Interior permanent magnet motor – Rockwell Automation 20G PowerFlex 750-Series AC Drives User Manual
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Rockwell Automation Publication 750-RM002B-EN-P - September 2013
241
Motor Control
Chapter 4
rotor structures similar to BLDC motors which contain permanent magnets.
However, their stator structure resembles that of its ACIM cousin, where the
windings are constructed in such a way as to produce a sinusoidal flux density in
the air gap of the machine. As a result, they perform best when driven by
sinusoidal waveforms. However, unlike their ACIM relatives, PMSM motors
perform poorly with open-loop scalar V/Hz control, because there is no rotor coil
to provide mechanical damping in transient conditions.
Field Oriented Control is the most popular control technique used with PMSMs.
As a result, torque ripple can be extremely low, on par with that of ACIMs.
PMSM motors provide higher power density for their size compared to ACIMs.
This is because with an induction machine, part of the stator current is required
to “induce” rotor current in order to produce rotor flux. These additional
currents generate heat within the motor. In a PMSM, the rotor flux is already
established by the permanent magnets on the rotor.
Most PMSMs utilize permanent magnets which are mounted on the surface of
the rotor. This makes the motor appear magnetically “round,” and the motor
torque is the result of the reactive force between the magnets on the rotor and the
electromagnets of the stator. This results in the optimum torque angle being 90
degrees, which is obtained by regulating the d-axis current to zero in a typical
FOC application.
Interior Permanent Magnet Motor
P35 [Motor Ctrl Mode] induction motor options.
•
10 = “IPMn VHz”
Some PMSMs have magnets that are buried inside of the rotor structure. These
motors are called Interior Permanent Magnet, or IPM motors. As a result, the
radial flux is more concentrated at certain spatial angles than it is at others. This
gives rise to an additional torque component called reluctance torque, which is
caused by the change of motor inductance along the concentrated and non-
concentrated flux paths.
This causes the optimum Field Oriented Control torque angle to be greater than
90 degrees, which requires regulating the d-axis current to be a fixed negative
ratio of the q-axis current. This negative d-axis current also results in field
weakening, which reduces the flux density along the d-axis, which in turn
partially lowers the core losses. As a result, IPM motors boast even higher power
output for a given frame size.
Motor data and an autotune are required for correct operation in this mode.
Refer to
for details on the autotune.
These motors are becoming increasingly popular as traction motors in hybrid
vehicles, as well as variable speed applications for appliances and HVAC. In the
servo motor world more and more designs are shifting away from SPM to IPM to