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Brushless, slotless, and cogless

Brushless, slotless, and cogless

Cost reductions in brushless motor designs, coupled with a dramatic decrease in the cost and size of electronic components, have resulted in wide acceptance of these motors in applications once dominated by brush motors. And advances in brushless motor technology continue apace.

Slotless stator construction, which can offer significant performance and operating improvements over conventional slotted brushless motors, is one example.

In a traditional slotted brushless motor, copper wires are wound through slots in a laminated steel core. A slotless, brushless design features copper wires wound against the laminations and held with adhesive. This design yields smooth rotation and virtually eliminates cogging.

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. The magnets are responsible for producing torque. As the flux density of the magnet material increases, the amount of torque available from the rotor assembly increases.

The stator features a group of slotted steel laminations (0.0004 to 0.025 inches thick), which are fused to form a solid uniform stack and create 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. Compared with brush-commutated motors, the brushless design offers several advantages, including high speed and fast acceleration, higher continuous torques thorough 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.

A solution: slotless designs. While advances in electronics are beginning to be applied to reduce normal cogging in slotted products, greater improvement in smooth operation is still needed. As brushless technology and manufacturing methods have improved, slotless stator designs have emerged as a solution to the cogging problem with conventional brushless motors. The slotless stator and refinements to the lamination process are the keys to smooth performance.

Instead of winding copper wires through slots in a laminated steel stack as is the case with conventional brushless motors (see diagram), 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 and results in 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.

Slotless motors are typically designed with sinusoidal torque output that produces negligible distortion, rather than 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, which is the maximum torque a motor delivers when de-energized. In addition, low magnetic saturation allows the motor to 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, improving current bandwidth for fast response and acceleration. In a slotted motor, the teeth naturally cause more inductance. Also, the coils of copper wire around these teeth interact with the iron, which tends to send the current back on itself. This phenomenon results in more damping (or dragging), impacting negatively on slotted motor response and acceleration.

Use of a samarium cobalt rotor in a slotless construction usually offers excellent resistance to demagnetization under most operating conditions. Rare earth magnets, in fact, are ideally suited for use within a slotless brushless motor, allowing the design to have 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 the ability to accommodate more magnet material. For applications with high-inertia loads, the slotless design therefore offers an advantage.

Slotless dc motors can also be customized to meet specific requirements and enhance their performance. For example, spur gearheads for the motors can be configured for specific torque and cost targets; planetary gearheads offer a higher-torque alternative. Slotless motors can be further customized with optical encoders, which provide accurate position feedback. Other options include connectors, custom cables, shaft modifications, shaft-mounted pulleys and gears, special bearings and windings, and electromechanical brakes.

How slotless stacks up. While specific motor customization for a particular application precludes a true "apples-to-apples" performance comparison between slotted and slotless designs, given the same rotor size, windings, housing, and other parameters, a conventional slotted motor would be more powerful than a slotless motor. This is because the "teeth" of a slotted motor (around which the copper wire is wound) place the iron closer to the magnets, so the magnetic circuit is completed more quickly. By reducing the air gap between iron and magnets, slotted designs also have higher torque.

However, manufacturers of slotless brushless motors routinely compensate for this shortcoming by utilizing high-energy, rare-earth magnets such as samarium cobalt. These more powerful magnets maximize the strength of the electromagnetic field for optimum power output, effectively enabling the same (or better) torque performance for slotless motors.

All things being equal, slotted motors will typically cost less than slotless motors for two reasons: use of these high-energy, rare-earth magnets and a more time-intensive manufacturing process due to the atypical configuration of wound copper wire.

However, for many applications, and in particular those in which cogging is detrimental, slotless brushless motors will be the preferred choice, regardless of the cost differential.

Typical applications for slotless, brushless motors

  • Computer peripherals, mass storage systems, and test and measurement equipment

  • Medical equipment that requires precise control, including machines that meter and pump fluids into delicate areas, such as eyes

  • Laser beam reflection and radar antenna rotation

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