Motion System Design
Slotless motors smooth the way

Slotless motors smooth the way

Eliminating the teeth on brushless motors removes a source of cogging, improving efficiency, torque ripple, and precision

Designers have usually chosen brushless motors for low power pumps, fans, and turntable drives because they eliminated the cost of electronics and the mechanical brush-commutator mechanism. Little consideration was given to optimizing the motor's functional parameters.

As industry needs evolved, however, the requirement to optimize motor performance has moved to the forefront. Today's brushless dc motors are so sophisticated that they can meet and exceed the functional characteristics of traditional brush-commutated dc servomotors.

The introduction of slotless stator construction is one example of recent advances. Slotless motor designs offer significant operating and performance advantages over slotted brushless motors.

Conventional approach

Most brushless motors use electronic commutation, usually Hall-effect sensors and magnets, in place of brushes. The motor's rotor consists of a steel shaft with permanent magnets or a magnetic ring fixed around the circumference of the shaft. It transmits torque.

The stator features a group of slotted steel laminations (0.004 to 0.025- in. thick) that are fused to form a solid uniform stack, which creates a series of teeth. Wound copper coils, which produce an electromagnetic field, are then inserted into each of the slots. Together, the laminated stack and wound copper coil form the stator assembly. The return path completing the magnetic circuit consists of the laminated material outboard of the copper windings in the stator, and the motor housing.

Conventional brushless dc motors are typically available with ratings up to 1.0 hp. Some motors produce even more power.

Slots pose a challenge

Compared to brush-commutated motors, the brushless design offers several advantages, including high speed and fast acceleration, higher continuous torques through heat dissipation, less audible noise, and less electromagnetic interference. However, the slotted stators in brushless motors cause cogging, which adversely affects motor performance and efficiency.

Cogging occurs when the permanent magnets on the rotor seek a preferred alignment with the slots of the stator. Winding copper wires through the slots tends to increase this effect. As magnets pass by the lamination teeth, they have a greater attraction to the iron at the ends of the teeth than to the air gaps between them. This uneven magnetic pull causes cogging, which contributes to efficiency loss, motor vibration, and noise. Cogging also is a component of torque ripple, which prevents smooth motor operation at low speeds.

Smooth stators

As brushless technology and manufacturing methods have improved, slotless stator designs emerged as a solution to cogging in conventional brushless motors. The slotless stator and refinements to the lamination process are the key to smooth performance. Instead of winding copper wires through slots in a laminated steel stack as in conventional brushless motors, slotless motor wires are wound against silicon-steel laminations. Then they are encapsulated in a high-temperature epoxy resin to maintain their orientation with respect to the stator laminations and housing assembly. This configuration, which replaces the stator teeth, eliminates cogging for smooth, quiet operation.

The slotless design also reduces damping losses related to eddy currents. These currents are weaker in a slotless motor because the distance between the laminated iron and magnets is greater than in a slotted motor. With low damping losses, slotless brushless motors achieve more efficient operation and run cooler.

Slotless motors are typically designed with sinusoidal torque output that produces negligible distortion, rather than a trapezoidal voltage output. The sinusoidal output reduces torque ripple, especially when used with a sinusoidal driver. Because the slotless design has no stator teeth to interact with the permanent magnets, the motor does not generate detent torque. Further, low magnetic saturation lets the motor operate at several times its rated power for short intervals without perceptible torque roll-off at higher power levels.

Slotless construction also significantly reduces inductance to improve current bandwidth for fast response and acceleration. A samarium cobalt rotor usually offers excellent resistance to demagnetization under all operating conditions.

In addition, high-energy rare-earth magnets have allowed designers to relax air gap tolerances, which traditionally dictated that stator teeth be in close proximity to the magnets. Eliminating these teeth and using stronger magnets will maximize the strength of the electromagnetic field for optimum power output.

Rare-earth magnets are ideally suited for the slotless lamination assembly, as they enable a motor design with optimum functional characteristics, including low electrical resistance, low winding inductance, low static friction, and high thermal efficiency.

One of the crucial differences between slotless and slotted designs is the rotor diameter. Slotless motors have a larger rotor diameter for the same outside diameter and generate a higher inertia, as well as accommodating more magnet material. For applications with high-inertia loads, the slotless design offers an advantage. For applications requiring high acceleration of small inertia loads, the slotless motor is not a practical choice because most of the torque would be needed to turn the motor.

Slotless applications

Designers usually select brushless motors over conventional brush motors because eliminating the brushes extends motor life. Motor life is application specific, however, designers usually specify 10,000 hr. Other reasons for specifying brushless motors include a wide speed range, higher continuous torque capability, and faster acceleration.

Slotless versions of brushless dc motors suit applications that require precise positioning and smooth operation without cogging, which is more apparent at lower speeds. Typical applications include computer peripherals, mass storage systems, test and measurement, and medical and clean-room equipment.

For precise control, designers of medical equipment use slotless motors in machines that meter and pump fluids into delicate areas, such as eyes. In medical-imaging equipment, these motors provide smooth operation at low speeds.

By eliminating cogging, slotless motors reduce vibration associated with hand-held production tools. Other applications include robots for library data storage, a slowly rotating mirror that reflects a light beam, and a rotating radar antenna. Because there is no detent torque, the control system for such an application doesn't have to struggle with the motor seeking its own preferred position. Therefore, the control is easier to tune.

Custom options

Drives with slotless motors, as well as conventional brushless motors, can be customized to meet specific requirements and enhance performance. For example, spur gearheads used with the motors are configured for an application's specific torque and cost requirements. Planetary gearheads offer a higher-torque alternative.

Accurate positioning feedback from optical encoders provides an opportunity for enhanced motor control and an even wider range of applications. Two or three-channel optical encoders may be combined with differential line drivers to counter the effects of electrically noisy environments and ensure uncorrupted positioning feedback from the encoder to the control circuit. Also, encoders are now available with commutation tracks along with the standard two or three-channel output. This allows a more accurate commutation.

Other options for particular applications include custom cables, shaft modifications, shaft-mounted pulleys and gears, special bearings and windings, and electromechanical brakes.

Randy Rhood is an applications engineer -- brushless motors at Pittman, Harleysville, Pa.

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