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New BLDC motors fit servo applications

Sept. 1, 2000
The challenge for an equipment designer is to determine if the application requires a high-performance motor or if a conventional motor can do the job.

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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Winding construction in a BLDC motor is similar to that of a three-phase ac motor. The major difference between the motors is in rotor construction and the addition of rotor position sensors such as encoders or Hall Effect devices. These sensors provide electrical signals the control uses to sequentially energize the three-phase windings to produce maximum rotor torque and desired rotation direction.

Regardless of construction, a servomotor is normally part of a complete servo control system. The system, Figure 3, includes a system controller, possibly an interface, an amplifier or speed control, a motor, and a feedback device for speed or position, or both.

Sometimes a servomotor may be selected because computer control is needed and not necessarily because “servo” performance is needed. On lowvolume applications, if sophisticated control is needed, you are probably better off from a total-cost standpoint to buy the motor that the control vendor offers or recommends. The cost differential between a servomotor and a comparably rated conventional motor rarely justifies the development time needed to prove out the system. On the other hand, a designer working on a high-volume application should look at as many options as possible to make sure the final design is cost-effective.

Servomotors can be classified as highperformance motors. For example, BLDC servomotors give tremendous performance and are relatively costly. Note the low time constants in Table 1. However, conventional BLDC motors have been designed into ceiling fans, computer peripherals, and a variety of other cost-sensitive, high-volume applications. It is safe to say that a properly sized servomotor can drive just about anything a conventional motor can drive. The challenge for equipment designers is to determine whether the application requires a highperformance motor or if a conventional motor is adequate.

Motion control: Three groups

In general, a motion-control application falls into one of three broad groups: velocity, positioning, or profiling. For both the velocity and positioning application groups, there are many examples showing where a BLDC motor can do the job of a more expensive servo.

A velocity application is one in which the motor rotates to drive the load, and the stopping position is not of major importance. Examples are a fan, a centrifuge, a continuously running conveyor, and a machine-tool spindle drive. The first two often use conventional BLDC motors because of their high speed capability, high power relative to size, and low maintenance. Servomotors are sometimes selected for conveyors when the speed of one conveyor must be synchronized with that of another or when set speed must be held closely. Both of these are cases where the control requirements imply that a servomotor is needed when, in fact, a conventional BLDC motor will work. Spindle drives must hold very tight speed tolerances even if the load changes rapidly. Servomotors can respond quickly to changing conditions and continue to be the best choice for this type of application.

In a positioning application, the motor rotates to drive the load from Point A to Point B. Examples are a garage door opener, a supermarket checkstand, a parts elevator, and a pick-and-place robot. The first two examples often use conventional ac motors, and limit switches or photoelectric sensors to control stopping position. Position tolerance is usually in the range of ¼ in.

Step motors often serve in positioning applications when high torque and low speed are required. Step motors may be used without external encoders in many situations. However, for tighter tolerances and feedback on position, encoders are often added. A conventional dc motor could be used on the parts elevator if rapid acceleration and deceleration is not a requirement. But if it is, a BLDC motor is the better choice. Servomotors are used on pick-and-place robots, which require both rapid movement and tight tolerance on stopping position.

In a profiling application, both speed and position are important at all times. An example is a machine to laser-cut shapes. Cutting complex shapes, such as a map of the United States, on a two-axis system means both motors must be rotating at a predetermined (and possibly changing) velocity at any point during operation. Servomotors are nearly always used when speed and position must be controlled simultaneously.

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Check the specs

A comparison of the requirements of an application with the features or properties of several motor types will help determine which type is best for the job. Properties appear in motor manufacturers’ literature as specifications.

The most important properties to compare are usually electrical and mechanical time constants. Electrical time constant is the measure of the time it takes for the motor to produce torque. It is measured in terms of seconds by dividing motor winding inductance by resistance. Mechanical time constant refers to the time it takes the motor to get up to speed. Also in terms of seconds, the mechanical time constant is proportional to the product of inertia and resistance divided by the product of the torque constant and voltage constant. Torque constant is the amount of torque generated per amp of motor-winding current. Voltage constant is the amount of back EMF produced per thousand rpm of motor speed. Obviously, for both time constants, the smaller the number, the better.

As Table 1 shows, the new conventional BLDC motors have a mechanical time constant significantly lower than that of brush-type servomotors. The conventional BLDC motor accelerates and decelerates quicker than the brush-type servomotor and costs up to 30% less.

As the cost of BLDC controllers continues to fall, you will see more of the new breed of BLDC motors in applications that may have used conventional PM dc motors or brush-type servomotors in the past.

Servomotor features include low inertia — the rotating part of the motor is smaller or weighs less, or both, than those of conventional motors with comparable output power. This is important to a designer only if:

• The load also has relatively low inertia. Low rotor inertia is useless if load inertia is high.

• Fast acceleration and deceleration is needed. If there is no such requirement, consider a conventional motor.

A low-inertia motor can be produced in a number of ways. Most brush-type servomotors use rotors that are long relative to their diameters. Inertia is directly proportional to weight and proportional to the square of diameter. If a long skinny rotor weighs the same as a short fat one, the long skinny one will have lower inertia and will accelerate and decelerate more quickly. Other brush-type servomotors use “coreless” armatures, which are basically coils of wire held together by epoxy resin. Both of these methods make the motors more difficult and expensive to produce. In recent years. motor manufacturers have introduced conventional BLDC motors with inertias lower than those of brush-type servomotors.

The low mechanical time constant of servomotors results from combining lowresistance, low-inductance windings with low-inertia rotors. The net benefit is the ability to quickly accelerate and decelerate motor and load. If acceleration and deceleration times in the order of a few milliseconds are not needed, consider a conventional PM dc motor.

To stay competitive, equipment designers are constantly under pressure for new ways to improve the cost and performance of their designs. The new breed of conventional BLDC motor gives designers a cost-savings option for applications that have definite performance requirements, but do not demand top-of-the-line servos.

Thomas J. Batek is Manager, Technical Resource Group, Bodine Electric Co., Chicago.

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