Rockwell Automation SD3000 Drive Configuration, Programming - Manual (Page 103)

Appendix F
Armature Current Feedback Resolution
Current feedback normally has between 12 and 13 bits of resolution. This resolution may be degraded if a
Power Module is misapplied. The Programming Executive software will warn you if this can happen because
of any parameter entry errors you make.The following explains how current feedback scaling is done and
what you can do to avoid this problem.
Data Acquisition
Current feedback is measured by a pair of current transformers (CTs) located in the Power Module. The D-C
Power Technology Module rectifies this signal and measures the voltage drop across a 10.2 ohm burden
resistor. The value of this burden resister may not be changed; therefore, to compensate for this inflexibility, a
programmable gain amplifier (PGA) is used to scale the voltage across the burden resister. The scaled
voltage is then applied to a voltage to frequency converter (V/F) that produces a range of 0 to 2 MHz. The
output of the V/F increments a counter at the rate of the frequency produced. The number of counts is
accumulated from SCR firing to SCR firing to provide a time-averaged current signal.
Valid Gain Test
The programming terminal software calculates the gain value that will be written to the PGA. The gain
depends on three parameters entered during configuration: the current transformer’s turn ratio, the full load
current, and the maximum current limit. It is calculated by the following formula:
Gain = (CT * 255) / (10.2 * I-full * (I-limit + 75%))
where: CT = current transformer turns ratio
255 = maximum gain allowed
10.2 = burden resister value
I full = full load current
maximum current limit
75% = over current limit
The amount of gain is limited to a range of 26 to 255. A gain of less than 26 will not be accepted and the user
will be required to change the value of one of the three parameters (as allowed by the application). If the gain
is greater than 255, the programming terminal will display a warning that the maximum gain value has been
exceeded. The gain will be limited to 255 and the resolution of current feedback will be reduced.
For example, if the CT ratio is 2000:1, full load current is 100 amps, and maximum current limit is 150%, then:
Gain = (2000 * 255) / (10.2 * 100 * (1.50 + 0.75)) = 222
Since the gain is within the acceptable range, it is not limited. When the gain is not limited, the V/F full scale
of 2 MHz represents maximum current limit plus 75%. With a line frequency of 60 Hz, this results in a
maximum of 5555 counts per SCR firing. With a line frequency of 50 Hz, this results in 6666 counts per SCR
firing.
As the full load current value decreases, the gain must be increased to maintain the same resolution of
current feedback. If the example above were changed so full load current were 10 amps, the calculated gain
would be 2222. However because the gain is limited to 255, the V/F can never reach 2 MHz. In this example,
the 10 amps * 225% results in 637 counts, or approximately 9 bits of resolution. When the gain is limited, the
maximum number of counts that will be read can be found with the following formula:
Maximum Counts - I-full * 2.25 * 10.2 * 5555 / CT
This assumes current limit is 150% and the A-C line frequency is 60 Hz.
F-1