F07802 — “Drive: infeed or power unit not ready” — is one of the most common alarms we get called for on SINAMICS S120 systems, and one of the most misdiagnosed. The fault text points at the motor module; the cause almost never lives there.
What the fault actually says
On an S120 drive lineup, motor modules expect the infeed (ALM/SLM/BLM) to report ready before they will enable. F07802 on a motor module simply means: “I was asked to enable, but the DC bus feeding me isn’t ready.” It is a downstream symptom of an upstream condition.
The field checklist
Work top-down, and resist swapping hardware until step 5:
- Is the infeed itself faulted? Check the ALM/SLM for its own fault (F30027 precharge, F06310 supply voltage, mains phase alarms). Fixing the infeed fault usually clears F07802 everywhere.
- Precharge circuit. If the DC bus never reaches ~1.35 × line voltage, look at precharge resistors and bypass contactor. A tired precharge contactor produces intermittent F07802 that “goes away” after a power cycle — until it doesn’t.
- Enable chain and 24 V. The infeed needs its own enables (EP terminals, OFF1). A dropped safety relay or a browning-out 24 V supply during contactor pull-in mimics a drive fault perfectly.
- DRIVE-CLiQ topology. If the infeed’s ready signal never reaches the motor module (p0864 interconnection), modules report not-ready with a healthy DC bus. Check this after any component swap or firmware mismatch.
- Only now suspect hardware. Measure the actual DC bus voltage, compare with r0070, and check the infeed’s temperature and fan before condemning a module.
The pattern behind the pattern
Roughly in order of what we actually find on site: supply-side issues (sagging mains during heavy starts, blown line fuses on one phase), then 24 V/enable chain problems, then precharge wear, and only rarely a genuinely failed power unit. The expensive mistake is the reverse order: replacing a motor module because its display showed the fault.
FAQ
Can I reset F07802 and keep running? You can acknowledge it once the infeed reports ready — but if it recurs, treat it as an early warning. Precharge and contactor wear are progressive.
Does F07802 exist on G120? G120 units have an integrated rectifier, so this specific infeed-handshake fault belongs to the modular S120 world. Comparable G120 symptoms usually surface as F30027 (precharge) or supply-voltage faults.
Our fault only appears on cold mornings. Really? Yes — marginal 24 V supplies and tired electrolytic capacitors both behave worse cold. Log r0026 (DC bus) and the 24 V rail at power-up and the pattern usually shows itself within a week.
Zone Otomasyon retrofits and services SINAMICS drive systems — including the 650 mm slitting line overhaul with regenerative S120 infeed described in our projects. Facing a recurring drive fault? Contact us for remote diagnosis.