PWM Topology

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This is the most popular power topology for driving an ac induction motor. Each of the three half-bridges are pulse width modulated independently. A three phase ac sine wave is applied to the three phase motor. Modern control DSP or MCU have timers especially designed to drive an ac induction motor using this topology.

This power topology can also be used to drive a Permanent Magnet Synchronous Machine PMSM using sine wave excitation.

A Brushles DC Machine BLDCM can be driven using this basic topology and a combination of six-step and PWM control. In simple control schemes the upper IGBTs are used for commutation, switched fully on, and the lower IGBTs are controlled by a PWM signal. Each winding is successively energized in this manner. It is also possible to apply PWM to two of the three half-bridges to provide four quadrant control.

Pulse Width Modulation

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Three phase pulse width modulation consists of three signals which are modulated by a sine wave. The duty cycle or high time is proportional to the amplitude of the sine wave. The effective average voltage over one cycle is the duty cycle times the peak to peak voltage. Thus, the average voltage follows a sine wave. In fact. this method depends on the motor inductance to integrate out the pwm frequency. These waveforms are edge aligned. The leading edges are all align. Thus, the top switches all turn on at the same time. This movie is ideally what you should see on the oscilloscope. The frequency is very low and the modulation depth is 100% to permit easy viewing.

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Another method is to align the centers of the three wave forms. This has several advantages. The motor will actually respond to the difference between two phases. By aligning the centers of the three pwm waveforms, the three motor windings are switched more often. This reduces the level of harmonics, reducing audible noise. This method also can provide twice the resolution for the same switching frequency. However, the digital timer is a bit more complex.

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