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Chalmers team developing new high-power integrated motor and battery charger; “rotating transformer”

Model of the integrated motor drive and battery charger. The image shows a plug-in hybrid electric vehicle, which also has a fuel tank and a combustion engine, but the technology system works equally well with a purely electric vehicle. Source: Chalmers. Click to enlarge.

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).




This is a way to complicate battery charging.

A simple, low cost wall mounted higher efficiency DC charger, will do a better job, without an on-board charger.

Road side ultra high speed charging stations will become common place in the not too distant future.

A portable very low cost 115/230 VAC charger could be used as an alternative support system, when and if required.


It looks like someone re-invented AC Propulsion's reductive charging system, only for PM motors and not as good.

Dave R

Yep, Brusa already has a 22 kW onboard charger in a small package.

Tesla is shipping 10 kW onboard chargers in small packages.

Both of which undoubtedly have much higher efficiency and can be stacked to provide even higher output if desired.

I would love to see 3-phase AC charging in the US - the cost of stations would be significantly reduced - see Mennekes type 2 systems in use in Germany - good for up to 44 kW...


I think you guys really get it.

The EV auto industry has embarked on an effort to morph the EV into an complex vehicle that they are more comfortable with and that will remain expensive.

Doesn’t this, along with Volvo's 4 speed dual-motor Vocis eDCT concept represent a giant leap forward by Big Oil, Big Auto and Big Electric Company to maintain the high cost of EVs?

They tried with the Fisker Karma and it failed; but now they are again pushing ahead – they are relentless.


The trend is toward smart charging circuit. The charger would have to be able to handle AC or DC charging, and accommodate adapters to SAE or Chademo plugs. There are also European and pending Chinese standards to worry about.

Jim McLaughlin

The AC Propulsion patents cover a non isolated system. The goal here is an isolated charger with similar component reuse as the reductive charging approach.

There are a few more contactors here and the clutch is required because the motor rotates while charging. I wonder what you could do with an asynchronous motor though...

I have read AC Propulsion's patent on the single phase reductive charging, and it is a clear simplification at the cost of being non isolated. I have not read the three phase patents held by AC Propulsion, I wonder if they need a clutch also?

It is one thing to double isolate the High Voltage system in the Mini E, where you can force an Emergency Power Off event if someone opens the hood with the system energized. That is another matter in a heavy truck.


An induction motor can be used as an inductor with the rotor stationary, so no clutch is required.  (The analysis of induction motors views them as slightly peculiar transformers.)

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