Autophasing
Autophasing is an automatic measurement routine to determine the angular position
of the magnetic flux of a permanent magnet synchronous motor or the magnetic axis
of a synchronous reluctance motor. The motor control requires the absolute position
of the rotor flux to control the motor torque accurately.
Sensors like absolute encoders and resolvers indicate the rotor position at all times
after the offset between the zero angle of rotor and that of the sensor has been
established. On the other hand, a standard pulse encoder determines the rotor
position when it rotates but the initial position is not known. However, a pulse
encoder can be used as an absolute encoder if it is equipped with Hall sensors,
albeit with coarse initial position accuracy. The Hall sensors generate so-called
commutation pulses that change their state six times during one revolution, so it is
only known within which 60° sector of a complete revolution the initial position is.
The drive monitors the motor status continuously, also during flux braking.
Therefore, flux braking can be used both for stopping the motor and for changing the
speed. The other benefits of flux braking are:
• The braking starts immediately after a stop command is given. The function does
not need to wait for the flux reduction before it can start the braking.
• The cooling of the induction motor is efficient. The stator current of the motor
increases during flux braking, not the rotor current. The stator cools much more
efficiently than the rotor.
• Flux braking can be used with induction motors and permanent magnet
synchronous motors.
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