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  1. #1
    Join Date
    Oct 2006
    Posts
    2

    Power requirements

    Hello,
    I am trying to figure out what my power supply requirements are. I have been reading all over the place and are pretty confused by now.
    Here are the stepper specs:

    Part No.: 85BYGH450C-012
    Frame Size: NEMA34
    Step Angle: 1.8 degree
    Voltage: 5.7V
    Current: 3.5A/phase
    Resistance: 1.9 Ohm/phase
    Inductance: 22 mH/phase
    Holding torque: 11.3N.m / 1600oz-in
    Rotor inertia: 3600g-cm2
    Number of wire leads: 4
    Weight: 5 kg
    Length: 151mm

    Currently I have a single 48V/7.3A supply which seems undersize for those motors.

    Here are my calculations:
    3.5 A/phase x 2 phases/motor x 4 motors = 28 Amps

    Are my calculations correct, and if so should I use multiple supplies or a single large one?

    Thanks in advance!

  2. #2
    Join Date
    Dec 2007
    Posts
    2134

    Re: Power requirements

    Wow, that's seriously high inductance steppers there! They'll need a considerable power supply to run optimally.

    The generally accepted rule for calculating how much voltage the steppers will need is 32 x square root of the stepper inductance. In your case a 150VDC power supply is the recommended voltage. AT 48VDC your sorely underpowering them.

    The calculation to work out current requirements depends entirely upon whether they are wired for parallel or series windings, in your case I would hazard the stepper is internally wired as a series bipolar, so it will use on average around a third of the current rating, roughly 1.2A each stepper, 3.6A total. Having said that, most people multiply this figure by 0.67, as generally it's pretty unusual to have all three steppers fully loaded down and running simultaneously. If your supply can put out at least 2.5-3A that should be adequate, but add at least %25 head room in my opinion. The power rating will also depend upon how efficient your drivers are.

    This link has some really great info from the guys that really know about driving steppers:

    http://www.geckodrive.com/support/st...or-basics.html

    I have to ask the obvious though, what house sized machine are you planning to run these steppers on? They are not only insanely high inductance, which will greatly affect performance, but they're also insanely large.

    Stepper torque is proportional to current and inversely proportional to speed, and stepper power output is a relationship of the supply voltage and stepper inductance, while these are grunty, they may possibly not be at all suitable for a general cnc machine due to the size and rating? For example if you were using these for a small 2' x 3' machine, a 280-380oz/in at 2.2mH-2.8mH would deliver more speed and power than these large ones you have using the same 48VDC power supply.

    cheers, Ian
    It's rumoured that everytime someone buys a TB6560 based board, an engineer cries!

  3. #3
    Join Date
    Dec 2007
    Posts
    2134

    Re: Power requirements

    This link has some great info further explaining why sometimes bigger isn't better with steppers, the relevant section is as follows:

    "Inductance affects high speed torque. Inductance is the reason why motors don't have a high degree of torque out to infinity. Each motor winding has a certain value of inductance and resistance. Inductance in henrys, divided by resistance in ohms, gives us a value of seconds. This amount of seconds (time constant) is the amount of time it takes the coil to charge up to 63% of its rated value. If the motor is rated for 1 amp, after 1 time constant, the coil will be at 0.63 amps. After about 4 or 5 time constants the coil will be up to 1 amps. Since torque is proportional to current, if the current is only charged up to 63%, the motor will only have about 63% of its torque after 1 time constant.

    At low speeds, this is not a problem. The current can get in and out of the coils fast enough, so the motor has rated torque. At high speeds, however, the current can't get in fast enough before the next phase is switched. Torque is reduced.

    Driver voltage plays a large part in high speed performance. The higher the ratio of drive voltage to motor voltage, the better the high speed performance. High voltages force the current into the windings at a faster rate than the 63% mentioned above"

    So to simplify the above from the site, you'll get loss of torque the faster you go, but as your power supply is rated at 48VDC, a third of the optimal voltage, current isn't being forced into the windings, affecting the charge rate.

    The Basics of Stepper Motors

    cheers, Ian
    It's rumoured that everytime someone buys a TB6560 based board, an engineer cries!

  4. #4
    Join Date
    May 2005
    Posts
    1397

    Re: Power requirements

    So, I want to start by saying that I totally agree with everything aargh posted... but I would like to... what's the word? Reframe? It a bit.

    You don't HAVE to run 150VDC into those motors. You can run them on 48VDC. Yes, you will lose some speed, but that doesn't mean the speed you get won't be "enough".

    One thing I know: Stepper motor systems are very hard to engineer and predict accurately .Gecko-drive and their documentation come closer than anyone else, but they don't tell you exactly how fast your motors will run, or rather how much SLOWER your motors will run at 48 vs 150 volts. It's quite possible that you will be just fine at the lower voltage.

    So how to be sure without spending gobs of money? Well... you can't, really. But following someone elses example is a good guess. And always buy used stuff when you possibly can (especially motors) and be ready to re-sell it if it doesn't work for you.

    Your motors are a good choice for a really big, slow, heavy machine. If you are building a small, fast, light machine, sell those motors and buy something else. Let the machine desired machine guide your design; don't start with the motors, or worse, with the driver as so many seem to do.

    But in any case, FIRST build the mechanical parts of the machine, and since you already have the motors and 48V power supply, try them out. Might as well.
    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
    Join Date
    Oct 2006
    Posts
    2

    Re: Power requirements

    Thanks everyone for the excellent advice.

    To answer the main question, yes this is a huge machine for cutting granite slabs into counter tops etc. I am helping someone to get it up and running.
    In therms of the voltage, I believe I am limited to the 80V max of the Gecko G201X drivers.
    Speed seems to be ok. The machine uses Rack and Pinion drive.

    My main concern is whether a single 48V/7.3A supply is sufficient considering that frequently 3 motors will run (2 on the X-gantry axis and 1 in the Y axis), or if I should add additional power to the system. If I understand correctly, the single power supply should be sufficient.

    Thanks again!

  6. #6
    Join Date
    May 2005
    Posts
    1397

    Re: Power requirements

    I personally like battery stacks with low power re-chargers. Massive peak current and the chargers will usually keep up because the average power use isn't really that much.

    From:
    techref.massmind.org/techref/io/stepper/power.htm

    One of the best power supplies for stepper motors (or any application where peak current is much more than average) is a stack of batteries. Stack up 12v car or SLA batteries to make whatever voltage is needed, and then buy or build a charger for that voltage. The charger won't need to supply more than an amp or two to keep the batteries charged, and the batteries can provide nearly unlimited current on demand. Even old batteries seem to work nicely. Some people have had excellent luck with free used car batteries from automotive repair businesses, or SLA's from old UPS units. They don't have to be full strength, since they are always charging.

    12 volt chargers are availalbe everywhere for next to nothing. 12, 36 and 48 volt chargers are commonly available for those "mobility carts" for under $50.

    Of course, fuses are /very/ important when working with batteries. Fuse each battery and the entire pack. Put the car batteries in battery boxes to keep any leaks or fumes out of the work area.
    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)

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