Understanding Speed-Torque Characteristics of Miniature DC Motors (Part 2 of 3)

Read: Part I  |  Part III

[continued...]

Now we shall consider the motor speed against an application. We need a motor such that higher limit of its permissible speed should be higher than the operating point of the application. Maximum permissible limit corresponds to upper limit of operating range diagram. This speed limit can be found in our catalogue.

Let us now consider the torque. Actually, all the operating points can be covered in few crucial values, and one of them is the average load torque. First point is at the end of the operation or near the extreme operating point where acceleration may end. At this point we have the higher speed at maximum torque. The second important point is the time required for motor acceleration, or time span to run the motor at very high torque values. Thus, we require a motor that can produce “rms torque” or an average load torque. In short, we need a motor with higher maximum continuous torque than a load torque at the same time, or we need a motor that can produce some short term torque required for acceleration. Maximum continuous torque can be obtained from our catalogue page. It actually limits the continuous operating range.

Maximum continuous torque of a DC motor

 

Maximum continuous torque defines the thermal limit of motor. Up to this limit, the motor can be continuously loaded without over heating the motor or its windings. Beyond the maximum continuous torque, there is a possibility of having a higher torque for a short period of time. This time span is known as the “temporary working range”. Maximum continuous torque is a parameter for a motor of a given design and size.  Actually, the maximum continuous torque is defined vis-a-vis a standard condition as shown in our catalogue. Though these conditions are indicative, a motor’s thermal capability can be enhanced by either mounting the motor on a metallic ring or tube, or by having air flowing over externally, or by adding cooling fins on tube or by any other means. With this, we can improve the heat dissipation of motor, and thus the continuous operating range can be extended to a higher value – possibly up to 25% more than maximum continuous torque.

 

Tips for Heat Dissipation

Read: Part III