Starting a sensorless BLDC motor

Starting a sensorless BLDC motor

Access to highly integrated MCU-less motor control solutions allows that permanent magnet synchronous motor (PMSM) operation can be achieved without requiring rotor sensors.

With a mechanically commutated brushed DC motor the commutation point is physically defined, meaning that simple reapplication of power will ensure the right coils are engaged to start the motor correctly. However, in sensorless BLDC operation, novel techniques are needed to determine the stationary rotor’s angle before energising the coils to ensure the rotor does not suddenly move in the wrong direction.

Toshiba’s TC78B011FTG sine wave pre-driver IC for sensorless three-phase brushless motor control negates the need for MCU use. It is very straightforward to configure, requiring just a few simple steps. After power is applied, the TC78B011FTG retrieves the stored configuration from NVM. A brake sequence, (generated by shorting the appropriate coils) guarantees that the rotor is stationary before attempting to start rotation. Once this is complete, the driver enters an idle state and awaits the next command.

When the speed control command is received, either via I2C or as a PWM or analogue signal applied to the SPD pin, the motor start-up sequence is engaged. There is DC excitation of the motor coils to determine the starting position of the rotor and then, forced commutation of the motor starts. Here, a rough electrical field is applied in 120° commutation to generate an initial back-EMF. A configurable soft start feature is also included, limiting the current drawn when spinning up the motor. All speed control at this stage is open loop.

Once the motor rotates fast enough to generate a back-EMF usable for the control algorithm, the TC78B011FTG switches to sensorless control with the current limit set for normal operation. If selected, closed-loop speed control is also engaged at this point.

Should the rotor already be rotating (known as idling or windmilling), due to an external mechanical force such as air passing over a fan’s blades, the motor driver will skip the initial excitation and forced commutation steps and proceed directly with sensorless operation. However, there are challenges with this approach so, to avoid this the motor controller can be configured to apply its braking sequence after leaving standby or power on, enabling the rotor to always start from a stopped state.

Toshiba has created a white paper specifically on MCU-less BLDC control. You can download your copy here:

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