Power loss ride through – Rockwell Automation 20B PowerFlex 700 AC Drives Vector Control (v4.001 and up) User Manual
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Rockwell Automation Publication 20B-UM002G-EN-P - July 2014
133
Application Notes
Appendix C
Power Loss Ride Through
When AC input power is lost, energy is being supplied to the motor from the
DC bus capacitors. The energy from the capacitors is not being replaced (via the
AC line), thus, the DC bus voltage will fall rapidly. The drive must detect this fall
and react according to the way it is programmed. Two parameters display DC bus
voltage:
• [DC Bus Voltage] - displays the instantaneous value
• [DC Bus Memory] - displays a 6 minute running average of the voltage
All drive reactions to power loss are based on [DC Bus Memory]. This averages
low and high line conditions and sets the drive to react to the average rather than
assumed values. For example, a 480V installation would have a 480V AC line and
produce a nominal 648V DC bus. If the drive were to react to a fixed voltage for
line loss detect, (that is, 533V DC), then normal operation would occur for
nominal line installations. However, if a lower nominal line voltage of 440V AC
was used, then nominal DC bus voltage would be only 594V DC. If the drive
were to react to the fixed 533V level (only –10%) for line loss detect, any anomaly
might trigger a false line loss detection. Line loss, therefore always uses the 6
minute average for DC bus voltage and detects line loss based on a fixed
percentage of that memory. In the same example, the average would be 594V DC
instead of 650V DC and the fixed percentage, 27% for “Coast to Stop” and 18%
for all others, would allow identical operation regardless of line voltage.
The PowerFlex 70 uses only these fixed percentages. The PowerFlex 700 can
selectively use the same percentages or the user can set a trigger point for line loss
detect. The adjustable trigger level is set using [Power Loss Level], see
.
Figure 16 - Power Loss Mode = Coast
Figure 17 - Power Loss Mode = Decel
Nominal
73%
Bus Voltage
Motor Speed
Output Enable
Power Loss
Nominal
82%
Bus Voltage
Motor Speed
Output Enable
Power Loss