Motion System Design, Stepper Motor Sizing

Sizing: stepper motor physical size

Author: Bob Parente | February 11, 2015

Our last posting talked about applications that had marginal torque and that caused our “Hybrid” motor to correct its position.

These applications were essential “path critical” and the correcting moves the “Hybrid” control did cause the path or the move time to vary. Proper motor sizing avoids this issue.

An important point to remember with a “Hybrid” motor or for that matter, any motor is that it can’t produce more torque than it’s rated for.

Yea, yea servos can produce more torque than its continuous rating, but then you need to pay attention to how far and for long the motor is in the peak torque area so that it doesn’t exceed its duty cycle. Some AC motors have very high starting torque compared to their running torque, but the bottom line is the motor can’t operate continuously in these high torque areas.

To choose a properly sized motor we need to figure out the torque requirement, then we can choose the motor technology that best suits the application, and then which motor in that technology family of motors produces enough torque to meet those needs.

Stepper motor sizing and NEMA standards

I’m going to focus on sizing steppers, (like you haven’t figured that out yet) but remember that torque is torque and you’ll be able to apply the information we’re going to cover to any rotary motor technology.

Figure 1: NEMA motor frame sizes
Figure 1: NEMA motor frame sizes

So how do you go about sizing a motor? Well, you could pop out a set of calipers, a tape measure or a scale depending on how accurately you want to “size” it.

(That was a feeble attempt at some motor humor)

Figure 2: Motor flange and shaft attributes defined by NEMA ICS 16-2001
Figure 2: Motor flange and shaft attributes defined by NEMA ICS 16-2001

 

 

But seriously, the physical size of the motor is important. Steppers have been defined by a NEMA (National Electrical Manufacturers Association) standard, NEMA ICS16: Industrial Controls and Systems. This document defines the standards for Motion/Position Control Motors, Controls and Feedback Devices and is available as a free download at https://www.nema.org.

NEMA size 14 (36 mm)

Starting with the smallest size offered by SEM, we have a NEMA 14. The “14” means that the mounting face of the motor is approximately 1.4” square. If you looked at the mechanical specs (https://motion.schneider-electric.com/products/nema14.html) you’d see that it’s actually 1.39” square.

This dimension may vary between manufacturers as the NEMA standard only declares that square flange motors fit inside a maximum circular dimension.

In addition to the mounting face, motors that meet the NEMA standard should have the same mounting holes diameters, the same spacing between those mounting holes, the same shaft centering boss size, the same shaft diameter and length.

So in theory, if you purchased a NEMA motor from one manufacturer it would physically fit into the same location as any other manufacturer. However, there may be some mechanical differences, especially with the diameter and length of the shaft. And there is a high probability there are differences between the manufacturer’s motor winding specs.

 The next size up is the NEMA 17 (42 mm)

How quick a learner are you? What is the approximate size of a NEMA 17 mounting face? I hope you said 1.7”. If you didn’t, you’d better reread the above paragraph.

The actual size is 1.67”. The NEMA 17 introduces different motor lengths. The mounting face of the motor stays the same, but the length of the motor now varies. This is done because the manufacturer has added magnet stacks to the rotor shaft. Additional magnets increase the motor’s output torque. A single stack motor would have less torque than a double stack and a double stack would have less torque than a triple stack.

The additional magnet stacks also increase the motor’s rotor inertia. Rotor inertia, we’ll see in later postings, is an important parameter that is used when sizing a motor.

 The next size up is the NEMA 23 (56 mm)

Again the 23 is the approximate mounting face size in inches. And again this series of steppers has three stack lengths. The MDrive Plus versions have four stack lengths. There is a point, however, where adding to the stack length gets prohibitive. This is because the manufacturing process to make longer and longer motors become difficult. To get more torque, it’s more easily accomplished and more economically done by increasing the diameter of the motor and adding stack lengths to that motor series.

 Then we have the NEMA 34 (85 mm)

And again the 34 is the approximate mounting face size in inches. And this series has three stack lengths for both the motor only and the MDrive versions.

And beyond

While stepper motors are readily available in NEMA size 42, beyond NEMA 34 using steppers become cost-prohibitive, especially when there are other, more, practical motor and drive technologies available to get the job done.

We’ll have more on sizing next time.

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