Silent motor start-up: Reducing noise in sensorless field-oriented motor control – even at low speed
01 March 2021
The move to electrically commutated motors has brought about significant energy savings & reduced maintenance demands compared to brushed motors. However, without the mechanical brush-ring combination to provide a commutation point, motor drive developers must resort to alternative commutation approaches.
The full version of this article was originally featured in the March 2021 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
Ideally, Hall-sensors, a resolver or an encoder provide the angle information required for optimal commutation. But there are many applications where, due to cost, reliability or space constraints, mounting a sensor is not possible. As Patrick Osterloh, Senior Manager at electronic components firm, Toshiba Electronics Europe explains here, over the years, there have been many innovative approaches applied to this challenge that determine the rotor angle, primarily relying on the back-EMF (electromotive force) of the motor windings.
In the case of a permanent magnet synchronous motor (PMSM), the permanent magnets of the rotor generate a back-EMF in the stator windings when passing the stator poles. The amplitude of the back-EMF voltage depends on the rotational speed and can be measured using the analogue-to-digital converter (ADC) within the microcontroller (MCU) being used to electrically commutate the motor. The issue with this approach is that, when the rotor is stationary or moving slowly, there is no or little back-EMF signal available to measure.
As a result, when the motor is stationary, the position of the rotor (and its magnets) is unknown in sensorless systems, complicating the optimum control of the motor. A forced commutation (such as six-step commutation) can still be applied to start the motor. This generates a rotating magnetic stator field in the hope that the rotor will eventually follow until the speed is such that a usable back-EMF signal is available. While the approach is valid, due to the mismatch between the rotor and rotating magnetic field, the torque ripple that results can be quite significant, leading to vibration and irritating motor noise. Combined with the resultant mechanical stresses, applications such as compressors, pumps and variable frequency drives are looking for better approaches that yield an improved user experience and product reliability.
Field-oriented control: a brief overview
The core aim of field-oriented control (FOC) algorithms is to achieve, then maintain, a 90° lead angle between the stator and rotor magnetic fields, thereby generating the optimum possible constant torque. With FOC, the motor driver circuit generates a three-phase sinusoidal output to excite the stator windings and maintain the appropriate lead angle. The sinusoidal output of the phases are at 120° to one another, resulting in a smoothly rotating stator field. The generation of the sinusoidal excitations is typically controlled by pulse-width modulated (PWM) outputs of the MCU, and today’s devices offer a range of motor-control-optimised peripherals to achieve this goal with minimal software development effort.
Read the full article in EPDT's March 2021 issue...
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