To experiment in implementing DC motor controllers with FPGAs and Microcontrollers I recently bought a 12V DC motor from eBay. Now, normally, one would have a target application in mind and peruse the motor's datasheet to determine if the motor is fit for purpose.
However, at this stage of my experiments with DC motors I am only interested in developing the electronics to drive a motor efficiently, as well as gather useful telemetry to send back to a host PC. Once this initial phase has been successfully achieved I will look at using DC motors in real-world applications like robotics and quadcopters.
What was quite surprising about the motor I bought was the fact that I actually received a datasheet, after emailing the eBay seller. My analysis of the motor's datasheet is the focus of this blog post.
The motor, which can be seen in the image above, is approximately 60mm in length and has a shaft length of about 30mm. The motor mount, seen in top right-hand corner, I made from Balsa wood with the mounting dimensions firstly being transferred to a drawing package before being printed, cut and attached to the front of the mount.
Attached to the shaft of the motor is a 3.18mm by 4mm CNC plum coupler, which was also bought from eBay. The black and red wires consist of 22AWG wire and have approximately 4mm-5mm spade terminal crimp connectors at their ends. I am yet to purchase the mating halves. I could, of course, just cut the ends off, but they seem like good things to leave on. Connected to the other end of the coupler is a 4mm metal rod.
Some of the DC performance characteristics are listed in the Table below.
|Circuit resistance||0 Ohm||0 Ohm|
|No Load Speed||3000 rpm||4580 rpm|
|Torque||50.7 mN.m||77 mN.m|
|Current||1.82 A||2.76 A|
The datasheet also specifies that the Torque constant as 28.85 mN.m/A, the Dynamic resistance as 5.11 Ohms and the Inductance as 2.85mH.
The rotational speed vs torque curve can be seen in the Figure above, as well as the power vs torque curve. The Stall Torque occurs when the output rotational speed is zero or point (1) in the Figure. Conversely, the No Load Speed is the maximum speed that the mechanical shaft can rotate without driving a load, at a given voltage. There is an inverse relationship between the torque and the rotational speed. Hence, the faster the motor spins less is the amount of torque produced. The Power of the drive is determined by the product of the torque and the rotational speed.
The current vs torque curve can be seen as the set of reddish/pinkish dots in the Figure above. This should prove to be quite an important curve when high-side or low-side current measurements are taken. Likewise, telemetry measurements of the current consumption vs rotational speed can be corroborated by this graph.
The black curves in the three Figures in this blog post relate to the performance of the motor when it is operated at 8.5V. Likewise, the motor's 13V performance is represented by the red curves. For any given set of performance curves the maximum power dissipated is given by
Maximum power dissipated = 1/2 torque x 1/2 rotational speed
as can be seen in the Figure below.