Rockwell Automation 1901 SyncPro User Manual

Page 29

Setup and Commissioning

4-3

1901-UM020

&-EN-P – -XQH

1 & R

2 Resistor Setup (cont.)

These settings must be made prior to any start attempt.
Determining the induced voltage which will appear across the
discharge resistor during starting can be done two ways:

a) 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
i.e. induced current @ 0 % speed

–

20 Amps.

induced current @ 95% speed

–

12 Amps.

discharge resistance

–

50 ohms

therefore,
induced voltage @ 0 speed - 20A x 50 ohms

1000V

induced voltage @ 95% speed - 12A x 50 ohms =

600V

b) A measurement can be taken using a storage oscilloscope or a

strip chart recorder, refer to Rockwell Automation 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.

NOTE: When doing this, a portion of the discharge resistor only
should be used, 1 ohm can then be used to determine the value
which will be on the entire resistor.

For example, a strip chart recording is taken across a 1 ohm
portion of a 50 ohm discharge resistor. The following peak to peak
values are obtained:

0 speed

– 56 V P-P

95% speed

– 34 V P-P

0 speed rms voltage across 1 ohm

= 56/2.828

= 20V

95% speed rms voltage across 1 ohm

= 34/2.828

= 12V

0 speed rms current across 1 ohm

= 20V/1ohm

= 20A

95% speed rms current across 1 ohm

= 12V/1ohm

= 12A

Once the induced voltage has been determined, make the appropriate
selection from Table 4.A. Wires from each end of the discharge
resistor should then be determined to the appropriate taps on the
R

1 and R

2 resistors. Both the 0 and 95% speed induced voltages

must fall between the upper and lower limits defined on the chart.