SINAMICS S120 Control Panel Motor Run Failure Troubleshooting

Problem Overview

During commissioning of a SINAMICS S120 drive line-up with a CU320 DP Control Unit, an Active Line Module (ALM), and 1PH7 main-spindle motors, a recurring field issue is the inability to run the motor from the Drive ES or AOP30 control panel. Typical symptoms reported by commissioning engineers are:

• The motor hisses audibly but only turns through a small angle (a few electrical degrees) before stopping.
• The control panel intermittently returns the status string "Switching on inhibited" even after the operator clicks Assume priority and Enable.
• No fault or alarm code is present in the diagnostic buffer; only the operating state is wrong.
• Torque feedback is alive (r0080 updates) but the speed actual value (r0061) stays at zero.
• When the shaft is rotated manually, r0061 moves but with the wrong sign relative to the rotation direction.
• When p1300 is forced to 0 (V/f open-loop), the motor runs at the commanded frequency, but SERVO closed-loop control still fails.

The text "Switching on inhibited" is a status indication generated by the SINAMICS state machine, not a fault. It is the surface symptom of one or more missing enable signals in the drive's enable chain. Treating it as a fault code and clearing it without reading the underlying bits in r0046 will keep the commissioning team stuck in a loop.

SINAMICS S120 System Architecture Context

The SINAMICS S120 is a modular, multi-axis drive system in which the CU320 DP Control Unit is the central intelligence for one or more Motor Modules and one infeed (Line Module). In a typical machine-tool or test-bench topology, the CU320 DP is connected to:

• An Active Line Module (ALM) for regenerative line-side energy recovery and active grid filtering.
• One or more Motor Modules (Single Motor Module, Double Motor Module, or Booksize Compact) supplying the motors.
• 1PH7 asynchronous main-spindle motors wired to the Motor Module power output, with motor encoders fed back through DRIVE-CLiQ or through a Sensor Module (SMC/SME).
• DRIVE-CLiQ cables carrying encoder data, electronic type plates, and module temperatures between the CU, Motor Modules, and Sensor Modules.

The 1PH7 motor family is Siemens' asynchronous main-spindle motor, available with and without encoders. Because 1PH7 supports both VECTOR and SERVO control, the choice of control mode during commissioning decides which parameter set, which controller structure, and which encoder evaluation path is active. Selecting the wrong mode — for example, running a closed-loop SERVO configuration on a VECTOR-style data set — produces exactly the symptom of "hiss without rotation": the inverter is building up flux, but the controller is not closing the loop correctly.

DRIVE-CLiQ is the proprietary digital interface used between SINAMICS components. It carries the motor's electronic type plate, encoder data, and the temperature of the power module. Swapped, damaged, or poorly seated DRIVE-CLiQ cables are the single most common cause of control-panel run failures on multi-axis S120 systems, especially when one motor responds to torque commands but the speed feedback is wrong, inverted, or assigned to a neighboring axis. For general product information, see the official SINAMICS S120 product page and the Siemens Industry Online Support portal.

Root Cause Analysis

The reported incident shows two interacting root causes, both of which are common during first commissioning of multi-axis S120 systems with 1PH7 motors.

Root Cause 1: Wrong Control Mode or Incomplete Motor Data

When the motors are initially configured in VECTOR control mode, the drive assumes a certain encoder evaluation strategy and motor data set. If the motor data has not been entered from the electronic type plate (or has been entered for a different frame size), the motor will build up flux, attempt to close the current loop, and stall in a controlled oscillation that sounds like a hiss. The drive stays in the "Switching on inhibited" state because the inner control loop reports it is not ready.

Switching to SERVO control and running the automatic motor identification (Motor ID, p1910 = 1 and p1960 = 1) restores basic rotation, but the motor then runs at a fixed speed regardless of the control panel slider, and r0061 stays at zero. This is the second-layer symptom: the power stage is healthy, the inverter is correctly energised, but the encoder path is broken. The drive silently falls back to a coarse setpoint because the actual speed value is not trustworthy.

Root Cause 2: Inverted or Swapped Encoder Feedback

Manual rotation of the motor shaft produces an r0061 value that is inverted with respect to the rotation direction (clockwise from the motor side produces a negative r0061). This sign inversion is the unambiguous signature of one of the following:

• The DRIVE-CLiQ cable from the motor encoder is plugged into the wrong port on the Sensor Module or on the Motor Module.
• The motor power phases U, V, W are wired correctly, but the encoder's count direction is reversed.
• A second motor on the same Control Unit is using the encoder that should be assigned to the first motor (cross-wired DRIVE-CLiQ).

Setting p1300 = 0 (V/f open-loop) confirms the power stage is good: the motor runs at the speed set in the control panel. The magnetic field "hiss" is the audible side effect of the high PWM carrier frequency in open-loop mode. SERVO and VECTOR modes both require a valid encoder; V/f does not. The fact that V/f runs cleanly while SERVO does not is a strong indicator that the fault is in the encoder path and not in the power stage.

Contributing Factors

• Brake solenoid state. 1PH7 motors are often delivered with an optional holding brake. If p1215 is configured for a brake but the brake is not releasing (r1229.1 = 0), the motor cannot move freely. A short test with the brake released manually will rule this in or out immediately.
• Infeed enable chain. With an ALM present, p0864 (infeed line supply, OK) must be set so that the Motor Module knows the infeed is healthy. If p0864 is not wired to the ALM's operating signal, the drive will be in "Switching on inhibited" even though the ALM itself is fine.
• Duplicate drive object numbers. On a CU320, each Motor Module is assigned a drive object number. If two axes end up with the same DO number, the wrong drive object may receive the run command and produce motion on the wrong motor, leaving the intended motor stationary.

Diagnostic Procedure

Use the following non-destructive sequence to localize the fault before changing parameters. The sequence is designed to be performed from the AOP30 operator panel or from STARTER / SIMATIC Drive ES.

1. Read the current state. Open the drive diagnostics and confirm r0002 displays "Switching on inhibited". Read r0046 (missing enable signals, bit-coded). Each bit names a specific enable that is missing. Common ones are bit 06 (infeed enable), bit 08 (speed setpoint from setpoint channel), bit 14 (OFF2 / OFF3 active), and bit 31 (safety enable missing).
2. Verify the infeed is running. The ALM must reach r0002 = "Operation" before any Motor Module can be enabled. Check r0863 (status word of the infeed). If the ALM is not in operation, "Switching on inhibited" on the Motor Module is the correct behaviour.
3. Check the holding brake. If the motor has a brake, verify r1229.1 (status, brake released). With the brake closed, the drive limits torque to p1532, which is usually too low to move the motor.
4. Inspect the DRIVE-CLiQ topology. In STARTER, open the topology view (Project → Drive → Topology) and confirm that the actual wiring matches the offline configuration. Pay particular attention to: the correct motor encoder assigned to the correct axis, the DRIVE-CLiQ port numbers on each Motor Module, and the cable integrity (no kinks, no loose connectors, no oxidation on the contacts).
5. Manually rotate the motor and observe r0061. With the drive disabled and the Safety Integrated functions released, rotate the motor shaft by hand. Monitor r0061 (speed actual value, unsmoothed) and r0063 (speed actual value, smoothed) on the relevant drive object. The value must change smoothly, with the correct sign, and only on the expected axis. A reading on a neighbouring axis, or an inverted sign, indicates a wiring issue.
6. Toggle the control mode temporarily. Set p1300 = 0 (V/f open-loop) and re-attempt a run command from the control panel. If V/f mode runs but SERVO does not, the fault is in the encoder path. If neither mode runs, the fault is upstream in the enable chain or in the power section.

Step-by-Step Resolution

The procedure below covers the full resolution path. Adjust p1300 only as a diagnostic; return the drive to SERVO or VECTOR control for production use.

1. Reset the Drive to Factory Settings (Optional)

If the commissioning history is unclear, perform a factory reset before recommissioning:

p0009 = 30     // device commissioning, factory reset
p0976 = 1      // parameter reset to factory setting

Wait for the drive to complete the reset (the BOP/AOP will display the appropriate completion code), then power-cycle the Control Unit.

2. Recommission with the Correct Control Mode

For 1PH7 main-spindle duty with encoder feedback, SERVO control (p1300 = 20 for speed control with encoder, or p1300 = 21 for torque control with encoder) is the correct choice. Open the STARTER commissioning wizard, select the 1PH7 motor from the motor list (or enter the motor code if it is a non-standard variant), and let the wizard populate the motor data set. The motor's electronic type plate is read automatically over DRIVE-CLiQ when the encoder is connected.

3. Run the Automatic Motor Identification

After saving the motor data, the wizard prompts for Motor ID. Select the appropriate identification routines:

p1910 = 1      // motor data identification, rotating
p1960 = 1      // speed controller optimization, rotating

Both routines require the motor to rotate freely. Confirm safe mechanical conditions, ensure personnel are clear, and provide a safety watch. The Motor ID routine applies test signals, measures the motor's electrical parameters, and optimises the current and speed controllers. The values are written back into the motor data set automatically.

4. Configure the Infeed Enable

If the system uses an Active Line Module, configure the infeed enable so the Motor Module can see that the infeed is healthy:

Confirm that the ALM's own enable chain is closed. On the ALM drive object, check p0840 (ON/OFF1), p0844 (OFF2), p0848 (OFF3), and the corresponding binector inputs. If any of these is missing, the ALM will not reach "Operation" and the Motor Modules will stay in "Switching on inhibited".

5. Verify the Encoder Direction

With the drive enabled in no-load SERVO mode, rotate the shaft manually. r0061 must follow the rotation with the correct sign. If the sign is reversed, the corrective action depends on the encoder type:

• DRIVE-CLiQ encoder (e.g., from the motor's electronic type plate): swap the encoder assignment in the topology by editing p0141 / p0142 (Sensor Module assignment), or invert the encoder using p0410 = 1, then re-run Motor ID.
• Non-DRIVE-CLiQ encoder (e.g., resolver via SMC10/SMC30): swap the A and A-not signals at the encoder connector, or use p0410 = 1 to invert the encoder in software, then re-run Motor ID.

6. Test the Control Panel Run Command

From the AOP30 or Drive ES control panel, click Assume priority, enable the drive, and apply a small positive speed setpoint (e.g., 100 rpm). The motor should accelerate smoothly along the configured ramp, and r0061 should track r0062 (speed setpoint) within the configured ramp time. Reduce the setpoint to zero, then disable the drive. If the run is smooth and r0061 follows the setpoint, the axis is operational.

Parameter Reference

SINAMICS State Machine (Reference)

Verification

1. No fault state. r0002 transitions to "Operation" when the control panel run button is pressed and held.
2. Speed follows setpoint. r0061 tracks r0062 within the configured ramp time, with no oscillation and no saturation.
3. Correct direction. A positive speed setpoint produces a positive r0061 when the motor is viewed from the shaft end. This is the verification that the encoder sign is correct.
4. Smooth torque. r0080 is within the motor's rated torque and does not oscillate at standstill.
5. Infeed operating. r0863 from the ALM reports "Operation" or "Ready" before the Motor Module is enabled.
6. Multi-axis independence. Enabling axis 1 does not cause motion, fault, or any change in r0061 / r0080 on axis 2.

Field-Proven Tips

• "Hiss without rotation" is a flux, not a fault. It almost always points to flux being built up but the rotor not moving because of (a) a closed holding brake, (b) a missing encoder signal, or (c) an inverted encoder signal. Listening to the motor is often a faster diagnostic than reading the parameter list.
• "Switching on inhibited" is a status, not a fault. Always read r0046 to identify the specific missing enable. Treating it as a fault and clearing it will leave you in the same state.
• p1300 = 0 is a diagnostic, not a fix. Use V/f open-loop to prove the power stage, but return the drive to SERVO or VECTOR for production. 1PH7 main-spindle duty with encoder feedback belongs in closed-loop control.
• DRIVE-CLiQ topology is the most common commissioning issue on multi-axis S120. A 30-second online/offline topology compare in STARTER identifies swapped motor encoders in seconds. Always re-run Motor ID and speed controller optimization after any topology change.
• After any DRIVE-CLiQ cable change, accept the actual topology. If the wiring is the source of truth, accept the new topology in STARTER and let the wizard update the parameter set.
• Multi-motor systems need a per-axis checklist. When two or more 1PH7 motors share one CU320, repeat the diagnostic procedure on each axis. Do not assume that fixing one axis fixes the others; the same root cause can be present on multiple axes if they were wired at the same time.
• The non-Siemens motor on the same CU is a separate engineering task. A non-Siemens motor on the same Control Unit that produces a very high magnetic field and runs at variable speed depending on the slider position is a strong indicator that the motor is being treated as a generic asynchronous machine without a proper motor data set. For third-party motors, enter the equivalent circuit data manually and run Motor ID; do not rely on the auto-detect wizard.

Safety Considerations

The procedures above require the drive to be in commissioning state. During Motor ID (p1910, p1960), the motor rotates at controlled speed and applies test signals. Verify the following before starting:

• The motor shaft is mechanically free to rotate (no mechanical coupling that could fail under test).
• Personnel are clear of the driven machinery, with the area marked off.
• The holding brake (if present) is released manually or by the drive before Motor ID.
• The emergency stop circuit is functional and tested.
• Safety Integrated functions (STO, SS1, SLS, etc.) are configured and tested before the drive is allowed to run in production.
• A second person is present as a safety watch when running rotating Motor ID.

Parameter numbers and commissioning workflow referenced in this article correspond to the standard SINAMICS S120 firmware releases documented in the Siemens Industry Online Support portal. The List Manual and Function Manuals for the S120/S150 family are available from the same portal and should be cross-referenced against the firmware version installed on the CU320 DP Control Unit. For product-level documentation, see the SINAMICS S120 product page on the Siemens corporate site.