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Electric Variable Transmissions: Frank Rentmeester about his project @ TNO

Frank Rentmeester works as a hardware design engineer in a large project at TNO Automotive in Helmond. TNO is renowned for innovating with concrete targets. His project, Electric Variable Transmission (EVT), is a good example of TNO’s focus on useful applications

EVT technology can be classified as hybrid transmissions. Normal hybrid transmissions are located between the internal combustion engine and the differential gear. They consist of one or more electric motors in combination with other standard transmission components like clutches, gears or planetary gear sets. All these parts can be replaced by the EVT, including the electric motors. Just one compact device with just two rotating parts will do the job.

Another application of this technology is that the EVT can be used as a smart electric motor/generator for range-extended electric vehicles (like the Opel Ampere/Chevrolet Volt). Using the EVT, the electric motor and generator are integrated into one concept. This brings a more compact solution, which, in combination with better efficiency, extends the electric driving distance and lowers the vehicle’s fuel consumption.


The machine is basically an advanced electric motor. This motor has one stator and two rotors (see the picture below). One of the rotors rotates inside the other one. The inner rotor is built up from a three phases coil, also called a turning stator (of a outer rotor machine). The rotor, located between the inner rotor and stator (the outer rotor), can have several implementations, for instance as a double cage based on induction machine technology or as a permanent magnet in combination with additional DC coils.


Especially the configuration with permanent magnets and DC coils has much potential. It enhances great power densities and has good flux weakening possibilities. It is currently developed at our department in TNO and with additional partners. Not surprisingly, some patents have been granted to the concept. Finite Element calculations at electromagnetic and thermal design have been done, as well as calculations for mechanical strength. Eventually, this leads to the construction of a prototype that is going to be tested soon.

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