Reference Manual

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PMAC 2 Software Reference

108 PMAC I-Variable Specifiation

Ix78 Motor x Induction Motor Slip Gain

Range

0 .. 8,388,607 {PMAC(1)}

0.0.. 1.0 (24-bit resolution) {PMAC2}

Units

(electrical cycles/update)/DAC bit {PMAC(1)}

Unitless (ratio of times) {PMAC2}

Default

Remarks

Ix78 controls the relationship between the torque command and the slip frequency of

magnetic field on the rotor of an AC asynchronous (induction) motor. While it is usually

set experimentally, it can be calculated as the ratio between the phase update period and

the rotor (not stator) L/R electrical time constant.

Ix78 is only active if Ix01 is set to 1 to specify PMAC2 commutation of Motor x. It should

be set to 0 for AC synchronous motors such as permanent-magnet brushless motors and

switched (variable) reluctance motors.

Ix78 operates slightly differently on PMAC(1) and PMAC2 boards.

PMAC(1) computes the slip frequency each phase update by multiplying the torque

command from the position/velocity-loop servo (or O-command magnitude) by Ix78. The

optimum value of Ix78 is dependent of the value of the Ix77 magnetization current, so if

Ix77 is changed (e.g. for field weakening), Ix78 should be changed in opposite proportion

so that the product of Ix77 and Ix78 stays constant.

PMAC2 computes the slip frequency each phase update by multiplying the torque

command from the position/velocity-loop servo (or O-command magnitude) by Ix78 and

then dividing by the magnetization current value controlled by Ix77. This makes the

optimum value of Ix78 independent of the value of Ix77, so changing the value of Ix77 for

field control does not require changes in Ix78.

Ix78 is typically set on a PMAC2 through use of the P2SETUP expert system program

running on a PC. P2SETUP excites the motor and analyzes its response to derive an

optimum Ix78 value.

Ix78 can also be set experimentally by giving the motor an O-command and watching the

velocity response, probably with the data gathering feature. As the velocity saturates

because the back EMF reaches the supply voltage, the velocity should fall back about 5%

to reach a steady-state value. If it falls back more than this, the slip time constant is too

high; if it falls back less than this, or not at all, the slip time constant is too low.

On a PMAC(1), 1200 is a typical value of Ix78 for an standard induction motor at a phase

update rate of about 9 kHz.

On a PMAC2, 0.00015 is a typical value of Ix78 for an standard induction motor at a phase

update rate of about 9 kHz.

If Ix78 is greater than zero, no power-on phasing search will be done (because the rotor

field is not fixed to the rotor).