“22B” motor and gearmotor represent the new breed of brushless dc devices intended to serve in many cases where only servomotors could go in the past.

A new breed of small, brushless dc (BLDC) motor has been introduced by several manufacturers in recent years. These motors are designed with features and performance benefits that can bring savings in applications that would have required expensive servomotors only a few years ago. On a high-volume application, the amount saved may be significant.

The new generation of BLDC motors was designed primarily to make available at a competitive cost the advantages of brushless dc to users of conventional permanent magnet (PM) dc motors. Because BLDC motors have no brushes, they have the benefits over brush-type motors of no brush maintenance, brush dust, or brush-generated electromagnetic interference. And there is no brush noise. Moreover, BLDC motor construction makes the motors more thermally efficient, resulting in greater power from a smaller package. All those characteristics tend to bring longer life for a BLDC motor than for a comparable PM dc motor. In addition, the continually decreasing cost of control electronics keeps narrowing the price differential between BLDC and conventional PM dc systems. Because these new BLDC motors are constructed with components (magnets, bearings, laminations, shields) and processes similar to many widely available PM dc motors, they typically compare favorably in cost to PM dc motors. However, because of the special performance capabilities inherent in a BLDC design, these new-generation motors can be used in selected applications that previously needed a high-cost servomotor if it was needed only to satisfy a few design parameters.

Motion-control applications run the gamut from fans to machine tools. The best type of motor is often obvious. For example, an inexpensive induction motor normally drives a fan. Conversely, a highspeed profiler normally requires a highperformance multiaxis servo control system. Historically, high-performance servomotors had to be used for applications in the vast “gray area” between the two extremes. But now, because of their capabilities, the new generation of BLDC motors can satisfy some of these applications. A basic understanding of the new breed and the fundamental requirements of those servo applications in which they can be applied will let you make an intelligent selection for a given application.

BLDC motors combine certain characteristics of both three-phase ac induction motors and dc motors. They are similar to these ac motors in that a moving magnetic field causes rotation. They are similar to dc motors in performance because they have linear torque-vs.-speed characteristics. Figure 1 shows cross sections of ac, permanent-magnet (PM) brush dc, and BLDC motors. The BLDC winding in Figure 1, when combined with a separate electronic commutator and control, form the basic components of a BLDC system. The new breed of BLDC motor has been designed without high-resolution feedback devices or expensive magnets. Most importantly, performance can be significantly better than that of a PM dc motor but at only slightly higher cost.

A little background

Originally, servomotors were two or three-phase ac induction motors. Motors had solid rotors, and windings on the outside, Figure 1. As electronics and magnet technology progressed, brush-type servomotors appeared. Such a motor has windings on the rotor and permanent magnets or field windings on the outside. The brushes are pieces of carboncopper composite graphite that rub on a portion of the rotor called the commutator to electrically connect it to the power source. The commutator segments, Figure 2, are located so that, as the rotor turns, current flows in the proper direction in the rotor winding to keep the motor going in the desired direction.

New, more powerful magnets gave the new designs “better response” than earlier designs. “Better response” can be defined roughly as quicker acceleration and deceleration. BLDC motors have permanent magnets on the rotor and windings in the stationary portion of the motor. Normally, BLDC designs give better response than brush-type designs.

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