IndustrialDriveDataDrive Board & Diagnostic Database
Siemens fault record

Speed feedback / tachometer instability: Speed Feedback or Tachometer Signal Instability

The 6RA70 becomes unstable, overspeeds, hunts, fails speed optimization, reports speed-feedback-related behavior, or runs differently after tachometer, encoder, motor or CUD1 work.

Symptom route10 min read

Scope of this technical record

Speed feedback, tachometer and encoder route for 6RA70 machines that hunt, overspeed, optimize poorly or show unreliable actual-speed behavior.

Safety boundary

Qualified personnel only. Run only controlled low-speed checks when feedback is suspect.

6RA70 speed feedback route

1Feedback source
→
2Polarity
→
3Scaling
→
4Cable / shield
→
5Actual speed
→
6CUD1 input

Feedback evidence comes before speed-controller tuning or CUD1 replacement.

6RA70 speed feedback route

Siemens 6RA70 speed feedback route source coupling polarity scaling cable CUD1 input
Speed feedback is proven before tuning or CUD1 replacement.

Feedback source comes before controller tuning

A speed-controller problem can be created by the feedback signal itself. The first decision is whether the drive uses armature-voltage feedback, analog tachometer or pulse encoder feedback. Each choice has different wiring, scaling and parameter evidence.

If tachometer polarity is reversed, scaling is wrong, coupling slips or an encoder channel is missing, optimization can fail and the motor can behave as though the controller is unstable.

Speed feedback evidence

Feedback typeEvidence to collectBoard boundary
Armature-voltage feedbackMotor voltage and field contextNot suitable for every regulation case
Analog tachometerPolarity, voltage at source and terminal, scalingCUD1 analog input only after wiring proof
Pulse encoderPulse count, channels, shield and interfaceOption / CUD1 route after device proof
Intermittent signalCable movement, shield, terminal and couplingDo not retune until signal is stable

Optimization failure can be a feedback failure

When F050 appears during speed optimization, speed feedback must be credible before the optimization result is trusted. Verify actual-speed behavior at standstill, jog and low speed before changing speed-controller dynamics.

Field record checklist

  • Feedback source
  • Tach / encoder identity
  • Polarity
  • Scaling
  • Terminal measurement
  • Mechanical coupling
  • Actual-speed value
  • Parameter set

Technical basis and reference documents

This is an independent editorial technical reference. Original manufacturer documentation remains controlling for installation, repair and commissioning decisions.

FAQ for 6RA70 DC MASTER and SIMOREG CM fault diagnosticsSiemens Industry Support

OEM troubleshooting reference for 6RA70 fault values, tachometer checks, CUD1 / power-interface context and optimization-run faults.

SIMOREG DC-MASTER 6RA70 operating instructionsSiemens / SIMOREG documentation archive

Base operating and commissioning context for SIMOREG DC-MASTER converters, parameterization and speed-sensing setup.

Diagnostic workflow

Workflow
Siemens SIMOREG 6RA70 Speed Feedback / Tachometer Diagnostic Workflow

The DC motor hunts, overspeeds, fails speed optimization, shows unreliable actual speed or behaves differently after tachometer, encoder, motor or control-board work.

Open →
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
Prepare request →