Brushless Mortor|Basic

Driving 3-Phase Full-Wave Brushless Motors: 120° Commutation Linear-Current Driving with Sensors


Points of this article

・In 120° commutation driving, each phase is shifted by 120°, to drive the motor repeating a 120° on (H), 60° off, 120° on (L), 60° off cycle.

Starting with this article, methods for driving 3-phase full-wave brushless motors will be explained. In general, 3-phase full-wave brushless motors are driven by using control and driving circuits for motor commutation. Commutation methods include 120° commutation and sinusoidal commutation. Each method has its advantages and drawbacks. To sum up, sinusoidal commutation is superior with respect to control precision, efficiency, and noise, but entails a complex system and results in increased costs. In contrast, 120° commutation is inferior to sinusoidal commutation with respect to control precision, efficiency, and noise, but the system for this driving is simple, and costs are lower. Each commutation method will be explained in turn; we begin with 120° commutation linear-current driving using sensors.

Driving 3-Phase Full-Wave Brushless Motors: 120° Commutation Linear-Current Driving with Sensors

In 120° commutation, driving is performed by control and driving circuits, with drivers comprising high side and low side switches for each of the 3 phases. Below, operation is explained, referring to an example of a driving circuit for 120° commutation and input/output waveform diagrams.

To begin with, please examine the current waveforms for each coil. With the coil phases shifted by 120°, cycles are repeated in which current flows into a given coil over the 120° turn-on interval, followed by a 60° turn-off interval, then another 120° turn-on interval in which current flows out from the coil, and another 60° turn-off interval (vertical dashed lines appear at 30° intervals). Because the commutation interval is 120°, this is called 120° commutation.

H1P/H1N to H3P/H3N in the driving circuit are Hall element voltage inputs; signals from the Hall elements are received as differential signals (see the Hall element voltage waveforms in the waveform diagram).

Hall element voltages are shaped into square waves by a differential amplifier (see the square waves H1 to H3).

In next-stage logic operations, the square waves become driving signals for the high side switches (transistors) and low side switches, to drive the high side and low side switches via current driver amplifiers (see the synthesized waveforms M1H/M1L to M3H/M3L and the current waveforms for coils 1 to 3).

Put simply, during the 60° intervals in which the coil current is turned off, a coil voltage should not occur; but in actuality, because the motor is rotating and induced voltages occur in the coils, rising and falling voltage gradients occur during off intervals, and at the points at which coil currents change in steps (change suddenly), spike voltage noise occurs, indicated by the arrows.

The next article will explain sinusoidal commutation driving.

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