Finding the right motor is one of the core elements in the selection of dynamic drive components. Due to their high starting torque maxon DC motors with or without brushes are the perfect matches for highly dynamic servo applications. Once the requirements of the application are specified and a general motion control strategy is established, there are a few rules to be respected to get the optimum motor.
By Dr. Urs Kafader, Technical training maxon motor ag, Switzerland
Often motors are selected from the calculated mechanical power, i.e. from the speed and torque requirement. This can lead t o totally wrong results. As an example, take an application running at 25W mechanical power. The maxon EC 22 with a rated power of 40 W would perfectly fit. The power rating of the motor is based on a high speed of approx. 22’000rpm and a nominal torques of about 18mNm. However, the motor overheats if the required load torque is 40mNm (together with a load speed of 6000rpm this results in 25W!) which is more than twice the nominal torque of the motor.
This leads us to rule no. 1: Select a motor type big enough to fulfill the torque requirements. If necessary use a gearhead to reduce the torque to an acceptable level for the motor. Essentially, there are two torque values to check. The first one is based on thermal considerations. Make sure that the average load torque of the application lies below the rated or nominal torque of the motor. In applications where a working cycle is constantly repeated, the RMS average of the load torque including dwell must be contained within the continuous operating range.