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  1. #1
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    Too much voltage?

    Hi guys I am currently running 1200oz in steppers I ignorantly bought as a kit a long time ago before even building my machine. Trying to gain some rpm so I'm looking at some 435oz in steppers. My current motors are rated at 5.6amps. surprisingly these particular smaller nema34 435oz in are rated at 6 amps. They are 8 wire motors though but will be used bipolar parrelel never the less. I have 60 volt power supplies. Is that too much for the smaller motors? I can reduce the current on the drives if needed but I don't want to have to replace my power supplies if I don't have to

  2. #2
    Community Moderator Jim Dawson's Avatar
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    Re: Too much voltage?

    The stepper system input voltage is limited by the drives not the motors. In other words, feed them with the voltage your drives will take. I'm running mine at 75 and 80 volts depending on the machine. Running NEMA 23s and 34s, all 4 wire.
    Jim Dawson
    Sandy, Oregon, USA

  3. #3
    Community Moderator ger21's Avatar
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    Re: Too much voltage?

    I'm running Nena 23 motors @ 60V, with no issues.
    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  4. #4
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    Re: Too much voltage?

    As Jim said, its up to the driver, not the motor.
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  5. #5
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    Re: Too much voltage?

    It's not because your drive can provide some power rating that it is the best choice for the motor.

    You need to ensure that you are not driving the motor winding into magnetic saturation for a sustained period at low revs or on on hold. Look at the current rating of the motor and the static coil resistance. Over stepping the current rating will just heat up the iron core in the motor without producing an ounce more effort. They will stand it for a short time but it is doing nothing but heating the motor.

    The coil windings should be able to stand the rated current indefinitely and hopefully the motor will have been designed to produce max. magnetic field ( ie magnetic saturation ) at that current.

    don't stick 80V across a 1 ohm coil rated at 3A , just because you can !

  6. #6
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    Re: Too much voltage?

    Yeah... no. Hang on... That's mixing things up reg.

    Yes, you set the driver to the current rating of the motor (so looking at the motor current rating is right) but the OP was asking about voltage, and looking for speed. No one said anything about "overstepping" the current rating of the motor.

    The drive will regulate the voltage going to the motor in order to provide the correct current flow in the motor coil. Putting 80 volts into a 1 ohm 3 amp coil is exactly what you want to do for a faster step to the next position, but only for a fraction of a second. And the driver will do exactly that, if it has an 80 volt power supply feeding it (and is rated for that supply, of course) but then it will stop down the voltage quickly as the coil current reaches it's rated maximum (which it will do faster with the higher initial voltage) and will NOT saturate the motor. So increasing the voltage gets you more speed, as does moving to lower inductance motors, because those things get the coil to the rated voltage faster.

    Voltage = speed. Current = torque.

    Too slow? Reduce microstepping, Raise voltage (not more than the /driver/ rating), Move to lower inductance motors.
    Missing steps at low speed? Raise current setting on driver (not more than the /motor/ rating)
    Stalling at midrage speeds? Lower current, More microstepping / better driver, Add a vibration damper.

    To that standard list, I would add:
    Missing steps after the machine has been on for a while? Add heatsinks to the motors. (someone will give me crap about this, but I've seen it work too many times), Automatic drive disable after job end.
    Missing steps only when raising the Z axis? Add a spring / gas strut or pulley and weight for "anti-gravity", Increase current, Move to higher inductance motors.

    Correct me if I'm wrong, but that list would probably answer 3/4 of the questions on here, wouldn't it? Other than the "I'm new, what should I buy for this machine that I haven't told you anything about" questions.
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  7. #7
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    Re: Too much voltage?

    Yes James. I don't disagree with any of that. But beyond a point extra voltage gives diminishing returns and just produces more heating in the driver. The first thing is to ensure that you are not pushing worthless current into the motor. How that works is not obvious, so I was explaining what that was about.


    Current = torque is only true until you hit saturation at which point it changes to current = heat, with zero increase in torque.

    Driving steppers is a complex subject and can not be covered properly in a few lines.

    Since you seem to have a quite a bit of knowledge, maybe you can help me understand the odd waveforms I'm getting from some STMicro driver chips:

    http://www.cnczone.com/forums/steppe...1-drivers.html

  8. #8
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    Re: Too much voltage?

    But beyond a point extra voltage gives diminishing returns and just produces more heating in the driver.
    Don't worry about it. Let the driver worry.
    DO make sure the driver is rated for whatever voltage you have. Got 60 VDC and the driver is rated for 80 V? Fine. Other way around? Dead driver.
    Then set the current limit on the driver for the rated current for the motor - or maybe a bit less.
    Enable 'current reduction when motor idle' if available.
    If the driver you have is antique and does not mention these features, buy a new driver. Seriously.

    Cheers
    Roger

  9. #9
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    Re: Too much voltage?

    Where does one find 60v power supplies? I have not found a decent price from a reliable source.

  10. #10
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    Re: Too much voltage?

    If you are trying to buy a switched-mode 60 VDC power supply for the drivers on your CNC,
    DON'T
    It won't work.

    Buy a 40-42 VAC output transformer (a big one), a high-current diode bridge, and a large capacitor rated at 80 VDC or higher (or several of them). This combo should give about 60 VDc - unloaded it will be a shade higher than this.

    To explain: SMPS units may produce a fairly stable output voltage at high current, but most of them will also have a lot of noise on the output - RF interference type of stuff. This RFI will play merry hell with your drivers (if they are modern ones). Modern drivers are designed to work with very simple transformer/bridge/cap power supplies, and the doco will normally tell you this.

    Cheers
    Roger

  11. #11
    Community Moderator Jim Dawson's Avatar
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    Re: Too much voltage?

    Quote Originally Posted by ShallowPass View Post
    Where does one find 60v power supplies? I have not found a decent price from a reliable source.

    I've had good luck with these guys https://www.automationtechnologiesin...ower-supplies/
    Jim Dawson
    Sandy, Oregon, USA

  12. #12
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    Re: Too much voltage?

    Jim is right! They would do.
    (Note the comments in the specs - clean unswitched power.)

    Cheers
    Roger

  13. #13
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    Re: Too much voltage?

    I also think that a lot of switch mode power supplies are designed to drive electonrics, not large inductive loads and they are not stable with that kind of load attached.

    I'm considering a bank of lead-acid batteries as used for UPS units. These can be recharged later or probably charged concurrently and effectively providing a large , stable ballast.

  14. #14
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    Re: Too much voltage?

    Quote Originally Posted by RCaffin View Post
    If you are trying to buy a switched-mode 60 VDC power supply for the drivers on your CNC,
    DON'T
    It won't work.
    Really? Then how come it does on my CNC and countless other CNC's out there. While I agree that an unregulated PS as you describe may be ideal, but saying that a SMPS won't work in bold letters is just plain false. You mention that the RF plays havoc with "modern" drives without even knowing what drives he has. He already has the power supplies as he mentioned in post #1 so you would have him buy an unregulated power supply before even trying the ones he has? In regard to the RF from a SMPS, yes it has some, but with proper design, shielding, filtering, etc. it is not a problem and it WILL[ work

  15. #15
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    Re: Too much voltage?

    Probably less dogmatic statements would bemore appropriate. There are many drivers and far more many switch mode PSUs all designed to fit certain specs. Trying to make blanket statements like things DONT or WILL work is by definition wrong.

    I found HP printer sw mode PSUs worked rather will as a cheap solution ( you can pick them up 2nd hand for about ten eurobucks ). They deliver 3A which is not too bad but the 32V was bit light for my needs of driving chinese motors which have unnecessarily high resistance and impedance. They are FCC approved for office and home use thus have reasonably good shielding.

  16. #16
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    Re: Too much voltage?

    not all switch mode power supplies are equal
    while power supplies from Meanwell for example usually work as expected

    with some other brands you can find the voltage regulation circuit can become unstable when the supply is connected to stepper drivers that use a "chopper" current control to limit the motor current

    switch mode power supplies are unable to deal with either the returned energy when stepper motor is decelerating
    or
    when two power supplies are connected in parallel (unless they are designed to run in parallel)

    usually one power supply will attempt to provide all the current - the one set to a slightly lower output voltage will see the higher voltage from the other supply and switch off

    at work nobody would question spending £90 for a 12V 200W power supply that will meet a guaranteed specification, from an industrial component supplier
    instead of a £16 power supply from ebay

    once you want to be able to hot swap power supplies that are connected in parallel it gets more expensive

    John

  17. #17
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    Re: Too much voltage?

    OK, DOES and DON'TS

    IF you are an experienced EE then that simple rule does not apply - but then you would not be asking the question.
    If you have little knowledge of SMPS design, then it is a good rule to follow, and will always work for you.
    John's comments are very much to the point.

    If you already have an SMPS unit you may be able to make it work in a particular situation by placing a lot of inductive filtering on the downstream side. This isolates the PS from the driver as far as RFI goes. Things like an HP printer PS or a PC PS have to meet FCC requirements, and they do this by adding the filtering - before the output leaves the metal case. The RFI stays inside. This does raise the cost of course. However, cheap generic SMPS units from eBay are unlikely to have this sort of filtering. Caveat emptor.

    I will add that if you want to be able to hot-swap part of a dual PS, it is fairly easy to do IF they are both simple transf/bridge/cap units and the transformers have the same outputs. - but I would recommend diode-isolating each unit.

    Example: I use a MOSFET SMPS to drive my spindle, but when I first installed it (replacing the dead previous supply) everything else stopped working reliably, and a nearby FM radio went bezerk. Massive RFI. I added a large dual-winding (ie balanced) output choke right at the output terminals, and harmony was restored.

    Cheers
    Roger

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