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| E motion hybrids use three-phase, permanent-magnet, pulse-width-modulated motors. The rotor is fixed to a shaft supported by a tapered roller-thrust bearing at each end. A ring of powerful permanent magnets is arrayed around the rim of the rotor. The magnets are an alloy of neodymium, iron and boron (NdFeB). In a conventional motor, the rotor is wound with coils of copper wire, which must be energized with electricity to create a magnetic field. In a permanent magnet motor, the only windings are those that make up the stator, which sits inside the motor's external casing. A digital controller turns the continuous DC from the batteries on and off, creating a series of pulses that pass through the stator windings. The pulses create expanding and collapsing magnetic fields. The magnetic fields alternatively attract and repel the poles of the permanent magnets on the rotor, causing it to spin. The controller regulates the frequency and duration of the pulses to control the amount of current passing through the stator coils. Each pulse gives the rotor a push. The longer the pulse lasts, the longer the push lasts and the more current passes through. Represented on a graph, the electrical pulses form a series of rectangular peaks. Voltage is on the Y axis, and time is on the X axis. The peak of each rectangle is at the output voltage of the battery pack, 144 volts. The width is the length of time each pulse lasts. Since the controller regulates the width, i.e., the duration, of the pulses, the control technology is known as pulse-width-modulation, or PWM. Although voltage from the battery pack stays relatively constant at 144 VDC, the average effective voltage applied to the stator windings varies with the pulse duration. When the pulses are on 25% of the time – known as a 25% duty cycle - the motor is effectively receiving 25% average voltage. If the pulses are on a 75% duty cycle, the effective average voltage is 75%. "Hall-effect" sensors in the stator windings provide feedback to the controller to regulate motor speed. As the electromagnets pass by the sensors during each revolution, they send a signal back to the controller. The controller compares the sensor signal rate to the rate of the pulses it sent through the stator windings. If it's not the same, the pulse duration, or width, is increased or decreased to keep the rotation at the desired speed. E motion PWM motors are very efficient because electricity flows only during the pulse. When the pulse is completed, there's no electricity consumed. With narrow pulses, the electricity is off more than it's on. Speed control is very precise because the digital circuitry can make minute changes in pulse duration, effectively creating a smoothly varying speed from minimum rpm to maximum. There's no more electricity wasted at low speeds than at high, since electricity is simply turned off for a longer period of time for low speed operation. By contrast, a conventional AC motor is much less efficient because it's regulated by controlling the voltage to the windings with resistors. To reduce rotational speed in a conventional AC motor, more resistance is put into the circuit. Voltage to the windings is reduced, but the same amount of electricity is consumed. The resistance has simply converted part of it to useless heat, which is very wasteful. Speed control in a conventional AC motor, such as the low, medium and high settings in a window fan, is extremely crude. Since voltage is reduced by switching resistors into the circuit, the speeds available are limited to the voltage variations possible from different resistors or resistor combinations. |
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