Rockwell Automation 1902 Syncpro II - Manual (Page 32)

34 Rockwell Automation Publication 1902-IN001B-EN-E - April 2013
Chapter 4 Setup and Commissioning
Determining the induced voltage which will appear across the discharge resistor
during starting can be done two ways.
1. If motor data is available the voltage can be determined by multiplying the
discharge resistance by the induced currents at zero and 95% speed as given
by the motor manufacturer.
2. A measurement can be taken using a storage oscilloscope or a strip chart
recorder, see publication 1900-2.10 for correct set point values. The
waveform obtained will have a peak value which must be converted to an
rms value. This is done by dividing the peak-to-peak value by 2
√
2 or 2.828.
When doing this, a portion of the discharge resistor only should be used, 1
Ω can
then be used to determine the value which will be on the entire resistor.
Once the induced voltage has been determined, make the appropriate selection
from
Table 1 on page 36 . Wires from each end of the discharge resistor should
then be determined to the appropriate taps on the R F 1 and R F 2 resistors. Both
the 0 and 95% speed induced voltages must fall between the upper and lower
limits defined on the chart.
EXAMPLE Induced current @ 0% speed: 20 A
Induced current @ 95% speed:12 A
Discharge resistance: 50 Ω
Therefore:
Induced voltage @ 0% speed: 20A x 50 Ω = 1000V
Induced voltage @ 95% speed: 12A x 50 Ω = 600V
EXAMPLE A strip chart recording is taken across a 1 Ω portion of a 50 Ω discharge
resistor. The following peak to peak values are obtained:
0 speed: 56V p-p
95% speed: 34V p-p
Therefore:
0 speed rms voltage across 1 Ω 56 / 2.828 = 20V rms
95% speed rms voltage across 1 Ω 34 / 2.828 = 12V rms
0 speed rms current across 1 Ω 20V / 1 Ω = 20A rms
95% speed rms current across 1 Ω 12V / 1 Ω = 12A rms