Application Note

Selecting stepper motor stack size

Description

Tips on selecting a stepper motor stack length for optimal perfomance.

Topics

  • General Motion Control
  • Mechanical
  • System Design

Applicable Product

  • MForce
  • Motion, Drives & Controllers

Requirements

Resource Files

Details

Stepper Motors
Figure 1: Stepper Motors

The capabilities of the different stack length motors are often confusing to people specifying them and designing them into systems.  The best way to think of the capabilities of the different stack lengths of a particular frame size is to think of the trade-offs.  The smaller MDrive stack length has lower torque that persists to a higher speed.  The larger stack length MDrives have the much higher torque that falls off much more rapidly as speed increases.  This is due to substantially higher inductance in the longer stack lengths.  This higher inductance results from the two things that create the inductance in the first place: iron and copper.  Longer stack length motors have more of both.

Iron and copper are also great at producing magnetism and thereby torque.  By their nature, steppers require the interruption and reversal of motor phase current. The higher the inductance, the more time this takes.   The result is that the current in a phase will be switched before it has been able to rise to the nominal value.  Since full current is never reached, the torque output is lower.

This is all very important when gearing is being selected.  There are times when the application needs more torque than a triple stack can provide, so gearing is added. Since the torque falls off with speed is so severe with the triple, some applications that use gearing are done with single stack motors since they can run faster which is required by the gear ratio.

Steppers are different from other motors in that they produce great torque at low speeds, but at higher speeds, that torque starts to fall off quickly (more so in larger stack sizes).  In other motors, like AC or DC motors, the torque is quite constant from low speeds up to some base speed like 1800 RPM or 3600 RPM.  Above that speed, the torque will then start to fall off.  Since they hold their torque to this higher RPM, they have much higher power because power is the product of both speed and torque.