User's Manual

PMAC2 User Manual

40 Setting Up PMAC2 Commutation (Direct PWM or Sine Wave)

Correcting Polarity Mismatch, Synchronous and Asynchronous Motors

To correct a commutation polarity mismatch, there are two possible options:

• Reverse the feedback direction sense by changing I9n0. However, this changes the direction sense of

the axis, which may not be tolerable.

• Reverse the output direction sense. For analog sine-wave output, this can be done by changing Ix72,

for example from 171 to 85, or by exchanging two phase leads between amplifier and motor. For

direct PWM, this must be done by exchanging phase leads. Usually, this is done at the screw

terminals on the amplifier. Exchanging any two phases will change the polarity in the same way.

However, the relationship between the sensor zero position and PMAC2’s commutation cycle zero

position is dependent on which two phases are exchanged.

After changing the polarity match by one of the above three methods, repeat the test to make sure that the

problem has been solved.

Establishing A Phase Reference (Synchronous Motors Only)

Purpose

Warning:

It is important for the safety of the machine that a reliable phase referencing

method be used, whether with an absolute or incremental sensor. If the phase

reference is incorrect by more than 1/4 of the phasing cycle, runaway will occur

when the servo loop is closed. Test the phase referencing carefully with a bare

motor before attaching a load, to make sure the method is reliable. Before

attaching a load, make sure that the PMAC2 fatal following error limit parameter

Ix11 and the amplifier overcurrent fault are active and working properly. Also

make sure that required mechanical protections are in place.

When commutating a synchronous multi-phase motor such as a permanent-magnet brushless motor, the

commutation algorithm must know the absolute position of the rotor. With an absolute sensor such as a

resolver, the phase referencing must just be done once, on assembly of the system. With an incremental

sensor such as an incremental optical encoder, the phase referencing must be done every time the system

is powered up. If incremental sensor power on signal is lost, even if controller power is retained, the

phase referencing must be done again before enabling the signal.

Hall-effect commutation sensors, or their equivalent on an optical encoder, are absolute, but of a very low

resolution (+

e). In a high-performance application, they are suitable to create a rough, temporary

phase reference, permitting movement until a more accurate reference is established.

The index pulse on an incremental encoder is absolute with high accuracy, but in general, there must be

movement before this pulse is reached. This requires at least a rough phasing, either from a low-

resolution absolute sensor such as hall-effect, or from a power-on phasing search.

Preparation

These tests require that both the commutation and current loop be working properly. Double-check that

the setup variables are correct for these actions, especially ones that may have been changed for earlier

tests. For motor 1, make sure:

• I100=1 to activate the motor

• I101=1 to enable commutation

• I170 and I171 are set to their proper value

For these tests, we will want access to the motor phase position register, where PMAC2 keeps track of

where it is in the phase cycle. The phase position register is 48 bits long, using both X and Y memory.