Intelligent control mechanisms
Developers have many options for how to control each type of motor, depending upon what operating range at which the motor needs to be efficient (i.e., low/high speed, high torque) and how much precision is required (i.e., position, speed, torque). Each control mechanism balances cost, power efficiency, accuracy and performance.
Simple scalar control (also referred to as V/f or Volts per Hertz) is a popular method for driving ACI motors given its straightforward implementation and correspondingly low processing requirements. Speed is managed by changing the frequency of the sine waves used to drive the motor with no effort made to control current or optimize torque. However, simple scalar control tends to deliver inefficient torque at low and high speeds, offer poor dynamic performance, react slowly to changes, overshoot the set point, and has high internal power losses at low speeds.
Field-oriented control (FOC) is a more intelligent control mechanism than simple scalar control and, depending upon the application, can offer comparatively substantial cost savings, power efficiency, and higher precision and performance in exchange for its added complexity. Also known as Vector Control, FOC provides optimum control over the full torque and speed ranges for ACI and PMSM motors (see Table 1). FOC not only increases starting torque while minimizing torque ripple, it efficiency supports maximum torque at all speeds. With fast response to changes and the ability to hold zero speed at full load, performance is stable across the entire speed range of the motor. As FOC is current-controlled, developers can optimize power inverter circuitry as well as motor size to the particular application.
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Table 1: Field-oriented control (FOC) provides full control over the full torque and speed ranges while responding quickly to changes in the set point or load.
Trapezoidal and sinusoidal control are the two primary choices for BLDC motor control. Trapezoidal control has traditionally been the method of choice due to its simplicity and low cost. Many developers are moving to sinusoidal control, however, to achieve smoother operation, better torque response, and lower electrical noise. The resulting gains in performance and efficiency, as well as the ability to work with distributed windings and exhibit better control at higher speeds, allow OEMs to differentiate their systems. The higher EMI of trapezoidal control, for example, can introduce instability into motor systems, significantly reducing performance and adding intrusive audible noise.