ElmoMC SimplIQ Software Manual - Manual (Page 122)

SimplIQ Software Manual Commutation
MAN-SIMSW (Ver. 1.4)
8-12
Digital Hall sensors have evolved to support six-step commutation.
The crude six steps produce an approximately 13% ripple torque when used with sinusoidal
motors, and must less ripple torque when used with trapezoidal motors (refer to
section
8.6 ). The main drawback of six-step commutation is the need to abruptly switch phase
currents, which imposes an extreme bandwidth demand on the current controller. If the
bandwidth of the current controller is less than satisfactory, noticeable “knocks” will occur
at commutation switching points.
SimplIQ drives use six-step commutation if no commutation encoder is available (CD[21]=0).
In such a case, the Hall effect sensors are also used as position sensors for speed and
position control.
SimplIQ drives also use six-step commutation immediately after motor on, and before the
first Hall sensor transition is encountered. Afterwards, the high-resolution commutation
sensor (encoder) can be homed, and commutation may proceed in the continuous mode.
8.5.2 Continuous Commutation
With continuous commutation, all three motor coils are powered simultaneously to yield a
magnetic field exactly in the direction of the rotor. This continuously brings
ε
θ
near zero .
with minimal torque losses and ripple torques.
The continuous commutation mode is native to the SimplIQ drive and is used most of the
time. This mode of commutation is much more complex than the six-step commutation. In
fact, it requires two independent current controllers for controlling both the amplitude and
the direction of the windings magnetic field.
Continuous commutation reduces the dynamic demands from the current controller,
because such demands are rarely switched abruptly.
8.6 Winding Shapes
For a general motor, the following algorithm applies:
)) 240 ( ) 120 ( ) ( (
o o − + − + ⋅ = θ θ θ
h I h I h I K T
c b a
where:
T is torque.
K is a constant.
h is the windings shape function.
I
a
, I
b
and I
c
are the A, B and C phase currents, respectively.
For optimal efficiency, the phase currents must be:
) 240 ( h I I ), 120 ( h I I ), ( h I I
0 c 0 b 0 a
o o − θ = − θ = θ =
for some value I
0
. (1)
In other words, the phase currents must be proportional to the corresponding commutation
function values. If (1) is satisfied, the magnetic field produced by the winding currents is
perpendicular to the rotor magnet.