Chalmers team developing new high-power integrated motor and battery charger; “rotating transformer”
Researchers at Chalmers University of Technology in Sweden have developed a new type of high-power integrated motor drive and battery charger for electric vehicles. Compared to today’s electric vehicle chargers, the new system could shorten the charging time from eight to two hours, and to reduce the cost by around $2,000, according to the developers.
Dr. Saeid Haghbin at Chalmers proposed the system which uses the components in the traction circuit—such as the electric motor and the inverter—in the charger circuit to reduce the size, weight and price of the on-board charger. In essence, the proposed system uses the motor as a grid-connected generator with extra terminals.
The ideal scenario would be to have a charger powerful enough to charge a car in five to ten minutes, but this would cost over $100,000, which is more expensive than the car itself. The question we posed was: how can we reduce the size, weight and price of the on-board charger?—Saeid Haghbin
In his dissertation—in which he reviewed a number of other approaches to integrated motor/charger systems—Haghbin presented an integrated motor drive and isolated battery charger based on a split-phase PM (permanent magnet) motor. The charger uses an extra clutch and a relay-based switching device to reconfigure the traction circuit from the traction mode to the charging mode.
The main idea is to introduce a multi-terminal device called motor/generator set to act like a motor in traction mode and like an isolated generator/transformer in charging mode. The so called motor/generator acts as an isolated three-phase power source after synchronization with the utility grid in charging mode. All machine windings are used in traction mode and are then reconnected in charging mode through a simple switching device.—Haghbin (2013)
|Circuit diagram of the proposed isolated integrated charger for a conceptual two pole motor: (a) traction mode and (b) charging mode. Haghbin Click to enlarge.|
To reduce the magnetization current due to the motor airgap, the motor rotates at synchronous speed during the battery charging. An extra clutch is used in the system to disconnect the motor from the vehicle transmission during the charge operation.
Motor system efficiency in the charging mode is lower than the efficiency in the traction mode. On the other hand, a separate battery charger with a high power is eliminated from the system.
The motor first synchronizes itself to the grid and then starts to charge the battery. Haghbin designed and implemented two experimental setups to verify the proposed scheme. There first was a 20 kW PM motor with a charging power of 3 kW and an efficiency close to 80%. The second system is based on a split-phase PMSM (permanent magnet synchronous motor) motor with two identical windings that are not shifted in the space; Haghbin found that integrated charging is feasible in this scheme as well.
The Chalmers integrated charger is still at the laboratory level. The main challenge for the proposed isolated integrated charger is system efficiency, Haghbin notes. If the motor in its classical shape is used as a transformer, there is an airgap in the magnetization path leading to a reduced efficiency. If the motor is used as a rotating transformer or motor/generator, the loss due to the rotation decreases the efficiency.
To achieve a more optimal system, further investigations and experimentation are necessary. However, the product has resulted in both a Swedish and an international patent. Chalmers is trying to find a potential industrial user, and Volvo AB is working on the concept for further enhancement to be used in its systems.
The research was funded by The Swedish Hybrid Vehicle Centre (SHC).