The motor stacks are connected in two groups of six stacks. The CEMF (counter-electromotive force) on each group is 63V at 45km/h.

In each group, 16 switches are in series, introducing a series resistance of 3.2 Ω. The current flowing in each stack is (130 – 63) / 3.2 = 21A, generating a motor torque of 260 Nm.

The modulation of this torque can be done by:

1. Powering only two wheels
   
2. Powering only one of the two wire networks in each motor (red or green wires in motor cross section, Part 1)
   
3. Powering only one of the two groups of six stacks
   
4. PWM (pulse-width modulation).

This set up can be used up to 90 km/h (56 mph) . For faster speeds, three groups of four stacks have to be used (up to 135 km/h (84 mph)), then four groups of three stacks (up to 180 km/h (112 mph)).

This concept controls the motor torque at any speed, just by introducing switches between the battery and the coils. Losses are limited to joule effect in the switches (and the coils).

Optimizing energetic yield
Careful selection of the switches is key. In this exemple, they are dominant in the generation of the losses, and in the limitation of the torque.

The figure below shows the electric command circuit for all applications (motor and brake). This circuit has to be duplicated to power the two networks in each motor, (shown in Part 1); only one circuit has been drawn here to clarify the picture).

Electrical schematic for motor and generator usages (to be duplicated for each of the two wire networks in each motor.

This figure shows an internal combustion engine and an electrical generator used to charge the battery (hybrid electric vehicle in series; the electric motor is the only driving mechanical energy source).

To read the entire article, which describes the calculations required to dimension the motor, please click here. Courtesy of EE Times USA.