Mathematics of Electronic Motor Control

Chuck Lewin, President & CEO of Performance Motion Devices (download as PDF)

Introduction

In the last ten years several trends have been at work to drive the use of sophisticated, complex motor control algorithms. The most important trend is the desire for lower energy consumption and the need for higher performance. As it turns out, these two are often interrelated. For example lower energy consumption may come not only from more efficient motor control techniques, but also from increased functionality. An example is the ability to reverse the direction of an AC induction motor used to drive a washing machine. Adding this capability allows the load to be automatically balanced, thereby allowing the spin rate to be higher, thereby reducing the amount of energy consumed by the dryer.

Another major trend driving the use of sophisticated control algorithms is the availability of low cost microprocessors and DSPs (digital signal processors) which can perform advanced vector control of multi-phase motors. The AC induction motor is the workhorse of most household goods because it is easy to control and is very low-cost. In its simplest configuration it plugs directly into the wall with a minimum of electronics in the motor. But to increase motor efficiency and improve control performance, a substantial amount of electronics must be introduced into the controller. Only relatively recently has the cost of these electronics been low enough for consideration in high volume white good applications.

Brushless DC motors are also multi-phase devices, however they tend to be used in motion control applications such as medical automation and robotics. In these applications cost is often not as critical as the desire for performance. In particular, smoothness of motion and a large dynamic operating range are often key requirements. There is one other characteristic of brushless DC motor that have made them attractive to new applications such as electric and hybrid vehicles, which is their efficiency. High performance brushless DC motors driven with advanced control techniques can have efficiencies of 95% or higher. This is substantially above the best efficiency for an AC induction motor, which is in the range of 85%.

In this article we will introduce the major techniques used to control multi-phase motors, both brushless DC and AC induction. Step motors, which are also multi-phase devices, will not be discussed because they are generally used for lower speed, low power applications. Although wildly popular for applications such as printers, they are not used in applications which consume large amounts of energy.