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DC Servo Motor Sizing Made Easy

15 March 2013

Finding the right motor is one of the core elements in the selection of dynamic drive components. Dr. Urs Kafader, technical training manager of maxon motor in Switzerland covers a few of the basic rules to follow to get the optimum motor - be it brushed or brushless – and avoid costly oversized motors

Often motors are selected from the speed and torque requirement, but this can lead to the wrong answer. As an example, for an application running at 25W, the maxon EC 22, with a rated power of 40W would perfectly fit.

The power rating of the motor is based on a high speed of approximately 22,000rev/min and a nominal torque 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.

Therefore the motor type must be big enough to fulfil the torque requirements. If necessary a gearhead can be used 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 – the average load torque of the application must lie 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.

The second torque restriction oncerns the peak load. For short periods of time, the motor can be overloaded without the risk of overheating. Although the details of this overload characteristic depend on the motor design, overloading a DC motor by a factor of two to three for several seconds is possible. The larger the motor, the longer the overload may be.

At given voltage applied to a DC motor, the speed decreases linearly with increasing motor torque. The motor speed is highest at no-load. The produced torque is largest at start-up, allowing for very high acceleration rates and resulting in the dynamic behaviour.

Changing the applied voltage produces a parallel shift in the speed-torque line. Hence, any required point of motor operation can be achieved simply by varying the applied motor voltage. In servo applications, that’s what the motion controller that takes care of.

It is important to cover all the operation points with the maximum available voltage at the motor. Typically, the most critical point is at the end of an acceleration process where the speed and the required torque are highest. The maximum motor voltage may be limited by the power supply used, the maximum supply voltage of the controller and the voltage drop in the controller.

For a given motor type or size, the mechanical characteristics are quite uniform while the electrical properties can vary a lot depending on the winding used. With a low resistance winding a wide range of operation points can be covered with the motor voltage available – but only at the cost of current needed to produce the torque.

A high resistance winding on the other hand requires a higher voltage but lower currents. Since "current is expensive, voltage is for free”, opt for the winding with the lowest current consumption. "Current is expensive” means that a higher current consumption needs a larger power supply, bigger controller and more elaborate shielding of cables which all adds to the costs. In this context, the available current from power supply and controller is high enough to cover the maximum required torque.

In motion controlled dynamic servo applications, brushless motors will normally be selected. They exhibit very high service life and reliability as well as allowing higher speeds.

Because brushed motors run without electronic commutation, the control electronics can be made simpler. If the service life require- ments are not extremely high, brushed motors can still have advantages for small size. Graphite brushes are found in larger motors and are better adapted to frequent start-stop applications. Precious metal brushes are used in small size motors below 10W and are best used for continuous operation at moderate load.
  • Selecting motors from the speed and torque requirement can lead to the wrong specification
  • If necessary a gearhead can be used to reduce the torque to an acceptable level for the motor
  • Overloading a DC motor by a factor of two to three for several seconds is possible