Siemens MM430 BOP '----' Display: 24VDC PSU Drop on Frame C

Problem Summary

A 15 kW Siemens MICROMASTER 430 (MM430) variable frequency drive, frame size C, exhibits a recurring fault pattern where the Basic Operator Panel (BOP) display shows four dashes ("----") during normal running condition. When the dashes appear, the drive drops out of run and ceases to control the motor. Field measurements confirm that the internal 24 VDC control supply at terminals 9 (+24 V) and 28 (0 V) drops from the regulated 24.0 V to approximately 17-18 V DC coincident with the BOP indication. The fault recurs approximately 7-8 hours after each repair attempt, indicating a progressive hardware degradation rather than a one-time transient event.

Substituting the Basic Operator Panel (BOP) and the I/O board with units from a known-good running drive produces no change in the symptom, which excludes those modules as root cause and localises the fault to the main control/power supply board. The 415 V AC incoming supply is verified stable. Field wiring has been tested by replacing the Siemens drive with an alternative manufacturer's drive on the same wiring, which operates without fault. The problem is therefore internal to the Siemens MM430 itself, specifically in the power supply unit that derives the 24 V DC rail from the DC link.

This article provides the engineering field reference for that specific failure mode, including the internal 24 V architecture, the diagnostic measurements that confirm the failure, the root cause analysis, and the available service options for a frame size C chassis.

Affected Drive Configuration

The affected unit is part of the 6SE6430 series MICROMASTER 430 product family. For 15 kW at 380-480 V three-phase, the catalog number is 6SE6430-2UD42-5GA0 (or the regional -5GB0/-5GC0 variants). Frame size C chassis covers the 7.5 kW through 15 kW power range in the 400 V class, with the following envelope:

Confirm the exact variant by reading the product code on the drive's nameplate, including the firmware version (for example, V2.0, V3.0, V3.2). MICROMASTER 430 firmware versions are not always firmware-compatible across the product life, and the parameter list manual differs by version. Reference the operating instructions and parameter lists for the specific firmware on the unit. Operating instructions and parameter lists are available from Siemens Industry Online Support by searching the catalog number.

Symptom Pattern and Timeline

The reported failure is not a single-event trip. The chronology is reproducible and that is the most useful diagnostic data:

1. Drive runs normally for 7 to 8 hours after power-up or after the most recent local repair.
2. Without any change in process load or command state, the BOP display changes from the running frequency (for example, 50.00 Hz) to "----".
3. Simultaneously, the drive removes its run command and the motor coasts to a stop.
4. Voltage measured between terminals 9 and 28 falls from a regulated 24.0-24.5 V DC to approximately 17-18 V DC while the BOP dashes are visible.
5. Removing input power (415 V AC off, wait for DC link discharge) and re-applying power restores normal display and operation for another 7-8 hours.
6. After two local repairs of the same drive, the fault recurs on the same timescale.

This pattern is inconsistent with a software fault, parameter fault, or input-side power-quality issue. It is consistent with a thermally progressive component failure: a capacitor with elevated ESR (equivalent series resistance) that drifts further as it warms, or a magnetic component (transformer core) that saturates as the converter runs and heats. The 7-8 hour window typically aligns with the time for the drive's internal ambient to reach steady-state operating temperature inside the enclosure.

Internal 24 V DC Architecture in MM430

The MM430 does not use an external 24 V supply for its own control electronics. The internal 24 V rail is generated on-board by an isolated DC-DC converter that takes its input directly from the DC link. The path is:

1. Three-phase 415 V AC input → input rectifier bridge → DC link capacitors → DC link rail (typically 540-620 V DC, with switching ripple superimposed).
2. DC link rail → high-voltage bus to the IGBT inverter module and to a separate auxiliary SMPS (switched-mode power supply) that generates the 24 V rail.
3. Auxiliary SMPS: a flyback or forward converter using a high-voltage switching transistor, an isolating high-frequency transformer, a secondary rectifier, output filter, and a regulation loop with optocoupler isolation.
4. Regulated 24 V DC output drives the control board (microprocessor, memory, gate drivers), the BOP interface, the digital inputs via optocouplers, and the digital/analog output circuits.
5. Terminal 9 (+24 V) and terminal 28 (0 V) expose this internal rail to the field. Maximum available current is typically 100 mA for the frame C MM430, which is sufficient to source a small number of digital inputs and a BOP, but not sufficient to source relay coils or field loads.

When the auxiliary SMPS can no longer hold regulation under load, the rail droops. As soon as the rail falls below the brown-out threshold of the microprocessor (typically 4.7-5.0 V on the internal 3.3 V or 5 V post-regulator, which corresponds to a 24 V rail droop to roughly 18-19 V), the processor resets. The BOP, which is connected to the same 24 V rail and exchanges data via a serial link, loses the link to the control board and displays "----" in lieu of a normal value.

DC Link to 24 V Conversion Path

The auxiliary SMPS is the component that has failed in this case. The following sub-circuits are present and the failure modes of each are well documented in field service:

The most common aging-related failure on a unit of this age and duty cycle is the output electrolytic capacitor bank. As the capacitor dries out, ESR rises. The output impedance at the ripple frequency increases, the regulation loop loses headroom, and the rail droops under transient load. The BOP, which is the most spike-sensitive load on the 24 V rail, is the first to lose the link - hence the "----" indication.

The secondary suspect is the optocoupler in the feedback path. As the CTR degrades (a normal aging mechanism, accelerated by heat and by current above the recommended operating point), the regulation loop sees less feedback signal. The controller compensates by increasing the primary duty cycle. Once the controller reaches maximum duty cycle, the rail droops and brown-out follows.

The third suspect, less common but observed, is the HF transformer core. If the core is mechanically loose or has been thermally cycled excessively, the saturation flux density drops, the primary current ramps sharply, and the controller enters current-limit, again reducing the available output.

Diagnostic Procedure and Measurements

The following procedure is non-destructive and can be run with the drive installed in the panel, with the motor mechanically isolated if necessary. Use a true-RMS digital multimeter with at least 10 MΩ input impedance and, ideally, an isolated oscilloscope for the ripple measurement.

1. Verify supply at the drive terminals. Measure L1-L2, L2-L3, L1-L3 at the input terminals. All three voltages should be within ±10 % of nominal (374-457 V on a 415 V system), and the unbalance should be below 2 %. Any unbalance greater than 2 % drives negative-sequence currents in the rectifier that can produce DC link ripple that stresses the SMPS.
2. Measure DC link voltage. With the drive in stop, measure between DC+ and DC- (the terminals behind the removable shroud). For a 415 V system, expect 580-620 V DC. If the DC link is low or unsteady, the SMPS is being fed an under-voltage condition and the droop at the 24 V output is downstream of the input, not a fault of the SMPS.
3. Measure the 24 V rail with the drive in stop. Between terminals 9 (+24 V) and 28 (0 V), expect 24.0-24.5 V DC. A reading above 25.0 V or below 23.5 V in stop already indicates the regulation loop is not healthy.
4. Measure the 24 V rail during run, immediately after start. Expect 24.0 V ±0.2 V.
5. Measure the 24 V rail over a 30-minute soak at full load. A drift to 23.5 V or below within 30 minutes indicates an output capacitor or optocoupler that is not holding up under heat soak.
6. Measure the 24 V rail over an 8-hour soak at full load. A drift to 17-18 V within 8 hours, as observed in this fault, confirms a thermally progressive failure in the SMPS.
7. Scope the 24 V rail. Use an isolated scope probe. The high-frequency ripple should be below 100 mV peak-peak. A ripple above 200 mV peak-peak is a clear sign of dried-out output capacitors. Capture the rail at the moment of the "----" event - the waveform will show a sudden droop to 17-18 V and a large low-frequency component riding on top of the HF ripple.
8. Check the current sourced from the 24 V rail. If the panel is using terminal 9 to source field DI and the field wiring is long or has a partial short, the 24 V supply can be loaded to the point of droop even with a healthy SMPS. Disconnect all field wiring from terminals 9 and 28 and re-measure. If the rail recovers to 24.0 V with no field load, the SMPS is fine and the issue is field loading.
9. Swap test the BOP and the I/O board. Already performed by the operator. Because the symptom persists, those two modules are exonerated as the root cause.
10. Check the cooling fan. A failing fan will not produce this exact symptom on its own, but an overheating drive will accelerate SMPS degradation. Listen for bearing noise, feel for airflow at the heatsink fins, and confirm the fan runs whenever the DC link is charged.

When the readings above are taken in stop and the rail is 24.0 V, the issue is not "is the SMPS dead" - the SMPS is clearly producing output. The issue is "the SMPS is producing output that is within regulation in cold conditions but drops out of regulation as the converter warms". This points squarely at the output capacitor and the optocoupler feedback path.

BOP Communication Link and "----" Indication

The BOP (Basic Operator Panel) is a 4-character 7-segment display with a small rotary encoder and a few keys. It connects to the MM430 control board via a 4-pin connector carrying power, ground, and a half-duplex serial link. The serial link is not standard RS232 or RS485; it is a Siemens-proprietary asynchronous protocol running at 19.2 kbps in the standard BOP variant.

The control board is the BOP master. It transmits display updates (frequency, current, parameter values, fault codes) and receives key presses. When the control board resets, loses its 5 V internal rail, or experiences a sustained brown-out, it stops transmitting. The BOP, finding no incoming frames, displays "----" rather than the last value, to avoid showing a misleading stale value.

This is an important distinction. If the drive were simply running into a fault such as overcurrent (F0001), overvoltage (F0002), or motor overload (F0005), the BOP would show a fault code, not "----". The "----" indication should be read as "the BOP has lost the link to a working control board", not as a particular fault. The control board itself, when interrogated over RS485 via the USS/Modbus terminals (29, 30) with a PC running STARTER or DriveMonitor, may still respond and may show no fault, or it may show a fault that occurred at the moment of the brown-out (often a comms-loss to the BOP, sometimes F0085 or similar, depending on firmware version).

Always cross-check using r0947 (fault number) and r0948 (fault value) via a PC connection, especially if the BOP is showing "----" and the drive appears to be in a fault state but no code is visible.

Root Cause Analysis

Combining the symptom pattern, the field measurements, and the failure modes of the auxiliary SMPS, the root cause is one of the following (in order of probability for a unit of this age and duty cycle):

1. Output electrolytic capacitor degradation (most likely). ESR has risen, capacitance has fallen, and the regulation loop can no longer hold 24.0 V under the BOP load when the drive is hot.
2. Optocoupler CTR degradation in the feedback path (likely co-failure). Even with replaced output capacitors, the rail may droop if the optocoupler is no longer passing enough current to the primary-side error amplifier.
3. HF transformer saturation (less likely). Only if the unit has been subjected to severe over-temperature events or mechanical shock (shipping damage, panel impact).
4. Secondary rectifier diode (possible). Forward voltage has increased with thermal aging, eating into the regulation budget.
5. PWM controller IC (rare). The UC3842-class device at the heart of the SMPS rarely fails except as a secondary effect of another failure above.

The 7-8 hour recurrence after repair is the most diagnostic data point. The unit does not fail immediately after power-up; it fails after reaching thermal steady-state. A "failed and now fixed" component cannot be the sole cause unless the repair itself was marginal. In practice, this means the local repair likely replaced one or two obviously stressed components (the most visible electrolytics, for example) and missed the less obvious aged component (typically the optocoupler or a less accessible capacitor). A second pass at component level is needed, or a replacement of the entire SMPS subassembly if one is available as a service part.

For a frame size C unit, the SMPS may be implemented as part of the main control board rather than a separate plug-in card. This is the case on most frame A-C MM430 units. On frame D and larger, the power supply is a separate subassembly that can be replaced in the field. On frame A-C, the SMPS components are soldered to the main control board, and repair requires component-level rework by a skilled technician.

Frame Size C Service Considerations

Siemens' official service position for MICROMASTER 4 frame sizes A through C is that the unit is not economically viable to repair at factory level. The manufacturer will typically offer replacement with a new or refurbished unit rather than a board-level repair. The practical implications for the end user are:

• OEM repair path: The factory will not repair. The factory will quote a replacement unit (new or refurbished) on an exchange basis. Lead time is typically 5-15 working days depending on stock and region.
• Local repair path: A third-party repair house with component-level capability can replace the failed SMPS components on the existing control board. This is faster (typically 1-3 days) and cheaper (typically 20-40 % of the cost of a replacement unit), but the long-term reliability depends entirely on the skill of the technician and whether all aged components are replaced preventively, not just the failed one.
• Upgrade path: For new installations or where the existing unit is at end of life, replacement with a SINAMICS G120 (the current Siemens general-purpose drive family) is the strategic choice. The G120 is form-fit-function compatible with the MM430 in many applications, although the parameter mapping and BOP are different. Re-commissioning is required.

Field Wiring and EMC Considerations

The operator's test of swapping the Siemens MM430 with another manufacturer's drive on the same field wiring, and confirming that the other drive operates without fault, is a strong negative result for the field wiring. It demonstrates that the wiring, the field devices, the start/stop pushbuttons, and the 24 V sourcing for the DI are not the proximate cause of the fault on the Siemens drive. That said, the following field-side practices are worth applying as a hygiene measure, even if they are not the root cause of the current fault:

• Do not source field wiring from the drive's internal 24 V if the field wiring is long or distributed across the panel. The MM430's terminal 9/28 supply is rated for the drive's own DI optocouplers, not for distribution. If a wire short or partial short occurs in the field, the SMPS will droop and the symptoms will look like the current fault. Use an external 24 V supply (for example, a Siemens SITOP PSU) and let the drive's DI sink from that.
• Use relay interposing between field devices and the drive DI. A 24 V relay coil is a much more benign load than a long cable run. The relay contact is then wired to the drive DI.
• Keep DI cables physically separated from power cables inside the panel. Cross at 90° if they must cross. Do not run DI in the same cable tray as the motor output cables (U, V, W) or the input supply cables.
• Check for noise sources. If the panel also contains contactors, chokes, or transformers, verify that the DI cables do not run parallel to the noise source for any significant length.

These measures will not fix the current PSU fault, but they will eliminate a class of noise- and loading-induced false positives that can mask the real fault and lead to repeated unsuccessful repairs.

Parameter Investigation

The following parameters are relevant when investigating a "----" on BOP event. Always verify against the parameter list for the specific firmware version installed on the unit. The MM430 parameter list manual is the 6SE6400-5AD00-0BP0 reference at the time of writing, and is available from Siemens Industry Online Support:

Use a PC connection over RS485 (terminals 29 and 30, USS protocol) with the Siemens STARTER commissioning tool or DriveMonitor. This bypasses the BOP entirely and provides stable access to r0019, r0947, r0948, r0954, and the rest of the parameter tree, even while the BOP is showing "----".

Repair, Replace, or Local Service Options

For a frame size C MM430 with a confirmed internal 24 V SMPS fault, the three practical paths are:

Recommendation: If the application is non-critical, go with the local component-level repair and demand a written 8-hour thermal soak test certificate from the repair house. If the application is critical, go with the SINAMICS G120 upgrade path and budget for a re-commissioning. The Siemens exchange path is a middle ground that preserves the existing wiring and parameter set (with a parameter upload/download via STARTER) but does not future-proof the installation.

Verification and Commissioning

After any repair or replacement, the following verification steps should be completed before returning the drive to production service. The first three steps are mandatory; the remaining steps are recommended for critical applications.

1. 24 V rail measurement in stop. 24.0-24.5 V DC between terminals 9 and 28.
2. 24 V rail measurement in run, at zero speed. Same range, no droop.
3. 24 V rail measurement in run, at full load, over 8 hours. Must stay within 23.5-24.5 V DC for the full 8 hours. If the rail droops at any point in the 8-hour soak, return for further repair.
4. 24 V rail ripple check (oscilloscope). Less than 100 mV peak-peak HF ripple on the 24 V rail at full load.
5. BOP link stability check. Run the drive for 8 hours and confirm the BOP never displays "----" or stutters, and the displayed frequency does not freeze.
6. Fault history check. Read r0947 / r0948 / r0954 after the 8-hour soak. The list should not have grown.
7. DC link voltage check under full load. The DC link should be stable within ±5 % of nominal under all load transitions (no-load to full-load, full-load to no-load).
8. Thermal check. After the 8-hour soak, measure the heatsink temperature with an IR thermometer or thermal probe. The heatsink should be below 80 °C at rated load. If the heatsink is above 90 °C, the cooling fan or the panel ventilation is suspect and should be addressed.
9. Insulation test. With the drive isolated from the supply, perform an insulation resistance test between each input phase and earth at 500 V DC. Expect >1 MΩ. A low reading suggests ingress or contamination.
10. Functional test of all DI/DO/AI/AO. Force each input and read each output, and confirm the expected drive response (start, stop, setpoint scaling, output indication).

If any of the above fails, return the drive to the repair house. Do not put a drive with a marginal 24 V rail into production service - the next failure is what causes the unscheduled downtime that the marginal repair was supposed to prevent.

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