ElmoMC SimplIQ Software Manual - Manual (Page 121)

SimplIQ Software Manual Commutation
MAN-SIMSW (Ver. 1.4)
8-11
To optimize the torque, it is necessary to maintain o
90
r F
≈ θ − θ
.
The commutation error can be defined as follows:
( )
r F
90
θ − θ − = ε
θ
o . Ideally, . 0
= ε
θ
The torque production is not very sensitive to ε
θ
. Writing equation (1) as:
T = KT * I cos ( ε
θ
), it can be seen that a discrepancy of 5° can result in a loss of about 0.4% of
the torque. With a miss of 30°, 13.4% of the torque is lost, as illustrated in the following
figure:
-100 -50 0 50 100
0
20
40
60
80
100
Commutation miss angle, degrees
Lost torque, %
Figure 8-2: Loss of Torque due to Commutation Miss
Two main methods — six-step commutation and continuous commutation — are used to
keep
ε
θ
near zero.
8.5.1 Six-step Commutation
With six-step 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 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 8-4: Six-step Commutation