Motor Technology Selection: Evaluation Factors
Motors are usually an indispensable element of an application’s motion system. However, selecting the motor is rarely straightforward. Motor technologies all have characteristics that can impact a design in different ways, and will determine whether you choose a brushless DC, coreless DC or stepper motor. Here are some key considerations to help you choose an optimal motor for your application.
Motor Speed and Output Torque
Brushless DC motors are well-suited for higher speed operation, while higher speeds with coreless DC motors cause the brushes to wear and can lead to a shorter lifetime. Stepper motors are electronically commutated, and their higher number of pole pairs makes them suitable to run at lower speeds.
Another factor to consider is the output torque requirement of your motion system, with a focus on both the continuous torque and any peak torque for a limited time during operation. Different motor technologies are characterized by different maximum continuous output torque capabilities.
Brushed DC motors have a mechanical commutation system that wears over time, limiting their lifetime. Brushless DC and stepper motors are electronically commutated and do not have any of the associated wear, giving them a longer expected lifetime. Bearings also influence motion system lifetime: Sleeve bearings will provide a few thousand hours of life, while ball bearings will generally provide more than 10,000 hours of life.
Mechanical Envelope and System Accuracy
To achieve your required mechanical envelope, first confirm that the motor technology you choose is available in a diameter and length that will fit in its intended space. The motor’s power capabilities must also be sufficient for the application.
With respect to system accuracy, 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 varying application requirements. You can also increase resolution by adding a gearbox to the front of the motor. The resolution is multiplied by the gear ratio, so combining the encoder and gearbox multipliers can give you precise positioning.
Stepper motors are constructed to provide positioning accuracy. 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 via half-stepping or micro-stepping.
All motors have a maximum rated temperature and operating the motor above that temperature can lead to damage of internal components. The amount of current the motor draws will influence the temperature rise; the higher the current, the faster the temperature will increase.
Since current is proportional to the torque output of the motor, you may choose to increase the current to increase the torque, keeping the motor’s overall size as small as possible. The on-time requirements for the cycle must be balanced so the current you draw doesn’t cause the motor to exceed its maximum operating temperature.
Stationary Versus Portable
A fixed location gives you greater flexibility in regard to motor size and weight so you can focus more on performance characteristics. For battery-powered mobile or ambulatory products, the lower the motor’s current draw, the longer the battery can run between charges.
As we have seen, every application presents many factors to consider during motor selection. Be sure to weigh all the factors carefully and determine how they will impact motor performance so you can choose the motor that will help you develop an optimal product.
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