SINUMERIK 840D sl Alarm 207901: Sensorless Spindle Overspeed Tuning

The SINUMERIK 840D sl alarm 207901 (Motor overspeed) frequently appears when commissioning third-party spindles that operate without an encoder (sensorless / open-loop V/f mode). This technical reference documents the parameter set, current-derating analysis, and gain adaptation required to clear the alarm and stabilise current above 24 000 rpm on a 30 000 rpm / 13 kW / 44 A induction spindle driven by a SINAMICS S120 Single Motor Module 6SL3120-1TE28-5AA3. Field-proven values for p1800, p1461-p1465 and p1082 are extracted from commissioning traces and verified against the Siemens SINAMICS S120 List Manual and the SINUMERIK 840D sl Diagnostics Manual.

Alarm 207901 - Definition and Cause

Alarm 207901 is generated by the SINAMICS drive when the actual speed exceeds the configured monitoring limit. The alarm is raised for both positive (M03) and negative (M04) overspeed conditions and indicates that the rotor has left the controlled region of the V/f curve. According to the SINAMICS S120/S150 List Manual, the monitoring limits are:

p2162 is the hysteresis allowance for overspeed detection. Its default is 0 rpm, meaning any transient exceeding p1082 immediately triggers 207901. When the spindle is commanded via CI:p1085 (the NC setpoint source) and that value falls below p1082, the effective ceiling becomes the lower of the two. Always cross-check r1084 (positive smoothed actual speed) and r1087 (negative smoothed actual speed) before adjusting p1082, because overspeed may be caused by V/f slip instability rather than a wrong setpoint ceiling.

Hardware Configuration

The S120 Motor Module 6SL3120-1TE28-5AA3 is a booksize 85 A unit from the 6SL3120-1TE series. When configured for the standard 4 kHz pulse frequency it delivers the full rated 85 A; at 8 kHz the available continuous current drops because of switching losses, and at 16 kHz the derating is severe enough to disable sensorless operation on a 44 A motor.

Speed Limit Calculation

The maximum speed that the SINAMICS current controller can resolve depends on the current controller cycle time and the motor pole pair count. The SINAMICS parameter manual gives two equivalent forms of the ceiling formula:

This form is used when p0115[0] (current controller cycle time) is left at its default. With p0115[0] = 125 µs and r0313 = 3 (p313 = 3):

This would limit a sensorless spindle to 16 000 rpm. The alternative form uses p0112 (sampling-time preset index) instead of p0115[0]:

When p0112 is set such that the effective current controller time becomes 62.5 µs (p112 = 62.5 µs), the maximum speed roughly doubles:

This matches the configured p1082[0] = 30 000 rpm. The alarm in this case is therefore not the result of a hard mathematical ceiling violation; it is downstream of the ceiling math, in the current capability of the drive module.

Root Cause - Pulse Frequency Derating

The 207901 alarm in this configuration was traced to p1800 (pulse frequency of the IGBT bridge). The drive had been initialised with p1800 = 16 kHz, which forces severe current derating on the booksize 85 A module:

The field-proven fix is to reduce p1800 to 8 kHz, which raises continuous current capability to about 60 % of rated (≈51 A on this module). 8 kHz is the recommended upper limit for booksize modules that drive motors in the 40-60 A class when no encoder feedback is available. Reducing further to 4 kHz is allowed but raises audible switching noise and is rarely necessary for a 30 000 rpm sensorless spindle.

V/f Control Without Encoder - Parameter Map

Sensorless spindle operation on SINAMICS S120 is implemented as V/f open-loop control, p1300 = 20 ("Sensorless vector control with V/f characteristic" / "V/f control without encoder"). Key drive parameters for a third-party induction motor:

The motor-code parameter p500 maps to a stored motor data set. When the spindle is third-party, p500 must still be set so that the inverter calculates the correct magnetising current and slip; setting p500 to 102 (induction motor) is the correct starting point for an externally-defined induction motor.

V/f Slip and Gain Adaptation (p1461-p1465)

With p1300 = 20 the drive uses a linear V/f curve and a slip model. Below the field-weakening range the rotor slip is small and the model is robust. Above the rated frequency the available voltage is reduced and the slip must be increased to deliver torque. If the slip-controller gain is too low, current oscillates, the rotor speed wanders, and the drive trips 207901 even though the setpoint is within p1082. The p1461-p1465 parameter group is the slip-controller gain adaptation that stabilises this region:

The adaptation works as a piecewise gain. Below p1464 the slip controller runs at the base gain (defined by p1460 / p1461), and above p1465 it scales by p1463. Setting p1465 above the spindle rating provides headroom so the gain does not saturate when the rotor is accelerating past the rated point. If p1465 is set lower than the operating point, the slip-controller gain collapses and the current oscillates - this is a frequent cause of 207901 between 20 000 and 30 000 rpm on a sensorless spindle.

Step-by-Step Tuning Procedure

1. Back up the actual drive parameters with STARTER / Startdrive (Upload from drive to PG). Record p1800, p1461-p1465 and r0027 for the existing configuration as a rollback baseline.
2. Confirm p1300 = 20 (V/f without encoder). p1300 = 21 (sensorless vector) is unsuitable for a true open-loop spindle and may trigger 207901 when the model breaks down in the field-weakening region.
3. Set p500 = 102 (or the induction-motor code matching the spindle rating) and download to the drive. Verify p310 (frequency reference) and p311 (speed reference) match the motor nameplate; otherwise the V/f curve mis-scales.
4. Set p2000 to the spindle rated speed (30 000 rpm in this case). The 100 % setpoint then corresponds to the spindle maximum.
5. Set p1082[0] to the maximum mechanical speed, not the synchronous speed. Verify p1082 ≤ 60 / (10.0 * p0115[0] * r0313) or p1082 ≤ 60 / (10.0 * p0112 * r0313) depending on which parameter you read.
6. Set p1800 = 8 kHz. Do not exceed 8 kHz on the booksize 85 A module for sensorless operation unless the derating curve in the S120 Equipment Manual is checked against the continuous current demand.
7. Run a slow speed command (e.g. M03 S500) and capture r0027 (actual current smoothed) and r0021 (actual speed smoothed). The current must remain stable with a steady load.
8. Sweep speed in 5 000 rpm steps to the maximum. At each step capture r0027, r0021 and r0060 (DC-link voltage) for at least 5 seconds. Look for current oscillation > ±10 % of r0027 or speed oscillation > ±200 rpm.
9. If oscillation appears above 20 000 rpm, adjust p1461 to p1465. Increase p1463 (adaptation factor) first to widen the high-speed gain; if that produces hunting, increase p1462 (filter time).
10. Set p1715 = 2 if the drive supports the second current-controller adaptation set; this is documented to stabilise the high-speed region on booksize modules. p1715 changes which adaptation curve the controller uses, not the gain.
11. Run a 60 second dwell at M03 S30000. Confirm no 207901, no 7900 (overcurrent), no 7902 (speed controller at limit) and no 7905 (encoder fault).
12. Save parameters to the CF card with p0977 = 1 so the change survives a power cycle.

Verification

The 207901 alarm is cleared when all three conditions are met:

• Actual speed r0021 stays within min(p1082, CI:p1085) + p2162 for at least 10 seconds.
• Drive DC-link voltage r0060 remains above the undervoltage threshold (560 V on a 600 V DC link).
• Motor current r0027 stays below the module derated continuous rating for the configured p1800 (≈51 A at 8 kHz for the 6SL3120-1TE28-5AA3).

A passing commissioning test cycle:

M03 S1000    ; dwell 5 s
M03 S10000   ; dwell 5 s
M03 S20000   ; dwell 5 s
M03 S25000   ; dwell 30 s
M03 S30000   ; dwell 60 s
M05

If 207901 reappears during any segment, return to the last stable speed and re-tune p1461-p1465 before continuing. Recording STARTER traces at 250 µs sampling makes the r0027 oscillation pattern visible - 10 Hz hunting points to V/f gain, 100 Hz hunting points to p1800 / current-loop interaction.

Troubleshooting Matrix

Acceleration Ramp and NC-side Limits

V/f control without encoder cannot deliver the same torque transient as closed-loop vector control. The NC-side acceleration ramp must respect this. Two machine data items control the ramp on the SINUMERIK side:

If the NC clamps the setpoint to MD36900 before the drive limit p1082, the alarm logic on the SINAMICS side never sees the overspeed - but the NC ramps up too quickly and the drive trips 7902 (speed controller at limit) instead of 207901. Check that MD36900 is consistent with p1082.

FAQ