Driving 3-Phase Full-Wave Brushless Motors: Advance Angle Control

In the previous article, in the explanation of “Sinusoidal Commutation PWM Driving”, an operation called “advance angle control” was mentioned. In this article, the nature of this “advance angle control” is explained.

Driving 3-Phase Full-Wave Brushless Motors: Advance Angle Control

The maximum motor torque is obtained when the phase of the magnetic field of the magnet (rotor) lags the phase of the magnetic field of the coils (windings) by 90°. The phase of the phase induced voltage leads the phase of the magnet (rotor) magnetic field by 90°, and the phases of the phase current and the coil magnetic field are the same, and so when the phases of the phase induced voltage and the phase current are the same, this condition is satisfied, and the greatest torque is obtained.

However, as indicated in the diagram below, if a voltage (red) is applied having the same phase as that of the phase induced voltage, in the expectation that the phase of the phase current (yellow) will be the same as that of the phase induced voltage (blue), the inductance of the windings causes a phase lag (indicated by the red arrows) to occur in the phase current (yellow). The torque for the phase is equal to the product of the phase induced voltage and the phase current, but there is a part of the cycle in which the product is negative (on the left side of the waveform diagram below, the intervals indicated by the gray bands); during these intervals, a negative torque occurs, and efficiency is reduced.

In order to alleviate this problem and raise the efficiency, there is a correction method in which the phase of the phase current is advanced by advancing the phase of the phase applied voltage, bringing the phases of the phase induced voltage and the phase current into agreement (on the right side of the diagram above, green arrow), and eliminating the negative torque interval. This is what is called advance angle control; the angle by which the phase of the phase applied voltage is advanced is called the advance angle.

The optimum value of the advance angle varies depending on the motor characteristics, rotation rate, and load torque (current value), and so the appropriate value must be set according to the state of motor use. The following are the main methods of advance angle control used by motor drivers.

• ・Fixed
• ・The phase of the phase current is detected and compared with the phase of a rotor position signal, and the two are brought into agreement
• ・Changed according to the rotation rate
• ・Changed according to a torque command value