Rockwell Automation 7000 PowerFlex Medium Voltage AC Drive (B Frame) - Classic Control - Manual (Page 274)

5-20 Functional Description
7000-UM150I-EN-P – June 2013 7000 “B” Frame
Off line Detection of Failed SCRs/SGCTs
• Line Converter – 6P-SCR, 18P-SCR and PWM
The rectifier diagnostics are performed when medium voltage is
first applied by closing the input contactor and when the drive
receives a start command. From 4.001 onwards, the drive also
performs off line diagnostics when a drive reset command is
issued. These diagnostics are capable of detecting a bad device,
loss of feedback fiber optic and loss of gating fiber optic. The
diagnostics consist of two stages. A passive diagnostic test
followed by an active diagnostic test. In the passive diagnostics
test no devices are gated.

SCR Rectifier Passive Off-Line Diagnostics
For SCR rectifiers, when the line voltage is applied to the drive
but the drive is not running, the voltage across the line converter
thyristors is high and positive for half cycle except during
intervals around the zero of the line voltage. The gate driver
transmits light whenever the device is forward biased with a
large enough voltage as shown in Figure 5.9. Since the drive is
not gating (no light) the feedback normally toggles state every
cycle of the utility voltage. However the feedback will not
toggle state if the device is shorted, or if the feedback fiber-optic
path is incomplete. If this occurs, the drive faults and issues an
Offline SC fault for the device.

PWM Rectifier Passive Off-Line Diagnostics
For PWM rectifiers the feedback for these devices should always
be high. However a high feedback level will not be received if a
device is shorted or if the feedback fiber-optic path is incomplete.
The drive presumes the device has failed and determines which
devices would be safe to gate for the more detailed active offline
diagnostic test.
Power Semiconductor
Diagnostics (cont.)