Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 271)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 271
Motor Control Chapter 4
element set the speed. Configuring the drive for torque regulation requires P309
[SpdTrqPsn Mode A] to be set to 2 “Torque Ref.” In addition, a reference signal
must be linked to the torque reference. For example, when Analog Input 0 is used
for the torque reference, P675 [Trq Ref A Sel] needs to be configured for “Anlg
In0 Value.”
When operating in a Torque mode, the motor current is adjusted to achieve the
desired torque. If the material being wound or unwound breaks, the load
decreases dramatically and the motor can potentially go into a runaway
condition.
Speed Limited Adjustable Torque (SLAT) Modes
The SLAT minimum and SLAT maximum modes are for applications that
require a smooth transition from a torque mode to a speed mode operation and
vice versa. When operating in a Torque mode, the motor current is adjusted to
achieve the desired torque. For example; web handling, center winders and center
unwinders or other mechanical drive train where the drive is normally following a
torque reference, but a break, disruption in flow or slippage could occur causing
the need to prevent a runaway situation, which is best controlled in speed mode.
Direction of the applied torque and direction of the material movement
determine whether SLAT minimum or SLAT maximum mode should be used.
SLAT Minimum
Choose SLAT minimum mode when material direction and speed reference is
considered “Forward” and a positive speed reference value for the Speed
Regulator. The Speed Regulator output then creates a positive Torque Reference
command value.
Typically configure a positive speed reference value slightly greater than what is
equivalent to maintain planned material line speed. This will in turn force the
speed regulator into saturation (the speed reference is slightly above the speed
feedback) commanding a “more positive” torque reference than what the torque
mode torque reference value is. In this scenario the drive would follow the torque
reference until there was a breakage or slippage in the application.
When the drive is following a torque reference (torque mode in SLAT minimum
mode), either one of two conditions will force the drive into following the speed
reference in (speed mode):
• The output of the speed regulator becomes less than the torque reference.
The reaction when triggered at the very point that the torque reference
value in speed mode is mathematically less than the value in torque mode,
generally results in greater velocity overshoot. This is the same condition
that would exist in minimum torque /speed mode without SLAT features.
The following plot represents the result without using SLAT features.