Process PI Loop 2-121
Pre-load command may be used when the PI has exclusive control of
the commanded speed. With the integrator preset to the commanded
speed there is no disturbance in commanded speed when PI is
enabled. After PI is enabled the PI output is regulated to the required
level.
When the PI is configured to have exclusive control of the
commanded speed and the drive is in current limit or voltage limit the
integrator is preset to the commanded speed so that it knows where to
resume when no longer in limit.
• Ramp Ref - The PI Ramp Reference feature is used to provide a
smooth transition when the PI is enabled and the PI output is used as a
speed trim (not exclusive control),.
When PI Ramp Reference is selected in the PI Configuration
parameter, and PI is disabled, the value used for the PI reference will
be the PI feedback. This will cause PI error to be zero. Then when the
PI is enabled the value used for the PI reference will ramp to the
selected value for PI reference at the selected acceleration or
deceleration rate. After the PI reference reaches the selected value the
ramp is bypassed until the PI is disabled and enabled again. S-curve is
not available as part of the PI linear ramp.
• Zero Clamp - This feature limits the possible drive action to one
direction only. Output from the drive will be from zero to maximum
frequency forward or zero to maximum frequency reverse. This
removes the chance of doing a “plugging” type operation as an
attempt to bring the error to zero.
The PI has the option to limit operation so that the output frequency
will always have the same sign as the master speed reference. The
zero clamp option is selected in the PI Configuration parameter. Zero
clamp is disabled when PI has exclusive control of speed command.
For example, if master speed reference is +10 Hz and the output of
the PI results in a speed adder of –15 Hz, zero clamp would limit the
output frequency to not become less than zero. Likewise, if master
speed reference is –10 Hz and the output of the PI results in a speed
adder of +15 Hz, zero clamp would limit the output frequency to not
become greater than zero.
Pre-load to Command Speed
PI Enabled
Spd Cmd
PI Output
Start at Spd Cmd
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