Bus Voltage High
The DC bus voltage in the power supply has exceeded the normal operating range, typically above 410VDC for 400V bus systems. This condition affects the PFC stage regulation and can damage downstream components if not corrected promptly.
Error Code
F905
Severity
Fault
Equipment
Power Supply
Manufacturer
ABB
Red fault LED illuminated continuously on power supply front panel
DC bus voltage measurement reads 420-450VDC when normal is 385-405VDC
Connected motor drives displaying overvoltage alarms F0001 or similar codes
Audible clicking from internal protection relays cycling on/off
Output voltage regulation becomes unstable with ±5% fluctuations
PFC stage switching frequency increases to 85-90kHz from normal 65kHz
Failed voltage feedback optocoupler U3 in control circuit providing incorrect reference signal
Defective PFC control IC UC3854 with damaged voltage error amplifier section
Open circuit in voltage divider resistors R15/R16 (typically 100kΩ/10kΩ) in feedback network
Shorted PFC boost diode D5 causing uncontrolled voltage rise during PFC operation
Input line voltage exceeding specification at 520VAC when rated for 480VAC maximum
Failed bus voltage sensing circuit with damaged precision reference IC LM431
Implement lockout/tagout procedures and verify zero energy state with calibrated multimeter
Discharge DC bus capacitors through 10kΩ 25W resistor until voltage reads below 50VDC
Measure DC bus voltage at test points TP1/TP2 and compare to nameplate specifications
Test PFC control IC UC3854 pins 1,2,8 for correct reference voltages of 7.5V, 2.5V, 5V respectively
Check voltage feedback optocoupler U3 with ohmmeter - should read 1-2kΩ LED side, infinite photodiode side when energized
Replace faulty PFC control IC and associated feedback components per manufacturer service bulletin
Energize system and verify DC bus voltage stabilizes at 400VDC ±3% with oscilloscope monitoring for 15 minutes
Safety First: Always isolate power and follow LOTO (Lockout/Tagout) procedures before working on electrical components. If unsure, contact a certified repair technician.
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