The basic principle behind a vehicle equipped with in-wheel electric motors is simple. The internal combustion engine normally found under the hood is simply not necessary. It's replaced with at least two motors located in the hub of the wheels. These wheels contain not only the braking components, but also all of the functionality that was formerly performed by the engine, transmission, clutch, suspension and other related parts.
Although the concept is relatively simple in theory, in-wheel motors pose a number of questions about performance, function and efficiency. We'll take a look at all of these questions and more beginning on the next page.
In a direct drive hub motor, the axle that passes through the center of the motor is actually the axle of the motor itself, with the copper windings fixed to the axle. This whole axle assembly is called the “stator”. The magnets are mounted to the outer shell of the hub motor. When electricity is applied to the stator a magnetic field is induced that causes the magnets to move. This in turn makes the whole shell of the motor turn and propels the ebike forward.
Inside a gearless direct-drive hub motor
Geared hub motors, on the other hand, have their cases connected to the stator through a planetary gear reduction system. For every rotation of the case, the motor inside actually turns many times faster. This allows the motor to work at higher and more efficient speeds, while still allowing the wheel to spin at a comparatively slower driving speed.
One common issue with hub motors is broken or shorted wires at the point where the wires exit the motor axle. If the wires are tugged or twisted the sharp edges of the hub motor axle can cut through the wire insulation or completely sever the wires. Most hub motors employ hall sensors to tell the controller the motor position. Hall sensors are easily burnt out if the signal wires short to power or ground. The motor shown below stopped functioning after the wires were damaged. A quick test showed that the hall sensors were burnt out. (Testing Hall sensors article to come)
Though hub motors tend to be fairly well behaved and can run for years without problems, they do occasionally go wrong. If the bike suddenly stops working or develops problems, you often end up in a position where you need to find out if it’s the controller or the motor that’s at fault.