Permanent magnet motor control – Rockwell Automation 20G PowerFlex 750-Series AC Drives User Manual
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Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 4
Motor Control
In large horsepower sizes applied to industrial loads, synchronous AC motors
serve two important functions. First, AC motors provide highly efficient means
of converting AC energy to mechanical power. Second, AC motors can operate at
leading or unity power factor, thereby providing power-factor correction.
There are two major types of synchronous AC motors: non-excited and direct-
current excited electric motors. Application of a VFD is to vary the desired
synchronous speed of the machine.
Permanent Magnet Motor Control
Permanent magnet motor control is selected by setting P35 [Motor Ctrl Mode]
to the appropriate choices of motor type. Refer to the PowerFlex 750-Series
Programming Manual, publication
, Appendix D for compatible List
of Allen-Bradley Servo motors and resolution criteria.
Surface Permanent Magnet Motor (SPM) or Permanent Magnet
Synchronous Motor (PMSM)
P35 [Motor Ctrl Mode] induction motor options.
•
4 = “PM VHz”
•
5 = “PM SV”
•
6 = “PM FV”
SPM or PMSM is a rotating electrical machine that has the stator phase windings
and rotor permanent magnets. The air gap magnetic field is provided by these
permanent magnets therefore it remains constant.
The conventional DC motor commutates itself with the use of a mechanical
commutator whereas SPM / PMSM needs electronic commutation for the
direction control of current through the windings. Because the SPM/PMSM
motors in effect have their armature coils at the stator, they need to be
commutated externally with the help of an external switching circuit. A three
phase PWM inverter (VFD) topology is used for this purpose.
The torque is produced because the interaction of the magnetic fields causes the
rotor to rotate. In permanent magnet motors, one of the magnetic fields is created
by permanent magnets and the other is created by the stator coils. The maximum
torque is produced when the magnetic vector of the rotor is at 90 degrees to the
magnetic vector of the stator.
Motor data and an autotune are required for correct operation in this mode.
Refer to
for details on the autotune.
The permanent magnet synchronous motor (PMSM) can be thought of as a cross
between an AC induction motor and a brushless DC motor (BLDC). They have