Slotless motors are typically designed with sinusoidal torque output that produces negligible distortion, rather then 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. 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.

Compared with slotted motors, slotless construction also can significantly reduce inductance to improve current bandwidth. The teeth in a slotted motor naturally cause more inductance; the coils of copper wire around the teeth interact with the iron in a slotted motor, and this interaction tends to send the current back on itself, resulting in more damping (or dragging) and impacting negatively on slotted motor response and acceleration.

In terms of delivering power, conventional slotted motors used to enjoy the advantage over slotless types, due (as noted) to the proximity of iron and magnets and the reduced air gap. However, this advantage has virtually evaporated, in large part due to the utilization of high-energy, rare-earth magnets (such as samarium cobalt and neodymium iron boron). By incorporating these magnets, manufacturers of slotless brushless motors have been able to routinely compensate for the greater air-gap distance. These more powerful magnets effectively enable the same (or better) torque performance for slotless products compared with slotted. Eliminating the teeth and using stronger magnets both serve to maximize the strength of the electromagnetic field for optimum power output. Rare-earth magnets, along with the fact that fewer coils or “turns” of the wire are required in slotless motors, also help contribute to low electrical resistance, low winding inductance, low static friction, and high thermal efficiency in slotless motor types.

One more important difference between slotless and slotted designs is the rotor diameter. Slotless motors have a larger rotor diameter than slotted construction for the same outside motor diameter, and will generate a higher inertia, as well as accommodating more magnet material for greater torque. For applications with high-inertia loads, the slotless product is more likely to be specified.

Slotless Motor Applications

In general, brushless motors are usually selected over brush-commutated motors for their extended motor life. (While motor life is application-specific, 10,000 hours are usually specified.) Other reasons for specifying brushless motors include a wide speed range, higher continuous torque capability, faster acceleration, and low maintenance.

Figure 4. Slotless motor smoothness.
In particular, slotless versions of brushless DC motors will suit those applications that require precise positioning and smooth operation (Figure 4). Typical niches for these motors include computer peripherals, mass storage systems, test and measurement equipment, and medical and clean-room equipment.

For example, designers of medical equipment can utilize slotless motors for precise control in machines that meter and pump fluids into delicate areas such as eyes. In medical imaging equipment, slotless brushless DC motors decrease banding by providing the smoother operation at low speeds. Airplane controls supply smoother feedback to pilots. And, by eliminating cogging and resulting vibration, these motors can reduce ergonomic problems associated with handheld production tools. Other appropriate applications include scanners, robots for library data storage, laser beam reflector rotation, and radar antenna rotation equipment.

Customization Options

Slotless brushless DC motors, as with most motors today, feature a modular design so they can be customized to meet specific performance requirements. For example, spur gearheads can be integrated on motors for an application’s specific torque and cost requirements; planetary gearheads offer a higher-torque alternative. Slotless motors can further be customized with optical encoders, which provide accurate position, velocity, and direction feedback that greatly enhances motor control and allows the motors to be utilized in a wider range of applications. Choices for optical encoders include having either two- or three-channel output, several line count options, and the availability of built-in commutation tracks. Standard optical encoder resolutions range from 96 to 1024 CPR for two-channel versions, and from 96 to 512 CPR for three-channel configurations. As a low-cost alternative to optical encoders, rotor position indicators can be specified.

When using optical encoders, differential line drivers can be utilized to eliminate the effects of electrically noisy environments. Differential line drivers are designed to ensure uncorrupted position feedback from the encoder to the control circuit.

Other options that can be selected and customized for particular applications include connectors, custom cables, shaft modifications, shaft-mounted pulleys and gears, special bearings and windings, and electromechanical brakes. Each can deliver specific performance benefits and ensure application requirements are satisfied.

Motor Selection Guided by Application

Despite the overall design and performance comparisons reviewed here for slotless and slotted brushless DC motor types, one should remain cautious in drawing any conclusion that one type is the ultimate choice over the other. There are simply too many variables that must be evaluated, ranging from rotor size and windings, to housing and special components. A given application and its requirements should (and will) be the guiding factors in selecting a particular motor type and the customized components to be incorporated.