If you run KUKA robots with a KRC4 controller, sooner or later you'll come across a DC bus fault. It's one of the most frequent failures of this generation and, although it sounds alarming, it has a fairly systematic diagnosis.
What the DC bus is on a KRC4
On the KRC4, the power supply (KPP) rectifies the mains and generates a direct-current bus. The servo amplifiers (KSP) that drive the motors are powered from that DC bus. A DC bus fault means the bus voltage is not what it should be: neither too low, nor too high, nor unstable.
Common symptoms
- An error referring to the DC link or DC bus on the smartPAD.
- The robot won't start or the drive enable drops out.
- Intermittent trips under load or when accelerating several axes at once.
Most common causes
- KPP (supply): degraded capacitors, damaged rectifier or braking resistor.
- KSP (servo pack): a short-circuited module pulling down the bus voltage.
- DC wiring and connectors: loose or oxidised connections on the DC bus.
- Motor or motor cable: a short to ground reflected on the bus.
How it's diagnosed
The key is to isolate the source instead of swapping parts blindly. The typical process: review the controller's event history, check whether the fault appears with all KSPs or just one, measure the bus and verify the KPP's condition. Only when the responsible component is identified is it replaced.
Replacing the KPP when the problem is in a KSP (or vice versa) is the most expensive mistake: the fault persists and downtime doubles.
Solution and parts
Depending on the diagnosis, the KPP or KSP is replaced or the wiring/motor is repaired. To resume production as soon as possible, we work with both new and reconditioned and tested parts, under warranty. See the detail of our service on KUKA robot maintenance.
Frequently asked questions
What is the DC bus on a KRC4?
The DC bus that powers the KSPs from the KPP. A fault means its voltage is not correct, due to the KPP, the KSP, the wiring or a motor.
Do I replace the KPP or the KSP?
It depends on the diagnosis. You must isolate the responsible component; swapping blindly increases the cost.
Are reconditioned parts OK?
Yes. Reconditioned and tested KPP/KSP let production resume immediately, with warranty and at a lower cost.