User's Manual

PMAC2 User Manual
22 Setting Up PMAC2 for Direct PWM Control
If a non-zero value of Ix77 magnetization current will be used, for induction motor control or for field
weakening of a permanent-magnet brushless motor, then Ix69 should be replaced in the above equation
by
.
In early testing, it may be desirable to set Ix69 to an artificially low value to prevent accidental
overcurrent commands into the motor.
Continuous Current Limit: Ix57
Ix57 specifies the continuous current limit for the motor/drive system for I
2
T protection. Almost always
it is the continuous current rating of the drive that is used for an axis. Even if its continuous current rating
is somewhat higher than that of the motor, its thermal time constant is usually so much shorter that the I
2
T
parameters are set to protect it first. Ix57 is calculated in a manner similar to Ix69:
768,32*
urrentFullRangeC
tLimitContCurren
57Ix =
Integrated Current Limit: Ix58
Ix58 sets the permitted limit of the time-integrated current over the continuous current value. If the time-
integrated current exceeds this threshold, PMAC2 will kill this axis as it would for an amplifier fault.
Typically, this parameter is set by noting the drive specification for time permitted at the instantaneous
current limit, and using this specification in the following equation:
(sec)imePermittedT*)Hz(eRateServoUpdat*
2
768,32
2
57Ix
2
77Ix
2
69Ix
58Ix
−+
=
Refer to the section Making the Application Safe in the User’s Guide for a more detailed explanation of
I
2
T protection.
Commutation Cycle Size: Ix70, Ix71
Caution:
It is very important to set the value of Ix72 properly for the system; otherwise the
current loop will have unstable positive feedback and want to saturate. This could
cause damage to the motor, the drive, or both, if overcurrent shutdown features do
not work properly. If unsure of the current measurement polarity in the drive,
consult the Testing PWM and Current Feedback Operation section of this manual.
Ix70 and Ix71 define the size of the commutation cycle (electrical cycle). The cycle is equal to Ix71
divided by Ix70, expressed in encoder counts (after decode). Ix70 and Ix71 must both be integers, but the
ratio Ix71/Ix70 does not have to be an integer. Typically, on a rotary motor, Ix71 is set to the number of
counts per mechanical revolution, and Ix70 is set to the number of pole-pairs (half of the number of poles)
for the motor, which is equal to the number of commutation cycles per mechanical revolution.
Commutation Phase Angle: Ix72
Ix72 controls the angular relationship between the phases of a multiphase motor. When PMAC2 is
closing the current loop digitally for motor x, the proper setting of this variable is dependent on the
polarity of the current measurements.
If the phase current sensors and ADCs are set up so that a positive PWM voltage command for a phase
yields a negative current measurement value, Ix72 must be set to a value less than 128: 85 for a 3-phase
motor, or 64 for a 2- or 4-phase motor. If these are set up so that a positive PWM voltage command
yields a positive current measurement value, Ix72 must be set to a value greater than 128: 171 for a 3-
phase motor, or 192 for a 2- or 4-phase motor. The testing described below will show how to determine
the proper polarity.