Notice that a modified square wave spends a short amount of time at the zero crossing point, rather than abruptly changing from positive to negative. Instead, modified square, or quasi-square current is used. Trapezoidal drives are sometimes referred to as square-wave drives, although true square waveforms are rarely used due to their sharp transition between positive and negative values. The windings are wound in a trapezoidal fashion and produce a trapezoidal back EMF.įor the best performance, the drive current should match the back EMF waveform, so BLDC motors should be driven with trapezoidal waveforms (direct current). In a BLDC motor, the rotor consists of permanent magnets, and the stator has steel laminations with windings through axial slots. The shape of a motor’s back EMF is important, because it determines what type of drive current and commutation method should be used. BLDC motors typically have a trapezoidal back EMF, while BLAC motors (also referred to as permanent magnet synchronous motors, or PMSM) have a sinusoidal back EMF. The primary distinction between brushless DC (BLDC) motors and brushless AC (BLAC) motors is their back EMF waveform. The geometry of the windings in a BLDC motor give it a trapezoidal back EMF waveform.
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