You will constantly be hearing about Nema 8 gearbox, NEMA14 gearbox…
NEMA is the acronym for National Electrical Manufacturers Association.
When it says national, it really means USA. It is quite a paradox that every NEMA you buy uses the ‘national’ standard, but they are Made in China.
The number will indicate the size of the motor. To be more precise it will specify the front face, where the screws are located. NEMA17 means it’s dimensions will be 1.7x1.7 inches.
It’s implied that usually the bigger the more torque will provide. Although it doesn’t have to be like that. It is totally feasible for a NEMA14 to over-match a NEMA17. It depends on the manufacturer and the specs of the motor design.
If the motor will be in a dynamic part of the machine, like the head or an extruder, then the size will really matter. The bigger the more weight, that will generate more momentum.
The NEMA11 motor are very light, but they are difficult to obtain with the necessary torque. The NEMA17 Motor are the easiest to find and the most common ones for Arduino projects.
Which motor to buy?
Álvaro Rey, from blog http://www.stepperchina.com: If you are in doubt about which NEMA17 to choose, always buy the biggest. If you are afraid of not having enough torque get one of big 70oz.
Heat
Bear in mind that motors tend to get into quite a temperature. If they work at top of their specs, it is not uncommon for them to get to 80º degree Celsius. And if you happen to use PLA plastic for your machine…
You might end in big trouble!
As an example, let’s imagine we find a NEMA14 motor which works with the required torque, but we want to get it to its limits, it is going to overheat a lot. In that case, if we try to hold it in place using PLA parts, we have to take into account that fact or use a NEMA17 motor which will not reach as much temperature.
Resistance
One of the motor characteristics is the internal resistance. How this influence us?
On one side it is the heat released. Due having more resistance, the motor will heat more.
And on the other side, being a high resistance could lead to not getting enough current for the motor to turn (this is the most serious problem when using motors with higher voltage like 24v or 36v). Anyway, I’ve never encountered this problem before.
Voltage
This value also generates lots of confusion. It is very common for the motor to have a value like 3.6V, but we obviously have a 12V power supply. What can we do?
The motor is just a copper coil, the value of the voltage is a way of telling you amperage. Since the resistance is a constant, and by Ohms law with constant resistance amperes are proportional to voltage.
Alright, in simple motors, as the unipolar, where drivers only let current through, it is key to not let too much voltage in, otherwise the motor will overheat.
But for bidirectional motors, it is very common to use advanced drivers, which manage correctly power consumption and they have a maximum power limitation. The driver will measure how much current is passing through and will cut power when we cross the limit.
That’s why the voltage is not what is relevant to us in those cases, we will manage the power by calibrating the driver.
Motor Voltage and the power supply
It totally different the maximum motor voltage than the driver voltage.
Shaft
It is important to check the shaft diameter and the length.
Make sure you are going to use the gears correctly and to secure the setscrews firmly.
Also, make sure the gear will fit in the motor and the shaft to not be too long.
I am quite pedantic over it since it is not the first time I suffered that problem, or that i have to short the length of a shaft.
Shaft types
There are 3 types
- Round shaft. The most common one.
- Flatted section shaft. It is round but it has a flat section. Therefore we can use them with a setscrew easily.
- Threaded shaft. Since it is also very common to attach a threaded shaft to the motor, there are motors which already come with a threaded shaft installed to be used with a nut. It is even possible to order them to specs.
- https://www.steppernews.com/2020/01/a-simple-guide-to-identify-stepper.html
- http://www.stepperchina.com/2020/01/04/size-and-nema-standard-of-stepper-motor-you-should-consider/
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