Guest Post by Pollen Robotics
While working with DC motors, you’ll be more than likely to deal with datasheets/specifications files. Many retailers provide them to define the specifications of their motors, in order to properly use them.
This is how it may look:
Here I want to divide this datasheet into 3 sections.
Section 1: General dimensions of the motor. This part is very useful for motor integration in the robot environment. You can see where to put screws, what length and diameter is the shaft, etc. Weight is also a good thing to know, and often a critical issue in robotics. I won’t talk more about this section here, because, well, dimensions are dimensions and that’s it.
Section 2: Here are the specifications of the motors, sometimes presented as a table. Again according to which retailer provides the information, you can find either many things (not always useful though) or either almost nothing. But some of these specs are necessary for the good understanding of the motor. We will see in a short while which specs are the most important.
Section 3: Characteristic curves. Sometimes you find them, sometimes not. They are useful to have a global view of your motor’s performances. I will explain them later in this post as well.
First, the basics
Some useful facts, always good to keep in mind:
- A motor absorbs energy in the form of current and voltage, so a motor’s datasheet will provide various electronic specifications.
- It delivers energy in the form of rotational movements (and a bit of heat). The movements imply speed and torque.
Note: A torque is a rotational force. It means a force applied at a distance from a pivot. Its expression is a force multiplied by a distance. A simple way to put it is the force you apply to a screwdriver while screwing a screw.
- A DC motor has two main ranges of use: continuous use and intermittent(or short-term) use (a third one is a no-go zone). The first one allows you to make it rotate during long periods of time, while the second one only allows short periods of time rotating until it heats too much.
Now, the minimum spec list
There are a minimum of three critical specs in section 2 you will need if you want to know well your motor and properly use it:
- Nominal voltage (Unom)
- No-load speed (S0)
- Stall torque (Tstall)
Why only these three are the most important, while a bunch of other strange words and values are orbiting around them?
Because every result you want to produce with a motor is dependent to speed or torque — or ultimately to both of them. And at a given voltage, speed and torque are tightly linked together. We will come back to that in a minute.
- Nominal voltage: This is at the same time the voltage at which the other specs were measured, and the suggested voltage at which the performances are the best, most of the time. You can consider using the motor with the nominal voltage without any problem, or at tvalue above it. Be advised that a too high voltage will result in damaging the coils.
Also, the voltage is directly proportional to the speed of the motor (as you read in previous posts).
- No-load speed: Exactly as it is named, this is the rotational speed of the motor’s output when no load is applied to it, i.e. when nothing is linked to the output. This is the maximal speed the motor can reach a given voltage.
- Stall torque: It is the maximal torque that can be applied to the rotor until it stops spinning.
The faster a motor rotates, the lesser torque it provides — and vice versa.
There is a simple experiment to check that deep truth at home: take a small DC motor and apply a low voltage to its terminals. Now grab the shaft and try to stop it spinning: the more “force” (actually torque) you apply to the output, the slower the rotor turns; and finally, it stops, until you release that poor fellow.
Note: Don’t try this a too long time — i.e. no more than a second, or two. A powered motor that is not rotating is like a power supply connected to a coil: the wires will quickly heat, its insulating sleeve will melt, the whole thing will expand a bit and might burn a lot.
What do these three specs tell you? They give you the theoretical range of use of your motor. You know that for optimal performance, the motor must be supplied by the given nominal voltage. Also, you know what are its maximal speed and the maximal load it can bear (1). Of course, no-load speed and stall torque are extreme values (theoretically impossible to reach), and it’s better not to push the motor close to these limits if you want to ensure a good dynamic. A motor never works well at its extreme values.
The basic curve and the ranges of use
According to what we just learned, this is how it looks like on a simpler characteristic curve:
Note: This curve is shown for a given fixed voltage. If you change the voltage, it will appear parallel to the original, but above it for higher voltage or under it for lower voltage:
If there is what I called earlier a section 3 on your data sheet, then it must provide this particular curve (or part of it at least). Some other curves can appear as well, we will see that later in this post.
As you can see, the rotating speed is at its maximum when there is no load on the shaft, this is the no-load condition. Then it decreases while the load increases. At the right of the curve, a maximum load implies no speed at all. This is the stall condition.
The curve represents actually a lot of functioning points associated with the motor. For example, a motor at 12 V with a given load of 5 mN.m will have a given speed of 400 rpm (2):
Note: Keep in mind that this is a theoretical behavior; it means that there always will be some small divergence if you try this with an actual motor, due to external conditions, building quality, ranges of precision.
Finally, a motor can’t virtually be used on its whole speed-torque curve. There is a virtual limit separating the continuous to the intermittent ranges. While you can use your motor as much as you want on the first range, the second one is likely to make your motor heat and to damage it if you keep too long into this range. This limit, called maximum continuous torque, can be observed most of the time around the value of Stall torque / 3, but this isn’t a golden rule.
You can read the full-length guide by Pollen Robotics here.