Siemens 1FT5 servo motor runs at full speed when enabled: causes and checks

The Siemens 1FT5 is an older synchronous servo motor with a built-in resolver, usually run on SIMODRIVE 611A or 611U drives. When the drive is enabled and the motor takes off at full speed, the speed loop has lost its feedback: the drive thinks the motor is not turning, so it keeps asking for more speed.

This often shows up right after a motor has been repaired, swapped or re-cabled. Replacing the U, V, W power cables rarely fixes it, because the power side is not the problem.

• Resolver feedback lost or wrong. A failed resolver, or one that was disturbed during a motor repair, gives no speed signal or the wrong one.
• Feedback cable damage. A broken conductor or a poorly connected shield in the resolver cable, often where it moves in a cable track.
• Connector problems. Bent pins, corrosion or moisture in the round signal connector on the motor.
• Drive measuring circuit. A fault in the resolver evaluation on the drive's control card gives bad speed feedback even with a good motor.
• Controller settings. On analog 611A axes, gain and reset-time settings that were changed can make the axis unstable on enable.

1. Lock out the machine. Disconnect the resolver cable at both ends and check every conductor and the shield for continuity.
2. Inspect the motor-side connector for bent pins, corrosion and water.
3. Check that the cable shield is connected the way the drive documentation specifies. A missing or doubled shield connection causes noise in the feedback.
4. If you have a known-good motor or cable, swap one at a time to see whether the fault follows the motor, the cable or the drive.
5. Only then look at the controller settings, starting from conservative values and following the Siemens documentation.

A motor that runs away at full speed can damage the machine and hurt people. Never test with anyone near the axis, keep the load disconnected or the axis secured where you can, and follow your lockout procedure and the Siemens manuals.

1FT5 motors and SIMODRIVE 611 drives are discontinued, but both are repairable. We check the resolver and its alignment, the windings and bearings, and test the motor under drive control on our own jigs before it ships with a test report and 12-month warranty. If the fault is in the drive, we repair the control card too. Servo motor repair · Request a repair.