Basics of the Electric Servomotor and Drive - Part 3: Brushless PM Motors

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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.]

15.6 Brushless PM Motors
The weaknesses of the brush PM motor have caused the brushless DC motor (sometimes referred to as synchronous AC PM motor) to dominate many servo-motor markets. The brushless motor replaces the mechanical commutator with electronic commutation, eliminating the brushes and their problems. However, brushless motors are more difficult to control.

Brushless controllers must sense the electrical position of the motor with a feedback device, such as a resolver or encoder, or, in some cases, with coarse digital sensors called Hall-effect sensors. In many nonservo applications, the BEMF of the motor is used to measure position; this is called sensorless control. In all cases, the electrical position is used to calculate commanded phase currents, with the goal being to maintain the commutation angle (θ_E in Equation 15.7) at or near the optimal 90°.

15.6.1 Windings of Brushless PM Motors
Windings of brushless PM motors are distributed about the stator in multiple phases. Usually there are three phases, each separated from the others by 120° (electrical). Brush motors can have many more phases, but a large number of phases in brushless PM motors is impractical because each phase must be individually controlled from the drive, implying a separate motor lead and set of power transistors for each phase. A simplifed winding diagram of a three-phase motor is shown in Figure 15-20.

Figure 15-20. Simple winding set for a three-phase four-pole motor.

Brushless motors rely on electronic commutation. The drive monitors the rotor position and excites the appropriate winding to maintain a 90° commutation angle. Consider Figure 15-21, which shows a brushless rotor in a sequence of three positions as it rotates counterclockwise. The large arrows show the flux created by the windings. To simplify the drawing, the field flux is not shown, but recall that it points out of the north poles and into the south poles. Notice that the winding flux in each of the three motor positions is maintained in quadrature.

Figure 15-21. Commutation sequence maintaining the winding flux between the magnet poles.

In brush motors, the commutation angle is maintained by mechanically switching phases in and out. Because the brush motor has many phases, each phase represents only a few electrical degrees of rotation and the torque from a brush motor is smooth. An equivalent technique is used on brushless motors in a commutation method called six-step, but it produces large torque perturbations at each transition because brushless motors usually have just three phases.

15.6.2 Sinusoidal Commutation
Unlike a brush motor controller, a brushless motor controller controls current in multiple phases independently. This allows the controller to move the winding flux (Φ_T) angle in small increments. Figure 15-21 shows how flux created from the three windings interacts with flux from the rotor magnets. Were the position of the rotor in Figure 15-21 midway between positions 1 and 2, flux from the windings could be positioned properly by placing equal current in phase A and phase B. In general, quadrature can be maintained precisely by independently regulating the phase currents according to Equations 15.14 - 15.16:

I_A = I_S × sin(θ_E)                                   (15.14)
I_B = I_S × sin(θ_E - 120 °)                        (15.15)
I_C = I_S × sin(θ_E - 240 °)                        (15.16)

where I_S is the magnitude of current in the motor and sin(θ_E) is the electrical position of the motor. This is called sinusoidal commutation.

Sinusoidal commutation provides smooth, efficient operation of the brushless motor. Torque is approximately proportional to I_S. In fact, brushless motors are usually given a torque constant based on I_S, so T ≈ K_T × I_S, assuming that commutation is performed correctly.