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The lowdown on motor control: Historical foundations

Posted: 12 Mar 2015     Print Version  Bookmark and Share

Keywords:electric motors  brushes  commutator  Thyristor  DC motor 

For over a century, electric motors have been an essential part of our lives. Without them, there would have been no Industrial Revolution, and your lifestyle would be quite different from what you have come to enjoy and expect. Motors are often unknown and unseen, quietly doing many of the mundane chores that we take for granted, but they are an important element of the system designer's toolkit.

To illustrate my point, I recently went on a quest around my house to see how many electric motors I could find. I didn't count any of the motors in my automobiles, since I really don't know how to find them all, nor did I count any motors in my home office lab, since that would skew the results. Still, I found 114 motors; ranging from the 230V motor in my well pump to the small fan inside my laptop keeping its processor nice and cool. (If you ever get bored after dinner some night, you should try counting motors. I think you will be astonished to find that your number is very close to mine. If it isn't, well, you're probably not looking hard enough.)

While motor designs haven't changed much over the past century, the way we control them certainly has. We can control the motor's speed by adding electronic components to modulate the motor's voltage. For many years, though, variable speed applications were the sole province of brush DC motors, where changing the voltage could be done simply by putting an electronic switch in series with the motor winding. But DC motors come with a lot of baggage, not the least of which are the brushes and commutator required to keep their magnetic fields properly aligned. Not only do the brushes and commutator generate a lot of heat and electrical noise, their use also limits the life of the motor (in some cases by quite a bit).

AC induction motors don't have these problems. However, they pretty much run at only one speed, which is determined by the frequency of the voltage waveforms you drive them with. If you want to run an AC motor at a different speed, you could try calling the power company and asking them to give you a different frequency today. Or ... you could find a way to synthesize your own AC waveforms.

The earliest attempts to synthesize AC waveforms for motors were thyristor-based controllers, which utilized phase control to vary the AC voltage and frequency delivered to the motor. Thyristor controllers were soon replaced with PWM-based designs, which allowed tighter control of the motor waveforms. These PWM-based AC drives were called "Volts-per-Hertz" controllers because they made sinewave voltage waveforms whose amplitude was directly proportional to their frequency. Although such AC drives required more electronics than brush DC motor drives, the cost of the electronic components dropped over time, and the system cost eventually became much more competitive.

There was just one problem. Although you could achieve variable speed control, the torque response was rather lethargic compared to brush DC motor drives. That's because there is no way to directly control the torque in a Volts-per-Hertz drive. For a given speed, the only knob you can adjust is the motor's voltage, which is not an effective way to control torque.

A variation of the Volts-per-Hertz drive was developed (called the Slip Controller) that automatically adjusted motor voltage to regulate slip, which is fairly proportional to torque on an induction machine. While some applications were okay with this, others still needed faster torque response and remained shackled to brush DC motors. But this was about to change.

In the late 1960s and early 70s, a researcher at Siemens named Felix Blaschke was experimenting with ways to achieve torque response from AC induction motors similar to what could be obtained from a brush DC motor. He realized that the key to the problem was determining the angle of the rotor flux.

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