Authored by:
John R. Casey
Project engineer and designer
Independent Engineering Concepts
Appleton, Wis.
Edited by Robert Repas
robert.repas@penton.com
Key points:
• Electric machines with noncoaxial rotors date back to the 19th century, but today’s electronic controls now make them practical.
• Each rotor assembly may have a different winding type to optimize efficiency.
• Contrarotating rotors can cancel torque effects, producing finer control.
Resources:
Independent Engineering Concepts, johncasey@tds.net

An electric motor designed for railway applications that used coaxial contrarotating rotors was granted a patent in 1890. The original design called for chain drives on the two output shafts with one chain imparting a 180° twist to rotate in the same direction at the driveshaft. Later inventions described multirotor electric machines for railway, marine, and automotive propulsion. Today, thanks to rapid improvements in control technology and lower costs, there’s a renewed interest in multirotor electric machines for electric-vehicle drivetrains. The twisted chain has been left in the past, replaced by high-technology motor-control systems.

Until recently, multirotor-differential electric machines fell into two design categories: a radial layer design with the rotors concentrically sandwiched, or side-by-side with axially adjacent rotors. For the most part, these designs had rotors coaxial or in-line with each other. However, a case can be made for a noncoaxial-differential electric machine for use in automotive drivetrains, marine propulsion, chemical blending, compact machining units, material separation, and diverting conveyors to name just a few.

An electric machine with noncoaxial rotors contains at least two rotors with axes of rotation not coincident on a common geometric line. Any type of rotor is suitable, including induction, wound coil, permanent magnet, and reluctance. Typically they operate in either a radial or axial flux field. The housing assembly forms a rigid framework that holds the rotor axes in a fixed orientation to the other, as well as at least one stator winding. An electronic control manages the transfer of electric power to or from the stator windings. Although there may be electromagnetic interaction between the rotors, they are not mechanically coupled within the electric machine. The rotors may rotate at the same or different speeds and in the same or opposite directions.

For example, a noncoaxial-differential electric machine with radial flux can function as a differential drive beween left and right wheels of a vehicle. This may be advantageous for body design, ground clearance, and driveshaft angles. Other benefits can include reduced costs and improved performance over traditional mechanical-differential transaxles.

With a mechanical differential, a wheel that loses traction spins with low applied torque, limiting the torque delivered to the other wheel. To overcome this design deficiency, mechanical differentials are sometimes equipped with mechanical-locking, slip-limiting, or electronic traction-control systems. Unlike mechanical differentials, which always apply equal torque to both wheels, a differential electric machine can deliver higher torque to the wheel that maintains traction.

Consider an electric machine having identical asynchronous induction-type rotors, and an electronic control with the two stator windings powered by a single-frequency ac-voltage source. If one wheel starts to spin while traction holds the other, minimal torque is applied to the spinning wheel while torque delivered to the stationary wheel increases as the output frequency from the controller rises. If the second wheel loses traction as well, it’s probably time to call the tow truck.

In general, if output frequency from the controller produces a synchronous speed faster than both rotor speeds, then both rotors will be driven as motors. If the synchronous speed of the supply is slower than both rotor speeds, then both rotors act as brakes. The regenerated energy returns through the controller to the power-storage device. If the synchronous speed of the supply is between the two rotor speeds, then the faster rotor functions as a generator while the slower rotor acts as a motor.

However, this single-frequency control scheme has one drawback in a vehicle differential. During cornering, more forward torque gets delivered to the slower inside wheel as compared to the faster outside wheel. Unfortunately, that means the less-tractive wheel due to body lean gets more torque. This situation is self-correcting in the sense that if the inside wheel should start to slip, energy delivered to that wheel drops (reducing torque) while energy delivered to the outside wheel remains unaffected, maintaining the same torque.

It is possible to configure the control to separately supply each set of stator windings with its own independent frequency. This splits the energy delivered to each wheel to adjust torque as desired based on feedback from wheel-speed sensors.

A noncoaxial-differential electric machine with axial flux and disc-type rotors can include planetary-reduction gearing at the output shafts. For synchronous-type rotors, such as permanent magnet, electromagnet, or reluctance, the electronic control needs a position sensor for each rotor. The straightforward method for generating differential speeds with two synchronous rotors is to supply each set of stator windings with its own independent current. However, if the synchronous rotors have a different numbers of poles, they can be driven at different speeds by supplying a single current that superimposes two alternating currents with different frequencies.

In general, induction motors operate more efficiently under high-speed, low-torque conditions, while permanent-magnet motors operate more efficiently under low-speed, high-torque conditions. Both types may be built into a single noncoaxial-differential electric machine.

For example, the machine may contain one squirrel-cage induction rotor and one double-shell synchronous rotor. The induction rotor drives the rear wheels of a four-wheel-drive vehicle through a driveshaft and mechanical differential. The synchronous rotor drives the front wheels in a similar manner. The electronic control receives torque feedback from each rotor. When the vehicle operates under high-speed, low-torque cruising conditions, only the stator for the induction rotor is energized. When torque demand exceeds a set level due to starting or heavy load conditions, the electronic control synchronizes its output frequency and energizes the stator for the synchronous rotor. When torque demand drops below the preset level, the stator for the synchronous rotor turns off.

For marine applications, a noncoaxial differential electric machine can work as a bow thruster. The machine mounts in a noncoaxial tunnel through the bow of the vessel. Because noncoaxial tunnel openings follow the contours of the hull, there is less resistance to forward motion compared to a straight-through tunnel with an exposed forward-facing wall. Controlling propellers mounted to the rotors of each electric machine provide variable-speed flow in either direction through the tunnel. The propellers may be identical and rotate in the same direction, or of opposite-pitch and counterrotating. The latter cancels torque and provides finer control.

Another use for noncoaxial-differential electric machines involves chemical-blending applications. The common housing assembly fixes the rotor axes at an angle for blade separation while maintaining a compact motor-head assembly. Although the rotor axes could be parallel, as in a traditional kitchen mixer, angled or skewed axes provide more-thorough mixing.

A third rotor and blade would add to the mixing action. The electronic-control programming can include mixing cycles with blade reversals and speed changes as needed.

Other applications for noncoaxial motors include machining operations. For example, machines with perpendicular rotors can create a compact machining unit for producing a cylindrical part with a groove. The workpiece is inserted through the support bearing and clamped in the collet chuck. Electronic controls monitor the workpiece rotor and the tool-spindle rotor while engaging a solenoid to extend the tool spindle and contact the workpiece. This is similar to extending the pinion shaft in an automotive starter motor. In this case, the electronic control mounts to the housing assembly. A proximity switch senses when the machining cycle is complete.

The final example is an electric machine with noncoaxial rotors for a material-separation application. Rollers mount to each of the rotors. If the noncoaxial rollers steer a fibrous web, they provide a tearing action to aid in slitting and separation. A knife positioned ahead of the housing assembly initiates the splitting action.

Alternatively, an electric machine with noncoaxial rotors could provide a compact drive for diverging conveyor belts.

© 2011 Penton Media, Inc.