User's Manual

PMAC2 User Manual

24 Setting Up PMAC2 for Direct PWM Control

If there is only a permanent magnet field, the armature current reading can either be fed into ADC A or

ADC B, with Ix82 telling PMAC2 which one is used. If ADC A is used, the serial digital input for ADC

B (+signal only) should be tied to GND so a zero feedback value is forced (alternately a background PLC

can periodically zero the direct current integrator register). If ADC B is used, then PMAC uses the

Compare A read/write register for the (non-existent) direct current feedback; in this case, a zero value

should be written into this register on power-on/reset, and no other value should be written to it during the

application.

If there is a wound field, it must be commanded from PMAC2’s B phase, and the current feedback must

be brought back into PMAC2’s B-phase. If only uni-directional voltage and current are required for the

field, the field windings can be commanded from a single half-bridge. If bi-directional voltage or current

is required, a full H-bridge must be used, with PMAC2’s B-phase commanding the two half bridges in

anti-phase mode (the command for the top of one half is also used for the bottom of the other half).

I-Variable Setup

To set up a motor for this technique, the following I-variable settings must be made:

• Ix00 = 1 to activate the motor

• Ix01 = 1 to activate commutation algorithms

• Ix02 should contain the address of the PWM A register for the output channel used (this is the

default), just as for brushless motors

• Ix70 = 4, Ix71 = 4: This defines a commutation cycle size of 4/4 = 1 count. The use of 4/4 instead of

1/1 allows us to rotate the angle +

for test and tuning purposes.

• Ix72 = 64 or 192 for 1/4 or 3/4 cycle between the armature (rotor) and field (stator). If a positive

voltage output number creates a negative current feedback number, use 64; otherwise use 192.

• Ix73 = 0, Ix74 = 0: No power-on phase search will be required

• Ix75 = 0: Zero offset in the power-on phase reference

• Ix77 = 0 for motors without wound field. With a wound field, Ix77 determines the strength of the

field; with field weakening functionality, Ix77 will be a function of motor speed.

• Ix78 = 0 for zero slip in the commutation calculations

• Ix81 = $80770: This tells PMAC2 to read the low eight bits of Y:$0770 for the power-on phase position.

This register is forced to zero on power-on/reset, so this setting forces the phase position to zero.

• Ix82 should contain the address of ADC B register for the feedback channel used (just as for

brushless motors) when the ADC A register is used for the rotor (armature) current feedback. If there

is a wound field, the stator field current feedback should be connected to ADC B. If there is a

permanent magnet field, there will be no feedback to ADC B.

• Ix82 should contain the address one greater than that of the ADC B register for the feedback channel

used when the ADC B register is used for the rotor (armature) current feedback. This is suitable for

motors with only a permanent magnet field.

• Ix83 does not really matter here, because the commutation position is defeated by the single-count

cycle size. However, it is fine to use the default value.

• Ix84 is set just as for brushless motors, specifying which bits the current ADC feedback uses.

• Ix61, Ix62, and Ix76 current loop gains are set just as for brushless motors. Before using the current

loop tuning aids for the rotor current, force the commutation phase angle to 90

by setting the phase

position register Mx71 to 1 (out of 4). Remember to set it back to 0 before actual operation.