2-172 Speed Reference
DPI
See the DPI on page 2-83 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.
Using the above formula, calculate the Speed Reference sent from a
network using a DPI adapter.
SpeedRef =
CommandFreq
[Maximum Freq]
x 32767
SpeedRef =
60 Hz
130 Hz
x 32767 = 15123
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