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Commutation
Any time electrical current flows in a wire or coil of wires, a magnetic
field is created. In a motor, this magnetic field is used to generate
torque at the motor shaft output. In a permanent magnet motor,
this occurs when a magnetic field caused by current flowing in the
motor windings attracts the permanent magnets in the motor.
Maximum torque is generated when the internal magnetic field
generated by the motor windings is at a right angle (90 degrees) with
respect to the permanent magnet’s magnetic field. By controlling
the magnitude and direction of the current flowing in the motor
windings, the magnetic field generated by them can be made to
rotate around the motor axis. The attraction between this rotating
field and the permanent magnet’s magnetic field is what causes
motor shaft rotation.
The necessary switching of current between the motor windings to
cause this magnet field rotation is what is referred to as
commutation. This switching can be done electronically or
mechanically.
In a conventional DC brush servo motor, this switching is done
mechanically. By incorporating a number of motor windings with
their current switched through carbon brushes riding on copper
commutation bars, effective commutation is accomplished at
speeds (typically) of 4000 RPM or less. The servo drive is only
required to control overall current.
Brushless servo motors are a better choice when higher speed, rapid
acceleration, higher bandwidth and compact size are needed. They
are commutated electronically. By controlling the magnitude and
direction of current in the three phase windings, high bandwidth
speed and torque control can be accomplished. This means the
motor drive must control both the commutation and the motor
current to provide servo performance.
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