Siemens 3RW4426-1BC44 Retrofit-Compatible Soft Starter

Siemens 3RW4426-1BC44 Retrofit-Compatible Soft Starter for Legacy Systems

The Siemens 3RW4426-1BC44 is a SIRIUS series electronic soft starter engineered for controlled motor starting and stopping in industrial automation environments. With a rated current capacity suited for medium-to-large motor loads and integrated bypass contactor functionality, this unit is a proven solution for upgrading aging motor control centers, replacing discontinued soft starters, and extending the operational life of legacy control cabinets. Whether you are modernizing a production line, replacing an end-of-life drive component, or restoring a critical automation system, the 3RW4426-1BC44 delivers reliable performance with minimal disruption to existing infrastructure.

This unit is supplied with a 12-month warranty and undergoes pre-shipment functional testing to verify output voltage ramp, current limiting, and bypass relay operation before dispatch. Inventory is maintained in-stock to support urgent retrofit and emergency replacement requirements.

Upgrade Compatibility Table

Retrofit Planning for Existing Automation Systems

Integrating the 3RW4426-1BC44 into an existing control system requires a structured approach to avoid commissioning delays and unplanned downtime. Before installation, engineers should confirm the available power supply capacity in the control cabinet — the soft starter’s control circuit requires a stable 110–230 V AC or 24 V DC auxiliary supply, and the main circuit must be sized for the motor’s full-load amperage with appropriate upstream protection via a Siemens 3RV2 or 3RV1 series motor protection circuit breaker.

Terminal wiring should be mapped against the existing schematic. The 3RW44 series uses a standard L1/L2/L3 input and T1/T2/T3 output terminal layout, but if the legacy installation used an older 3RW3 series soft starter or a third-party equivalent, terminal pitch and torque specifications may differ. A 3RT2 series contactor is typically used upstream for isolation, and its auxiliary contacts may need to be rewired to interface with the 3RW4426-1BC44’s control terminals.

For systems using a Siemens S7-300 or S7-400 PLC as the master controller, the soft starter’s run/fault status outputs should be mapped to the existing digital input module — typically a SM 321 or SM 323 I/O module. If the legacy system used a PROFIBUS DP network for drive coordination, verify whether the 3RW4426-1BC44 requires the optional PROFIBUS add-on module (3RW4900-0KC00) to maintain network communication, or whether hardwired I/O is sufficient for the application.

Backplane and rack space should be confirmed before ordering. In compact control cabinets, the integrated bypass contactor of the 3RW44 series eliminates the need for a separate bypass contactor assembly, reducing overall footprint compared to older 3RW3 installations. However, cabinet depth and busbar clearance must still be verified, particularly in retrofit scenarios where the original layout was designed around a different component profile.

If the system includes a Siemens TP700 or KTP900 HMI panel, confirm that the motor status and fault screens reference the correct PLC data blocks after rewiring. Parameter changes to ramp-up time, current limiting threshold, and motor protection class should be documented and cross-referenced against the original commissioning report to ensure the replacement unit operates within the same performance envelope as the legacy soft starter.

For applications involving signal isolation between the soft starter control circuit and the PLC I/O, a Siemens SITOP power supply module or a dedicated signal isolator may be required to prevent ground loop interference, particularly in older installations with mixed AC/DC control wiring. Inventory of associated components — including spare 3RW4900 series parameter modules and 3RT2 contactors — is recommended to support future maintenance cycles and reduce lead time risk on critical spare parts.

Downtime Control During System Migration

Minimizing production downtime during a soft starter replacement requires advance preparation and a disciplined changeover sequence. Before the maintenance window opens, the replacement 3RW4426-1BC44 should be bench-tested against the motor’s nameplate data, with ramp time and current limit parameters pre-configured to match the legacy unit’s settings. This eliminates on-site parameterization time and reduces the risk of commissioning errors under production pressure.

The original PLC program logic — including motor start/stop sequences, fault handling routines, and interlock conditions — should be backed up and verified before any wiring changes are made. If the legacy soft starter used hardwired control signals, the new unit’s terminal assignments should be pre-labeled and the replacement wiring pre-fabricated to minimize time in the cabinet. For systems where the soft starter is integrated into a PROFIBUS DP or AS-Interface network, the network address and GSD file configuration should be confirmed before the maintenance window to avoid bus communication faults on restart.

A staged restart sequence is recommended: energize the control circuit first, verify fault-free status on the PLC HMI, then apply main circuit power and execute a supervised test start at reduced load before returning the motor to full production duty. This approach protects the original program logic, maintains field control continuity, and allows any wiring or parameter discrepancies to be identified and corrected before full-load operation resumes. Total changeover time for a prepared team is typically 2–4 hours, depending on cabinet accessibility and network complexity.

Retrofit Support FAQ

Q1: Is the 3RW4426-1BC44 a direct drop-in replacement for the 3RW4426-1BC34?
The 3RW4426-1BC44 and 3RW4426-1BC34 share the same mechanical footprint and terminal layout within the 3RW44 series. The primary difference is the control supply voltage specification. Confirm the control voltage available in your cabinet before ordering to ensure compatibility. No mechanical modification is required for the main circuit terminals.

Q2: What commissioning steps are required after installation?
After mechanical installation and wiring, set the motor full-load amperage using the front-panel adjustment or parameterization interface. Configure ramp-up time, current limiting factor, and motor protection class to match the original settings. Perform a no-load test start to verify control signal response and bypass relay operation before connecting the motor under load. Document all parameter settings for future reference.

Q3: Does this unit support PROFIBUS DP communication?
The base 3RW4426-1BC44 operates as a standalone unit with hardwired I/O control. PROFIBUS DP integration requires the optional 3RW4900-0KC00 communication module. If your existing system uses PROFIBUS for soft starter control, confirm whether the add-on module is already installed on the legacy unit or whether it needs to be ordered separately.

Q4: What does the 12-month warranty cover, and is pre-shipment testing included?
All units supplied by NINERMAS COMPANY LIMITED carry a 12-month warranty covering manufacturing defects and functional failure under normal operating conditions. Each unit undergoes pre-shipment functional testing — including voltage ramp verification, bypass relay operation, and current limiting confirmation — before dispatch. In-stock inventory is maintained to support urgent retrofit and emergency replacement requirements with short lead times.