Axle Drive for electric Vehicle

Antriebe Elektroauto

“Choos­ing the right dri­ve archi­tec­ture for elec­tric vehi­cles to max­i­mize their range.”

Axle Drive with Planetary Gear

Selection of the right type of electric Motor and its Inverter

Axle Drive with Planetary Gear
A major chal­lenge with axle dri­ves is to select the design of the elec­tric motor and the invert­er so that they do not over­heat dur­ing oper­a­tion. The two main oper­at­ing states are short-term pow­er (5s-30s) to accel­er­ate the vehi­cle and con­tin­u­ous pow­er (30min-60min). For the short time pow­er very high cur­rents are need­ed for a few sec­onds, so the invert­er reach­es its lim­its very fast. Due to its high ther­mal mass, the motor is rel­a­tive­ly insen­si­tive to short-term over­load. If the motor and its cool­ing is not suf­fi­cient­ly dimen­sioned, the elec­tric motor will be destroyed dur­ing con­tin­u­ous power.
The accel­er­a­tions of a vehi­cle are rel­a­tive­ly low in the WLTP cycle. Accord­ing­ly, the torques required by the elec­tric motor are also small, if a trans­mis­sion ratio of 1:10 of the gear­box is assumed. The prob­lem here is that most elec­tric motors are not par­tic­u­lar­ly effi­cient at these oper­at­ing points. This is one rea­son why con­ven­tion­al elec­tric vehi­cles have a short range in the cycle.

Torque in WLTP cycle

Vehicle Speed and Torques of the Electric Motor

Torque in WLTP cycle

Axle Drive with switchable Gearbox

Gearbox with variable Transmission Ratio

Axle Drive with switchable Gearbox
Axle dri­ves for elec­tric vehi­cles usu­al­ly have a gear­box with a fixed trans­mis­sion ratio. If a trans­mis­sion is used that has two gears, the elec­tric motor has two ratios to con­vert its torque. Depend­ing on the trans­mis­sion and elec­tric motor com­bi­na­tion, this can improve the effi­cien­cy of the axle dri­ve and increase its max­i­mum speed. How­ev­er, such trans­mis­sions are slight­ly more expen­sive, because an addi­tion­al actu­a­tor is required.

Direct dri­ves have no gear­box and are nor­mal­ly used where high dynam­ics are required. Because direct dri­ves also have a very com­pact design, they are often used for autonomous vehi­cles. In autonomous bus­es, wheel hub motors are usu­al­ly used for this application.

Direct Drive for electric Vehicle

Electric motor for autonomous bus

Direct Drive for electric Vehicle

WLTP Operating Points in the motor map

Operating points of the driving cycle in the map of the electric motor

WLTP Operating Points in the motor map
If you add the oper­at­ing points from the WLTP cycle to the char­ac­ter­is­tic map of an elec­tric motor, you can see that none of the points are in the area of high­est effi­cien­cy. By switch­ing the trans­mis­sion ratio, the effi­cien­cy in the oper­at­ing points can be improved.