How to size and select a dc motor: A motion engineer’s guide

There are many motor choices on the market today, from heavy-duty AC motors to tiny DC brushless and stepper motors. To select the right motor for the application, users must have a full understanding of the application parameters, including power, speed, torque, physical size, efficiency, lifetime expectations, and other requirements. There have literally been books published about each of these aspects, and to define them all accurately in a short article would be difficult to say the least.

So here the focus is on a few of the primary differences between DC brushed and brushless motors to help you decide which to use in your application. For example, many machines today operate in a clean environment as opposed to being in a harsh industrial application. This would mean that long life and low noise would become key characteristics for drives used in these applications.

DC Motor Characteristics

Key criteria for selecting a DC motor includes first finding out what voltage is readily available for the application and what physical size the motor needs to be. Speed and torque can then be considered once these first two parameters are determined.

1. Voltage availability is a critical element in motor selection. Remote applications or portable devices, for example, are battery operated, while many rack-mounted devices and tools operate from a 24V power supply. DC motors are available for use at voltages as low as 1.5V and as high as 48V dependent on required power.

2. Physical size is often one of the limiting factors in motor selection because more and more applications have smaller footprints, like desktop 3D printers, portable medical devices, and hand tools. Often a compromise needs to be made between which motor to use and the available space it needs to fit into. Efficiency becomes a primary concern when you need to worry about power consumption to maximize battery life in a surgical tool or unmanned security drone.

3. As mentioned before, torque and speed also have an effect on motor frame size. High torque motors are often larger in size than their low-torque counterparts, which means that larger mounting hardware and larger housings may be an important machine requirement. For example, it takes a larger motor to rotate the magnets in an MRI than it does to run the windows in the doors of an automobile.

Although speed and torque are independent requirements in many applications, typically speaking when the torque increases the speed will decrease – if the voltage stays the same. This connection is based on the slope of the speed/torque curve.

4. Motor duty cycle could be one of the most telling aspects of a lot of semiconductor production machines. Intermittent operation not only reduces the wear and tear on the motor and increases the life of the motor, but it also means that a smaller motor size can be used without depleting the positive characteristics of the machine itself.

Brushed or Brushless

Key specifications quickly show that brushless motors last much longer than brushed motors, which rely on a mechanical connection for operation. And brushless motors run much faster as well. If you’re using a brushless motor for reliability, you won’t want to add a gearhead to the mix, though. The mechanical nature of a gearhead automatically means that it’ll have a shorter life cycle. Using a gearhead with a brushless motor will only negate the longevity of the combined system, and therefore reduce the longevity of the machine it was designed into. On the other hand, there are times when using a gearhead on a brushless motor is advised. For example, if the environment is such that noise is a concern or that a higher torque is needed, a gearhead will do the job.

Don’t use gearheads to increase the speed of brush motors. Using a gearhead with a brushed motor won’t change the life cycle to any great extent. Both are mechanical components that are subject to wear and tear. A real issue in selecting between a brushed and brushless motor is the expertise of the machine builder. Brushless motors either come with built in electronics or with external electronics to operate the motor. It takes some experience to provide the custom electronics many machine builders choose to provide. But for high sales volumes, the costs are easily regained.

Brushed motors, on the other hand, don’t need electronics to run the motor, offering a plug-and-play option to the designer. This means that if the machines are expected to sell in low quantities, a brushed motor will save on the overall cost of the system. A final concern is the power needed for the motors. In working with a motor manufacturer, be sure that their motors are available in a wide variety of power ratings, such as up to 250W for brushed motors and 400W for brushless motors.

Overall, many machine builders are electing to use brushless motors whenever possible. Long life and high speeds make these motors applicable to a broader array of applications.