Producers of drive technology have long tried to create motors without encoders. But approaches to date, says Günter Grossmann, head of Stöber motors product management, had a serious drawback, “In the range zero to 1000 rotations per minute the rotor no longer produces any useful position signals. And you can only hope that it‘s turning. In other words, in this range you must always work controlled – virtually in a stepper motor mode, where exact control isn‘t possible.“ The new LeanMotor changes that. Here development engineers came up with an anisotropic method. Meaning that, depending on inductance, the motor takes a relatable position that can be measured, and thus allows a conclusion at any time of where the rotor is. Based on this, speed and torque can be controlled even without an encoder, and continuously from standstill through maximum speed with full torque control. Speed deviation stated by Stöber is less than 1%, and positioning accuracy ±1°. 

Günter Grossmann, Stöber: “In the LeanMotor we‘ve saved on everything - except performance.“

© Stöber Antriebstechnik

This is due to what is called interior permanent magnet synchronous motor (IPMSM) technology with ‘buried‘ magnets. Positive features of reluctance motors also went into the design. “In that way we also succeeded, despite using less permanent-magnet material, to get more torque out of the motor“, enthuses Grossmann. That means designers can make drives smaller, or scale the machine itself smaller, saving floor space into the bargain. Given the very much reduced flywheel mass, control of the LeanMotor is also more dynamic and efficient compared to a standard asynchronous motor with incremental encoder. With efficiency of as much as 96% it even betters energy efficiency defined for IE4. Not enough - for the versatile LeanMotor there is an alternative, some 30% more cost-attractive and much more robust because neither sensors nor fans are built-in. That suits it for the kind of automation encountering heavy vibration and oscillations. Replacing a high-performance servomotor by a LeanMotor is not the ultimate purpose however. “The measuring accuracy for positioning isn‘t enough, so we don‘t get 500,000 items of information per rotation out of the motor“, states Grossmann. “Still, it doesn‘t need an encoder.“

On the subject of ‘doing away‘ - because the motor manages entirely without sensors, you only need a single standard power cable without extra wires or shielding for connection to a controller. “I can‘t imagine a simpler solution with just one cable for a motor that feeds back a position“, reckons Grossmann. Other conventional encoder-supported and single-cable solutions would always call for a special cable and connectors. The new motor is due to come to market in 1Q/2018, in three sizes and different lengths. That will cover a range 0.75 through 8 kW. With specified torque rated 2.4 through 26 Nm.