Delta Tau GEO BRICK LV User Manual

Commutation Phase Angle: Ixx72 
Ixx72 sets the angle from Phase A to Phase B as a fraction of the commutation cycle.  Turbo PMAC splits 
the commutation cycle (360

o

e) into 2048 parts.  For a 3-phase motor, the angle from A to B is either 1/3 

of a cycle (Ixx72=683) or 2/3 of a cycle (Ixx72=1365).  For a 2-phase or 4-phase motor, the angle from A 
to B is either 1/4 of a cycle (Ixx72=512) or 3/4 of a cycle (Ixx72=1536). 

The proper choice of Ixx72 is dependent on the commutation feedback encoder’s direction sense as 
determined by its wiring and the encoder decode variable I7mn0, and on the wiring of the phases of the 
motor.  This choice is generally determined experimentally through a test explained below.  Changing 
Ixx72 between, say, 683 and 1365 is equivalent to exchanging two phase wires of the motor. 

A quick test can establish basic operation of the commutation outputs, the drive and motor, and the 
feedback.  The test uses the output-offset variables Ixx29 and Ixx79 to force current directly into the 
particular phases and drive the motor like a stepper motor.  The test can be used to verify that – 

• 

Output voltages are obtained from the Acc-8E board 

• 

Currents flow in the phases of the motor 

• 

The currents cause the motor to lock into a position (it only does this well for a synchronous motor), 
and to set the proper polarity of the Ixx72 commutation phase angle parameter (it only does this well 
for a synchronous motor; for an asynchronous induction motor, the polarity of Ixx72 may have to be 
determined by trial and error). 

Executing the Test 

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.  It should first be done 
on a bare motor with no load for safety reasons: 

M101->X:$078001,24,S 

; Encoder 1 phase position register 

#1O0 

; Command zero output 

I129=2000 

; Positive offset of 2000 bits on 1st phase 

M101 

; Request position (after motor settles) 

382 

; PMAC responds with position 

I179=2000 

; Positive offset of 2000 bits on 2nd phase 

M101 

; Request position (after motor settles) 

215 

; PMAC responds with position 

If the servo-loop feedback has been established already with Ixx03, the position query P command or the 
position window in the Executive program can be used instead of the Mx01 encoder position register. 

Verifying Basic Operation 

Setting a non-zero value for Ixx29 should force a voltage on the A-phase DAC, which can be read with a 
voltmeter or oscilloscope.  It should also force current in the matching phase of the motor, which can be 
measured with a current probe.  Setting a non-zero value for Ixx79 should do the same for the B-phase DAC. 

A synchronous motor should lock into a position and hold it when an Ixx29 offset is given.  An induction 
motor may lock in briefly for a brief period of time due to short-term eddy currents in the rotor. 

When the Ixx79 offset is added, a synchronous motor should lock into a new position a fraction of a cycle 
away from the earlier position.  An induction motor may do this also, but probably not as strongly.