Thread: NEMA sizes, why one over the other

1. NEMA sizes, why one over the other

Given motors are rated similarly, and the physical size doesn't matter, what are the reasons to choose say a NEMA 23 rather than a NEMA 32 stepper motor? OR a NEMA 23 over a NEMA 17?

They would normally use the same driver and power supply I assume.

Just curious.

After that, how do I know (rather than 'gut feel' or 'everyone else does it') what size of motors I should use for an application (say light duty CNC machine) or laser cutter, or plasma cutter?

The proper way to choose a motor, is to set performance goals for your machine, and choose a motor and drive system to meet those goals. Motor manufacturers have motor sizing software, where you enter parameters and it tells you what motor to use. Or, do your own calculations to see how much torque you want at a given rpm, and choose a motor that will give you what you need. You'll usually want to try to achieve a certain acceleration rate, as higher acceleration is usually more desirable than using a low acceleration to achieve higher speeds.

You choose a motor based on the torque curve, which tells you how much torque a motor has at any given rpm.
Smaller frame motors tend to have higher maximum rpm's, which give them a flatter torque curve. Larger motors have steep torque curves, with very little torque at higher speeds.
Generally, the lower the motors inductance, the more torque it will have at higher speeds.

Nema 17 size motors really don't have enough torque for anything but very small machines. Generally, Nema 23 motors tend to be used if the required torque is up to 400oz-in or so. Above that, a low inductance Nema 34 will generally give better performance.

3. Given that you know what torque and PPS you need yes you could well find a similar stepper in another size. However there is a bit more to the choice and that is the inductance of the winding. Steppers fall in output torque as the PPS rises and it is the self inductance of the coils that sets the limit. Often the physically bigger motor will have thicker wire and hence can accept higher current (pulses) and this can to some extent the lower inductance of the smaller motors windings. But you need to carefully look at the curves showing the PPS v torque is also required as some designs are better than others. In general stepper motors are low speed high stall torque. For applications needing high speed then brush less motors are much preferred but there is the added expense of positional feed back and a more expensive (more complicated) driver circuit. The brush-less motors can be had in the same sizes and these also come in a variety of technologies. So as you have guessed it is horses for courses but the speed torque requirement is the starting point in any design unless it is just a cut ans see design!

If the interest is in an actual machine do as Gerry has indicated work out how much torque is required under ALL conditions including acceleration and cutting tool reaction. Do not forget that micro-stepping reduces available torque dramatically. Then short list motors that can supply the torque required at the maximum PPS you need taking into account the fall in torque that micro stepping causes. Then from the short list of motors look at the inductance and pick the one with the lowest.

Always ensure you have adequate torque margin as stepper motors are usually used in open loop systems so losing steps means errors in the work produced.

As you have seen there is a wide range of specifications available from a singe manufacturer in the same mounting flange size as they add more sections increasing the length of the motor. There are also different coil connections and internal layout as well as the way the magnetic system works. Never ever open a motor to see what is inside as removing the rotor will inevitably ruin the permanent magnet system. Repairs are a specialist job and the motors are cheap enough to be regarded as disposable.

Hope this satisfies your curiosity - regards - Pat

PS if the manufacturer or stockist can't give you the torque speed curves and the inductance don't buy.