User's Manual

PMAC User Manual
Setting Up PMAC Commutation 89
Permanent Magnet Brushless Motor Commutation
When commutating a permanent magnet brushless motor (often called a DC brushless motor, sometimes
called an AC synchronous motor), very little beyond the basic commutation cycle parameters noted above
must be specified.
Getting the Polarity Right
For proper commutation, it is required that the feedback polarity, as determined by the encoder wiring and
the Encoder Decode I-variable (I900, I905, etc.) match the output polarity, as determined by the amplifier
and motor wiring, and by Ix72. That is, when PMAC issues a positive command through its commutated
outputs, it must cause the encoder counter to count up. If the polarity is mismatched on a permanent-
magnet brushless motor, the motor will lock in quickly and refuse to move.
Testing the Polarity
A quick test early in the setup of the motor can verify whether the polarity is correct. The test uses the
output offset variable Ix29 and Ix79 to force current directly into the particular phases and drive the motor
like a stepper motor. Observing the direction the motor position counts as different phases are driven, we
can tell whether the polarity is correct, given the present encoder wiring and decode variable, and the
present motor wiring and output phasing variable.
This test, which can be done easily from the terminal window of the Executive program by typing in a
few simple commands, is best illustrated by an example, which will use Motor 1:
#
1O0 ; Command zero output
I129=2000 ; Positive offset of 2000 bits on 1st phase
P ; Request position (after motor settles)
382 ; PMAC responds with position
I179=2000 ; Positive offset of 2000 bits on 2nd phase
P ; Request position (after motor settles)
215 ; PMAC responds with position
Polarity Rule
With I172 equal to 64 or 85, the motor should have moved in the negative direction when the second-
phase positive offset was added on top of the first-phase positive offset (as it did in this example). With
I172 equal to 171 or 192, the motor should have moved in the positive direction. Alternatively stated, if
the motor counted down in the test, I172 should be set at 64 or 85; if it counted up in the test, I172 should
be set at 171 or 192.
For the motor in this example, we conclude that we want a value of 64 if it is a 4-phase motor or 85 if it is
a 3-phase motor. If the encoder direction is changed for system reasons, I172 should be changed as well,
to match.
Power-on Phasing Search
Warning:
An unreliable phasing search method can lead to a runaway condition. Test the
phasing search method carefully to make sure it works properly under all
conceivable conditions. Make sure the Ix11 fatal following error limit is active
and as tight as possible so the motor will be killed quickly in the event of a serious
phasing search error.