Magnetics Business & Technology - Fall 2013 - (Page 12)

APPLICATION FEATURE Magnetic Continuously Variable Transmission At the heart of many hybrid vehicle systems, such as the Synergy Drive found on the Toyota Prius, is an electronic continuously variable transmission (eCVT). An eCVT is a combination of a planetary gear and a motor/generator and is also found on other Toyota hybrids, including the Camry and Highlander and the luxury Lexus brand. A similar configuration is used by Ford in their Escape and C-Max Fusion Hybrid vehicles. Magnomatics’ Magsplit, is an integrated magnetic equivalent of a planetary gear and a motor/generator, and just like the mechanical system, Magsplit’s gear ratio can be controlled over a broad range and is capable of fully disconnecting the output drive and of driving in reverse. However, it offers a 3 to 5 percent fuel economy increase in a more compact device than the mechanical system. Its inherent compliance, or elasticity, also enables the removal or a significant reduction in complexity, of both flywheel and torsional damper components. Magsplit is currently being considered by a number of global car, bus and truck manufacturers. Ever more efficient hybrid powertrain configurations are essential if manufacturers are to meet regulatory targets and fulfill increasing customer demands for low carbon vehicles. But how does this magnetic continuously variable transmission work? Magnetic Gear Operation - Magnomatics’ magnetic gear consists of two rings of permanent magnets with a ring of steel pole pieces in between. These steel pole pieces act as flux paths between each of the rings of magnets. This has the effect of creating harmonics in the fields produced by each ring of magnets. By careful selection of pole numbers one can couple to the harmonic field and this creates a gear ratio. For example, if there are 23 pole pairs of magnets on the outer array and 27 steel pole pieces then this will produce a dominant four pole field at the inner, high speed rotor. The gear ratio between the inner and outer magnet arrays will be 23/4 or 5.75:1. The behavior of the magnetic gear is exactly the same as a mechanical epicyclical gear. One element of the gear is usually held static while the other two rotate with a fixed gear ratio. In many applications the outer ring of magnets is the static ring. A comparison between the magnetic and mechanical devices is shown in Figure 1. Magnomatics’ magnetic gears are currently in production for downhole pumping applications in the oil and gas sector. Magsplit Operation - In the Magsplit device shown in Figure 2, a wound stator is introduced and the outer ring of magnets is free to rotate. Uniquely, this outer rotor now becomes the high speed rotor of the gear system (equivalent to the sun gear). This would be impossible with a mechanical gear. By controlling the rotational speed of this outer ring of magnets one can change the gear ratio between the inner magnetic rotor and the steel pole pieces. The ratio can be continuously adjusted and even set such that the output shaft is stationary or reverses, ideal for a hybrid vehicle. This is not a passive device since power flow is always required through the stator, either as a source or sink. Typically, the maximum power required in the control stator is 25 percent of the power being transfer through the powertrain. Magsplit would typically be located between the engine output and final drive, as shown in Figure 3, and provides a first stage of gearing. The steel pole pieces and inner magnetic rotor are the primary means of transmitting torque into the differential and then the wheels. The gear ratio, and hence mechanical power delivered by the engine to the wheels through the mechanical path, is controlled by varying the electrical power flow through the control rotor. This control rotor can operate as both a motor, using power and as a generator, extracting power. This power flow into and out of the Magsplit, means that a second motor/ generator must exist in the drive train to ensure that the power is circulated. Typically an energy storage system such as a battery is also used and this enables a pure electric mode. The magnetic gear provides a wide range of gear ratios allowing the engine to operate over a narrow speed band equating to its peak efficiency while delivering a variable speed drive to the wheels. It can fully disconnect the output drive (de-clutching operation) and provide sufficient torque to crank the engine, when switching from all electric to hybrid modes for example. The inherent compliance of the magnetic gear can reduce engine torque pulsations down the drive line eliminating the torsional damper arrangement and contributing to reduced system cost. 12 Magnetics Business & Technology • Fall 2013 Figure 1. Epicyclical and Magnetic Gear Figure 2. Section Through MAGSPLIT Figure 3. Typical Hybrid Power Train Configuration Figure 4. eCVT Section www.MagneticsMagazine.com http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Fall 2013

Editor's Choice
Nanocrystalline Materials for the Next Generation of Noise Suppression Solutions
Magnetic Continuously Variable Transmission
Magnets • Materials • Measurement
Simulation Accuracy Helps Pioneering Superconducting Generator Developments
Application • Component Developments
Research & Development
Industry News
Marketplace/Advertising Index
Spontaneous Thoughts: NdFeB: The First 30 Years and the Next 30 Years

Magnetics Business & Technology - Fall 2013