What kinds of motor in e-bikes?

What kinds of motors does an e-bike have?

The short answer is BLDC where BL means “brushless” and DC means “direct current”.

Direct current means that the current always goes in one direction, that is from positive (red) to negative (black) as opposed alternating current (AC), as in household supply, where the current reverses direction 60 times per second. DC motors can use power from a battery or AC to DC transformer.

To make an electric motor turn the current has to go to different segments of the ring of wire coils inside the motor. Until recently most DC motors had a “brush” to do the switching. The brush is in contact with a metal ring with 2 segments and so the brush switches current to two halves of the coil every revolution of the motor. Anyone with an old power tool or car starter motor may know the brushes wear out.

Brushless means without brush, of course, and brushless motors are different inside. There are many more segments of the coils and they are wired as groups of three. Power is sent to the groups in turn by an electronic switch within what is called a “controller”. That is usually a separate metal box. The current going to the motor coils needs to be “in sync” with the motor and for that there is feedback from 3 sensors within the motor to the controller. These are called Hall sensors. The controller has other functions than to send power to the motor coils. It interprets signals from the throttle or pedal assist system (PAS). Some controllers also send signals to (and get signals from) a control panel – for instance, motor speed (hence bike speed) and the level of assistance.

In Asia many of the cheaper electric bikes have brushed motor. Some older electric bikes here may have a brushed DC motor.

There are two types of BLDC motors powering electric bikes. Ungeared (or direct drive) and geared. The difference being having gears (usually internal) or not having gears.

The ungeared motors are larger diameter (usually about 250 to 350mm and pancake shaped). The larger diameter enables the motor to have more power and rotate slower. The motor rotor is fixed to the axle and its speed IS the wheel speed and there are no other moving parts inside other than the rotor. Hub motors more than about 350 watts are usually ungeared motors.

The geared motors have smaller diameter and spin much faster. Because the motor spins 5 to 10, or more, times faster than the wheel (or the equivalent for motors driving the chain) there needs to be a gear system to reduce the speed of the power output shaft. Most geared motors will have internal gears. Some of the motors driving the chain or pedal axle will have external gears outside the actual motor case.

You may see mention of “torque”. Sometimes that is confusing, and meant to exaggerate the claimed power of the bike. It is irelevant. A large ungeared slow revving motor may have 10 times the torque of a small geared fast revving motor, yet both having same power output. That is because power = torque X motor speed. That is 10 times the torque with one tenth of the speed is the same power.

In nearly all cases a bike that’s legal in Australia will have a small geared motor. Either in hub of front or rear wheel, or centrally located near the pedals. (That’s a separate question). There are few ungeared hubmotors with 200 or 250 watts.

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