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Gearbox vs. direct drive: a comparison

Traction drives for on- and off-road vehicles can be designed with or without a gearbox; both variants can be useful in practice. The following text is to illustrate the respective advantages and disadvantages as well as give specific application examples of when it is advantageous to use a gearbox in an electric drive train or when a direct drive is the better choice.

The purpose of gearboxes

The purpose of gearboxes is to transfer the motor power to the wheels as well as to reduce this power in order to achieve more torque and less speed. In comparison to electric motors, combustion engines have a comparably low torque in the lower speed range. Without a gearbox, this would result in insufficient power transmission for starting, acceleration, and ascents. In addition to the goal of achieving the best possible efficiency with the largest share in the optimum speed range, this is the reason why gearboxes are always used in traction applications with combustion drives. The gearbox, which connects the motor with the drive train, adapts the torque to the respective load situation that is called up. This allows the motor power to be used efficiently. The gears are also adapted to the torque, which every vehicle needs for starting and acceleration – the gearbox converts enough in 1st gear that the motor does not die or over-rev. The gear spread defines the ratio range of the gearbox, thus creating the ratio between the individual gears.

How is it possible to operate electric drives without a gearbox?

The most important difference between a combustion engine and an electric motor is that an electric motor already has the full torque available upon starting – almost independently of the speed. This advantage of electric motors is additionally supported by the VECTOPOWER devel-oped by ARADEX. This means that the gearbox is not necessary for power transmission in itself, unlike in the case of combustion engines. However, it can still be useful to equip electric drive trains with a gearbox as well.

Gearbox and direct drive

The use of a gearbox as well as a solution using a direct drive have various advantages and dis-advantages, which must be weighed against each other in individual cases.

Field weakening in electric drives

To simulate the so-called gear spread, electric vehicles can also use something called field weakening. Field weakening is a method in which the magnetic flux between the rotor and stator is deliberately weakened using currents in the stator in order to change the torque constant. Field weakening allows higher speeds at lower torques with the same terminal voltage. The function of field weakening is thus comparable to a switchable gearbox and allows an economical design of the drive train with a high starting torque as well as high final speeds.

In the following, three vehicle categories with possible drive configurations are presented as examples from our projects.