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  1. #71
    Quote Originally Posted by Clive S View Post
    A small problem with wiring the secondary in parallel is that if the amount of turns on them is not exactly the same the transformer is not as efficient.
    I got around this by rectifying the two windings separately then combining them at the DC side.
    Just a thought. ..Clive
    If they're mismatched that configuration will still draw less current from the winding with a lower emf.

    The following quick simulations I just did demonstrates this:

    Click image for larger version. 

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    Click image for larger version. 

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    The first graph shows the current drawn from each winding for the aforementioned mismatched transformer with a light load and the second graph shows the same supply with a greater load. This shows that the effect if greatest when the load current is low, so although the currents are mismatched they will both be within the rating for a light load. Similarly, the mismatch is smaller when the load is greater, so if the rating of one winding is exceeded it is unlikely to be by enough to cause a problem. Also, there are a lot of other factors besides load current involved here, so the above simulation is by no means comprehensive.

    In short it's extremely unlikely for the windings to be mismatched enough for either method to cause problems, so although I'd advise buying a transformer rated for half the voltage so you can put the secondarys in series, if you find one for the right price which has to be wired in parallel I wouldn't worry.
    Old router build log here. New router build log here. Lathe build log here.
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  2. #72
    Quote Originally Posted by Rogue
    Also intriguing, though how would you establish that? Would it be as straightforward as checking the voltage from each output separately? I suspect I will take the easy way out and just use the one rectifier, but I'm intrigued by the idea.
    I have just joined the two outputs from the caps together with no problems at all. ..Clive
    Last edited by Jonathan; 15-08-2013 at 10:25 PM. Reason: Fixed quote formatting

  3. #73
    Quote Originally Posted by Rogue View Post
    Is it better to match up the driver output with the motor rating exactly, or leave a bit of a margin?
    In theory you should be safe to run any driver up to its rating, since that's the whole point of a rating, if you can trust it. In reality however it's always best to overrate the drivers if you can, since then they will run at a lower temperature which prolongs the life of the components.

    Quote Originally Posted by Rogue View Post
    Also intriguing, though how would you establish that? Would it be as straightforward as checking the voltage from each output separately?
    Yes - the difficult bit is determining how much of a mismatch would pose a problem.
    Last edited by Jonathan; 15-08-2013 at 10:23 PM.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  4. #74
    Quote Originally Posted by Jonathan View Post
    In short it's extremely unlikely for the windings to be mismatched enough for either method to cause problems, so although I'd advise buying a transformer rated for half the voltage so you can put the secondarys in series, if you find one for the right price which has to be wired in parallel I wouldn't worry.
    Agree here and I've built both ways and never seen any difference to machine which is what matters at end of day. I buy what's cheapest or available and don't care which.

    Quote Originally Posted by Rogue View Post
    Intriguing. I've seen a bit posted about the AM882 drivers but a lot of it didn't seem positive. On some further reading, most of the issues seem to be down to user error and misconfiguration. The price difference at this stage is minimal as well.

    Is it better to match up the driver output with the motor rating exactly, or leave a bit of a margin?
    Yes there have been but that's mostly been because they have little experience or never used digital drives before but trust me they are fantastic drives and give super smooth performance with great resonance handling. The Fact you can configure them thru software helps if you have problems with resonance or just want to get the best performance you can.

    Yes it's always better to match current to motor ratings but not go higher. The only margin you want to leave is on the voltage but it's also always better if the drives are not max'd out on current IE max current setting = motor current so the drives not working flat out.!
    Last edited by JAZZCNC; 15-08-2013 at 10:32 PM.

  5. #75
    Quote Originally Posted by Jonathan View Post
    In theory you should be safe to run any driver up to its rating, since that's the whole point of a rating, if you can trust it. In reality however it's always best to overrate the drivers if you can, since then they will run at a lower temperature which prolongs the life of the components.
    Hi Jonathan, thanks for chipping in again. I'm still aiming to finish this before you finish at Uni so I'll need you to work towards a PhD, should give me enough time.

    I was thinking about this from the point of view of the stepper motors (4.2A) rather than the drivers. Either of the two drivers discussed are rated higher than the stepper (up to 5.2A for the DM856, up to 8.2A for the AM882) so there is plenty of margin. My question was whether it's better to run the steppers at their max or give them a bit of a margin. If your answer still applies (which logic suggests it does) then it seems reasonable to set the drivers for a little under 4.2A.

  6. #76
    Quote Originally Posted by JAZZCNC View Post
    Yes it's always better to match current to motor ratings but not go higher. The only margin you want to leave is on the voltage but it's also always better if the drives are not max'd out on current IE max current setting = motor current so the drives not working flat out.!
    That answers that question, I should have waited another minute or two before replying! There's approximately 10V difference between the transformer output and the driver max input which should hopefully be fine.

  7. #77
    Quote Originally Posted by Rogue View Post
    That answers that question, I should have waited another minute or two before replying! There's approximately 10V difference between the transformer output and the driver max input which should hopefully be fine.
    Yes 10V will be fine and the fact your using toroidal transformer helps further still because the caps will absorb back EMF better.

  8. #78
    Quote Originally Posted by Rogue View Post
    Hi Jonathan, thanks for chipping in again. I'm still aiming to finish this before you finish at Uni so I'll need you to work towards a PhD, should give me enough time.
    That's the plan, however at work they seem to want me to do the PhD whilst working them, in which case you've only got a year left!

    Quote Originally Posted by Rogue View Post
    My question was whether it's better to run the steppers at their max or give them a bit of a margin. If your answer still applies (which logic suggests it does) then it seems reasonable to set the drivers for a little under 4.2A.
    Yes my answer does still apply. As you increase the current in the motor windings, the copper losses increase by the square of the current, so the motor will get hotter. The degradation of the insulation on the motor windings is dependent on the temperature, as with most chemical reactions, so according to the Arrhenius equation we can say (very approximately) that if the motors run 10 hotter, their lifetime will be halved (assuming they fail due to insulation breakdown). This sounds bad, but we have to put it in perspective - if the motors last 20 years then halving this to 10 years probably isn't a big deal.
    I'd set the driver current as near as you can to the motor rating without exceeding it. You might be able to go lower, but I think it's better to ensure the machine will not stall by having a good safety margin, rather than worrying about the lifetime of the motors.

    Going back to me previous point about the power supply, I just ran the simulation again but this time with both configurations. The first graph is with the windings connected in parallel on the DC side and the second graph has them connected in parallel before the rectifier:

    Click image for larger version. 

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    I realise that there's not much point discussing this further, but it's interesting to note that the waveforms do look slightly better for the DC coupled version. There's still not much in it.
    Last edited by Jonathan; 15-08-2013 at 11:01 PM.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  9. AM882's are an excellent bit of kit and really are easy to use once you get around the chinese strange way of setting up the software for programing the drives. I use them and they really do make things run very very smoothly. -Michael
    CAD software Shark Pro v10, Also Aspire v9.0
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  10. #80
    Quote Originally Posted by Jonathan View Post
    That's the plan, however at work they seem to want me to do the PhD whilst working them, in which case you've only got a year left!
    Glad to hear it, it's not that common these days to get your foot in the door of a good job straight out of Uni. Make the most of it and don't forget us whilst you're busy making money!

    Quote Originally Posted by m.marino View Post
    AM882's are an excellent bit of kit and really are easy to use once you get around the chinese strange way of setting up the software for programing the drives. I use them and they really do make things run very very smoothly. -Michael
    From reading around a bit more I think I'm sold on the AM882 drivers. Currently got 3 of them (+ a lead) sitting in my basket at Zapp. Wisdom dictates that I don't pull the trigger until the end of the month, but my trigger finger is getting fairly itchy at this stage

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