Background info  

How much power can you send back?

This depends mainly on two values: torque and speed. The higher both values are, the higher the generated energy. There are mechanical and electrical losses, so the efficiency is never 100%. Mechanical losses are friction and slack, for example at the bike chain and between the wheel and the brake. Electrical losses are heat inside the motor and switching losses inside the electronic system. Heat and friction are the dominant values, slick and switching losses are less important.

How does the PowerBack system work?

The advanced motor brake of the new Genius Virtual Reality trainer has an optimized version of a very special feature: PowerBack. With this system you can generate your own electricity while riding on your trainer. A great invention, but how does it work?

For some years now the Fortius VR trainer has PowerBack as one of its outstanding features. With the replacement of the Fortius by the Genius as Tacx’s high-end Virtual Reality trainer, PowerBack has also been optimized. The Genius PowerBack system is much more efficient than the Fortius PowerBack, which means that the generated energy is also much higher.

The PowerBack system converts the mechanical work of the Tacx rider into electricity that is then, through the Genius motor brake, fed back into the power grid. This means that you are generating your own electricity. When you ride on the Genius, your PC shows the exact amount of energy that is fed back into the power grid.

You can compare the process to a dynamo that generates electricity for the bike lights. The rotational movement of the rider is driving the brake motor’s rotor. This generates AC current in the motor winding. The AC current is converted into a DC current, which is stored in a high voltage capacitor. This is comparable to charging a battery inside the brake. On the grid side, an electronic circuit (‘inverter’) converts the DC voltage back to a 50Hz/230V or 60Hz/110V AC current and feeds it into the power grid.

When do you send back more energy?
The higher the mechanical work, the higher the produced energy. Power calculates as: P = Torque x RPM x 2 x pi. This means that higher torque as well as higher speed generates more energy. Of course most energy is generated if both values are high. High mountain % means high torque, while high cadence means high speed. If one wants to maximize generated power, he should drive up the steepest mountains at the maximum speed.

So the best thing you can do is…
Climb steep mountains as fast as you can. The faster you go, the more energy you can send back into the power grid.

This powergraph shows how much power you deliver at different speed and slope settings and how much power will be sent back: