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  1. #11
    Quote Originally Posted by CharlieRam View Post
    This is part of my problem...the spreadsheet says the motor has a good margin but will it be ok on a 70v power supply?? Attachment 25523
    As long as you set the current limit on the drivers correctly, it didn't ought to be a problem. OK the current will rise quicker, but the peak current should be the same as it would be with a lower voltage, it'll just happen quicker.

  2. #12
    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 2,245. Received thanks 247 times, giving thanks to others 6 times.
    Quote Originally Posted by Voicecoil View Post
    As long as you set the current limit on the drivers correctly, it didn't ought to be a problem. OK the current will rise quicker, but the peak current should be the same as it would be with a lower voltage, it'll just happen quicker.
    The big issue is as the motor spins faster, the back emf increases (emf = Electromotive Force, essentially voltage). Back emf is why you need a higher voltage supply, as in order to over come the emf and 'force' enough current through the windings, you need a higher voltage.
    A modern drive only controls current which in turn limits voltage. The voltage over a motor coil, is related to the back emf being produced (higher inductance = higher back emf), with the highest voltage being the maximum supply voltage. As back emf reaches the point at which the supply voltage is unable to fully overcome the back emf, the coil current starts to decrease (and torque, as motor torque is proportional to current)

    Now the problem comes when you have too much speed, and too much voltage.
    Take the motors in question, with their recommended 57V, and 4.2A current. At a high enough speed to use the full 57V, the motor is using around 240W. Bump that to 70V, and power jumps to 294W, about a 20% jump.
    Now can the motor dissipate that much heat?
    (not all that power ends up as heat, but steppers are not that efficient)

    It's where you've got to judge where you want to compromise.
    If you never run the motors that fast, a higher voltage supply is not likely to be an issue.
    You could reduce motor heating by reducing the current, but then you loose torque.

    Ultimately, the best measure is how quickly the motors heat up in use.
    If they get too hot to touch within a couple minutes of running, you've probably already cooked and demagnetised the rotor, and now have a poorly performing paperweight. Rotor overheating is what will kill a stepper motor, and as it's got no direct cooling, you're relying on heat transfer to the motor body to cool it, and that heat transfer is pretty poor.
    If however after an hours running, they reach and plateau around 60-80deg C, you've probably got them on their ideal limit.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

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  4. #13
    Thanks for explanation :) it seems I have 3 options then...

    1. Risk the 3.1Nm (57v) steppers with the 70v PSU
    2. make a lower voltage supply for the 3.1Nm steppers
    3. choose the less powerful 1.89Nm stepper which is 81v?

    Option 3 seems like a step back to me though so I may try and build the supply and see what voltage I get and decide whether to change out the Toroidal transformer.

    Cheers Guys

  5. #14
    Quote Originally Posted by CharlieRam View Post
    Thanks for explanation :) it seems I have 3 options then...

    1. Risk the 3.1Nm (57v) steppers with the 70v PSU
    2. make a lower voltage supply for the 3.1Nm steppers
    3. choose the less powerful 1.89Nm stepper which is 81v?

    Option 3 seems like a step back to me though so I may try and build the supply and see what voltage I get and decide whether to change out the Toroidal transformer.

    Cheers Guys
    Hi charlieRam.

    I think what everybody is saying: there is no real risk.
    Else they would say:
    Dont do it, it is going to fail.
    People here will tell you if your going in the wrong direction.

    What i am reading between the lines is :

    - They key factor is limitting the max current in the stepper driver settings.
    - Higher voltage is an advantage to go fast.
    - To high voltage can become a problem. -
    - You need to check while running at max performance (rpm) for long periods.

    I guess you will not be running at max rpm all the time.

    Steppers are easily monitored by checking the temperature of the steppers.

    Do this while commisioning the setup and learn to know your machine.

    For me, motor calcs advised 57 volts. I run 4nm nema23 steppers, i have unregulated 72volts psu.
    Until now i run 15% under max current.

    I have to check software driver setup to set my drivers at the exact max.


    My steppers never get above 40 deg.
    My drivers reduce to 50% current at hold conditions

    I have no performance issues.

    I calculated for 2m/min cutting speed @50N cutting forces.

    I have been cutting mdf @ 3m/min.

    I am very happy with the guidance i got from here.

    Grtz Bert.



    Verstuurd vanaf mijn SM-A320FL met Tapatalk

  6. #15
    I think what everybody is saying: there is no real risk.
    Else they would say:
    Dont do it, it is going to fail.
    People here will tell you if your going in the wrong direction.
    Exactly All the machines I have done the motors have never gone above 40C. There is a big difference between theory and practice, too many variables.

    Very seldom will the motors draw full current.
    ..Clive
    The more you know, The better you know, How little you know

  7. #16
    Cheers guys 😁

  8. #17
    Quote Originally Posted by Clive S View Post
    Personally I would go with the 68V p/s with the drives above or https://www.ebay.co.uk/itm/2DM860H-2...frcectupt=true

    There are plenty of people using this setup without problems
    I was just looking at those drives and tried to search for ones in the european union to avoid any duty but are these fake? it says the max voltage on the drive is only 70v DC!
    https://www.ebay.co.uk/itm/Stepper-M...EAAOSwyHxcfEjv

  9. #18
    Quote Originally Posted by CharlieRam View Post
    I was just looking at those drives and tried to search for ones in the european union to avoid any duty but are these fake? it says the max voltage on the drive is only 70v DC!
    https://www.ebay.co.uk/itm/Stepper-M...EAAOSwyHxcfEjv
    Try this one. https://www.ebay.co.uk/itm/NEW-CNC-2...EAAOSwikJb7QmM
    Last edited by Clive S; 1 Week Ago at 11:36 PM.
    ..Clive
    The more you know, The better you know, How little you know

  10. #19
    What's the likelihood I'll get stung on import duties? What kind of cost should I expect to have to pay if I do get stung? ��

  11. #20
    It all seems to depend on what shipping method is used. Typically for stuff coming by the big carriers (Fedex, DHL, UPS etc.) it'll be 4 to 5% duty, a handling fee of between £10 and £15 and then 20% VAT on the whole lot.

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