Basics of the Electric Servomotor and Drive - Part 4: Brushless PM Motors (conc.)

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[Part 1 of this article covers basic magnetics, definitions of the motor control system elements, and an overview of electric servomotors. Part 2 examines the electrical and mechanical characteristics of permanent-magnet brush motors, as well as methods used in their control and their strengths and weaknesses. Part 3 begins a look at brushless permanent magnet motors and methods used in their control.]

15.6.4 DQ Control of Brushless PM Motors
A parallel method for phase control of brushless PM motors is direct-quadrature, or DQ, control. DQ control rearranges the system by placing commutation inside the current loop. This improves some aspects of controller performance because the commutation frequency does not pass through the current controller. Thus, phase lag caused by the current loop does not alter the commutation angle. Rather than regulating measured states (I_A, I_B, and I_C), DQ loops regulate calculated states, namely, the direct and quadrature currents, I_D and I_D.

Direct and quadrature currents produce flux in relationship to the rotor, whereas phase currents produce flux in relationship to the stator. For example, if a motor were rotating at a constant speed and with a constant torque, the phase currents (I_A, I_B, and I_C) would be varying sinusoidally, but I_D and I_D would be constant values. Phase currents are measured with respect to the stator frame of reference; a phase controller sees abundant activity in a motor spinning at high speed. DQ currents are measured with respect to the rotor frame; when DQ currents are measured on a spinning motor, there may be very little activity in the current controllers.

In DQ controllers, measured phase currents are combined to produce the state currents I_D and I_D using trigonometry to translate from the stator frame to the rotor frame:

I_Q = I_Asin(θ_E) + I_B sin(θ_E - 120°) + I_Csin(θ_E - 240°)                     (15.23)
I_D = I_Acos(θ_E) + I_Bcos(θ_E - 120°) + I_Ccos(θ_E - 240°)                (15.24)

The direct and quadrature currents are closely related to the winding and field fluxes that have been discussed throughout this chapter. In fact, I_D generates Φ_T and I_D generates Φ_F-WINDING. Figure 15-24b is redrawn in Figure 15-28 accordingly.

Figure 15-28. Flux generated by DQ currents shown in vector form.

To better comprehend the operation of DQ control, consider the diagram in Figure 15-29, which shows a rotor in two positions, with winding flux (Φ_T) moved so that it is 90° ahead of the field flux (Φ_F) in both positions. Here the torque producing current is equal in both positions and there is no field weakening.

Figure 15-29. Two rotor positions, with the controlled flux vectors remaining constant.

Note that after 20° of rotation, the Φ_T has moved 20° with respect to the stator. However, the position of Φ_T is constant with respect to the magnet position; in both cases it is correctly aligned between the magnet poles. So while the phase currents would vary between these two positions, the quadrature current (generating Φ_T) and the direct current (generating Φ_F-WINDING = 0) would not vary.

The DQ control system is shown in Figure 15-30. The torque command (T_C) is explicitly divided down by the estimated K_T (K*_T), although this step is implicit in most drives. The quadrature current loop is closed before commutation. The direct current

loop is commanded to zero current at low speeds and increased for field weakening at higher speeds.