User's Manual

PMAC User Manual
96 Setting Up PMAC Commutation
Switched Reluctance Motor Commutation
To the PMAC commutation algorithm, a switched (variable) reluctance motor can look the same as a
permanent magnet DC brushless motor. The difference is in the amplifier. Because the phases of an SR
motor are driven uni-directionally, the power stage can be simpler.
However, the analog pre-driver circuitry must convert the bidirectional nature of the PMAC outputs.
First, the extra phases is generated from the commands alone (not from any actual current information, as
is desirable for DC brushless and AC induction motors). Then each of the phase current command signals
must be half-wave rectified before being sent to the current loop, because in an SR motor, during half of
the commutation cycle, any current in either direction in a phase works against you.
AC Induction Motor Commutation
PMAC can drive standard AC induction motors as position servos by a technique known as indirect
vector control. PMAC continually estimates the orientation of the rotor magnetic field and orients the
current in the stator phases in order both to induce rotor current and to create torque.
The algorithm turns out to be the same as that for the DC brushless motor, but with the addition of two
more terms. One is the magnitude of the magnetization (inducing) current (Ix77), which is kept parallel
to the estimated rotor field, and a slip gain term (Ix78), which determines how much the estimated rotor
field angle is advanced in response to each unit of stator torque current (which is kept perpendicular to the
estimated rotor field).
Setting the Slip Gain
There is a simple technique for setting the slip gain for an induction motor. Slip gain is the constant of
proportionality between applied torque and slip frequency. If the matching values of slip frequency and
torque are known, divide the former by the latter, and convert units, to get the slip gain. Get all of the
information needed from nameplate information for the motor, amplifier, and controller.
Motor Information
From the motor, obtain the following information:
• Rated (full load) speed in RPM
• Rated line (field) frequency in Hertz
• Number of poles
• Rated (full load) current (RMS)
From this information, compute the rated (full-load) slip frequency of the motor as the difference between
the field frequency and the rotor frequency:
2
poles#
*
minsec/60
)RPM(speedRated
)Hz(freqField)Hz(freqSlip −=
Use the current information later.
Amplifier Information
From the amplifier, obtain the maximum (overload) RMS current as a percentage of the motor’s rated
full-load RMS current.
Controller Information
From PMAC, obtain:
• The frequency at which the phasing calculations are done
• Number of peak output (DAC) bits required to command the amplifier to its maximum RMS current
• The constant of proportionality in the equation relating slip frequency, applied torque, and slip gain.
The first piece of information will allow conversion of the rated slip frequency to units of electrical cycles
per phase update – the slip units PMAC uses: