Figure 4.73: PMSM drive mode (a) and BLDC drive mode (b).
4.4.1 What is the BLDC mode?
It has been mentioned in section 4.2.5 that PMACMs are categorized into two
groups according to the shape of their back-EMF waveforms. The PMACMs
whose back-EMF is trapezoidal with 120 ◦ fl attop are usually called brushless
DC (BLDC) motors , and the PMACMs with sinusoidal back-EMF are
usually named as permanent magnet synchronous motor (PMSM) . These
back-EMF waveforms are illustrated in Figure 4.73. It is a widely accepted
practice to drive a PMSM using sinusoidal drive current and a BLDC mo-
tor using 120 ◦ square current (Figure 4.73). For the sinusoidal drive current
(PMSM drive mode), a complex motor drive system is usually required as the
current must be adjusted instantaneously in small magnitude according to the
variation of the rotor position. Besides the PMSM drive mode requires con-
tinuous and accurate detection of rotor position, which adds to the diﬃculty
to the realization of PMSM drive mode. On the contrary, the BLDC drive
mode is easy to implement as only a few rotor positions are needed, and the
commutation of current is required only at these limited rotor positions. The
three phase back-EMFs and the desirable drive currents of the BLDC motor
are illustrated in Figure 4.74.
A BLDC motor is usually driven by an inverter shown in Figure 4.75. A
position sensor is needed to identify the required rotor position for current
commutation. From Figure 4.74, we can see that at any time instant, only
two phases are energized with drive current. The switching sequence of the
energized two phases are (AH, BL) (AH, CL) (BH, CL) (BH, AL) (CH, AL)
(CH, BL) according to the symbols shown in Figure 4.75. The procedure is
same as the one introduced in section 4.2.3. This drive mode with only two
phases energized at any time instant is called the BLDC drive mode, or in
short, the BLDC mode.