CPT's switched-reluctance motor-generator seen as path to 48-V systems

  • 12-Mar-2014 01:32 EDT
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CPT’s SpeedStart unit is shown on a 48-V test rig supervised by lead applications engineer John Kelly.

Controlled Power Technologies (CPT) will demonstrate two technologies in booth 422 (and in technical paper presentations) at the SAE 2014 World Congress April 8-10 in Detroit: SpeedStart torque assist and kinetic energy recovery system; and Tigers turbine integrated gas energy recovery system.

Both systems rely on switched reluctance (SR) motor-generator technology "developed for a new breed of 48-V mild-hybrid vehicles" and are, according to the company, “at a high level of automotive application readiness.”

At the heart of SpeedStart is a 48-V belt-integrated starter-generator—an SR machine able to handle the challenge of balancing the need to customize it for different applications against the need to maintain commonality to minimize cost, reduce risk, and accelerate development cycles.

Vehicle electrification at 48 V is a rapid growth area for the automotive industry,” said Peter Scanes, Senior Manager for SpeedStart. "The number of architectures under consideration for 48-V systems is still growing, particularly across different vehicle classes. This results in a range of system-level requirements from different manufacturers, dictated by vehicle targets and other issues such as battery state-of-charge and energy-storage capability.”

By maintaining a common core of rotor and stator laminations and a common coolant system for the unit, it is possible to accelerate the development of each application, he continued. “This common core also provides greater confidence in the robustness of prototypes, while minimizing the cost of variants further supports the development of 48-volt systems. SR machines are ideally suited to this concept of low-cost development, while reducing the cost of production components. Moreover, compared with other electrical machines, they have a high efficiency across a large speed range, good controllability, and reliability.”

For SpeedStart, CPT established the critical design rules covering, for example, the optimum packaging and overall geometry of a 12.5-kW belt-integrated starter-generator. “The commonality of machine variants allows relatively easy variation of performance for different applications including trade-offs between peak torque, peak power, steady state power and other requirements from the electrical and mechanical systems,” Scanes explained. “Sophisticated modeling of the control and power electronics, in addition to the electromagnetics of the motor, is essential to this capability and means our control strategy can allow each machine to operate closer to the real motor limits seen in varied automotive duty cycles.”

As with the SpeedStart device, the liquid-cooled switched reluctance generator is at the core of the Tigers technology, again offering many advantages over conventional permanent-magnet generators. Due to the absence of rotor windings, SR machines possess low rotating inertias, thus minimizing rotor losses. In addition, due to the exclusion of permanent magnets, there is no risk of torque loss from de-magnetization or uncontrolled generating modes at high-speed.

The SR type generator also facilitates cost savings across the design, including minimal tooling investment due to the straightforward motor construction and a reduction in the rating of the power electronics due to low switching frequencies. The 65,000-rpm high-speed capability of the Tigers electrical generator also facilitates a direct coupling to the turbine rather than a reduction gear set. This reduces the number of components in the device, making the unit design inherently lighter, cheaper to manufacture, and simpler to assemble in both its prototype and volume production form, according to CPT.

Tigers can be applied to naturally-aspirated or forced-induction engines, though most likely will be applied to downsized turbocharged engines as an additional unit to the existing turbocharger.

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