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Maximizing Motor Control Effectiveness at Lower Operational Speeds

Maximizing Motor Control Effectiveness at Lower Operational Speeds

BLDC motors have significantly better torque to weight ratios, higher efficiency levels and greater reliability than brushed motors, with there being increased use of them in contemporary electronic designs. As BLDCs cannot rely on the direct commutation function that is available to their brushed counterparts, electronic commutation methods are needed to control them. Often this means magnetic sensor devices are employed to measure the current passing through motor’s stator windings, in order to give ongoing status of the rotor’s position. Such arrangements do, however, have disadvantages in terms of cost and space utilisation.    

Undertaking this work without the use of sensors, by continuously monitoring the back EMF figures, has several major logistical benefits from an engineer’s perspective. The motor system can thus be more streamlined and less expensive to implement, as well as supporting a longer operational lifespan. Field-oriented control (FOC) is a popular technique via which this can be achieved - allowing the precise position of a BLDCs rotor to be calculated at any time. Though this is very effective when the motor is running at normal rotational rates, it is difficult for FOC algorithms to obtain accurate positioning data at lower rotational rates. Under such circumstances, the FOC setup must be supplemented by the applying of a high-frequency signal. The torque ripple resulting from this will unfortunately lead to acoustic noise and vibrations being present within the motor system.   

There are a wide range of different applications where vibration and noise generation of this kind would be a distinct operational disadvantage. These include the compressors used in domestic appliances, climate control systems, conveyor belts, etc.

Figure 1: The A-PMD function block incorporated into a Toshiba M4K MCU
Figure 1: The A-PMD function block incorporated into a Toshiba M4K MCU

The Advanced Programmable Motor Driver (A-PMD) technology featured within Toshiba’s M4K MCUs provides a way of solving this problem. A-PMD eliminates the need to apply high-frequency signals to FOC motor control systems when the BLDC is running at lower speeds. This strategy relies instead on symmetrical PWM carriers being assigned to each motor phase.

You can download a detailed Toshiba whitepaper that discusses the advances being made in FOC motor control below: