AC induction motors

The AC induction motor is the work-horse of white goods and other common applications because it is cheap, easy to build, and easy to operate. Most household AC induction motors are single-phase, and are driven directly by the 60 Hz AC wall current. In this mode they basically have one operating mode, on or off. Larger motors, and motors that are to be controlled in a more sophisticated way, are wired with three separate phases, much like a brushless DC motor. And like a brushless DC motor, the three motor coils generate a stator electrical field. However unlike the brushless DC motor, there are no magnets in the rotor. The rotor generates a magnetic field from current which is induced by the stator magnetic field.

The angle and magnitude of the induced current vector in the rotor depends on a number of factors including the stator winding frequency and magnitude, and the type of rotor iron material. Also, unlike the brushless DC motor, measuring the mechanical angle of the rotor does not tell us the angle of the rotor’s electric field. Since the current is induced, it typically lags the frequency of the stator current, and does not stay fixed relative to the mechanical rotor position. This difference is commonly referred to as the slip angle, or slip frequency.

Although not trivial, it is in fact possible to determine the rotor magnetic angle so that a high performance field oriented control approach can be used to control an AC induction motor, thereby allowing it to be used bi-directionally, as well as with variable torque and speed across the entire operating range.

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To determine the rotor electrical angle, one of two methods are used. The first is broadly known as flux vector control, and uses a measurement of the mechanical rotor, the stator excitation signals, and estimations of flux generation properties in the rotor to mathematically calculate the rotor magnetic angle. It is worth mentioning that “flux vector control” is a widely used term, and sometimes refers to this estimation technique, the overall technique of field oriented control, or any number of other proprietary multi-phase control approaches.

Besides flux vector estimation, another commonly used approach is to measure the back-EMF of the three coils to determine the rotor magnetic angle. Whether by flux vector estimation or back- EMF measurement, once we have the rotor magnetic angle we can apply a technique such as Field-oriented control to provide high performance control of the motor. One key difference however between controlling a brushless DC motor and an AC induction motor is that the D (direct) desired force can not be set to zero.

In the brushless DC motor rotor magnetic flux is generated by permanent magnets, and thus for control purposes we set the D command to zero. In an AC induction however we need to apply some amount of electrical energy to induce a magnetic flux in the rotor. And thus the desired value of D is generally set to a constant value which is characteristic of the motor’s rotor magnetic properties and the drive voltage used. For sophisticated applications, on-the-fly modification of this desired D value is generally referred to as “field weakening,” and can important for avoiding magnetic saturation.