User Manual

Table Of Contents
Sensorless Trapezoidal Commutation
Advanced Digital Motor Controller User Manual 123
0V
V+
ZC
Commutation
Integration Start
BEMF Integrator
Limit
Phase Voltage
Back EMF Integrator
FIGURE 8-19. Commutation based on Back EMF Integration
Detection of rotor position using the BEMF at zero and very low speeds is not possible.
At startup, the motor is therefore operated by applying a fixed commutation sequence
and runs as a stepper motor until sufficient speed is gathered to detect Back EMF and
Zero Crossings. Sensorless can therefore NOT be used if torque is required at stall or
very slow speed. Nevertheless, there are many applications (e.g., fans, pumps, watercraft
thrusters) that do not require high torque at very low speed.
Important Notice
Do not use sensorless commutation if torque is required at start or at very low
speed. Always use sensored motors when driving wheels. Sensorless works best
for driving propellers or fans.
Sensorless Parameters
The following parameters need to be tuned for trapezoidal sensorless to work optimally.
Startup Frequency (SSF): This is minimum frequency (electrical Hz) at which the
motor will always try to commutate during motor start or when slowed down.
Back EMF Integrator Limit (SVT): This defines the threshold at which the integrated
back-EMF of a given phase will trigger the commutation.
Back EMF Coupling Constant (BECC): In case of non-ideal back-EMF waveforms,
this number is required to compensate for back-EMF coupling due to other two
phases.
PWM Frequency: In order for BEMF Integration to perform optimally, the PWM
Frequency changes automatically dynamically within a min and max value..
The “Startup Frequency (Hz)” is set to 10Hz by default. This frequency works for practical-
ly all motors. Some motors with high friction, heavy load and/or high inertia may not start
at all, or start unreliably. If this occurs, try different Startup Frequency values.
PWM Frequency is set to 16kHz by default. Lowering the frequency helps the Back EMF
detection at low speed and can therefore help improve startup. See discussion below.