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Slotless servomotors suit high-speed applications

Many design engineers working in the field of motion control are familiar with slotless motor designs, but may wonder if these specialized motors significantly improve machine performance. In many cases, they can. Here's why: Slotless motors offer more torque per frame size, produce more power, run more smoothly, and achieve higher speeds than their slotted counterparts. Following is a closer look at the technology, plus realistic tips on evaluating whether or not your next application could benefit from a slotless motor.

Slotted servomotor construction

A traditional slotted brushless servomotor has a stator made of stamped metal sections called laminations that are stacked to form teeth. Wire is wrapped around these teeth; when current flows in the wire, an electromagnet is created in the stator. Permanent magnets are fixed to the rotor.

Slotless servomotor construction

As in slotted motors, the permanent magnets in slotless servomotors are fixed to the rotor. However, a slotless motor's stator is built without teeth. Motor windings are wrapped around a temporary mold and then encapsulated to hold them in place. Eliminating the teeth yields many benefits.

Higher torque: A slotless motor's redesigned stator allows the rotor to be significantly larger. Because torque increases proportionally to rotor diameter, torque from a given slotless unit is significantly higher than that from a similarly sized traditional motor. Due to the absence of teeth, the area available for windings is also greater — which further increases torque. More specifically, torque at a given speed can be increased by 10 to 25% compared to a slotted motor.

Higher speed: As the magnets pass by the teeth in a slotted motor, a change or modulation in the magnetic flux is created, which in turn induces voltage in the surface of the magnets per Faraday's Law — e = dø/dt. These magnets are conductive, so a current flows in them. These Eddy currents, as they're called, increase exponentially with speed and create heat in the magnets, which in turn diminishes their strength. Because slotless motors have no teeth, they can achieve speeds in excess of 4,000 rpm over slotted motors.

Higher power: Power is calculated by multiplying torque by speed. Because a slotless motor outputs both higher speeds and torques, it can produce more than twice (2x) the power of a slotted motor.

Smoother motion: As the magnets on a slotted motor's rotor move past the stator's iron teeth, they are magnetically attracted to the teeth. This creates a torque disturbance known as cogging. Because there are no teeth in a slotless motor, this cogging effect is eliminated — yielding smoother motion.

Easier tuning: Motors with larger rotor inertias can be easier and simpler to tune. If the load becomes momentarily decoupled from the motor (a common phenomenon) the servo loop is less likely to become unstable if motor inertia is high relative to the load. In fact, precise servo tuning and filtering, which can be difficult to achieve, may not be required with a slotless motor.

Better stiffness: A rotor with a larger diameter has greater stiffness because torque increases with rotor diameter and a higher-torque motor responds faster to any displacement from the commanded position. The torque displacement curve is steeper.

Higher efficiency: All of the above traits boost motor efficiency by 5 to 25% compared to slotted motors.

Disadvantages of slotless motors

Despite their benefits, slotless motors are not without drawbacks and are therefore not suited to every application. For any given size, slotless motors generally have larger diameter rotors, and because rotor inertia increases exponentially with the rotor's diameter, inertia can increase significantly. Consider an application in which load inertia is very low compared to motor inertia, and high acceleration is required. Here, a slotted motor may be able to accelerate faster than a slotless one — if the slotless motor's additional torque cannot compensate for the higher torque required to accelerate the system.