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Selecting Controllers for Brushless DC Motors

Selecting Controllers for Brushless DC Motors

Motor controls receive supply voltages and provide signals to drives interfaced to motors.

When selecting controllers for brushless DC (BLDC) motors, many variables will come into play for designers to consider within the context of an application. Some of the most important are described in this article.

Dynamic response
For optimum dynamic performance, the motor controller's dynamic response should exceed the electrical and mechanical time constant of the motor and load.

The motor dynamic torque response will depend on both the bandwidth of the controller's current regulating loop and the motor inductance (electrical time constant). Motor controllers with a motor current feedback loop will generally provide faster dynamic torque response, but they may require tuning the motor control current loop to achieve ideal performance. Low-inductance motors generally require high bandwidth to limit current ripple frequency and amplitude, along with prohibiting wide excursions during fault conditions.

The motor dynamic speed response will be dependent on the motor and load inertia (mechanical time constant). Motors with high inertia require low velocity loop bandwidth. Conversely, motors with low inertia require high velocity loop bandwidth to generate the appropriate changes in speed demand.

As in both instances of control, transducer type (current or velocity), accuracy, and resolution play a role in motor performance.

Dynamic speed
Designers should select a motor controller output frequency to match the maximum desired motor fundamental frequency (function of motor RPM and pole count). High-speed motors controlled via pulse width modulated (PWM) schemes require PWM switching frequencies of more than 10 times the fundamental frequency.

RPM range
For BLDC variable speed PWM motor controllers, the minimum motor RPM speed range will be limited by the minimum allowable PWM duty cycle (voltage) applied to the motor. The maximum motor RPM will be limited by the available motor controller rated output voltage, controller maximum output frequency, and motor controller motor field weakening capability (exceeding rated speed at reduced torque output).

Operating environment
Temperature tends to limit motor controller life, with expectancy halving with each increase of 10C. Designers should match the power/current rating with the expected temperature. (In helping to eliminate intermittent shutdown, drives often are equipped with temperature monitoring for self-preservation.) For applications requiring precise regulation of speed or torque, the best rule of thumb is to select a motor controller with low performance drift over temperature.

The effects of humidity and/or potential contamination will help determine the enclosure rating (IP classification) and/or the protective coatings (urethane, acrylic, or epoxy) applied to a controller's PC board assembly.

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