Brushless DC Motor COMMUTATION SEQUENCE

Figure shows an example of Hall sensor signals with

respect to back EMF and the phase current. Figure 8

shows the switching sequence that should be followed

with respect to the Hall sensors. The sequence numbers

on Figure correspond to the numbers given in Figure 8.

Every 60 electrical degrees of rotation, one of the Hall

sensors changes the state. Given this, it takes six steps

to complete an electrical cycle. In synchronous, with

every 60 electrical degrees, the phase current switching

should be updated. However, one electrical cycle

may not correspond to a complete mechanical revolution

of the rotor. The number of electrical cycles to be

repeated to complete a mechanical rotation is determined

by the rotor pole pairs. For each rotor pole pairs,

one electrical cycle is completed. So, the number of

electrical cycles/rotations equals the rotor pole pairs.

**BLDC Motor Waveforms**

Figure is a graphical representation of the BEMF formulas

computed over one electrical revolution. To

avoid clutter, only the terminal A waveform, as would

be observed on a oscilloscope is displayed and is

denoted as BEMF(drive on). The terminal A waveform

is flattened at the top and bottom because at those

points the terminal is connected to the drive voltage or

ground. The sinusoidal waveforms are the individual

coil BEMFs relative to the coil common connection

point. The 60 degree sinusoidal humps are the BEMFs

of the driven coil pairs relative to ground. The entire

graph has been normalized to the RMS value of the coil

pair BEMFs.

Notice that the BEMF(drive on) waveform is fairly linear

and passes through a voltage that is exactly half of the

applied voltage at precisely 60 degrees which coincides

with the zero crossing of the coil A BEMF waveform.

This implies that we can determine the rotor

electrical position by detecting when the open terminal

voltage equals half the applied voltage.

**BEMF waveforms and the zero-crossing points**

The fact that one of the windings is not energized during each sector is an important characteristic of six-step control that allows for the use of a sensorless control algorithm. When a BLDC motor rotates, each winding generates BEMF, which opposes the main voltage supplied to the windings according to Lenz’s Law. The polarity of this BEMF is in the opposite direction of the energizing voltage. Figure 2, below, shows ideal BEMF waveforms and the zero-crossing points.

**Waveform of a Brushless DC Motor ESC video**

E-flite EFLA311, 20 A ESC driving an E-flite 450 motor.

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