Diagnosing Siemens 3RW40 3RW44 Soft Starter No-Fault Shutdowns

Overview

A motor soft starter is a solid-state AC switching device used with three-phase induction motors to temporarily reduce the inrush current and locked-rotor torque at start. Unlike a VFD, which modulates both voltage and frequency, a soft starter only phase-angle controls the line voltage applied to the motor. Once the motor reaches its nominal speed, an internal or external bypass contactor is closed to eliminate the conduction loss of the thyristors. The Siemens SIRIUS 3RW40 and 3RW44 families implement this principle with a shared run-up detection algorithm, and the source conversation concerns a 37 kW, 400 V, three-phase motor that started with a peak current of 100 A, decayed to 67 A over 10 s, and then stopped suddenly with no fault message on the display. That behavior is the textbook signature of the run-up detection algorithm failing to close the internal bypass contactor. The sections that follow explain the internal architecture, the run-up conditions, the diagnostic procedure, and the parameter adjustments required to resolve the issue on a 37 kW load.

Working Principle of a Motor Soft Starter

A soft starter inserts two anti-parallel thyristors (SCRs) into each phase leg between the line connection (L1/L2/L3) and the load connection (T1/T2/T3). By delaying the firing angle alpha after each voltage zero-crossing, the starter clips the leading edge of every half-cycle and reduces the RMS voltage applied to the motor. The output voltage as a function of alpha is:

For alpha = 180 deg the output is the full line voltage; for alpha near 0 deg the output is near zero. The controller typically starts at alpha approx 110 deg (approx 30% voltage) and decreases alpha linearly over a configurable ramp time until full conduction is reached. Motor soft starters use this principle to limit the inrush current and the locked-rotor torque. The key advantages over a star-delta starter include continuous voltage control, configurable ramp time, and applicability to higher-inertia loads. Compared with a VFD, a soft starter is significantly cheaper, does not generate PWM harmonics, and does not require special motor insulation. Typical applications include centrifugal pumps (where a soft start prevents water-hammer), fans and blowers (where a soft start reduces belt slip and bearing wear), screw compressors (where a soft start prevents rotor seizure), and conveyors (where a soft start reduces belt stress).

At the end of the run-up, the soft starter closes a bypass contactor that shorts out the thyristors. The motor is then fed by a mechanical contactor with conduction losses below 1 W per phase, while the thyristors cool down. The bypass contactor is a wear part; the run-up detection algorithm exists to ensure it closes only when the motor is close to synchronous speed and the contactor will not be exposed to inrush current.

Siemens SIRIUS 3RW40 and 3RW44 Internal Architecture

The 3RW40 and 3RW44 share a common physical layout but differ in feature depth, communication, and diagnostic capability. The 3RW40 is the standard-performance family with a basic keypad and LED indication. The 3RW44 is the high-end family with a graphical display, Softstarter ES connectivity, integrated trend recording, and fieldbus communication modules. For a 37 kW, 400 V motor, the relevant 17-frame selection is:

Always cross-check the catalog number against the motor's full-load current and the network voltage on the Siemens Industry Online Support portal. The 3RW40 17 and 3RW44 17 are direct equivalents in I_e and I_lr; the 3RW44 adds the diagnostic features that the source conversation explicitly recommends.

Power Section

Each phase leg contains two anti-parallel thyristors mounted on a shared heatsink. The heatsink is sized for the I squared t of the worst-case locked-rotor condition. A thermistor on the heatsink provides overtemperature protection, and a thermal model in firmware extrapolates the thyristor junction temperature for a "thyristor overload" pre-trip. The thermal model uses an RC equivalent of the junction-to-ambient thermal impedance, charging during high current and discharging during low current, so the soft starter can predict the junction temperature in real time.

Bypass Contactor

For the 17-frame, the bypass contactor is integrated inside the housing. For larger frames (3RW40 3/4, 3RW44 5/6) the bypass is a separate component driven by the soft starter's output contact. The bypass contactor has a finite mechanical life (typically 100,000 operations for the integrated type) and is the most likely wear item. The contactor carries the motor's full-load current indefinitely, so its main contacts must be inspected during scheduled maintenance.

Control Electronics

A microcontroller generates the firing pulses, samples the phase currents via built-in current transformers, drives the bypass contactor coil, and manages the user interface. The 3RW44 adds a non-volatile log, a trend recorder, and the RS-232/fieldbus interface for Softstarter ES. The 3RW44 firmware is field-upgradable via Softstarter ES; the 3RW40 firmware is fixed. The control supply is galvanically isolated from the power section and is available in 24 V DC, 110 V AC, and 230 V AC variants.

Run-Up Detection Logic

The run-up detection algorithm is the heart of the soft starter. Its task is to decide when the motor has reached full speed so that the bypass contactor can be safely closed. Siemens' algorithm, common to the 3RW40 and 3RW44, requires two simultaneous conditions before closing the bypass contactor. A simplified state diagram is shown below.

Condition 1: Current Below 0.5 * I_start

When the soft starter first turns on the thyristors, it samples the peak current during the first few cycles. This captured value is stored as I_start. As the motor accelerates, the slip decreases and the current naturally falls. The first condition for run-up detection is:

For the 37 kW case study, I_start was captured at 100 A. The threshold is therefore 0.5 * 100 = 50 A. The motor must pull the current below 50 A for the condition to hold. If the motor is heavily loaded and the current never falls below this threshold, the bypass contactor is not closed.

Condition 2: Current Below 1.5 * I_n

The second condition prevents the soft starter from declaring run-up during a stalled condition that happens to be at half the starting current:

For the 37 kW case study, I_n was set to 67 A. The threshold is therefore 1.5 * 67 = 100.5 A. This second condition is almost always satisfied on a running motor, but it is critical when a motor is started with the load disconnected (a no-load start), where the current is far below 1.5 * I_n and condition 1 becomes the gating criterion.

Combined Logic

Both conditions must hold simultaneously. As long as either fails, the soft starter continues to control the motor through the thyristors and the bypass contactor remains open. On a typical 6-8x starting-current DOL motor, the 0.5 * I_start condition requires the current to drop from 6-8 * I_n to 3-4 * I_n, which is still well above the 1.0 * I_n steady-state value. This is why the 0.5 * I_start check is the tighter gate in practice.

Rotating-Motor Branch

If the motor is already spinning when the start command arrives (a fan still coasting down, for example), the algorithm switches to a different mode. The soft starter waits for the configured ramp time to elapse (so the thyristors reach full conduction) and then requires the current to be constant for at least 2 seconds and below the soft starter's rated current I_e. This branch is what allows a "flying restart" of a still-spinning load. If the current oscillates (e.g., a load with a cyclic torque demand), the 2-second constant-current check can prevent the bypass from closing for a long time.

37 kW Motor Case Study: Why the Soft Starter Stopped Suddenly

The source describes a 37 kW motor with the following observed behavior:

• Peak starting current: 100 A
• Steady-state current: 67 A
• Ramp time: 10 s
• Behavior: started normally, current decayed to 67 A over 10 s, then stopped suddenly with no fault message

Step 1: Verify the Motor Current Calculation

For a 37 kW, 400 V, 50 Hz, 3-phase induction motor with power factor 0.86 and efficiency 0.92, the expected line current is:

I_n = P / (sqrt(3) * V_LL * cos(phi) * eta)
   = 37000 / (1.732 * 400 * 0.86 * 0.92)
   = 37000 / 547.8
   = 67.5 A

The observed 67 A matches the calculated value, so the soft starter parameter I_n is set correctly. If the source means 400 V is line-to-neutral or the motor is wired in star at 400 V line, the same formula applies with the line-to-line voltage used. Always confirm the wiring from the motor nameplate.

Step 2: Verify the 100 A Starting Current

The peak starting current of 100 A is much lower than the DOL inrush (typically 6-8 * I_n = 405-540 A for a standard 37 kW motor). The reduction is the intended effect of phase-angle control: by starting at approx 30% of line voltage, the inrush is clamped at about 1.5 * I_n. If the source motor is a high-efficiency design (IE3/IE4) the DOL inrush can be higher, 7-9 * I_n, so the soft starter's 100 A clamp is even more significant.

Step 3: Apply the Run-Up Detection Conditions

At the end of the 10 s ramp:

• I = 67 A (motor at rated current)
• 0.5 * I_start = 50 A. Since 67 A > 50 A, Condition 1 FAILS.
• 1.5 * I_n = 100.5 A. Since 67 A < 100.5 A, Condition 2 PASSES.

Because both conditions must hold, the soft starter does not engage the bypass contactor. The motor continues to run on the thyristors after the ramp ends. When the firmware determines that the run-up conditions cannot be met within the configured window, it drops the output to protect the thyristors from continuous I squared t stress. The drop-out is not flagged as a fault because no hardware condition (overcurrent, overtemperature, phase loss) is violated; the firmware simply concludes that the motor has not accelerated to full speed. This matches the source conversation's note that "the motor does not manage to reach full speed at the end of the 'soft start'."

Step 4: Diagnose the Underlying Cause

If the motor draws 67 A continuously during startup and the run-up condition (I < 50 A) is never satisfied, the motor is essentially running at its rated current the entire time, which means it is not accelerating to full speed. Possible reasons include:

1. The load torque is too high: the pump is running against a closed valve, the conveyor is fully loaded, the fan damper is closed, the compressor has back-pressure. For a centrifugal pump, the most common cause is a closed discharge valve; for a fan, a closed inlet damper; for a compressor, a high back-pressure.
2. The voltage ramp is too aggressive: a high U_start or a short ramp time does not give the motor enough time to develop torque above the load torque.
3. The current limit is set too high: the soft starter permits the motor to draw more current than needed for the load, but the slip does not reduce quickly enough to lower the current below 0.5 * I_start.
4. The motor has a mechanical problem: bearing failure, coupling misalignment, or a partial short in the windings.

Verify the load condition before adjusting the soft starter parameters. For a pump, walk over to the discharge valve and confirm it is open. For a fan, check the inlet damper actuator. For a compressor, check the unload solenoid.

Why a Soft Starter Switches Off Without a Fault Message

The absence of a fault message in the 37 kW case is not unusual. Several legitimate shutdown conditions do not generate a fault entry, and the 3RW40/3RW44 have specific firmware behaviors that can produce a silent drop-out.

1. Start Command Removed

If the ON signal at terminal 1 (3RW40) or DI1 (3RW44) is removed, the soft starter simply turns off the thyristors. The firmware records no fault because the operator (or PLC) issued a legitimate stop. A hand-off-auto switch with a broken return spring, a PLC with a momentary contact bounce, or a control contactor with dirty contacts can all drop the start command mid-start. Check for a clean 24 V signal at the start input for the entire 10-15 second ramp.

2. Control Voltage Brownout

The 3RW40/3RW44 require a stable 24 V DC or 110/230 V AC control supply. If this supply dips below the undervoltage threshold, the control electronics reset and the starter drops output. No fault is generated because the firmware is in a brown-out condition. The threshold is approximately 20.4 V for 24 V DC variants, 85 V for 110 V AC, and 176 V for 230 V AC. A long cable run to a small PSU can cause a brown-out under the inrush of the bypass contactor coil.

3. Run-Up Conditions Not Met (Ramp Elapsed)

As described in the case study, if the run-up conditions fail and the ramp time elapses, the firmware may drop output to protect the thyristors. This is firmware-version dependent. Some firmware versions display a "soft stop" or "startup failed" message; others simply drop the output silently. Always check the fault buffer on the 3RW44, even if the main display shows no message.

4. Bypass Contactor Failure

If the bypass contactor is commanded to close but the contactor does not close (mechanical jam, coil failure, dirty auxiliary contact), the soft starter may interpret this as a fault. On some firmware versions, a non-closing bypass contactor causes a silent drop-out because the firmware cannot distinguish between a contactor that is closed and a contactor that has been commanded to close. This matches one of the source conversation's hints: "there is a problem with the bypass contactor." Check the contactor's coil resistance against the value in the manual and verify that the auxiliary contact toggles.

5. Motor Thermal Overload (Hidden)

The 3RW40 and 3RW44 implement an electronic motor overload protection based on a thermal model. If the cumulative I squared t exceeds the class 10 or class 20 trip curve, the starter trips with an overload message. However, if the operator clears the fault from the display before reading the buffer, the message can be missed. The 3RW44 retains the last 8 to 16 events in non-volatile memory; the 3RW40 only retains the most recent event until power is cycled.

6. Phase Loss or Phase Rotation

A blown line-side fuse causes an asymmetric current that triggers the phase-loss protection. A reversed phase sequence is configurable on the 3RW44; on the 3RW40, phase rotation is not monitored. If the motor is a reversible type and the rotation direction matters, a phase-rotation swap during maintenance can cause the soft starter to refuse to start.

7. Cooling Time Violation

If too many starts per hour are attempted, the thyristor's thermal model trips. The 3RW44 logs this as "thyristor overload"; the 3RW40 indicates it via LEDs. The thermal model is conservative; if the motor and starter are undersized for the duty cycle, the trip can occur even on the first start of the day.

Step-by-Step Diagnostic Procedure

1. Read the fault buffer. On the 3RW44, navigate to Menu → Diagnostics → Fault Memory. The last 8 to 16 events are stored with timestamps. On the 3RW40, the LED pattern on the front panel indicates the last fault (refer to the manual for the LED legend).
2. Verify the control voltage. Measure the 24 V DC (or 110/230 V AC) supply at terminals A1/A2 with a true-RMS multimeter. Verify that the voltage is stable under load and does not sag during the start.
3. Verify the start command. With an oscilloscope or a voltage monitor, observe the ON input (terminal 1 on 3RW40, terminal DI1 on 3RW44) for the duration of the start. A PLC with a bouncing output, a dirty contact, or a defective switch can drop the start command.
4. Capture the current waveform with Softstarter ES (3RW44 only). Connect a PC running Softstarter ES to the 3RW44 via the RS-232 port (use a 3RX1 200 cable) and start the trend recorder. Trigger a start and observe the per-phase current.
5. Measure the line voltage. Verify that L1, L2, L3 are at the expected line-to-line voltage at the soft starter terminals. A 10% undervoltage on the supply side forces the motor to draw more current for the same torque.
6. Inspect the bypass contactor. With the starter isolated and locked out, check the contactor for mechanical freedom, contact pitting, coil resistance, and auxiliary contact operation. Compare the coil resistance to the value in the manual.
7. Verify the motor and load. With the motor decoupled from the load, attempt a no-load start. If the soft starter completes the run-up on no-load, the problem is load-related. If the soft starter still misbehaves, the problem is in the starter or the wiring.

Softstarter ES Commissioning Software

Softstarter ES is the Siemens PC tool for the 3RW44. The 3RW40 does not support Softstarter ES; it is configured via the keypad and LED display. Softstarter ES provides:

• Online monitoring of all parameters in real time.
• Parameter upload, edit, and download from the PC.
• A 4-channel trend recorder for current, voltage, and status bits.
• A fault log with timestamps and event codes.
• A firmware update tool.
• A control panel for remote start/stop.

The connection is via the RS-232 port on the 3RW44 (a 3RX1 200 cable is required) or via PROFIBUS/PROFINET through a 3RW49 00-0KC00 (PROFIBUS) or 3RW49 00-0KC10 (PROFINET) communication module. Softstarter ES is the recommended first-line tool for any 3RW44 commissioning or troubleshooting task. The trend recording makes the 0.5 * I_start and 1.5 * I_n conditions directly visible. The 3RW44 parameter set, accessible via Softstarter ES, includes:

Parameter Tuning for 37 kW Motors

If the run-up detection fails on a 37 kW pump, fan, or compressor, the following adjustments should be considered in this order:

1. Reduce the starting voltage. Set P2 to 20% instead of 30%. This limits the inrush at the first firing instant and gives the current ramp more headroom to fall below 0.5 * I_start.
2. Extend the ramp time. Set P3 to 15-20 s instead of 10 s. This gives the motor more time to reach full speed before the run-up conditions are evaluated. This is the most common fix for high-inertia loads.
3. Reduce the current limit. Set P4 to 2.5-3 * I_n instead of 4 * I_n. This keeps the current under tighter control and improves the run-up detection margin.
4. Enable the run-down function. For pump applications, set P5 to 10-15 s to prevent water-hammer on stop.
5. Check the load. Verify that the pump discharge valve, fan damper, or compressor back-pressure valve is in the correct position for start.

After any parameter change, perform a full start and verify the current profile with Softstarter ES. The 3RW44 trend recorder will show the per-phase current in real time and confirm whether the 0.5 * I_start and 1.5 * I_n conditions are now satisfied. If the current still does not fall below 0.5 * I_start, the load torque is too high for the available motor torque and the mechanical installation must be inspected.

PLC and Fieldbus Integration

The 3RW44 integrates with PLCs via the following interfaces:

• Digital I/O: four 24 V DC inputs (DI1-DI4), three relay outputs (DO1-DO3), one analog output (AO). DI1 is the start command; DI2 is the stop command; DI3/DI4 are user-defined. DO1 is the run signal; DO2 is the bypass-closed signal; DO3 is the fault signal.
• PROFIBUS DP via the 3RW49 00-0KC00 module. The GSD file is "siem80d2.gsd". The process image includes the motor current (in 0.1 A steps), the status word, the control word, and the diagnostic buffer.
• PROFINET IO via the 3RW49 00-0KC10 module. The GSDML file is "GSDML-Vx.x-Siemens-3RW49.txt". PROFINET adds the IRT (Isochronous Real-Time) and MRP (Media Redundancy Protocol) features.

The 3RW40 supports digital I/O only; no fieldbus module is available for the 3RW40. For applications that need network control of a 3RW40, an external PLC must wire the start/stop signals to the soft starter's terminals. For Modbus TCP or EtherNet/IP integration, a 3RW55 soft starter (newer family) or a third-party gateway is required. The 3RW40 and 3RW44 do not natively support these protocols. A typical TIA Portal hardware configuration for a 3RW44 over PROFINET uses the 3RW49 00-0KC10 module in the device catalog, slot 0 (the module itself), with the standard motor-starter data structure in slot 1.

Standards, Coordination, and Selection

Soft starters for industrial motor applications are designed and tested to IEC 60947-4-2, "Low-voltage switchgear and controlgear - Part 4-2: AC semiconductor motor controllers and starters." This standard defines the thermal current I_e (the current the device can carry continuously with the bypass closed), the locked-rotor current rating I_lr, and the utilization categories. The 3RW40 and 3RW44 are Type 1 coordinated devices when used with a Siemens SIRIUS 3RV motor protector; Type 2 coordination requires a Siemens 3NE or 3NA fuses on the line side. For UL applications, the equivalent standard is UL 508, "Industrial Control Equipment." The 3RW40 and 3RW44 carry a UL listing for use in industrial control panels.

For the upstream circuit breaker, the selection must respect the soft starter's I_e and I_lr. The thermal-magnetic circuit breaker (e.g., 3RV) is sized for the motor's full-load current; the magnetic trip is typically set at 8-12 * I_n. The short-circuit current rating (SCCR) of the assembly must be coordinated with the available fault current at the line terminals. The typical 50 kA SCCR of a 3RW40 17 + 3RV combination is adequate for most industrial feeders; for higher SCCR (65 kA or 100 kA), use a 3NE fuse on the line side.

Verification and Acceptance Test

After any parameter change or maintenance event, perform the following verification tests:

1. Cold start. With the motor at ambient temperature, perform a start and verify the current profile with Softstarter ES. The expected peak is 80-120 A; the expected steady state is within 5% of 67 A.
2. Bypass engagement. Verify that the bypass contactor closes within 12 s of the start command. Listen for the contactor click and read the bypass status bit via Softstarter ES.
3. Hot restart. Immediately after a normal stop, perform a second start. The thermal model may permit a higher inrush; verify the current profile.
4. Soft stop. If run-down is enabled (P5 > 0), verify that the motor stops smoothly over the configured run-down time.
5. Fault log review. Inspect the fault buffer for any warnings or trip events during the verification. Clear the buffer before the final handover.

The acceptance criteria for the 37 kW case study are:

• Peak starting current between 80 A and 120 A.
• Steady-state current within 5% of 67 A (i.e., 64-71 A).
• Bypass contactor closes within 12 s of the start command.
• No fault messages in the buffer.

Troubleshooting Matrix

Frequently Asked Questions