Brushless Mortor|Basic

Driving 3-Phase Full-Wave Brushless Motors: Maximization of Motor-Applied Voltage

2023.08.09

Points of this article

・In sinusoidal driving of a 3-phase full-wave brushless motor, if the motor is driven using unmodified sine waves, the motor-applied voltage is 0.87 times the power supply voltage.

・There are two methods to maximize the motor-applied voltage.

  • -Add halves of a lower-side arc part of the sine wave to the upper sides of the other two phases to drive the motor.
  • -Add third harmonic waves to the fundamental sine wave.

In this article, methods to maximize the voltage applied in sinusoidal commutation driving of a 3-phase full-wave brushless motor are explained.

Driving 3-Phase Full-Wave Brushless Motors: Maximization of Motor-Applied Voltage

In this waveform diagram, the PWM signal in sinusoidal commutation PWM driving is replaced with a voltage, showing that a motor-applied voltage VM is applied using sine waves (broken lines) in three phases with the power supply voltage as the amplitude.

As one can probably grasp things from the waveforms, when unmodified sine waves are used for driving, VM is of the power supply voltage, so that put simply, only voltages up to 13% lower than the power supply voltage can be applied. In motor driving, maximization of the applied voltage is of course absolutely necessary; in actual applications using sinusoidal commutation driving, methods to maximize the applied voltage are employed.

Methods for Maximizing Applied Voltage in Sinusoidal Commutation Driving ①

In this waveform, halves of a lower-side arc part of the sine waves (broken lines) are added to the upper sides of each of the two other phases (green arrows). As can be seen in the waveform diagram, a VM that is greater than the unmodified sine waves can be applied.

The applied voltage waveforms are no longer sinusoidal, and are deformed, but the voltages actually applied to the motor are differential voltages between the phases, and adjustments are made such that the differential voltages are sinusoidal, to make the phase currents sinusoidal. Consequently there is no adverse effect on the high efficiency or smoothness that are advantages of sinusoidal commutation.

Methods for Maximizing Applied Voltage in Sinusoidal Commutation Driving ②

In this method, third harmonic waves are added to the fundamental wave (broken-line sine wave). The sine wave becomes a trapezoidal waveform, and as is again seen in the waveform diagram, a larger VM can be applied, and the phase current becomes sinusoidal.

The basic principles of these driving methods have already been explained, but in actual motor driving, such techniques for improving efficiency should be born in mind.

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