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Guidelines for current draw of stepper motors vs Drivers vs Power supply

Discussion in 'CNC Mills/Routers' started by Max Williams, Jan 5, 2017.

  1. Max Williams

    Builder

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    Hi!
    I'm building a standard C-Beam kit and have a question about the current draw of the stepper motors.

    I have 3x 4.2A NEMA 23s. It says 4.2A per phase. I also have a 24V PSU that can supply 15A.

    So my questions are:
    1. If it says 4.2A per phase and the motor has 2 phases (4 wires) then could it potentially draw 8.4A?
    2. Will the motors draw 4.2A all the time? Or just when moving? Or just when moving and facing resistance? Or is it like a sliding scale between 0 and 4.2A when it reaches its maximum torque?
    3. If I use a Synthetos gShield with TinyG, with DRV8818 drivers (2.5A per motor phase), will this supply up to 2.5A and then hold it? Or will it trigger overcurrent protection and just stop?

    I guess what I'm wonder is: if I'm not pushing it to the limit, will it run with these steppers, a 15A PSU and a Synthetos gShield? Or will it simply not run at all without smaller stepper motors (or larger PSU/drivers).

    Thanks! :)
     
  2. snokid

    snokid Master
    Staff Member Moderator Builder

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    Yes they will draw 4.2 all the time (well that depends on how you have your software set)
    yes the 8818 will work just supplying 2.5 amp....
    everything will be happy....

    kind of like driving a corvette with and egg on the peddle!!!
    lol
    Bob
     
  3. Joe Santarsiero

    Joe Santarsiero OB addict
    Staff Member Moderator Builder

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    1. It's a 4.2Amp motor.
    2. Steppers use the most current and are strongest while at a standstill. Both current and force reduce with increase in rotation. They have different characteristics which can be somewhat predicted if you can get a hold of a torque curve chart. These characteristics can be improved to an extent with better drivers. Steppers are easier to stall the faster they go. Some drivers are better than others. Some have a half current setting to reduce heating while the motors are not moving.
    3. The current limit pots on the gshield will determine what current gets to the motor.

    In my opinion this is not a good motor and driver combination. It's akin to putting substandard fuel in a high performance vehicle. You'll likely experience crunchy motors and hot drivers. You may want to consider another higher capacity driver or swapping those motors out for something with less current requirement. With drivers it's best to have a little more than you need for the motors, but the DQ542ma's in the storeroom would probably work very well for those guys.
    Just my 2c

    Here is a nice article that goes through the ins and outs of stepper motor and driver ratings and describes how the term phase is used regarding them; http://machinedesign.com/archive/stepper-phase-current-made-easy

    Joe
     
    GrayUK and Rick 2.0 like this.
  4. Max Williams

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    OK great, thanks for info Joe and Bob!
     

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