. .
Page 3 of 3 FirstFirst 123
  1. #21
    Anyone wanting to make one Jonathan is selling these http://www.mycncuk.com/threads/10475...al-transformer. A bargain I think.
    Last edited by Clive S; 06-01-2017 at 11:29 AM.
    ..Clive
    The more you know, The better you know, How little you know

  2. #22
    Quote Originally Posted by Neale View Post
    I don't want to get hung up on this transformer voltage business (except that I was surprised by the toroidal transformer I bought recently that measured about 5% above nominal voltage when off-load - the "regulation" factor that's mentioned above which I hadn't expected) but I am interested in the practical choice of smoothing capacitors.
    Another thing to play with is adding or subtracting turns from the transformer to tweak the voltage. But I wouldn't recommend that by design, only if you the insulation materials and enameled wire lying around... It's a neat way to make an auxiliary supply though - just add on an extra winding.

    Quote Originally Posted by Neale View Post
    Stepper drivers can cope with fairly wide voltage ranges. From memory, the EM806 which I am using is rated at 24-80V absolute min/max, for example, although the Leadshine recommendation is to stay at least 10% away from either extreme.
    Having scoped the turn-off overshoot voltage of the MOSFETs in a DQ860MA, I have no qualms about running them at 80V. They use 100V mosfets and the overshoot was only a few volts, so still some headroom. They do skimp on the high frequency decoupling, so I added an extra ceramic capacitor on the empty footprint. I can't comment on other drives though.

    Quote Originally Posted by Neale View Post
    We are not building audio amplifiers here, so we're not interested in hum levels on the output.
    Yes!

    Quote Originally Posted by Neale View Post
    The capacitors are being charged at 100Hz (assuming the usual bridge rectifier) but the steppers are pulsed at a few KHz, so any given pulse will see only the variation in voltage during that pulse - small compared with the variation across a half-mains cycle?
    Correct. The stepper driver is a current controller with a switching frequency of 10-20kHz, so the current control bandwidth will be plenty sufficient to deal with some input voltage ripple.

    Quote Originally Posted by Neale View Post
    On top of all that, one of the stepper driver's main jobs is to control and limit current through the stepper coils, so one thing it can do very well is cope with a range of input voltages while trying to maintain the target current through the stepper coils. So do I really care if the DC output voltage sags by a few volts, across a half mains cycle?
    Probably not, for a few volts...

    Quote Originally Posted by Neale View Post
    In practice, then, just what size capacitor will do an adequate job without going OTT? What is the effect of a too-small capacitor?
    That is the question everyone should be asking - the answer is not straightforward.

    Quote Originally Posted by Neale View Post
    Don't know the answer myself, but once my machine is back running I'll stick an oscilloscope on the PSU output and see what's happening, just in the interests of science.
    Maybe I'll try it this evening, if I can find that spare stepper driver...

    Meanwhile, I just dug out these graphs I captured whilst repairing a stepper driver, showing the voltage across one mosfet Vds (blue) and the line current through the same (yellow).

    The first graph shows a normal situation - we see the current rising when the voltage is applied, and falling otherwise with the inductance of the motor smoothing this current. Observe how little overshoot there is on the voltage.

    Click image for larger version. 

Name:	2016-09-26 00.04.49.jpg 
Views:	246 
Size:	409.0 KB 
ID:	20240

    For this one I removed the dc-bus capacitor (can't remember if it was the small or large one) and supplied from a lab PSU. You can see the voltage and current now oscillating horribly.

    Click image for larger version. 

Name:	2016-09-26 00.02.47.jpg 
Views:	239 
Size:	340.8 KB 
ID:	20238

    Same again, but with a lower supply voltage - notice the current is the same (so the regulation is working) and the duty cycle has increased.

    Click image for larger version. 

Name:	2016-09-26 00.03.06.jpg 
Views:	240 
Size:	377.7 KB 
ID:	20239

    This doesn't tell us anything about sizing the supply capacitor, still it's mildly interesting...

    Quote Originally Posted by m_c View Post
    The thing is, that 1000uF per A is an arbitrary figure provided as a guideline.
    I just had a quick scan through the Gecko Drive guide, and they list the following formula for minimum capacitance in uF-
    (80'000 * I)/V
    This is what I like to call a "Forum Formula": formula's commonly distributed with little concern for how they were derived. Often empirical, commonly including a seemingly arbitrary constant designed to make the user think they are learning something.
    Just like the formula's in the first post to estimate the supply current - these bare little relevance to reality.

    Where does the 80,000 come from? Which is the I, motor or supply current? etc...

    The 1000uF per amp is derived from .. The gecko formula will be something similar.

    There is some better information in this thread. Post #2 is pertinent.
    This one too.

    Quote Originally Posted by m_c View Post
    I'd imagine they've done some calculations and research to reach that figure, and it'll involve the maximum voltage ripple their drive internal capacitors can handle safely under various conditions.
    But do they apply to every driver - maybe close enough, maybe not?

    Quote Originally Posted by Clive S View Post
    Anyone wanting to make one Jonathan is selling these http://www.mycncuk.com/threads/10475...al-transformer. A bargain I think.
    Thanks Clive - I still have two left and more capacitors on the way.
    Last edited by Jonathan; 06-01-2017 at 03:47 PM.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  3. The Following User Says Thank You to Jonathan For This Useful Post:


  4. #23
    A few months ago I said that I would have a look at the output of my power supply with an oscilloscope to get a feel for what was happening to the supply voltage with the router in operation. I was able to do this recently. I had some gcode running that was winding X and Y backwards and forwards at full rapid speed (gantry probably weighs in at around 50Kg, rapid speed on X and Y 4.5m/min, NEMA23/ballscrews, to give an idea of mechanical load), and I hung the oscilloscope off the supply to one of the drivers. Basically, there was the best part of 10V mains frequency ripple, with maybe a couple of volts of high-frequency noise on top of that. Mains ripple was presumably what the smoothing capacitors could not remove, and the high-frequency noise was the combined result of four unsynchronised switching drivers. That's all on top of a nominal 68V DC supply - the output voltage was swinging either side of that. The amount of ripple didn't change very much when the steppers were stationary although the stepper drivers are set to half-current mode. Clearly that doesn't make much difference to the PSU ripple.

    Edit - have now seen that PSU has 2x6800uF smoothing capacitors, 100V/105C rated. Voltage limit is fine for this job and it's good to see the higher temp capacitors being used rather than the cheaper 85C - electrolytic capacitors are not the most reliable of electronic components but the higher-temp versions are better.

    On the face of it, that doesn't sound like a great result. I was surprised by the amount of ripple - much more than expected. I can't read the capacitor values where they are, unfortunately, so I'm not sure how generous they are (the original PSU is the PS806 from Zapp although with a replacement transformer). On the plus side, the stepper drivers clearly don't give a damn and just keep on doing what they are supposed to be doing, and do it very well. Maybe, just maybe, it's possible to get a bit too hung up on the details sometimes and worry unnecessarily.
    Last edited by Neale; 20-06-2017 at 09:38 AM.

  5. The Following User Says Thank You to Neale For This Useful Post:


Page 3 of 3 FirstFirst 123

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. 2 x Toroidal transformers question
    By Proteus in forum General Electronics
    Replies: 10
    Last Post: 20-10-2016, 10:28 PM
  2. What Toroidal parts do i need
    By amxen in forum General Electronics
    Replies: 11
    Last Post: 27-09-2014, 10:53 PM
  3. Toroidal Transformer
    By Carlb1058 in forum General Electronics
    Replies: 7
    Last Post: 17-08-2013, 06:54 AM
  4. sourcing transformers
    By adscnc in forum General Electronics
    Replies: 3
    Last Post: 20-11-2010, 11:05 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •