In 1831, Michael Faraday discovered that subjecting a coil to a moving magnetic field induces a current. Today, electromagnetic induction powers most electric motors, along with a phenomenon described in Ampere's law (involving the forces generated between the magnetic field surrounding a current-carrying conductor and an external magnetic field.) But the interaction of the magnetic and mechanical forces inherent to this design is not always the most efficient.
That's why some motors use Lorenz forces instead. Named after the Nobel laureate, this force is that experienced by a point charge moving along a wire in a magnetic field, at right angles to both the current and magnetic vector. Current in a Lorenz motor may be viewed as a procession of moving charges subject to Lorentz force, thereby transduced to a force on the conductor itself. This force is then multiplied by the number of coil conductors.
Note: Versions of Lorenz-force motors are sometimes also called pancake motors; Timken has used them in cutting-edge vehicle designs detailed here.
In short, Lorentz and Faraday motors both use electromagnetic properties to create mechanical work, but the way the forces are applied in a traditional Faraday motor means it is always trying to reshape itself. By contrast, the lack of steel in Lorentz-force motors allows application of electromagnetic forces with almost no mechanical energy losses — and no electrical lamination “buzzing” noise.
Q: How powerful are Lorenz-force motors?
A: It's been held that power density is not achieved by coils alone. However, the forming, interleaving, precision layer winding, and square magnet (or litz) wire in Lorenz stators eliminates the need for steel poles to concentrate flux created by current in the coil. Stacking of coil and rotor segments multiplies torque in a compact package, and extra multiplication is possible with integral planetary gearheads — in virtually the same footprint as an induction motor.
Q: How does the performance of Lorenz-force motors differ from traditional motors?
A: Eliminating poles to concentrate flux means the motors effectively eliminate core saturation, without exotic steels. No saturation means a linear current/torque relationship over an unusually wide range resulting in high torque at low rpm, including at stall. This allows the use of high current to produce proportionally high torque, for better efficiency and less waste heat. Together, extended torque linearity and higher torque translate into excellent damping and reduced overshoot in servo applications. (Because the controller doesn't have to compensate for saturation or iron losses, it gets back everything it puts in.) Performance is consistent in all modes: constant or variable speed, steady state, intermittent, and reversible.
Lorenz-force motors are also responsive, precise (in terms of speed and torque), and repeatable (to within two arc-seconds). Their low coil inductance and thus electrical time constant translate to better precision and bandwidth in servo applications.
Q: Why aren't Lorenz-force motors the norm?
A: The majority of motor applications today require only brute horsepower. And until recently, most motors only needed to provide rotation and torque — for which conventional motors are quite efficient under the right circumstances. But this is changing.
In a growing number of applications, motors are being called on to provide motion control in addition to horsepower. This has led to increased use of specialty motors that accelerate quickly, present little inertia, and produce torque proportional to current with no torque ripple. Although today's applications may justify the more costly winding configurations of Lorenz motors, there is one caveat. Lorenz motors have no core losses and thus no inductive heating, but the elimination of steel from the heat-generating portion of the motor makes it more difficult to remove the heat caused by winding I2R losses. Another concern is that off-the-shelf amplifiers may not be suitable for Lorentz force motors owing to the extremely low inductance due to the lack of iron.
This month's handy tips provided by J.E.Marth,V.P.of Engineering at Bodine Electric Co., and Edmund Glück. For more information, visit www.bodine.com.