Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 270)

270 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 4 Motor Control
P690 [Limited Trq Ref ] - Limited Torque Reference
Displays the torque reference value after filtering (P689), power limits, torque
limits, and current limits have been applied. This parameter is the most effective
VFD representative Torque Reference value to be monitored for motor load
assessment and to be passed on to other drives for load sharing applications
involving multiple drives. It represents the percent of the rated torque being
developed at the motor shaft.
For additional and expanded illustration of the Torque Control, refer to the
PowerFlex 755 Control Block Diagrams starting on
page 375 .
Speed Torque Position Modes
Zero Torque
Operation in Zero Torque mode enables the motor to be fully fluxed and ready to
rotate when a speed command or torque command is given. This mode can be
used for a cyclical application where throughput is a high priority. The control
logic can select zero torque during the rest portion of a machine cycle instead of
stopping the drive. When the cycle start occurs, instead of issuing a start to the
drive, a Speed Regulator mode can be selected. The drive immediately accelerates
the motor without the need for flux up time.
Speed Regulation
Operating as a speed regulator is the most common and simplest mode to set up.
Examples of speed regulated applications are blowers, conveyors, feeders, pumps,
saws, and tools. In a speed regulated application, the speed regulator output
generates the torque reference. Note that under steady state conditions the speed
feedback is steady while the torque reference is a constantly adjusting signal. This
is required to maintain the desired speed. In a transient state, the torque reference
changes dramatically to compensate for a speed change. A short duration change
in speed is the result of increasing or decreasing the load very rapidly.
For the PowerFlex 755 drive, the Inertia Compensation, Inertia Adaption, and
the Friction Compensation influence the output of the speed regulator.
Torque Regulation
A torque regulated application can be described as any process requiring some
tension control. An example is a winder or unwinder with material being drawn
or pulled with a specific tension required. The process also requires that another
IMPORTANT Zero Torque can excessively heat the motor if operated in this mode for
extended periods of time. A load or flux current is still present when the drive is
operating in Zero Torque mode. A motor with an extended speed range or
separate cooling methods (blower) can be required.