SINAMICS G120 CU250S-2 4-20 mA Speed Reference Troubleshooting

The SINAMICS G120 modular drive family with the CU250S-2 Control Unit is widely deployed on centrifugal pump skids, fans, and HVAC equipment. A recurring field complaint is a 4-20 mA speed reference that reads correctly at start-up but then decays toward the configured minimum speed, even though the loop-current measured at the terminal block remains stable. This article walks through the parameter chain that must be verified, the diagnostic read-outs that confirm each stage, and the field fixes that resolve the drift.

1. Problem Description

The reported behavior:

1. Power-on, run command issued.
2. Output frequency tracks the 4-20 mA reference at the analog input. r0752 shows the loop current correctly (e.g., 12.0 mA) and r0755 displays the corresponding percentage (e.g., 50.0 %).
3. After a few seconds to a few minutes of operation, the motor frequency ramps down to p1080 (minimum speed) and stays there, even though the analog input is still driven with the same or higher current.
4. No fault is latched, no warning is raised, and the drive remains in RUN state.

This pattern almost always points to one of the following conditions, in order of frequency observed in the field:

2. Root Cause Analysis

2.1 Analog Input Type Selection (p0756 and DIP Switch)

The CU250S-2 has two physical analog inputs, AI0 and AI1. Each input can be configured as a voltage or current input by:

• The hardware DIP switch on the front of the Control Unit (between the terminals and the status LEDs).
• The software parameter p0756[0] (for AI0) and p0756[1] (for AI1).

The two settings must agree. The DIP switch is read at power-on and overrides p0756 only at the next cold start. Changing p0756 online without flipping the DIP switch back to "I" leaves the analog frontend in voltage mode, where a 4-20 mA signal produces a 4-20 V sense voltage that the ADC interprets as a low reference. The drive may appear to start, but a software clamp or wire-break monitor will eventually pull the speed down.

For a 4-20 mA setpoint, p0756[0] = 3 and the DIP switch for AI0 must be in the I position. A value of 4 (bipolar voltage) is the most common commissioning error and produces exactly the symptom reported: a brief period of apparent correct speed, followed by collapse to minimum.

Reference: SINAMICS G120 CU250S-2 List Manual (parameter list p0756).

2.2 Wire-Break Monitoring and F03505 / A03505

When p0756[0] = 3 is selected, the firmware enables the wire-break / open-circuit monitor on AI0. If the measured current falls below the configured threshold (default 4 mA) for longer than the configured debounce time, the drive reacts according to:

On a wire-break, fault F03505 (analog input 1 wire break) is raised. The default fault reaction is OFF2, which removes the run command and ramps the drive to standstill. If the response has been reconfigured (e.g., to OFF1 only) or if only warning A03505 is active, the setpoint is replaced with the value set in p1077 (alternative setpoint / fail-safe setpoint) or with zero, which causes the speed to fall to p1080 (minimum speed).

To confirm wire-break activity without disturbing the running process, monitor r0752[0] over a few minutes. If r0752 holds the loop current steady, the wire-break monitor is not tripping. If r0752 momentarily drops to 0 mA, the transmitter loop, the shielded cable, or the terminal block is opening under vibration.

2.3 Reference Scaling (p0757 - p0760)

The CU250S-2 maps the measured input current to a percentage value r0755[0] (0 - 100 %) using a two-point linear characteristic defined by:

Common field errors include y1 set to a non-zero value (causing a deadband at low signal) or x1/x2 swapped, which inverts the characteristic. After the next ramp, the drive will head to minimum because the perceived reference is below the configured minimum.

2.4 Setpoint Source Selection (p1000)

p1000 defines the main setpoint. The default for CU250S-2 with analog input wiring is p1000[0] = 6 (analog setpoint). If a STARTER or Startdrive project inadvertently sets p1000[0] = 1 (Motor potentiometer) or p1000[0] = 3 (Fixed setpoint), the analog input is ignored after ramp-up and the drive falls to the value of p1080.

2.5 Ramp-Function Generator and Setpoint Processing

The output of r0755 is fed through the main setpoint processing chain. The relevant parameters to confirm the chain is healthy:

If r1170 is correct but r1199 is at zero, the ramp-function generator is being held or zeroed by an upstream inhibit bit (e.g., OFF1/OFF2 via STW1.0 or STW1.1 from a PLC that is releasing the line briefly).

3. Diagnostic Parameters and Read-Outs

Use the Starter commissioning tool, Startdrive in TIA Portal, or the IOP-2 / BOP-2 keypad to read the following parameters live while the fault is present:

The single most useful diagnostic in the field is to trend r0752[0] and r0021 on a laptop running Starter with a 100 ms trace. If r0752 holds 12 mA but r0021 collapses to zero, the problem is downstream of the analog input and is most likely a wire-break response that has been re-wired to warning-only.

4. Step-by-Step Resolution Procedure

Prerequisites

• Drive is in READY TO SWITCH ON state, motor stopped.
• Parameter access level p0003 = 3 (Expert) or 2 (Extended).
• Starter / Startdrive online connection via PROFIBUS, PROFINET, or USB to the CU250S-2.
• Calibrated mA source or known-good loop calibrator on the AI0 terminals.

Procedure

1. Power cycle the drive. Reading p0756 alone is not sufficient if the DIP switch was changed online.
2. Verify the hardware switch. Look at the DIP switch block on the front of the CU250S-2. AI0 switch must be in the I position (slide toward the terminals labeled "I"). If it was set to U, flip it and power cycle again.
3. Set p0756[0] = 3. Unipolar current input 0-20 mA with monitoring.
4. Set the scaling p0757 - p0760. Use the values in the table above (4 mA = 0 %, 20 mA = 100 %).
5. Disable / soften the wire-break monitor if the loop is noisy. Set p0761[0] = 2.0 mA, p0762[0] = 100 ms, and confirm the reaction in p2100 / p2101. If a vibration-induced trip is suspected, raise p0761[0] to 3.0 mA and p0762[0] to 500 ms. Do not exceed 4.0 mA; that would defeat the safety purpose of the monitor.
6. Confirm p1000[0] = 6. Main setpoint from analog input. If the project uses a different value, re-route the connector: p1070[0] = 755[0] (r0755[0] on connector).
7. Verify p1080 and p1082. A minimum speed of 0 Hz combined with a sticky fail-safe setpoint (p1077) will produce the reported symptom if p1077 has been left at default 0 %.
8. Inject test signal. Apply 12.00 mA at AI0. Read r0752, r0754, r0755, r1078, r1170, r1199. All six must read 50.0 % within the smoothing time constant.
9. Issue run command and watch r0021. Speed must remain at 50 % of p1082. If it collapses after 10-30 s, the wire-break monitor is the most likely cause, even with the DIP switch set correctly.
10. Save to non-volatile memory. Execute "Copy RAM to ROM" from the Starter project tree, or set p0971 = 1 and wait for the drive to finish saving.

5. Wire-Break Monitoring and Fault Handling

The CU250S-2 wire-break monitor is a high-priority diagnostic because it protects the process from a lost reference. The default behavior is correct for safety but is sometimes over-aggressive for installations where the transmitter is a low-power loop-powered device that needs a settling time after power-up.

To change the wire-break response from fault to warning, use the fault configuration in Starter or the parameter list:

p2100[3] = 3505
p2101[3] = 0

This demotes F03505 to a warning (A03505) with no OFF2 reaction. The drive then continues to run at the fail-safe setpoint p1077, which by default is 0 % and will pull the speed to p1080. Set p1077 to the minimum acceptable process speed (e.g., 30 %) for a graceful degradation.

Reference: SINAMICS G120 Function Manual - Faults and Alarms.

6. Setpoint Processing Chain

The complete setpoint path is:

When the wire-break monitor is the source of the speed drop, the diagnostic signature is unambiguous: r0752[0] momentarily goes to 0 mA at the moment of the trip, the firmware raises F03505 or A03505, and the setpoint collapses from the post-scaling percentage to p1077 (or zero) within p0762[0] ms.

7. Pump-Specific Considerations

Pump applications add two constraints that are not present in fan or compressor drives:

7.1 Quadratic-Load Derating

Pump torque rises with the square of speed, but hydraulic power rises with the cube. The motor current is usually very low at low speed, which can cause the drive's OFF1 low-speed timer or the load monitoring to interpret the low current as a no-load condition. Verify that p2180 (no-load speed) and p2181 (load monitoring) are appropriate for the impeller curve.

7.2 Minimum Speed p1080

For centrifugal pumps, p1080 is rarely 0 Hz. A typical field value is 15-20 % of rated speed, set to prevent bearing damage and to keep the pump in its valid operating envelope. If p1080 is left at 0 Hz and the analog setpoint decays to zero, the drive will report the speed as 0 Hz with the pump effectively dead-headed against a closed valve. Confirm the suction / discharge conditions and the IGV / throttle valve position before assuming a control fault.

7.3 Pipe-Fill Mode and Ramp Time

Many pump packages use a dedicated "pipe-fill" routine on first start. If the routine is left enabled, it will hold the speed at a low value for a fixed time, then release. Confirm p1130, p1131, p1132, p1133 (ramp-up / ramp-down times) and the value of p1134 (rounding type) match the application. A short ramp-up combined with a high derivative gain on the internal PI can cause the ramp-function generator to output a step change that is then clipped to p1080 by the minimum-speed limiter.

8. Verification and Commissioning Checklist

After completing the parameter changes, run the following verification sequence on the bench or with the pump uncoupled:

1. Power-on. Confirm r0752[0] reads the loop current.
2. Apply 4.00 mA, 12.00 mA, 20.00 mA in turn. Record r0752[0], r0755[0], r1078, r1199 at each step. All three should move proportionally.
3. Issue run command at 12 mA. Wait 5 minutes. r0021 must stay at 50 % of p1082.
4. Drop the input to 4.00 mA. Drive should ramp to p1080 within the configured ramp-down time. No fault.
5. Open the loop (simulate wire break). Drive should raise F03505 / A03505 and react per p2100 / p2101.
6. Restore the loop. Fault must auto-clear (A03505) or be acknowledged (F03505) before the drive will return to RUN.
7. Save to ROM. Power cycle. Re-confirm the parameter values.

9. Troubleshooting Matrix

10. Firmware and Catalog Number Notes

Parameter positions and default values are stable across the supported firmware versions for CU250S-2 (V4.4, V4.5, V4.7, V5.1, V5.2). Two firmware-specific caveats apply:

• V4.5 and earlier: p0762[0] debounce time minimum is 10 ms. Settings below 10 ms are clamped.
• V5.1 and later: The "Monitoring" variants of p0756 (values 1, 3, 6, 7) gained a per-channel smoothing parameter p0763. If the project was migrated from a V4.x project, the smoothing value may be inherited as 0 ms and produce visible ripple in r0752 under high-EMI conditions.

Common catalog numbers for the Control Unit (for spares and project documentation):

Reference: SINAMICS G120 CU250S-2 List Manual, SINAMICS G120 Operating Instructions, SINAMICS G120 Function Manual - Setpoint Processing.

11. Field Proven Cautions

• Always power-cycle after flipping the AI0 DIP switch. The firmware reads the switch state at cold start only.
• Reading p0756 in Starter or Startdrive will show the parameter value but not the physical switch position. The two can disagree.
• If the project is downloaded from a TIA Portal V15.1 (or earlier) Starter project, the analog input type may have been written only to the startup parameters. After a factory reset ("Restore factory settings"), the drive will boot with p0756 = 4 (bipolar voltage) and the original fault will return.
• Shielded twisted pair is mandatory for the 4-20 mA loop. Run the shield to the CU250S-2 PE terminal, not to a remote ground point.
• Do not use the drive's 24 V supply (terminal 9) to power a 4-wire transmitter if the loop is longer than 10 m. Use a separate DC supply or a 2-wire loop-powered transmitter.

FAQ