Electronic Design

Motor Basics

The wide range of electric motors can be a confusing topic even without considering intelligent motor control. This brief overview is for those who think motors are only divided into two categories: ac or dc.

A variety of motor types address a wide range of applications, from small, high speed drills to large motors that drive electric locomotives. Many are designed to run at a constant speed while others run at variable speeds over a wide range. Applications may also require different levels of torque at different speeds.

One common aspect between ac and dc motors is the number of windings. The electrical control is referred to as the phase, with single-phase motors being the simplest. Two-phase motors are common in low-cost solutions, such as motors used in automotive controls, while three-phase motors are most common in high-performance environments. Additional phases are often found in larger motors where efficiency and smooth operation are paramount.

There are two types of dc motors—brushless and brushed. Brushless dc (BLDC) motors employ permanent magnets in the rotor, while brushed dc motors use electrical power to generate a magnetic field in the rotor. Both generate a magnetic field in the stator, the outer stationary part of the motor. BLDC motors are popular because of their cost and reliability, as there are no brushes to wear out. In general, dc motors feature a high starting and low speed torque.

One of the more ubiquitous motors is the ac induction motor. It's essentially a transformer where the shorted secondary winding rotates. The other ac entry is the synchronous ac motor. It's typically a constant speed motor, but variable speed is possible with intelligent motor control. These motors come in non-excited and dc-excited versions that indicate how the motor rotation is initiated.

Universal motors run on either ac or dc power, though they tend to be more efficient using dc power. The advantage of these motors is high starting torque and a compact design, which is more characteristic of dc motors.

Motors can have one or more pairs of magnetic poles. The need for multiple poles depends on the type and characteristics of a motor and its power supply. For example, for some ac motors, the number of poles combined with the ac frequency will control the speed.

All types of motors can benefit from intelligent motor control, even if it's simply a matter of monitoring speed, power consumption, and temperature.

Still need more motor info on, say, stepper motors, servos, and linear motors? Visit the web site of Machine Design, Electronic Design 's sister publication. It has a plethora of information on the subject.

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