Motion System Design, Stepper Motor Sizing

Sizing: Load to rotor inertia matching ratios

Author: Bob Parente | March 17, 2015

We started our stepper motor sizing discussion with the last posting and introduced the NEMA rating for motors. The stepper sizes that we talked about ranged from the smallest NEMA 14 up to the largest the NEMA 34. Other manufacturers might have smaller (NEMA 8 &12) or larger sizes (NEMA 42), but we’ll let those folks write their own blogs.

Least expensive alternative

My experience has shown me that stepper systems are the least expensive positioning systems that produce highly accurate, repeatable positioning results. I start looking at steppers first as a possible motion control solution, because of that cost-effective reason.

Add the integral control to the back of the motor and you can save even more because the drive and intelligence are automatically installed when you mount the motor. You don’t need space in the enclosure for the control electronics, so don’t just look at the cost of the motor. Look at all the expenses associated with the installation.

If the application requires a faster move time or requires a higher torque solution, then I migrate over to costlier solutions like servo systems.

The typical qualifying metrics in choosing a motor is the torque required to move the load the desired move distance in the desired move time. Let me repeat that because that’s important.

The typical qualifying metrics in choosing a motor is the torque required to move the load the desired move distance in the desired move time. Another important parameter is the load-to-rotor inertia matching. We’ll get into why load-to-rotor inertia matching is important in a bit.

I remember some sales engineers that would just “wing it” when picking a motor size for their customer. They would inevitably size it too large. When asked why that size? They answered “You know, for insurance, to make sure it would work.” Well just have a competitor come along that was willing to do their sizing homework properly and they might be able to offer a smaller, less expensive motor that performed equally as well. What would you prefer if you were the customer?

Inertia matching: load to rotor inertia ratios

Load-to-rotor inertia is an interesting sizing parameter and the first one that I look at to size a motor.

  • Figure 1: Inertia matching load to rotor ratios
    Figure 1: Inertia matching load to rotor ratios

    I label a 1:1 load-to-rotor inertia as an ideal ratio.

  • Greater than 1:1 and up to 5:1, with the load inertia being greater than the rotor’s inertia, I label as a good.
  • Greater than 5:1 and up to 10:1 I label as okay.
  • Greater than 10:1 I try to avoid.

I’ve seen some articles where the authors felt that these ratios were not so important and I’ve seen many where the authors felt it was. Let me put my “spin” (a motor pun, but do I have to always point these out?) on it. Hey it’s my blog.

Without going through all the math to prove it, the 1:1 load-to-rotor inertia is similar to the “maximum power transfer” theory in an electrical circuit where the “generator” supplies the maximum power transfer to the load when the load’s impedance is equal to the generator’s source impedance. If you want to read more about that, then just Google “maximum power transfer”.

The other “spin” on these ratios that I have found to be true is the impact on the “ringing time” when the step motor stops.

More on this next time.

Register to receive updates

Absolute MDrive

Lexium MDrive rotary and linear integrated motor products - now with multi-turn absolute encoder in the same compact footprint.

Custom Products

When it comes to your form, fit and function requirements, don’t settle. Get precisely what you need working with us. We know motion.

> Learn more

Reach Us

Contact us with any questions about how we can help you with your motion application or for assistance with your SEM products.

> Contact Us

Need information?

Browse our resource section and find the most useful tools and documents for all our products.

> Browse