current = volume, same thing.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  8. #8
    Join Date
    Feb 2007
    Posts
    962

    Re: SCR filter?

    How many stall amps is the motor rated at ?
    I would think you could use a LC filter on each leg of the motor as long as you didn't go with too large an inductor. Just be sure to have the inductor before the capacitor.
    The SCR drive will make some growling at low speeds, but near max, it should be fairly quiet, except for a hum.

    also you could try a RC snubber at the motor to suppress the brush arcing. But it could be the brushes are arcing because they are near their wear limit too.
    Manufacturer of CNC routers and Viper Servo Drives
    www.LarkenCNC.com and www.Viperservo.com

  9. #9
    Join Date
    Jan 2005
    Posts
    238

    Re: SCR filter?

    Hey Larken, thanks for the advice. Just so happens I use Viper 200 drives for XYZ. Excellent drives BTW. They haven't failed me since I got them configured over 3 yrs ago. My Z axis recently is for some reason pulling more current, and I've had to readjust the current limit trim pot. But it's all good now. As for the 5hp DC Spindle motor, it is rated for 180VDC 25amps continuous, 230amps stall. The cables leading into the motor are 00AWG, and look like booster cables. It measures about 18-20 inches long by 9 inches in diameter and weighs a ton. The single phase(230V) is fed directly into the SCR, and only two wires going to the dc motor directly from the output of the drive. I am disappointed with the SCR drive only because of the jet engine like noise above 1500 RPM. The low growl is present only during start up. I recently checked the brushes and they are worn down about half ways. They were like new in 2010. I am seriously thinking of putting in a AMC 100A40 drive in the near future. It's too bad you don't offer anything that can push this motor. I like the Viper drives especially now that they can be programmed via a GUI. That's so much simpler and you can now get a visual of how the drive will perform with the motors. Good job on that Larken. For now though I will run what I've got until I find an alternative. Thanks.

    P.S. being that this is the oldest mill in the shop, an old Tree Journeyman 300, but still good for making complex 3D molds, plus we have so much tooling for it, I plan to keep using it for as long as it can hold tolerance.

  10. #10
    Join Date
    Jan 2005
    Posts
    238

    Re: SCR filter?

    It's been a while, but finally got my hands on an Advanced Motion Controls 100A40L. I'll see how that works once all hooked up. Paid $325 for it, looks brand new.
    Anybody know if I'm going to need a shunt for it or can it handle braking the motor? Downloaded the pdf but little info in it. Also, where does all the energy go when braking? My old massive drive, would dump all the energy into heat resistors and now I'm worried the AMC drive won't be able to handle it without some sort of dump circuit. There's tons of energy that needs to go somewhere when the motor goes from 3000RPM to 0RPM in 1 second.
    BTW, my KB drive blew an SCR just yesterday, after Mach3 sent an M3 S200 to it. I was running on 0-10V signal input and it died killing the power instantly to everything on the machine. I hope nothing else got fried.
    I'm kind of disappointed in the KB drive, but hoping that the AMC drive can make a go at it. The AMC drive outputs a 14.5khz pulse, so I imagine the old squeal will still be audible?
    Anyone know what the "L" stands for in 100A40"L"?
    Here's hoping for the best.

  11. #11
    Join Date
    Dec 2003
    Posts
    24216

    Re: SCR filter?

    Its a regenerative drive, so it should handle the braking energy.
    At 14Khz you should not hear anything, in fact one of the Microchip manuf recommend around 4-5Khz motor PWM frequency as the PWM noise becomes discernible.
    Not sure what the "L" suffix is for that drive?
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

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