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Electric Hub Motors

Electric Hub Motors - What are they and how do they work?

Motorized bicycles work by assisting your pedaling with electric powered brushless hub propulsion technology that utilizes incredibly strong rare earth magnets

If you own a conventional bicycle, you can easily replace your front wheel or rear wheel with an E-BikeKit™ brushless hub motor conversion kit wheel.

The E-BikeKit hub motor & wheel uses battery powered electric current to electrify rare earth magnets inside the hub motor to assist your natural pedaling. Using either a thumb or twist throttle you control the flow of electric current from the battery to the hub motor. With an intelligent 25 Amp controller regulating the current between the brushless hub motor and the battery you are in command of how much or how little propulsion power to use at any time.

How does a brushless electric motor work?
Brain, Marshall. "How does a brushless electric motor work?" 15 December 2006.
HowStuffWorks.com

In a typical DC motor, there are permanent magnets on the outside and a spinning armature on the inside. The permanent magnets are stationary, so they are called the stator. The armature rotates, so it is called the rotor.

Figure 9 The armature of a typical DC motor

 

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The armature contains an electromagnet. When you run electricity into this electromagnet, it creates a magnetic field in the armature that attracts and repels the magnets in the stator. So the armature spins through 180 degrees. To keep it in a brushless DC motor (BLDC), you put the permanent magnets on the rotor and you move the electromagnets to the stator. Then you use a computer (connected to high-power transistors) to charge up the electromagnets as the shaft turns. This is how the E-BikeKit brushless hub motors are made.

The Components of the E-BikeKit Complete conversion System

The motor has three phases, or power supply wires (think of this as a 3-cyclinder automobile engine). Each of these phases fires in succession with the others, determined by the "Central Brain", or what is called the "PWM (pulse width modulation) Controller." Inside the motor on the outer perimeter there are many "rare earth magnets". Closer to the center of the motor are the wire coils, also wired in three phases. As a pulse of electricity is sent to the motor from the controller the coils create a strong magnetic field, which repulses them from the magnets and causes the motor to advance (rotate) away from the magnets. The controller then sends another pulse of electricity and the next phase fires, rotating the motor even further. It's helpful to think of our automobile engine example: 3 cylinders - each firing one after the other. There are three power (phase) wires going into the hub motor, and five Hall Effects Sensor wires coming out of the hub motor. The Hall Effects wires are used only to help the motor start from a dead stop.

The controller determines the speed at which the motor rotates. The pulses of electricity sent to the hub motor are determined by the throttle, which is mounted on the handlebars of your bicycle. The throttle is actually just a magnet passing by a "Hall Effects Sensor." Power is supplied to the throttle from the battery. The throttle then rations a small amount of power: from 0 to 5 Volts, which it sends to the controller. The amount of voltage sent to the controller is determined by how much the throttle is twisted. As the throttle is twisted, "Hall Effects Sensors" read the amount of voltage, and send this amount of voltage to the controller. 5 volts means the controller does nothing: sends no pulses to the hub motor, 0 volts means the controller sends electrical pulses to the hub motor as fast as it can.

The battery pack is the power supplier for your kit. You must look after SLA battery packs carefully. SLA batteries need to be recharged immediately after EVERY use. They hate to be left in a discharged state, even if they are only slightly discharged. So, if you forget to recharge your batteries, leaving them drained for even one day, you WILL damage them, and they will soon begin to give you poor performance. LiFePO4 battery packs offer much more convenience and do not require charging immediately after use.

The Brushless Motor

There are a number of drawbacks to the brush and commutater mechanism used in a brushed motor: the brushes cause friction, there is some electrical resistance in the brush-to-commutater interface, and the mechanical switching of the armature current results in sparking, which can cause radio interference. Brushless motors do away with the brushes and commutater to get around these problems. The result is greater efficiency (more output power for a given amount of input power), and less electrical interference. The basic principles by which a brushless motor operates are exactly the same as those of a brushed motor. Figures 15 and 16 show two stages in the operation of a simple brushless motor.

The basic principles by which a brushless motor operates are exactly the same as those of a brushed motor. Figures 15 and 16 below show two stages in the operation of a simple brushless motor.

Figure 15 is the brushless motor equivalent of Figure 9. The electromagnets are fixed, and the permanent magnet rotates

 

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Notice that Figure 15 is almost identical to Figure 9 at the top of the page, except that there are no brushes and no commutater, and the types of the magnets have been exchanged. The permanent magnets have become electromagnets, and vice versa. The rotating permanent magnet is being repelled by the two electromagnets.

 

Figure 16. The motor from Figure 15, almost a full turn later. Notice that the electromagnets have changed their polarity.

 

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In Figure 16, almost a full turn later, the polarity of the left and right hand magnets has changed. The rotating magnet is now being pulled into alignment.

The problem to be solved here is how to cause the electromagnets to reverse their polarity at the right time. One could devise some sort of mechanical scheme controlled by the rotating permanent magnet, but this would nullify the main benefits of brushless motors.

Instead, the electromagnets are controlled by external circuitry. This circuitry monitors the current position of the rotating magnet, and energizes the external magnets appropriately to keep the motor turning. This circuitry is part of the brushless electronic speed control (ESC).
There are two ways for a brushless ESC to monitor the position of the rotating magnet. One is by way of magnetic sensors (based on the Hall-effect). These sensors report back to the ESC through a separate set of wires. The other method is known as "sensorless". Roughly, in this method the ESC monitors the three motor power wires for fluctuations caused by the spinning magnets.

The E-BikeKit brushless DC motor (BLDC) system has many advantages:

* Because a computer controls the motor instead of mechanical brushes, it's more precise. The computer can also factor the speed of the motor into the equation. This makes brushless motors more efficient.
* There is no sparking and much less electrical noise.
* There are no brushes to wear out.
* With the electromagnets on the stator, they are very easy to cool.
* You can have a lot of electromagnets on the stator for more precise control.

Electric Bike Hub MotorElectric bike hub motor 2

Links to originals
figure 1 http://static.ddmcdn.com/gif/motor.gif
figure15.gif   http://www.stefanv.com/quiet/2002-12/figure15.gif
figure16 http://www.stefanv.com/quiet/2002-12/figure16.gif

Text Links

Hall Effects Sensor http://en.wikipedia.org/wiki/Hall_effect_sensor

electronic speed control (ESC). http://en.wikipedia.org/wiki/Electronic_speed_control