R1 - 2017-03-19 22:48

Motor in electric unicycle - part 1

Recently, when I am meeting with other enthusiasts of electric wheels, I often hear questions about how does the motor work. Most people know that the motors used in unicycles are brushless DC motors, in short BLDC. But here often this knowledge ends. Because I am an electronic engineer and I have already designed control systems of this type and I am in a perfect position to explain how it all works in a simplified way. I want to focus on the general principles without going too much into the theory and details that not everyone may be interested in or that may be too difficult to be understood clearly by those not involved in physics or electronics.

The classic electric motor

The classic electric motor was invented by the English physicist and chemist Michael Faraday in 1831. The most common electric motors are commutator brush engines. Brushes are a special type of electrical contact between two or more moving parts. They have springs for better pressure, and abrasive elements of graphite.

The electric motor brushes provide a constant supply of power to the commutator through which the electric current goes to the rotor windings. While the current flow is generated, the electromagnetic field causes the phenomenon of attraction and repulsion of the coil windings mounted on the stator (fixed housing of the motor).

The biggest disadvantage of brushed motors are the brushes themselves. Because these elements have a permanent mechanical contact with the rotating commutator, they wear out and require replacement after some time.

Brushless DC electric motor (BLDC motor)

A BLDC motor (Brushless DC electric motor) is a motor with no brushes that is powered by direct current (DC). Instead of rotating coils, the BLDC motor uses whirling magnets. Coils are disposed on the stator only. Since the coils are stationary it is easy to connect them to the wires. The rotating part does not require any coils but only permanent magnets, which are subject to an electromagnetic field.

As a result, getting rid of the brushes makes the motor quieter and definitely more reliable. The only elements likely to be affected by wear and tear are the bearings, but they are present in every electric motor and have always limited lifetime.
BLDC motors have a much higher energy efficiency level, and most importantly, the user is free to control the rotational speed almost independently of the motor torque. Also, an important advantage of this type of motor is their smaller size in comparison to comparable conventional brushed electric motors.

If brushless motors have so many advantages, why are the standard brushed motors still in use today?

Normal brushed motors can operate with voltage directly from the AC power found in your wall sockets in every home. There is no need for any special controller to get the motor rotating. Just plugging in the power will make the motor start to rotate. In the case of BLDC motors, an electronic control unit is a must. Without it, the engine will not work at all. With this high level of technology comes a certain difficulty and an obvious cost. Another factor affecting the price of a brushless motor is the large amount of expensive ferro-magnets (rare Earth minerals). Often, even the ordinary brushed motor including the inverter (which enables smooth speed control) is less expensive than a BLDC motor with the control unit. This BLDC option has similar parameters when you have the necessary control unit in place.

Motor in an electric unicycle

The electric unicycle motor is integrated inside the wheel rim. On the inner side of rim there are strong, permanently fixed magnets (ferro-magnets) that revolve along with the wheel rim, and the coil windings are positioned closer to the center of the circle on the stationary motor shaft. The coils are connected together in such a way as to constitute only a couple of poles. It connects every third or every sixth coil.

Companies such as KingSong or Gotway use 3-pole control, while IPS, Ninebot or InMotion use a 6-pole configuration. The fewer poles, the less wires, and thus lower the cost of building the motor. This simplifies the way the motor is produced, therefore providing a cheaper electronic system for its control. However, the control of the motor from the algorithm becomes more complex. To maintain fluidity and precision of the wheel rotation (especially at low speeds) much more sophisticated control techniques need to be used, both in terms of selecting appropriate high-speed processors, the control unit and properly constructed software. This is the hardest part of all the fun.

In the next part I will tell you how the motor control is processed in our wheels.

The sources of the pictures:


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