Elmo HARmonica, HARSFEN0602 software manual 112, Continuous commutation

9.5.1 Six-step commutation

HARSFEN0602ElmoHARmonicaSoftwareManual			PRELIMINARY		DRAFT
100
80
60
40
Lost torque, %
20
0
-100	-50	0	50	100

Commutation miss angle, degrees

112

Figure 7: Loss of torque due to commutation miss

Two principal methods are used to keep ε near zero. The first is called "Six-Steps"

commutation, and the other is the continuous commutation.

With Six-Steps commutation, only two motor terminals are energized at each time instance. The third motor phase is open-circuited.

Six field angles are possible:

Current flows:			Field direction, degrees
C	A, B open	θ	30
C	B, A open		90
A	B, C open		150
A	C, B open		210
B	C, A open		270
B→A, C open			330

Table 9-2: Six-Steps commutation

The digital Hall sensors have evolved to support Six-Steps commutation.

The crude Six-Steps produces about 13% ripple torque when used with sinusoidal motors, and much less ripple torque when used with trapezoidal motors – see the section "Winding shapes" below. The main drawback of the Six-Step commutation is the need to abruptly switch the phase currents. This need imposes an extreme bandwidth demand for the current controller. If the bandwidth of the current controller is less than satisfactory, there will be noticeable "knocks" at commutation switching points.

The Harmonica uses six-step commutation if no commutation encoder is present (CA[21]=0).

In that case, the Hall effect sensors will be also used for speed and position control as the position sensors.

The Harmonica also uses six-step commutation immediately after motor on, and before a first Hall sensor transition is encountered. When the first Hall transition is encountered, the high-resolution commutation sensor (encoder) can be homed and commutation may proceed in the continuous mode.

9.5.2 Continuous commutation