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Shown here is the waveform of the terminal voltage and the back-EMF. The back-EMF information can be extracted from the terminal voltage of the floating phase. From T1 and T2, the winding is floating, from T2 and T3, the winding is conducting current and from T3 to T4, the winding is floating again. The engineer can get the back-EMF signal when PWM is “off”. If the back-EMF is negative, it is clamped by the diode paralleled with the switch. Between T1 and T2, rising edge of zero crossing is detected, and between T3 and T4, falling edge of the zero crossing can be detected. The red curve is the full shape of back-EMF. When executed for all three phases, the engineer will have the full back-EMF waveform.
PTM Published on: 2015-04-06
