Selecting the motor for the motion portion of an application is rarely a straightforward task, with different motor technologies all having characteristics that impact on the design in different ways. Here, Dave Beckstoffer of Portescap guides us through the factors to consider.
As a design engineer, there is always excitement when you get to work on a new product development. But, when it comes to motor selection, the challenge can be difficult, with factors such as speed, torque, lifetime, mechanical envelope, noise, weight, cost and precision all having an impact.
There are several technologies to consider, including brushless DC, coreless DC and stepper motors. So let’s work through the different application parameters and look at how these impact on the choice of motor technology.
Motor speed
One of the first factors to consider in a new product development is the output speed requirement of the motion system. Based on their construction and electronic commutation, brushless DC motors are most suited to run at higher speeds. In brushed DC motors, brush wear increases at higher speeds, resulting in shorter life. Steppers have a higher number of pole pairs so even though they are electronically commutated, they are not designed to run at high speeds.
Output torque
The next factor to consider is the output torque requirement of the motion system, focusing on both the continuous torque and any peak torque for a limited time during operation. The different motor technologies are characterised by different maximum continuous output torque capabilities.
Depending on the application, all three may be capable of delivering the required output torque. Alternatively, a gearhead could be added to increase the output torque capability of the motion system. Note, however, that there is a corresponding reduction in output speed according to the ratio.
Operational lifetime
The motion system has a job to perform which is typically defined in a number of cycles per day and time per cycle or hours of operation per day and duty cycle of operation. With these details, we can determine the approximate number of hours the motion system needs to complete over the anticipated product life.
This impacts on the motor choice. Brushed DC motors have a mechanical commutation system that wears over time, limiting the lifetime of the motor. Brushless DC and stepper motors are electronically commutated and therefore do not have any wear associated with commutation, giving them a longer expected lifetime.
Another factor to consider for lifetime of the motion system is the bearing system. Sleeve bearings will provide a few thousand hours of life while ball bearings will generally provide in excess of 10,000 hours of life. Of course, we are assuming that the radial and axial load applied to the motion system shaft stays within the design limits of the specification, and that there will not be elevated temperatures that will reduce the lifetime of the lubrication.
Mechanical envelope
A critical first action is to confirm that the chosen motor technology is available in a diameter and length that will fit in the allotted space within the application. While the speed and torque requirements can often be met by one or more motor technologies, and even by variations within a given technology, the power capability needs to be reviewed to ensure required mechanical envelope can be achieved.
System accuracy
Another factor to review is the accuracy required from the motion system. Both brushed and brushless DC motors require an encoder to track and control the position of the rotor. Standard encoders offer a range of resolutions within the same package size to meet the varying requirements of applications. Increased resolution can be gained by the addition of a gearbox on the front of the motor. The resolution is multiplied by the gear ratio, so combining the encoder and gearbox multipliers can achieve precise positioning.
Stepper motors provide positional accuracy based on their mechanical construction. The number of poles on the rotor will dictate the number of steps per revolution, providing a step angle for each pulse given to the motor. Drivers typically can increase this resolution by half-stepping or micro-stepping, creating intermediate electrical steps between the mechanical ones. The addition of a gearbox is an option here too, gaining the additional resolution of the gear ratio.
Duty cycle
Typically, the motor does not run continuously but rather operates for a period (on-time) and then rests for a period (off-time). The main consideration with the duty cycle is the temperature rise of the motor. All motors have a maximum rated temperature and operating the motor above that temperature can lead to damage to internal components. The amount of current drawn by the motor will dictate the temperature rise; the higher the current the faster the temperature will rise.
Since current is proportional to the torque output of the motor, it can be desired to get increased torque out of the motor via increased current, keeping the overall size of the motor as small as possible. But the on-time requirements for the cycle must be balanced such that the current drawn does not cause the motor to exceed its maximum operating temperature.
Duty cycle must also be considered in relation to environmental factors. Will the motor be mounted to a base which can conduct heat away? Is there air flow around the motor which will help keep it cool? Is the motor in an enclosure where other components are generating heat, raising the ambient temperature?
Fixed versus portable
The type of application being designed will dictate different factors be considered for the motion solution. If the product remains in a fixed location, the size and weight of the motor may be less important, allowing flexibility in the overall size of the motor for performance optimisation within the application.
A mobile or ambulatory product primarily draws power from a battery, making the current draw of the motor a critical consideration. The lower the current draw (while meeting the required performance), the longer the battery will last between charges. Also, size and weight become a priority.
Conclusion
As we have seen, there are many factors to consider when choosing the optimum motion technology for a given application. There may be one or more primary factors which would provide a starting point to determine the most suitable motor technology. But only by carefully reviewing all the factors and their impact will the most appropriate motor technology be settled on to optimise the performance of the product.
