SINAMICS S120 Torque Boost: Sizing 1FK7 Servo Torque Limits

1. Problem Statement and Engineering Context

A common commissioning question on SINAMICS S120 systems pairing a CU310-2 PN Control Unit with a 1FK7 permanent-magnet synchronous servo is: "How do I increase the torque of the motor to 200% of rated torque by parameter?" The question conflates two distinct engineering concepts that must be separated before any parameter is changed:

1. Torque limit setting – the maximum torque the drive is allowed to command. This is set via P1520/P1521 (torque limits) and P1530/P1531 (power limits). These are upper boundaries.
2. Torque availability – the actual mechanical torque the motor can deliver continuously or intermittently without exceeding thermal winding limits, magnetic saturation, or the power module's current capability.

Increasing a parameter does not create torque. It only raises the ceiling under which the closed-loop torque controller is permitted to operate. The actual delivered torque is still bounded by the inverter's maximum output current, the motor's thermal time constants, and the mechanical load demand.

Before any modification, confirm three things on the existing project:

• The reported actual torque r0078 is being limited by a parameter (not by the load itself).
• The line-side power module (e.g., 6SL3120-… blocksize or 6SL3130-… booksize) is rated for the desired overload duty cycle.
• The motor's thermal model in P0611 and P0612 is intact so a winding overtemperature fault (F07011, F07012, F07013) is not being masked.

2. Reference Hardware Identified in the Project

The 1FK7101-2AC71-1QB0 is a permanent-magnet synchronous motor (PMSM). It has no squirrel cage, no rotor bars, and no slip. Any parameter that influences induction-motor rotor flux (such as voltage boost at low speed) has no physical meaning on this motor. This single fact rules out several parameters commonly suggested for legacy induction-motor torque shaping.

3. Torque and Current Limit Architecture in SINAMICS S120

The torque limiting chain in S120 is hierarchical. The lowest of the following active limits determines the maximum current the closed-loop torque controller can command:

1. Current limit – P0640 (factor, default 100%), applied as a percentage of the maximum inverter current.
2. Motor current limit – derived from P0304 (rated motor voltage), P0305 (rated motor current), P0307 (rated motor power), and P0311 (rated motor speed). On a configured motor with DRIVE-CLiQ, these are auto-loaded from the motor electronic rating plate and are normally read-only.
3. Torque limits – P1520 (upper, motoring) and P1521 (lower, regenerating), expressed in Nm.
4. Power limits – P1530 (motoring) and P1531 (regenerative), expressed in W or in percent of rated motor power (unit switch via P1532).
5. System-level torque limits – P1522/P1523 (Cl: upper/lower torque limit) and P1524/P1525 (scale upper/lower). These accept connector inputs from safety (STO/SS1), telegram limit words, or TLC/TLI bits from the higher-level controller.
6. PROFIdrive telegram limit words – in Telegram 3/5, the controller can transmit M_LIM_P / M_LIM_N in STW2 or a dedicated supplemental telegram word.
7. Thermal model – P0611/P0612 I²t integration of stator current; will fault the drive (F07011, F07012, F07013) before winding damage.

Setting P1520 to "200%" does not literally write 200. The value is interpreted as the upper torque limit in the unit selected by P1522. For a PMSM, the practical interpretation is to convert "200% of rated torque" to a numeric Nm value using the motor's rated torque and write that to P1520.

4. Parameter Reference Table

Reference the SINAMICS S120/S150 List Manual (LH1) for the authoritative description of every parameter listed.

5. Why P1310, P1311, and P1312 Do Not Apply to a 1FK7 Servo

Similarly, P0338 is not a parameter in the SINAMICS S120 standard list; it originates in legacy MICROMASTER 4 parameter sets. On S120 the equivalent value is the rated motor current in P0305, which is read from the DRIVE-CLiQ motor electronic rating plate and is not normally writable. If P0338 appears in a third-party tool, treat it as a mapping onto P0305.

6. Prerequisites for Safe Torque-Limit Increase

1. Confirm motor type. P0300 = 2 (synchronous servo motor). P0301 = 1FK7101-2AC71-1QB0 (or the matching MLFB if STARTER auto-load failed).
2. Verify motor identification. Run p1900 = 2 (rotating measurement) or p1900 = 1 (standstill) once with the motor uncoupled, before relying on torque data.
3. Identify the power module. Note the MLFB (e.g., 6SL3120-1TE21-0AA4) and look up its overload capability in the datasheet. Typical SINAMICS S120 blocksize ratings: 200% rated current for 3 s with a 5 s cycle, or 150% for 60 s with a 300 s cycle, depending on the specific module.
4. Compute requested torque in Nm. "200% of rated torque" must be written as a numeric value, not as a percentage. For example, if the motor's rated torque is 22 Nm, the upper torque limit becomes 44 Nm. Cross-check this against the rated motor current: T_max_continuous = T_rated × (I_max_cont / I_rated).
5. Confirm mechanical margin. Gearbox, coupling, and shaft must accept 200% rated torque. A 34.8:1 gear ratio amplifies reflected inertia and torque transients. If the gearbox is rated for only 1.5× its nominal duty, raising the motor torque to 200% may mechanically saturate the gear first.

7. Step-by-Step Configuration in STARTER / Startdrive

7.1 Open the Drive Offline View

1. Launch TIA Portal with Startdrive (recommended for S7-1200 + S120 + CU310-2 PN) or the standalone STARTER / Scout.
2. Connect to the CU310-2 PN via PROFINET or Ethernet service interface (X127, IP 169.254.11.22 default).
3. Open the drive object DO1 (Servo) and navigate to Parameter list → Expert list.

7.2 Read the Motor Electronic Rating Plate

Navigate to Commissioning → Motor data. The following fields must match the MLFB and ambient conditions:

• P0304, P0305, P0307, P0310, P0311, P0335, P0341, P0342, P0344 – all read-only if the motor is recognized.
• P0611 thermal time constant and P0612 warning threshold.

7.3 Set the New Torque Limits

Calculate the desired upper torque limit in Nm. Example formula:

T_limit_upper_Nm = (rated_motor_torque_Nm) × (desired_factor)

For a 200% request with a rated torque of, e.g., 22 Nm:

P1520 = +44.00 (upper, motoring) P1521 = -44.00 (lower, regenerative, if regen is possible)

Notes:

• P1520 and P1521 accept a connector input through P1522/P1523. If those connectors are driven (e.g., from a Telegram 5 M_LIM_P), the online value at the connector overrides the P1520/P1521 numeric entry.
• If you want the PLC to control the limit online, leave P1520/P1521 as defaults and write P1524/P1525 scale factors (0–100%) from the S7-1200 program.

7.4 Set the Power Limits

1. Switch the unit: P1532 = 0 (kW) or leave the default.
2. Set P1530 to the desired motoring power. If you want 200% rated power, set the value to twice the value in r0333 (rated motor torque × rated motor speed × 2π / 60).
3. Set P1531 symmetrically if regeneration is allowed into the DC bus. If a braking resistor or active line module is absent, leave P1531 at the default to avoid over-voltage on the DC link.

7.5 Raise the Current Limit Only If Required

P0640 is normally 100% and rarely needs adjustment on a correctly sized system. Raising it allows the inverter to command currents above its continuous rating. The practical effect on the S120 PM module:

• If P0640 × I_rated exceeds the inverter's peak current capability, the drive will fault F30001 or F30002 (overcurrent).
• P0640 × I_rated is within peak but exceeds the I²t of the power module, you will see A05001 / F05000 warnings or faults from the inverter thermal model.

Prefer raising P1520/P1521 (motor torque limits) rather than P0640. The former limits the actual torque; the latter limits raw inverter current regardless of torque produced.

7.6 Save and Copy RAM to ROM

Use Online → Copy RAM to ROM in STARTER, or execute p0977 = 1 in the expert list. Without this step the changes are lost on the next power cycle.

8. Verification with the Trace Function

Open Commissioning → Trace and configure the following signals at a 4 ms or 1 ms sample rate:

Trigger the trace on a positioning move that requires high acceleration (typically the S7-1200 MC-MoveVelocity with a very high Acceleration parameter). Examine the resulting plot. A correctly configured system shows:

1. Torque plateau near the configured limit during acceleration.
2. No F07011, F07012, F07013 thermal faults.
3. No F30001/F30002 overcurrent faults.
4. r0034 motor load may approach 100% briefly, then recover during the next low-demand phase.

9. S7-1200 Integration and Telegram Considerations

When the S7-1200 controls the drive via PROFINET using a PROFIdrive telegram, the telegram structure constrains how limits are applied:

If the project uses the S7-1200 technology object TO_SpeedAxis or TO_PositioningAxis with the SINAMICS S120 drive object, the standard telegram is configured in the device configuration (Startdrive). To allow online torque scaling from the PLC:

1. Connect P1522 to the analog/connector source for upper torque limit (e.g., a free function-block output).
2. Connect P1523 similarly for the lower limit.
3. Drive P1524 / P1525 from the PLC with a percentage (0–100% or 0–199% for the boost case, but clamped to P1520/P1521).

For S7-1200 motion control, the higher-level SINA_SPEED or SINA_POS blocks from the SINAMICS support library handle the bits and scaling. See the SINAMICS Startdrive Communication Blocks for S7-1200 application example.

10. Hardware Overload Considerations

The CU310-2 PN is the control head; the actual power stage is a separate Line Module + Motor Module combination. The overload capability depends entirely on the Motor Module selected. Always consult the specific power module datasheet before requesting 200% torque:

• Blocksize (5–50 A) – typical overload: 150% rated for 60 s with 300 s cycle, or 200% for 3 s with 5 s cycle.
• Booksize (3–200 A) – typical overload: 200% for 3 s with 5 s cycle; higher for short pulses.
• Active Line Module (Smart Line, Basic Line) – limits DC-bus input current; relevant for sustained regen.

The "200% torque" objective is normally a transient requirement: acceleration of a high-inertia load through a gearbox with ratio 34.8:1. The reflected inertia at the motor shaft is J_reflected = J_load / N² + J_motor. With N = 34.8, even a modest load inertia appears as a very small value at the motor side, so 200% torque is generally available briefly during the accel phase. The danger is sustained 200% torque, which would not be supported by either the motor thermal model or the inverter thermal model.

11. Fault and Warning Mapping

Refer to the SINAMICS S120/S150 List Manual for the exhaustive fault list and remediation paths.

12. Commissioning Checklist

1. ☐ Confirm motor MLFB and electronic rating plate loaded correctly.
2. ☐ P0300 = 2 (PMSM) verified.
3. ☐ P1900 motor identification executed at standstill at minimum.
4. ☐ P1520 and P1521 set to target Nm values (not percentages).
5. ☐ P1530 and P1531 set to target power values.
6. ☐ P0640 left at 100% unless power module datasheet proves a higher value is safe.
7. ☐ Trace r0078, r0080, r0067, r0034 over a loaded positioning move.
8. ☐ Confirm r0080 reaches the requested factor (e.g., 200%) during acceleration.
9. ☐ No F07011, F30001, F30002 faults during the test cycle.
10. ☐ r0034 motor load returns below warning threshold between cycles.
11. ☐ Copy RAM to ROM executed and verified by power-cycle test.
12. ☐ Mechanical coupling, gearbox, and shaft rated for the new torque envelope.

13. Frequently Asked Questions