Bus Voltage High
The DC bus voltage in the servo drive system has exceeded the upper threshold limit, typically above 400VDC in a 380V system. This condition affects the servo amplifiers and spindle drives, causing protective shutdown to prevent component damage from overvoltage stress on capacitors and switching devices.
Error Code
E7848
Severity
Fault
Equipment
CNC Controller
Manufacturer
Bosch Rexroth
Red fault LED illuminated on servo amplifier display with voltage reading above 410VDC
CNC control panel displays 'SERVO ALARM 401 - BUS OVERVOLTAGE' message
All servo axes immediately stop with regenerative braking disabled
Spindle motor decelerates uncontrolled without dynamic braking
DC bus voltage meter reads 420-450VDC instead of normal 350-380VDC range
Audible relay clicking from main contactor cycling on overvoltage protection
Faulty regenerative braking resistor (>10% resistance increase) or open circuit in braking IGBT
AC input voltage fluctuation above 420VAC due to utility supply instability or transformer tap issues
Defective DC bus capacitors with reduced capacitance below 80% of rated value causing voltage spikes
Malfunctioning voltage feedback circuit in servo amplifier with drift in reference voltage divider
Rapid deceleration programming without proper ramp rates overloading regenerative capacity
Failed surge suppression devices allowing transient overvoltages from switching operations
Implement LOTO procedure - disconnect main power, verify zero energy state with multimeter on all circuits
Measure AC input voltage at main terminals - should be 380-400VAC, check all three phases for balance within 2%
Test regenerative braking resistor with ohmmeter - should read 27-33 ohms, replace if open or >35 ohms
Check DC bus capacitor bank with capacitance meter - replace if below 80% of rated value or ESR >0.5 ohms
Inspect and test braking IGBT module with multimeter diode test - replace if short circuit or open gate detected
Verify servo drive voltage sensing circuit calibration - adjust reference voltage to 2.50VDC ±0.02V if drifted
Power up system and monitor DC bus voltage under load - verify 350-380VDC range during normal operation and <400VDC during regeneration
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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