S-1FT7 Power vs Speed Curve: Siemens Simotics Servo Motor Sizing

Overview: Why the Power vs. Speed Curve Matters for S-1FT7

The SIMOTICS S-1FT7 is a permanent-magnet synchronous servo motor from Siemens used primarily in conjunction with the SINAMICS S120 drive system for high-dynamic positioning, robotics, machine tool feed axes, and packaging machinery. Unlike asynchronous induction motors that can be started direct-on-line (DOL), the S-1FT7 must always be operated through a closed-loop controlled drive. This has direct consequences for how the motor's performance envelope is documented and how it must be sized.

For a DOL motor, the catalog presents a single rated point and a thermal curve. For a servo motor such as the S-1FT7, the usable operating range is a continuous surface in the speed-torque (n, M) plane, bounded by:

• Stall torque M0 at zero speed with self-cooling or forced cooling
• The pull-out / breakdown torque limit at every speed
• The maximum mechanical speed nmax (bearing-limited)
• The field-weakening range where the back-EMF approaches the DC-link voltage
• The thermal (S1) limit at the rated cooling condition

Engineers accustomed to reading induction-motor data sheets often look for a "power vs. speed" curve as a single primary graph. For the S-1FT7, the primary curve published by Siemens is torque vs. speed, and the power vs. speed curve is a derived quantity computed from the torque curve and the rotational speed.

Document Location 1: 1FT7 Configuration / Operating Manual

The authoritative, manufacturer-controlled source for S-1FT7 performance curves is the operating manual / configuration manual for the motor family. Chapter 6 of that document is the chapter that contains the speed-torque and torque-current characteristic diagrams.

Official Siemens support entry for the S-1FT7 synchronous motors documentation:

• Manual landing page (ID 109482538): SIMOTICS S-1FT7 synchronous motors - Support
• Direct PDF (English, edition 03/2024): S-1FT7 synchronous motors - Configuration Manual (PDF, 03/2024)

Chapter 6 of this manual presents, for each shaft-height / winding variant:

1. Torque vs. speed (M-n) characteristic for self-cooled motors
2. Torque vs. speed characteristic for forced-cooled motors (where applicable)
3. Torque-current characteristic (M-I) derived from the standstill torque M0 and standstill current I0
4. Permissible S1 (continuous) operating region for the assigned cooling method

From M0 and I0 the user can compute the torque constant kT at standstill using kT [Nm/A] = M0 / I0. This is the same constant that, when multiplied by speed, gives the induced voltage (back-EMF) constant ke, and the relationship ke ≈ kT (in SI units with consistent rad/s and V·s/rad conventions) is what defines the start of the field-weakening region when the drive line voltage is reached.

Document Location 2: TIA Selection Tool Drive Dimensioning

The TIA Selection Tool (TST) is Siemens' central engineering tool for sizing drives, motors, and gearboxes. It is the recommended path for generating application-specific power vs. speed plots for the S-1FT7, because the tool:

• Combines motor data with the SINAMICS S120 line voltage, DC-link voltage, and current limit
• Reflects the actual control mode (servo with field-weakening, vector, or U/f where applicable)
• Applies thermal derating for the selected duty cycle (S1, S6, S3)

Although TST can also be reached by browsing Simotics LV induction motors (1LE0, 1LE1, 1MB1), the S-1FT7 does not appear under that catalog path. The 1FT7 is selected only through the synchronous-servo branch that lives under Drive Technology → Drive dimensioning → S120. Selecting the 1FT7 from the SINAMICS S120 drive dimensioning context is the path that surfaces the servo motor catalog inside TST.

Reference TIA Selection Tool build observed: 2023.11.0.03575. Newer builds follow the same selection logic but add motor variants and the latest SINAMICS firmware ratings.

Step-by-Step: Generate the Power vs. Speed Curve in TIA Selection Tool

The following sequence produces a usable power vs. speed diagram for an S-1FT7 motor matched to a SINAMICS S120 line module and Motor Module.

Prerequisites

• Installed TIA Selection Tool (current delivery; build 2023.11.0.03575 or later)
• Known supply: 3-phase 400 V, 480 V, or 690 V line voltage (as available on the plant)
• Known mechanical duty cycle: continuous (S1), short-time (S2), or periodic (S3/S6) with duty factor
• Known cooling method: natural convection (self-cooled / surface-cooled) or forced ventilation
• Shaft-height preference (e.g. 1FT7 032, 036, 040, 048, 063, 080, 100) based on torque class

Procedure

1. Open TIA Selection Tool and create a new project.
2. Navigate to Drive Technology → Drive dimensioning (do not use the Simotics LV motor configurator path; 1FT7 is not exposed there).
3. Select SINAMICS S120 as the drive platform.
4. Choose the line-module family matching the supply (e.g. Smart Line Module for unregulated DC-link, Basic Line Module, or Active Line Module for lowest harmonics and energy recovery).
5. Add a Motor Module sized for the expected peak current of the S-1FT7 shaft height.
6. In the motor selection list, filter on SIMOTICS S-1FT7. The synchronous-servo catalog appears because the S120 context enables it.
7. Pick the shaft height and the winding / rated speed variant that best matches the mechanical envelope.
8. Open the detail view of the selected motor. The default plot is Torque M vs. Speed n.
9. Use the chart's pull-down menu (y-axis selector) to switch the y-axis from Torque [Nm] to Power [kW]. The x-axis remains the speed axis; TST recomputes the displayed curve using P = 2π·n·M / 60.
10. Optionally overlay the S1 continuous-duty envelope, the S6 duty envelope (e.g. S6-25%, S6-40%, S6-60%), and the field-weakening limit on top of the operating points of the application cycle.
11. Export the diagram (PNG / PDF) and store it in the project documentation for the machine builder.

Verification

• Cross-check the TST power value at rated speed against the value obtained by manual calculation: P_rated = n_rated × M_rated / 9550. The two should agree within rounding (≤ 2 %).
• Compare the TST torque curve at the operating point against the corresponding M-n diagram in Chapter 6 of the S-1FT7 configuration manual. They must match within the cooling-method classification.
• Verify that the operating point lies inside both the thermal (S1) envelope and the mechanical (nmax) limit.

Reading the S-1FT7 Torque-Speed Characteristic

The published M-n diagram has four functional regions that the power vs. speed plot inherits:

When the y-axis is switched to power, the constant-torque region becomes a straight line through the origin. The constant-power region flattens out. This is why most engineers find the P-n view more intuitive for sizing a machine cycle: the S1 operating region becomes a rectangle in the P-n plane, and a duty cycle can be checked by simply asking whether the application's average power and peak power points fall inside that rectangle.

Deriving Power from Torque-Current Data

Chapter 6 of the configuration manual also publishes a torque-current (M-I) curve for the self-cooled variant. From this curve, plus the motor's rated speed, the entire performance envelope can be reconstructed. The procedure is:

1. Read the standstill torque M0 and standstill current I0 from the data block of the chosen motor variant.
2. Compute the torque constant: kT = M0 / I0 [Nm/A].
3. For any operating point (I, n), the developed torque is M = kT × I (modulo saturation, which Siemens linearises for the S-1FT7 in the published curve).
4. Compute the output power: P = 2π·n·M / 60 = (π/30) · n · kT · I.
5. Check that n ≤ nmax and that the point lies inside the S1 thermal envelope of the chosen cooling method.

This manual reconstruction is useful when the engineer needs to size a duty cycle against the published S6 curve but the S6 plot is not visible in the customer's project copy of TST.

Why the TIA Selection Tool Path Matters

The 1FT7 is filtered out of the Simotics LV motor catalog inside TST on purpose: TIA Selection Tool enforces the engineering rule that an S-1FT7 is always paired with a controlled drive. The "LV motor" branch expects induction motors that can in principle be started DOL or soft-started, and it does not carry the field-weakening, peak-current, and S120 DC-link data needed to draw a correct P-n plot.

If a user does not see 1FT7 under Simotics LV motor configuration, that is expected behavior. The correct entry point is:

• Drive Technology → Drive dimensioning → S120 → motor filter: SIMOTICS S-1FT7

The same logic applies to the 1FK7, 1FS6, and other SIMOTICS servo families: they appear under the S120 drive-dimensioning context, not under the LV motor catalog.

Matching the S-1FT7 to a SINAMICS S120

Before the P-n curve has engineering value, the motor must be paired with a Motor Module whose peak current (Imax) exceeds the S-1FT7's stall current I0 by the headroom required for the application cycle. Typical rule of thumb is Imax_drive ≥ 2 × I0 for a positioning axis with a 4-quadrant duty, and ≥ 1.5 × I0 for a continuous-feed application. The TST drive-dimensioning result already enforces this when the SINAMICS S120 is selected first, but it must be re-verified manually if the motor is being migrated from an older S110 or SIMODRIVE 611 converter.

Key parameters to capture in the project documentation:

Common Pitfalls When Sizing from the P-n Curve

• Confusing rated power and peak power. The continuous (S1) value is the point at which the motor can operate indefinitely. The curve's envelope at higher speeds is often a duty-limited (S6) value, not continuous.
• Ignoring the field-weakening knee. Above rated speed, torque drops roughly as 1/n. A load that demands constant torque above rated speed will exceed the curve and stall or fault.
• Using DOL-style motor catalogs. Sizing the 1FT7 from a generic LV induction-motor catalog ignores the field-weakening and SINAMICS S120 DC-link limits and overstates the achievable power at high speed.
• Using the wrong cooling method. A self-cooled S-1FT7 in a sealed cabinet can deliver only a fraction of its nameplate M0. Use the S1 envelope that matches the installed cooling.
• Plotting the application cycle on the M-n curve, not the P-n curve. For load profiles that are power-limited (compressors, conveyors, mixers), the P-n view catches the violation at a glance; for torque-limited profiles (acceleration ramps, lifts), the M-n view is the better check. Always overlay the application on whichever axis the load is constrained by.

Commissioning Notes

During commissioning, the SINAMICS S120 STARTER / Startdrive commissioning tool will display the S-1FT7's torque limit, current limit, and thermal model in the drive's online diagnostics (r0108, p1080 for speed limits, r0027 for actual current). After autotuning, the engineer should:

1. Run a no-load speed sweep from 0 to nmax and record the actual speed and current. The result should track the published M-n curve converted via P = 2π·n·M / 60.
2. Apply a load step at rated speed and verify that the motor settles inside the S1 envelope without invoking the thermal model (p0605 / p0611 thresholds).
3. Capture the drive trace and store the P-n operating envelope with the machine documentation.

These recorded traces are the practical proof that the catalog curve matches the as-built machine. The P-n plot from TST is the design-time artifact; the trace from the commissioned drive is the acceptance-time artifact.

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