Magnetic attraction

From the standpoint of torque generation, a good working model for most motors is the simple bar magnet. The bar magnet spins around its center (modeling the motor’s rotor) and interacts with magnet fields generated in the stator by fixed, nonmoving coils. For brushless DC motors the rotor magnetic field is generated by magnets mounted directly on the rotor. For AC induction motors the rotor magnetic field is generated by induction (therefore the name of the motor) from the magnetic fields in the stator. The direction of this magnetic field, unlike for the brushless DC motor, changes based on several factors including the stator excitation frequency and current, the rotor speed, and the torque experienced by the motor.

The stator windings for brushless DC motors generally come in a 3-phase configuration, as do the windings for AC induction motors used with FOC techniques. Particularly for AC induction motors it is worth noting that other winding configurations are also used, notably the single phase AC induction motor. This motor is the workhorse found in most family A/C units, refrigerators, washers, and dryers, but it does not lend itself to the most advanced vector control techniques because the stator windings can not be individually controlled.

In any case, the three stator phases are arranged to be 120 electrical degrees apart from each other. It is the sum of the force generated by these three phases that will ultimately generate useable motor rotation. Depending on how the individual magnetic coils are phased, they can interact to create force that does not generate rotational torque, or they can create force which does drive rotation. These two different kinds of force are known as quadrature (Q) and direct (D), with the useful quadrature forces (not to be confused with quadrature encoding scheme for position feedback devices) running perpendicular to the pole axis of the rotor, and the non-torque generating direct forces running parallel to the rotor’s pole axis. Figure 1 shows this.

The trick to generating rotation is to maximize Q (quadrature) while minimizing D (direct) torque generation. In the case of a brushless DC motor, this is, at least in concept, easy, because brushless DC motors have magnets mounted directly on the rotor. Thus if the rotor angle is measured using a Hall sensor or position encoder, the direction of the magnetic field from the rotor is known. Things get more interesting for velocity and torque control applications where sensorless control is attempted. Since there are no direct mechanical measurements available for the rotor position, the angle must be inferred from the back-EMF voltage profile at the three windings. Although not trivial, back- EMF control is fairly common these days. Remember though that back-EMF requires that the motor be spinning, so it is not appropriate for positioning applications that must hold at a steady position.

In the case of an AC induction motor, a similar approach is used, however because of an additional requirement to maintain some amount of inductive flux, the D force is not driven to zero, but instead to a small constant value characteristic of the motor. Also, measuring the location of the rotor using Hall sensors or an encoder is not sufficient to determine the rotor’s magnetic angle, because it does not tell us the effective magnetic field angle generated by the rotor. Recall that this magnetic field is induced, and thus changes continuously.

This difference between the rotor location and the rotor magnetic angle is called the slip angle. The greater the actual torque on the motor, the greater the amount of slip, and thus the greater the compensating torque drive by the motor. This equilibrium is not unlike the way a hydrostatic transmission works. The greater the difference in speed between the engine and the wheels, the greater the torque generated by the transmission. This means that the motor’s rotation speed will be less than the driven frequency at the stator.

For the kinds of applications that AC induction is commonly used in, such as A/C units, washers, dryers, etc., a slip-reduced motor speed is not a problem. But for positioning applications, or to run the motor at its highest level of efficiency, this slip must be explicitly controlled. There are a few ways to do this, but they all require a measurement, or an estimate, of the rotor's induced electric field. Once again, a common way to achieve this is by using back-EMF techniques. Another popular approach is known as flux vector control, which measures the mechanical rotor angle, and attempts to derive the rotor magnetic angle algorithmically using estimations for various characteristics of the motor.