User's Manual

PMAC2 User Manual
14 Setting Up PMAC2 for Direct PWM Control
In a motor, there are three frames of reference that are important. The first is the stator frame, which is
fixed on the non-moving part of the motor, called the stator. In a brushless motor, the motor armature
windings are on the stator, so they are fixed in the stator frame.
The second frame is the rotor frame, which is referenced to the mechanics of the moving part of the
motor, called the rotor. This frame, of course, rotates with respect to the stator. For linear brushless
motors, this is actually a translation, but because it is cyclic, we can easily think of it as a rotation.
The third frame is the field frame, which is referenced to the magnetic field orientation of the rotor. In a
synchronous motor such as a permanent-magnet brushless motor, the field is fixed on the rotor, so the
field frame is the same as the rotor frame. In an asynchronous motor such as an induction motor, the field
slips with respect to the rotor, so the field frame and rotor frame are separate.
Working in the Field Frame
The physics of motor operation are best understood in the field frame. A current vector in the stator that
is perpendicular to the rotor field (that is, current in the stator that produces a magnetic field perpendicular
to the rotor magnetic field) produces torque. This component of the stator current is known as quadrature
current. The output of the position/velocity loop servo algorithm is the magnitude of the commanded
quadrature current. For diagnostic purposes on PMAC2, the O command can be used to set a fixed
quadrature current command.
A current vector in the stator that is parallel to the rotor field induces current in the rotor that changes the
magnetic field strength of the rotor (when the stator and rotor field are rotating relative to each other).
This component of the stator current is known as direct current. For an induction motor, this is required
to create a rotor magnetic field. For a permanent-magnet brushless motor, the rotor magnets always
produce a field, so direct current is not required, although it can be used to modify the magnetic field
strength. On PMAC2, parameter Ix77 for motor x determines the magnitude of the direct current.
Analog Loops in the Stator Frame
In an amplifier with an analog current loop, the closure of the loops on the stator windings must be closed
in the stator frame. The current measurements are in the stator frame and analog circuitry has no practical
way to transform these. In such a system, the current commands must be transformed from the field
frame in which they are calculated to the stator frame, and converted to voltage levels representing the
individual stator phase current commands. These are compared to other voltage levels representing the
actual stator phase current measurements.
As the motor is rotating, and/or the field is slipping, these current values, command and actual, are AC
quantities. Overall loop gain and system performance, are reduced at high frequencies (high speeds).
The back EMF phase voltage, which acts as a disturbance to the current loop, is also an AC quantity. The
current loop integral gain or lag filter, which is supposed to overcome disturbances, falls well behind at
high frequencies.