Bus Voltage Low
The DC bus voltage has dropped below the minimum operating threshold, typically below 350VDC for 400V systems. This condition indicates problems with the PFC boost stage or input power quality affecting the ability to maintain proper DC voltage levels.
Error Code
024
Severity
Fault
Equipment
Power Supply
Manufacturer
Mitsubishi
Yellow undervoltage LED flashing at 2Hz rate on power supply status panel
DC bus voltage measurement reads 280-340VDC when specification requires 385-405VDC
Connected equipment shutting down intermittently with low voltage protection active
Input current drawn increases by 15-20% above nameplate rating during operation
PFC inductor L1 producing audible 120Hz magnetostriction noise
Output ripple voltage increases to 8-12V peak-peak from normal 2-3V measurement
Defective PFC boost inductor L1 with partial winding short reducing inductance by 30-40%
Failed PFC switching MOSFET Q1 with increased on-resistance above 0.8Ω specification
Degraded electrolytic capacitors C3/C4 in DC bus with ESR exceeding 0.5Ω at 120Hz
Input line voltage sagging below 440VAC during peak load conditions
Open PFC boost diode D2 preventing proper energy transfer to DC bus
Contaminated PFC control IC UC3854 with moisture causing erratic switching operation
Execute lockout/tagout procedure and verify all energy sources isolated with appropriate test equipment
Use insulated tools to safely discharge DC bus through appropriate load bank resistor
Measure AC input voltage at terminals L1, L2, L3 during load conditions with true-RMS meter
Test PFC inductor L1 inductance value with LCR meter - should read 1.2mH ±10% at 1kHz
Check PFC MOSFET Q1 with curve tracer for gate threshold voltage 2-4V and on-resistance specification
Replace failed PFC components including inductor, MOSFET, and boost diode with exact OEM parts
Commission system with gradual load application while monitoring DC bus voltage regulation and ripple content
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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