Rockwell Automation 20B PowerFlex 70, PowerFlex 700 Reference Manual User Manual
Page 168
Speed Reference
168
Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013
DPI
See the
for a description of DPI. One of the DPI ports can be
selected as the source of the speed reference.
In the PowerFlex 70, 700, and 700VC the speed reference from DPI is scaled so
that [Maximum Freq] = 32767. [Maximum Freq] is the largest output frequency
that the drive will deliver to the motor.
Additionally, the PowerFlex 70 and 700 drives have a parameter called
[Maximum Speed]. [Maximum Speed] limits the drive speed reference, such as
from a communication network or analog input.
PowerFlex drives contain the following necessary rule:
[Maximum Speed] + [Overspeed Limit] = [Maximum Freq]
[Overspeed Limit] allows the drive to operate above [Maximum Speed] for
certain functions such as bus regulation, current limit (during regeneration), PI
control, and slip compensation. It is important that [Overspeed Limit] is set to
allow enough headroom for the application. For example, let’s assume we have an
application where [Speed Mode] = “Slip Comp”. Slip compensation adds some
frequency to the commanded speed in order to compensate for slip in a loaded
motor. In this case, [Overspeed Limit] should not be set to 0. Otherwise, if the
drive is running with a commanded frequency of 60 Hz and the motor is loaded
at all, slip compensation will add some frequency and we would get a nuisance
“Overspeed” fault.
Defaults are as follows:
•
[Maximum Speed] = 60 Hz
•
[Overspeed Limit] = 10 Hz
•
[Maximum Freq] = 130 Hz (this is default so that users who want to go twice
base speed don't have to change it).
To send out a speed reference to the drive from a controller over RIO, you can
perform the following calculation:
For example, to send out a command frequency of 60 Hz on a PowerFlex 70 or
700 with default settings we would calculate the following:
The following example illustrates how a change in P55 [Maximum Freq] in the
PowerFlex 70 or 700 affects the speed reference scaling:
[Overspeed Limit] = 10 Hz (this is factory default)
[Maximum Speed] = 60 Hz (this is factory default).
[Maximum Freq] = [Maximum Speed] + [Overspeed Limit] = 60 Hz + 10 Hz = 70 Hz.
SpeedRef =
CommandFreq
[Maximum Freq]
x 32767
SpeedRef =
60 Hz
130 Hz
x 32767 = 15123