Functional path
Field supply source
Field fuse / protection
F+ / F- field terminals
Motor field winding
Field current / acknowledge evidence
F521 decision
Diagnostic signals and checkpoints
F521 at start requestNo field currentOpen field fuseWrong selected motorField wiring disturbedField unit ready but no acknowledge
Technician notes
- FieldAck depends on the selected motor and field circuit, not only the field-exciter electronics.
- Field circuit evidence should be collected with the equipment made safe; do not use run attempts as field proof.
- If field current is credible but acknowledgement is missing, move from field-power proof into communication or control interpretation.
Evidence intake
Turn this record into a qualified service request
A repair decision is much more reliable when the request includes the exact identity of the drive, the first fault evidence and the machine condition when the symptom appeared.
- Complete drive type code / MLFB or nameplate model
- Fault code, fault value and first event before reset
- When the event appears: power-up, enable, ramp, run, decel or stop
- Motor/cable connected or isolated during the symptom
- Visible board, option-card, module and connector identifiers
- Previous repair history, replacement parts and repeat-failure pattern