What is "NEMA"?

I see references to NEMA 17 & NEMA 23. What makes a stepper motor a NEMA?

Makerbot description for the NEMA-17 stepper they sell states:

"NEMA-17 is a standard motor mounting geometry. The outside of the motor housing is 1.7"x1.7"

So it seems to refer to motor geometry.

Edited 1 time(s). Last edit at 12/31/2009 08:33PM by D2TNJ.

Ok, so it's the size of the outside of the motor, and not the max torque rating, power usage, or anything else. I guess the NEMA 23 would be 2.3" x 2.3" then, eh?

Pretty much. I've been trying to find a good link to explain it, but it's proving harder than I thought. I did find this link which goes into "what is this NEMA size all about". It is interesting that the rule doesn't seem to hold for all the NEMA sizes. For instance, on their chart NEMA 24 is 2.3" square.

If you feel adventurous (I gave up) you could try searching the NEMA web site for a specification document.

You did better than I. My searching failed to even turn up the NEMA web site. The other link you posted was pretty good. Thanks.

A NEMA 24 is 2.4 x 2.4, whoever made the webpage made a mistake.

NEMA is a US based association that attempts to set standards
for manufacturing. For stepper motors, they have chosen a label
that lists the size first. So a NEMA 17 is a 1.7" cube and any
NEMA 17 motor will mount in the same mount holes of another NEMA 17.
This gives the manufacturers more flexibility with their designs because
they don't have to redesign the motor mount method just because they
changed motor supplier.

If you want a really boring read, go to the NEMA website, then go into
standards, search for motor. In the search results there should be a
tabel of contents or an index for motors and generators. Read through it
until you find the listing for motor labeling, look to the right of that
listing and it will tell you what standard(s) you need to look for, then
go to the standard(s) and start reading. If you make it past 5 pages
before falling asleep then you may want to seek out a mental health
professional.

It would be good to write it up here briefly. I've used cris's explanation as a seed text.

[objects.reprap.org]

I'll try and get it written up this week

Thanks!

ok, so here is a write up a did a few years ago for a hobby robotics group to cover the basics. Let me know if I need to add more details. If you want the design tables, you can download the standard for free from NEMA after you create
an account (free) with them.

NEMA is short for "National Electrical Manufacturers Association". The standards that NEMA creates are developed by the manufacturers through a voluntary consensus to provide descriptions of how things should be. Essentially, it is the manufacturers all agreeing that an apple is an apple and that a pear is not an apple. This makes swapping components easier, because you know ahead of
time that one NEMA 17 motor will fit into the mounts of another NEMA 17 without having to redesign anything

The NEMA standards for Stepper Motors are covered in NEMA ICS 16 (Motion/Position Control Motors, Controls, and Feedback Devices). The stepper motor NEMA nameplate information is covered in Section 4.3.1.1 of ICS 16. The NEMA stepper motor naming includes diameter DD (inches x 10), mount type MM (inches x 10), length LLL (inches x 10), phase current CCC (amps x 10), insulation class I, phase voltage VVV (rating x 10), steps SSS, and winding code W. The quantites
are written without a decimal point and a dash (-) to separate mounting characteristics from other data.

So the full NEMA description of a stepper motor is written as:
NEMA DDMMLLL-CCCIVVVSSSW

Examples of naming:
a 3.4" diameter stepper moter is a NEMA 34
a 3.4" diameter stepper motor with a flange is a NEMA 34D
a 3.4" diameter stepper motor with a flange and face tapped holes is a NEMA 34CD
a 3.4" diameter stepper motor that is 1.6 inches long with a flange is a NEMA 34D016
and the most obnoxious name of all is
a 3.4" diameter stepper motor with a flange that is 1.6 inches long, has a phase current of 1.6 Amps, class B insulation, 5.3 phase voltage, 200 steps per revolution, and type A winding connection is:
NEMA 34D016-016B053200A

For square stepper motors, the length of a side is used instead of the diameter.
So a square stepper motor that is 1.7" long on a side would be a NEMA 17 cube.

On to the boring details.

Flange (D) or Face (C) or Face Flange (CD)
A NEMA flange (D) is a flange on the end of the motor that has slots (not holes) in the flange to slide bolts through. The flange types are given by a Flange number from 17 to 56. Each flange has different values
A NEMA face (C) has tapped holes in the face of the motor even spaced around a pitch circle.
A NEMA face flange (CD) has a flange with holes through the outside of the
flange to allow for mounting. Note that these are through holes and not the slots
of the regular flange. The holes are even spaced around a pitch circle.
For all three of them,
With the NEMA standards all flanges are designed alike and all face mount holes are designed alike. So if you buy a NEMA stepper with a flange, any other NEMA stepper that is the same size with a flange will mount in its place without any problems. The same goes with the face mount and the face flange - the holes will be in the same place.
The exact size and hole locations of each can be referenced in tables on the web, but just about every manufacturer gives you the details of the mount type, so I am not including it here.

Insulation
Defines the maximum allowable operating temperature
Class A is 221 F
Class B is 266 F
Class F is 311 F
Class H is 356 F
In the US, Class B is the most common type for 60 cycle motors. Internationally
Class F is the most common type for 50 cycle motors. And generally speaking, going
10 degrees above the maximum temperature will reduce the motor life by half.

Winding Code
Describes how many internal wires the external wires are connected to.
Most stepper motors have 4 to 8 wires on the outside of the motor and on the inside of the motor those 4 to 8 wires are connected to 2 or more wires.
Winding type A is two wires, type B is 3 wires, type C is 4 wires, type D is 5 wires, type E is 6 wires, and finally type F is eight connected.

Tolerances
There are specific tolerances for each measurement listed and many, many details about each one. In general, the stated value is within +/- 0.01. So an inch may be 0.99 inches or 1.01 inches and 5 volts maybe 4.99 volts or 5.01 volts.

Thanks cris. I've posted that into:
[objects.reprap.org]

Editing the wiki is very straightforward (aside from a 90 second create-account-step), so one of the library gnomes reading this or
[objects.reprap.org]
will get around to it shortly. The wiki is open to everyone, so I guess we're all library gnomes.

Thanks very much for your explanation. Very helpful and informative.
Could you clarify one point for me : initially you were talking about the size of a cube as being the basic designation number, then in your fuller explanation, it switched to being diameter. Are we talking about the outside dimension of the case, or the diameter of something else (the mounting holes perhaps?), since the case is obviously not round?

The diameter/cube thing can be a bit confusing. The NEMA naming convention for stepper motors grew out
out of the naming convention for traditional round DC motors. NEMA kept the term diameter in the standards
and added a footnote to handle the cubes.

I mentioned it above under the examples of naming, but it is easy enough to overlook, so here it is again:

For a round NEMA motor the diameter is used. So a round NEMA 34 has a 3.4" diameter.

For a cube NEMA motor the length of the side is used. So a cube NEMA 34 has 3.4" length sides.

Sorry to update an old thread but a picture is worth a thousand works...enjoy. Dion

NEMA Motor Size 11 to 34 Drawing - Omega (PDF)