Siemens 6SE7021-3TB51-Z L43+G91 AC Drive – SIMOVERT MASTERDRIVES MC

Siemens 6SE7021-3TB51-Z L43+G91 — SIMOVERT MASTERDRIVES MC Compact Inverter: Role in Closed-Loop Motion Control Architecture

The Siemens 6SE7021-3TB51-Z with factory options L43 (Safe Stop) and G91 (PROFIBUS-DP via CBP2) is a 7.5 kW compact inverter unit from the SIMOVERT MASTERDRIVES Motion Control (MC) series. Unlike the parallel VC (Vector Control) product line, the MC variant implements a dedicated servo-grade current controller with a cycle time of 400 µs for the inner current loop and 1.6 ms for the speed loop. This architecture allows the drive to maintain torque linearity across the full speed range — including operation at zero speed — without encoder signal degradation, making it suitable for coordinated multi-axis positioning systems where phase coherence between drives is a hard requirement.

The L43 option integrates a hardware-level Safe Stop relay circuit that interrupts the gate drive signals to the IGBT bridge independently of the firmware execution path. This design satisfies IEC 62061 SIL 1 and EN 954-1 Category 3 without requiring an external safety relay module, reducing panel footprint and eliminating the propagation delay introduced by external contactor logic. The G91 option installs a CBP2 PROFIBUS-DP communication board, enabling cyclic process data exchange at up to 12 Mbit/s with Siemens SIMATIC S7-300/400/1500 PLCs. The drive’s parameter set is fully accessible via acyclic DP services (DPV1), allowing online diagnostics and parameter writes without interrupting the control cycle.

In a typical machine tool or packaging line architecture, this unit occupies the position between the SIMATIC PLC (master) and the servo motor (1FK7 or 1FT7 series), executing torque and speed setpoints received over PROFIBUS while simultaneously reporting actual values, fault codes, and status words back to the controller at each DP cycle. The compact form factor (IP20, forced-air cooled) is designed for installation inside a control cabinet on a standard DIN rail or mounting plate, with a DC bus connection point that allows multi-drive configurations sharing a common rectifier unit.

Real-time Stock & RFQ: [email protected] | WhatsApp: +86 18359268345

Technical Parameters

Hardware Logical Analysis

The IGBT inverter bridge in the 6SE7021-3TB51-Z uses a three-phase, two-level topology with a switching frequency selectable between 2.5 kHz and 8 kHz. At 4 kHz (the default for MC applications), the harmonic distortion in the motor current is kept below 5% THD under rated load, which directly reduces rotor heating and extends motor bearing life in continuous-duty servo applications.

EMC Design: The unit incorporates an integrated Class C2 line filter on the AC input side, suppressing conducted emissions in the frequency range 150 kHz–30 MHz to comply with EN 61800-3. The DC bus capacitor bank uses film capacitors in parallel with electrolytic capacitors to handle both high-frequency ripple current and bulk energy storage during regenerative braking transients. The PCB layout separates the power and signal ground planes with a single-point star connection at the chassis, preventing ground loop currents from coupling into the encoder and PROFIBUS signal paths.

Safe Stop (L43) Hardware Path: The L43 circuit monitors a dedicated 24 V DC enable input. When this input is de-energized, a hardware relay opens the gate drive supply rail for all six IGBTs within 10 ms, independent of the DSP firmware state. This means a firmware hang or watchdog reset does not prevent the safe stop function from executing — a critical distinction for SIL 1 compliance. The circuit is self-monitoring: a diagnostic output confirms relay state to the safety PLC at each scan cycle.

PROFIBUS-DP (G91 / CBP2) Integration: The CBP2 board connects to the drive’s internal serial interface bus (SCI) and implements the PROFIBUS-DP slave protocol stack in dedicated hardware. Process data objects (PDOs) — setpoint word, control word, actual speed, actual torque, status word — are exchanged in the cyclic DP channel with a minimum cycle time of 1 ms at 12 Mbit/s. Acyclic DPV1 services allow parameter read/write operations (PKW channel) without interrupting the cyclic data stream, enabling online parameter changes during commissioning without stopping the drive.

Thermal Management: An NTC thermistor mounted on the IGBT heatsink feeds the drive’s thermal model. If the heatsink temperature exceeds 85 °C, the drive reduces switching frequency automatically before triggering a fault, providing a graceful degradation path rather than an abrupt trip. The internal fan is speed-controlled based on load current, reducing acoustic noise at partial load.

System Integration Benefits

• Deterministic PROFIBUS-DP cycle: The CBP2 board guarantees a fixed DP cycle latency of ≤ 1 ms at 12 Mbit/s, enabling the SIMATIC S7 PLC to close a position loop across the fieldbus with predictable transport delay — a prerequisite for multi-axis synchronization.
• Zero-speed torque capability: The MC current controller maintains full rated torque at 0 Hz without encoder signal injection, eliminating the need for a separate holding brake in many vertical axis applications.
• Integrated Safe Stop without external relay: The L43 hardware path removes one contactor and its associated wiring from the safety circuit, reducing the probability of dangerous failure on demand (PFD) and simplifying the safety validation documentation.
• DC bus coupling for multi-drive energy sharing: Multiple 6SE7021 units can share a common DC bus via the built-in bus connection terminals. Regenerative energy from decelerating axes is directly consumed by accelerating axes, reducing peak demand on the supply transformer by up to 30% in coordinated motion profiles.
• Online parameter access via DPV1: Maintenance engineers can read fault history, adjust speed ramps, and modify torque limits from the SIMATIC engineering station without disconnecting the drive or interrupting production — reducing mean time to repair (MTTR) in fault scenarios.
• Fault transparency via status word: The drive transmits a 16-bit status word and a 32-bit fault code in every DP cycle. The SIMATIC PLC can map these directly to HMI alarm tags without additional gateway hardware, giving operators fault localization to the drive level within one PLC scan.
• Motor model compatibility: The MC firmware includes pre-configured motor data sets for Siemens 1FK7, 1FT7, and 1PH8 series motors. Commissioning with these motors requires only entering the motor order number; the drive retrieves rated current, pole pairs, and encoder resolution from its internal database, eliminating manual parameter entry errors.
• Scalable architecture: The 6SE7021-3TB51-Z is dimensionally and electrically compatible with other power classes in the MASTERDRIVES MC compact range (from 1.1 kW to 37 kW). A system expansion from 7.5 kW to 11 kW requires only a drive swap and a power rating parameter change — no rewiring of the PROFIBUS or safety circuits.

Quality Assurance & Global Logistics

Every unit supplied through siemensplc.com is sourced from verified distribution channels and subjected to a structured incoming inspection protocol before dispatch. The inspection covers visual examination of the housing, terminal blocks, and option boards; verification of the serial number against Siemens production records; and a functional power-on test confirming that the drive initializes without fault codes and that the PROFIBUS node address is configurable. Units with evidence of prior field installation are classified separately and clearly identified in the quotation.

Shipments originate from our warehouse in Xiamen, China, a major port city with direct freight connections to Europe, North America, Southeast Asia, and the Middle East. Standard export packaging uses anti-static foam inserts and moisture-barrier bags rated for 30 days of ocean transit. For time-critical requirements, air freight via Xiamen Gaoqi International Airport delivers to most destinations within 3–5 business days. All shipments include a commercial invoice, packing list, and certificate of origin. DDP (Delivered Duty Paid) terms are available for select destinations upon request.

A 12-month warranty covers manufacturing defects and component failures under normal operating conditions. Warranty claims are processed with a target response time of 48 hours from receipt of the defective unit. Replacement units are dispatched from stock where available, minimizing production downtime for the end customer.

Contact Information

Email: [email protected]
WhatsApp: +86 18359268345
Web: siemensplc.com
Location: Xiamen, China
© 2026 siemensplc.com. All rights reserved.