# Lorentz forces shrink motor size

Aug. 17, 2000
The ironless stator of the e-Torq motor eliminates the iron saturation common to most other types and lets the motor run smoothly at extremely low speed, even when powered by a standard drive.
 The ironless stator of the e-Torq motor eliminates the iron saturation common to most other types and lets the motor run smoothly at extremely low speed, even when powered by a standard drive. In addition, the high peak torque capability eliminates gearboxes for many applications, such as where a 14-in. e-Torq motor can deliver more than 700 N-m torque and speeds to 6,000 rpm.

Traditional brushless electric motors are designed around principles based on Faraday's Law which defines the behavior of attractive and repulsive forces between two inter-acting magnetic fields. For example, permanent magnets in a rotor generate one field while current in the field-coil windings generate the other. The reverse configuration also holds true, that is, where the rotor is electromagnetic and the stator is composed of permanent magnets. In both motor constructions, the rotor turns freely when the forces attract and repel numerous times in one shaft rotation.

By contrast, motors also may be designed around a similar principle attributed to the Dutch physicist, Hendrik Lorentz. The Lorentz law states that when electric current flows through a wire in a magnetic field, a force develops around the wire with direction and magnitude defined by the "right-hand rule," a concept widely taught in college physics courses. Mathematically, the cross product of the magnetic field vector and the current electric field vector produces the Lorentz force.

The brushless motor described here generates these Lorentz forces to produce shaft torque. The force comes from the cross product of the current vector in stator coil windings and the vector from the rotor's permanent magnetic field. The flat coil arrangement of the ironless stator, sandwiched between two closely spaced, permanent-magnet rotors (mechanically coupled to the output shaft), contain all the available flux. Although a variety of earlier actuators depend on the Lorentz force for torque, this new design provides several times more torque and higher efficiency than conventional motors in a similar-size package. Moreover, the combination of coil geometry and the iron-free stator eliminate the cogging typically encountered in most other permanent-magnet motors.

Information for this article was contributed by Sean Y. Kim, director, Bodine Electric Co., e-TORQ Systems Div., 2500 W. Bradley Place, Chicago, IL 60618, (773) 478-3515, www.etorq.com

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