Elmo HARmonica, HARSFEN0602 9.5.1 Six-step commutation , 9.5.2 Continuous commutation

HARSFEN0602
-100 -50 050 100
0
20
40
60
80
100
Commutation miss angle, degrees
Lost torque, %
Figure 7: Loss of torque due to commutation miss
Two principal methods are used to keep εnear zero. The first is called "Six-Steps"
commutation, and the other is the continuous commutation.
9.5.1 Six-step 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
CA, B open 30
CB, A open 90
AB, C open 150
AC, B open 210
BC, A open 270
BA, 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