1. #1
    I'm going to replace the original X, Y, & Z stepper motors on my BP Series 1 CNC with NEMA34 steppers. All electrics are being renewed. After a lot of homework, I *think* I've narrowed motor selection down to one of these two...

    1. NEMA34 1232 oz.in - 5.6amp, 5.5mH
    2. NEMA34 1100 oz.in - 4.0amp, 3.5mH

    Using the formula '\sqrt{mH} * 32' gives me a recommended max voltage for the power supply of 75v for the 1232 oz.in motors, or 60v for the 1100 oz.in motors.

    I'll make my own unregulated PSU so I've got some leeway in terms of optimising voltage and current to best suit the motors.
    Drivers are DM860A (input voltage: 24-80VDC).

    If I opt for the 1232 oz.in motors, a commonly available toroidal transformer is 100VA with 2 x 50v secondaries. Wired in parallel, that would give me ~70VDC and 20amps. Is that a suitable combination for the motors and DM860A drivers, or is a 70v power supply tak things too close to the 80v limit of the drivers?

    Alternatively, I could build a smaller power supply for the 1100 oz.in motors (eg. 50-60v), keeping me well within the drivers' voltage range.

    Of the two motor choices I've listed above, does one offer a performance advantage over the other? I'm leaning towards the 1232 oz.in motors but maybe I'm missing something?

    Probably worth mentioning the mill's original stepper motors have a 2.5:1 pulley ratio on the X and Y axis, and 2:1 on Z. I was planning to reuse the pulleys - unless that's a bad idea?

    Although I've done a lot of reading up on this stuff lately, I'm not confident I've got it all figured out correctly. I'd appreciate any advice or pointers as to whether I'm heading in the right direction re motor selection and power supply voltage. Cheers!

  2. #2
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 1 Day Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,193. Received thanks 243 times, giving thanks to others 5 times.
    I'd probably go for the 1100oz-in steppers, as the lower inductance should give better performance.

    50V may end up a bit too close to max voltage. You have to remember the 50VAC is at rated load, so at no load the voltage will be higher (a decent quality toroid manufacturer should publish a load regulation figure, which can be used to work out how much voltage will rise under load). Combine that with mains voltage fluctuations (officially mains can vary from 216 to 253V, and toroids will often be rated at 230V), and you can quite easily get a higher voltage than you expect.
    And that's before you consider the effects of back EMF during high deceleration of motors, which will result in the drivers dumping energy back into the supply and increasing voltage even higher.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

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  4. #3
    Quote Originally Posted by m_c View Post
    I'd probably go for the 1100oz-in steppers, as the lower inductance should give better performance.

    50V may end up a bit too close to max voltage. You have to remember the 50VAC is at rated load, so at no load the voltage will be higher (a decent quality toroid manufacturer should publish a load regulation figure, which can be used to work out how much voltage will rise under load). Combine that with mains voltage fluctuations (officially mains can vary from 216 to 253V, and toroids will often be rated at 230V), and you can quite easily get a higher voltage than you expect.
    And that's before you consider the effects of back EMF during high deceleration of motors, which will result in the drivers dumping energy back into the supply and increasing voltage even higher.
    I didn't know about the no load voltage rise. Thanks for the detailed reply.

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