At the heart of every robot and every axis of motion is a motor. If the robot needs to accelerate quickly, move at very high speeds, carry large loads, or run smoothly, it is the motor that makes this happen. If the motor is moving with the robot the weight of the motor contributes to the robot mass so the more torque and speed (power) the motor can produce in the smallest possible package size, the better. Typically these motors are traditional brushless servo motors because of their long life and performance benefits and now slotless technology is pushing brushless motor performance to the next level.
Slotless brushless motors are similar to traditional brushless motors but have one significant difference, there are no teeth in the stator of the slotless design. The windings are created with a mold and then inserted into the stator housing.
Eliminating the teeth is not trivial but the benefits are enormous. Firstly, eliminating the teeth creates space in the stator housing so a larger diameter rotor can be used creating higher torque. Secondly, the teeth are the weak link of the magnetic circuit and saturate first limiting the torque. Eliminating the teeth yields a much higher peak torque in the slotless design. Thirdly, the magnetic attraction between the stator teeth in a traditional motor and the magnets on the rotor cause Eddy currents to flow in the magnets. These currents cause the magnets to become increasingly hotter and in turn cause the magnets to lose strength at higher speeds.
The slotless design does not suffer from this limitation and can maintain torque to much higher speeds. Fourthly, this magnetic attraction causes rough motion particularly at slow motor speeds. For applications where smoothness of motion is important the zero magnetic cogging of the slotless design is a big benefit. Finally, by minimizing the wasted heat in the magnets and losses due to cogging the slotless motors are more energy efficient and can reduce power consumption.
Xtrafors Prime Series
The modular construction concept is the basis for the wide range of standard models. The slotless stator design results in outstanding technical properties such as high speed, high torque, zero cogging, minimal torque ripple, high efficiency and high rigidity. This technology makes these motors perfectly suited for various demanding servo applications.
In one very demanding warehouse application the robot design engineers began by testing several traditional brushless motors. They required very high peak torque for acceleration but also wanted to run at very high speeds when delivering product to the far end of the warehouse. The only solution they found to reach the required speed with a traditional motor was to use a clutch brake and special gearing. Because of the very high peak torque capability and usable torque at speed performance, the Xtrafors Prime slotless motor from Infranor could produce both the peak torque required for acceleration and the high speed torque needed allowing it to run at speeds above 8000 RPM with low battery voltages, and achieved this with a smaller motor package. The clutch and special gearing were eliminated saving weight and increasing reliability.