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Implementing Inverter Coast-to-Stop (OFF2) with Sinamics G120 and PCS 7 APL MotSpdCL

Implementing Inverter Coast-to-Stop (OFF2) with Sinamics G120 and PCS 7 APL MotSpdCL

To implement an emergency coast-to-stop (OFF2 / Coast Down / Impulssperre) on a SINAMICS G120 drive (CU240E-2 DP) controlled by SIMATIC PCS 7 APL block MotSpdCL using Siemens Telegram 352 (PZD-6/6): 1) In PROFIdrive architecture, OFF2 is controlled by Control Word 1 (STW1) Bit 1, which is low-active (1 = Operating condition / pulses enabled, 0 = Immediate pulse suppression / coast to stop); 2) When using the standard APL driver MotSpdCL coupled with a telegram adapter or Drive ES block, interconnect the emergency trip signal to the drive block's dedicated trip or coast-stop pin (e.g., Trip or ProtEn); or 3) In custom or direct telegram mapping, clear Bit 1 of STW1 upon safety interlock activation, causing the inverter to immediately de-energize its IGBT pulses and allow the motor to coast to a stop by mechanical friction.

Table of Contents

Executive Summary

In process drive applications—such as centrifugal decanters, high-inertia hammer mills, and agitators—different operational and safety stopping profiles are required depending on plant conditions. While a normal operational stop (OFF1) ramps the motor down along a controlled deceleration ramp, an emergency trip often requires an immediate coast-to-stop (OFF2 / Impulssperre) to decouple motor torque instantly without electrical regenerative braking. When integrating SINAMICS G120 variable frequency drives (utilizing Control Unit CU240E-2 DP) into SIMATIC PCS 7 via standard Telegram 352 (PZD-6/6), engineers frequently seek the correct CFC interconnection using the APL motor block MotSpdCL. This technical reference details the PROFIdrive control word bit definitions, telegram mapping mechanics, and CFC implementation procedures.

Architecture and Fundamental Mechanics

PROFIdrive Stopping Profiles and Telegram 352 Mechanics

+-------------------------------------------------------------------------+
|                    PROFIdrive Stopping Modes Comparison                 |
+-------------------------------------------------------------------------+
| Mode | Name       | STW1 Bit | Physical Inverter Action                 |
|------|------------|----------|------------------------------------------|
| OFF1 | Ramp Stop  | Bit 0    | Controlled deceleration along ramp (p1121)|
| OFF2 | Coast Stop | Bit 1    | Immediate pulse suppression; free coast  |
| OFF3 | Fast Stop  | Bit 2    | Emergency rapid stop along ramp (p1135)  |
+-------------------------------------------------------------------------+

TELEGRAM 352 (PZD-6/6) WORD STRUCTURE:
+-------------------------------------------------------------------------+
| Word 1 (STW1): Control Word 1                                           |
|   - Bit 0: ON / OFF1 (1 = Run, 0 = Ramp Stop)                           |
|   - Bit 1: Operating Condition / No OFF2 (1 = Enable Pulses, 0 = COAST) |
|   - Bit 2: Operating Condition / No OFF3 (1 = Enable Pulses, 0 = FAST)  |
|   - Bit 3: Pulse Enable                                                 |
|   - Bit 10: Control by PLC                                              |
| Word 2 (NSOLL_A): Speed Setpoint (16-bit normalized: 4000H = 100%)      |
| Word 3 to 6: Configurable Process Data                                  |
+-------------------------------------------------------------------------+
                                    ^
                                    |
+-------------------------------------------------------------------------+
|                         PCS 7 CFC Architecture                          |
|                                                                         |
|   [Emergency Interlock: Intlock / Protect]                             |
|                     |                                                   |
|                     v                                                   |
|   +-----------------------------------------------------------------+   |
|   | APL MotSpdCL / Drive Channel Driver (Drive / SinaG120)          |   |
|   |                                                                 |   |
|   | Standard Run: STW1 Bit 0 = 1, Bit 1 = 1, Bit 2 = 1              |   |
|   | OFF1 Command: STW1 Bit 0 = 0, Bit 1 = 1, Bit 2 = 1              |   |
|   | OFF2 Command: STW1 Bit 1 = 0  <-- [PULSES CUT IMMEDIATELY]      |   |
|   +-----------------------------------------------------------------+   |
+-------------------------------------------------------------------------+

Why OFF2 Differs from Standard APL Stop

  • Standard Stop (Stop pin on MotSpdCL): Clears STW1 Bit 0 (OFF1). The drive decelerates the motor along parameter p1121 (Ramp-down time). If braking energy is high, this may require braking resistors or can trip on DC-link overvoltage.
  • Coast Stop (OFF2): STW1 Bit 1 is active-low (No OFF2). Setting this bit to 0 immediately cuts off the gate pulses to the inverter power transistors (IGBTs). Motor torque drops to zero within microseconds, and the rotor spins freely until slowed by load resistance and friction.

Step-by-Step Implementation Procedure

Step 1: Configure Telegram 352 in HW Config and Drive

  1. In HW Config, insert the SINAMICS G120 (CU240E-2 DP) into the PROFIBUS-DP network.
  2. In the module configuration, select Standard Telegram 352, PZD-6/6.
  3. Note the I/O base addresses (e.g., IW 512 and QW 512).
  4. Using STARTER or Startdrive, parameterize the G120 drive:
    - Set p0922 (Telegram selection) = 352.
    - Ensure p0844 (OFF2 source 1) is mapped to r2090.1 (STW1 Bit 1 from PROFIdrive).

Step 2: CFC Implementation with PCS 7 Drive Solution / Drive ES

  1. In CFC, insert the APL motor block MotSpdCL (FB1854).
  2. Insert the telegram channel driver block (e.g., DRIVE FB286 or SinaG120).
  3. Connect the driver output word to MotSpdCL.
  4. Connect the process emergency trip or safety interlock to MotSpdCL.Protect or MotSpdCL.Trip.
  5. On the driver block, locate the specific input for OFF2 (e.g., Off2 or CoastStop).
  6. Connect the inverted emergency trip signal so that when a trip occurs, Off2 is asserted (0), clearing STW1 Bit 1.

Step 3: CFC Implementation with Standard APL Logic (Direct Word Assembly)

  1. If building custom telegram logic without Drive ES:
  2. Create an SCL or CFC bit-packing structure using StruDiOu or W_BO to assemble STW1.
  3. Map logic bits:
    - Bit 0 (ON/OFF1): Connected to MotSpdCL.Start.
    - Bit 1 (No OFF2): Connected to MotSpdCL.Protect.Value (1 = Healthy, 0 = Coast Stop).
    - Bit 2 (No OFF3): Connected to 1 (or emergency stop ramp).
    - Bit 10 (Control by PLC): Tied to constant 1.
  4. Route the assembled control word to the fieldbus driver Pcs7DiOu or directly to output peripheral address PQW 512.

Step 4: Configure Reset and Acknowledgment

  1. After an OFF2 coast stop, SINAMICS drives latch into an OFF2 inhibit state.
  2. To re-enable the drive, STW1 Bit 1 must be restored to 1, followed by a fault acknowledgment toggle on STW1 Bit 7 (Fault Reset), which MotSpdCL executes via its RstOp command.

Verification

  1. Normal Ramp Stop Test: Run the drive at 50 Hz. Issue a normal Stop command from the MotSpdCL faceplate. Observe that the motor decelerates steadily along its 10-second ramp to 0 Hz (OFF1 active).
  2. Emergency Coast Stop Test: Run the drive at 50 Hz. Trip the emergency input (Protect = 0). In STARTER or on the drive BOP-2 display, verify that drive status transitions immediately to n=0 / Coast Stop active (p0844), IGBT pulses cut off instantly, and the motor spins freely without electrical deceleration braking.
  3. Reset and Restart Test: Clear the trip condition, click Reset on the MotSpdCL faceplate, and confirm that the drive returns to Ready for Switching On status.

Limitations and Operational Precautions

  1. Safety Standards Compliance: Software-commanded OFF2 over standard PROFIdrive telegrams is operational coast-stopping; it does not fulfill certified Safety Instrumented System (SIL 3 / PL e) requirements. For functional safety, use certified PROFIsafe (e.g., STO - Safe Torque Off via Telegram 30/900) or hardwired safety terminals on the CU240E-2 DP-F.
  2. Free-Wheeling Hazard: High-inertia loads may take several minutes to coast to a complete stop during OFF2; ensure mechanical hazards are properly guarded or interlocked with zero-speed monitors.

Engineer FAQ

What is the difference between OFF1, OFF2, and OFF3?

OFF1 ramps to stop along p1121; OFF2 immediately cuts inverter pulses for free coasting; OFF3 performs rapid emergency stopping along fast ramp p1135.

Which control word bit controls OFF2?

STW1 Bit 1, which is low-active: 1 means operating condition (No OFF2), and 0 commands immediate coast stop.

Can MotSpdCL execute OFF2 directly?

Standard MotSpdCL controls run/stop via Bit 0; executing OFF2 requires mapping the trip interlock to STW1 Bit 1 via a drive driver block or telegram logic.

What is Telegram 352?

A standard Siemens PROFIdrive telegram consisting of 6 transmit and 6 receive process data words (PZD-6/6), including STW1, setpoint, and actual values.

Does OFF2 cause DC-link overvoltage trips?

No. Because pulses are cut off immediately, no kinetic energy is converted back into electrical energy, completely avoiding regenerative overvoltage.

How do I clear the drive after an OFF2 trip?

Restore Bit 1 (No OFF2) to 1, clear the trip cause, and send a fault acknowledge pulse via MotSpdCL RstOp.

Is OFF2 considered functional safety (STO)?

No. Standard telegram OFF2 is process-level pulse suppression. Functional safety requires PROFIsafe STO (Safe Torque Off) or hardwired safety terminals.

Why did the drive not restart after clearing OFF2?

The drive requires an OFF1 edge (0 to 1 transition on STW1 Bit 0) to restart after an OFF2 trip condition is cleared.

What drive parameter monitors OFF2?

Parameter r0046 (Missing enable signals) shows if bit 1 (OFF2 enable) is missing.

Where can I find Siemens Drive ES blocks?

Drive ES PCS 7 is an optional library package available through an authorized Siemens distributor or the Siemens Industry Mall.

Technical References

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Related Tags

  • Sinamics G120
  • MotSpdCL
  • OFF2
  • Coast Down
  • Telegram 352
  • PROFIdrive
  • Drive ES
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