S7-315-2DP and MICROMASTER 440 VFD System Design for 22 kW Motor

System Overview and Application Context

The reference design described in this article combines a Siemens SIMATIC S7-300 CPU 315-2DP, a SINAMICS / MICROMASTER 440 (MM440) inverter, and a SIMATIC Mobile Panel 170 (MP170) to drive a 22 kW, four-pole, 1500 rpm asynchronous induction motor through a 28.3:1 speed reducer. The gearbox output shaft runs at 53 rpm and produces 3960 Nm of torque at full motor load. The mechanical load is a bulk-fertilizer loading elevator that draws material from a conveyor and discharges it into a railcar. Because the bulk density of the fertilizer varies significantly from one chemical formula to another, the operator must continuously trade speed against torque to keep the elevator from stalling while still maximizing throughput. The control system must therefore expose an adjustable speed setpoint, protect the motor from thermal overload, and provide a clear visualization of the process on the mobile panel that the loader operator carries along the railcar.

All of the design numbers below come from the motor nameplate, the MM440 operating instructions (6SE6400-5AD00-0BP0), the S7-300 CPU 315-2DP manual (entry ID 12996906 in the Siemens Industry Online Support), and the SIMATIC HMI Mobile Panel 170 manual. Where the design assumes a value that is not stamped on a nameplate, the calculation path is shown explicitly so that you can substitute the actual project value.

System Architecture and Topology

Three logical networks are formed:

1. PROFIBUS-DP between the S7-300 CPU 315-2DP (master) and the MM440 (slave). The MP170 also drops onto the same DP segment as a slave and is wired in a daisy chain with a terminating resistor enabled at the physical end of the line.
2. 24 V DC power for the S7-300 PS307, the MP170, the encoder, and the I/O backplane. A SITOP PSU300M 24 V / 10 A is recommended for the control section so that the PLC and HMI remain powered through a short mains dip.
3. 400 V AC power from the factory feeder through a fused disconnect or molded-case circuit breaker (MCCB) to the MM440 line input (L1, L2, L3), then a 4-core shielded VFD cable from the MM440 output (U2, V2, W2) to the motor junction box. The motor is bonded to the building ground grid at the motor frame and at the VFD end of the cable shield.

Mechanical, Electrical, and Torque Analysis at 50 Hz vs 60 Hz

The motor nameplate defines the constant-torque operating envelope. With 50 Hz, 400 V, 22 kW, 1500 rpm the motor produces:

T_motor = P / ω = 22 000 W / (2π × 1500/60) = 22 000 / 157.08 = 140.05 Nm at the motor shaft

Through a single-stage helical-bevel reducer with a 28.3:1 ratio and ~95 % mechanical efficiency, the gearbox output torque is:

T_out = 140.05 × 28.3 × 0.95 ≈ 3763 Nm

For a 100 % efficiency assumption the manufacturer quotes 3960 Nm on the gearbox nameplate. Treat 3763 Nm as the design torque and 3960 Nm as a theoretical ceiling.

What happens at 60 Hz without voltage boost

If the line is 380 V at 50 Hz, the V/Hz ratio is 7.6 V/Hz. Pushing the drive to 60 Hz while keeping 380 V gives V/Hz = 6.33, a 16.7 % reduction. The motor's continuous torque capability drops to roughly 83 % of the design value:

T_motor_60 = 140.05 × (6.33 / 7.6) = 116.6 Nm
T_out_60 = 116.6 × 28.3 × 0.95 ≈ 3134 Nm

The synchronous speed of a 4-pole induction motor at 60 Hz is 1800 rpm, so the gearbox output rises to 1800/28.3 = 63.6 rpm. For a constant-torque load (an elevator bucket, not a centrifugal fan) the absorbed power scales linearly with speed, reaching 26.4 kW. This is above the motor's 22 kW thermal limit, so the motor will thermally overload within minutes at full current.

What happens at 60 Hz with voltage boost

If the supply is 460 V (standard in 60 Hz IEC environments such as Saudi Arabia, the Philippines, or the U.S. NEMA region) and the MM440 is parameterized to deliver 460 V at 60 Hz, the V/Hz ratio returns to 7.6 and the full 140.05 Nm of motor torque is available. The MM440 supports this through parameter P0210 (supply voltage) and the auto-detection of the motor's V/Hz profile in P2000 (reference frequency) and P2002 (reference current).

V/Hz profile curve (SVG)

Motor Thermal Protection and Circuit Protection Sizing

There are two distinct protective devices in this design: the MM440 electronic motor protection (which models the I²t heating of the motor windings) and the short-circuit / overload device on the line side of the VFD (which protects the VFD input rectifier, the cabling, and the panel).

MM440 I²t model parameters

Line-side protection (MCCB or fused disconnect)

The line-side device does not protect the motor — the MM440 does. The line-side device protects the rectifier input, the input reactor, the cabling from the panel to the drive, and the upstream busbar. A 22 kW MM440 (6SE6440-2UD34-5FA1) has a recommended input fuse of 63 A (gG/aR class) per the operating instructions. The molded-case circuit breaker should be selected at the next size above 63 A with a C or D trip curve so that the inrush of the DC-link capacitors (peak ≈ 4–6 × FLA for 1–2 ms) does not nuisance-trip:

VFD-to-Motor Cable Selection (20 m, 4-core Shielded)

At 20 m the cable is short enough that the reflected-wave effect (also called standing-wave or transmission-line effect) is usually negligible for 400 V class drives. EMC compliance, however, still requires a symmetric, shielded cable so that the common-mode currents from the IGBT switching do not radiate into the surrounding installation. A 5-wire cable (3 phases + N + PE) is not the right choice for a VFD output. The VFD does not need a neutral, and 3 phases + PE on a four-core shielded cable is the IEC standard arrangement.

Recommended cable construction: XLPE/Cu/SCR/PE, e.g., 2YSLCY-J or similar, with:

• 3 × power conductors sized for VFD current (45 A → 10 mm² Cu minimum, 16 mm² preferred for voltage margin)
• 1 × PE conductor (green/yellow, full cross-section per IEC 60364-5-54)
• Overall tinned-copper braid screen, coverage ≥ 80 %
• Optional: a smaller control pair inside the screen for the motor PTC and/or the thermal sensor

For the Siemens-branded equivalent, use 6FX8002-5DA-series MOTION-CONNECT cables, e.g., 6FX8002-5DA11-1AF0 (4G4 + 2 × 1.5) or 6FX8002-5DA31-1AF0 (4G6 + 2 × 1.5) depending on the conductor size you need. For this design with 45 A, the 4G6 cable is appropriate.

Shield termination

At the VFD end, expose the braid and clamp it with a 360° EMC gland into the VFD gland plate. At the motor end, terminate the screen to the motor's internal ground bar (or to a separate EMC clamp on the motor terminal box). Do not pigtail the screen to a PE terminal with a flying lead — that defeats the screen at high frequency.

PROFIBUS-DP Networking with the MM440 and MP170

The CPU 315-2DP (6ES7315-2AG10-0AB0) has an integrated PROFIBUS-DP master on the first MPI/DP port. The MM440 does not have PROFIBUS on board — you must add the PROFIBUS communication module:

• 6SE6400-1PB00-0AA0 — MM4 PROFIBUS module (DP-V0, 12 Mbps)

The MP170 has a 9-pin Sub-D PROFIBUS connector on the bottom. The bus is wired as a shielded 2-wire RS-485 segment, terminated at both physical ends with 220 Ω (bus termination) plus 390 Ω pull-up/pull-down for the idle state.

Set the MM440 PROFIBUS address with parameter P0918 (the rotary switches on the PROFIBUS module override P0918 if not all 10 positions are in the "0" state). Baud rate auto-detects up to 12 Mbps.

PROFIBUS telegram selection

For closed-loop speed control with a single control word and a single status word, select standard telegram 1 (PZD 2/2). For torque control, use telegram 2. For combined speed/torque with the Siemens PPO type 1 (4 PKW + 2 PZD), the MM440 supports it natively. Configure the telegram in the S7-300 hardware configuration by right-clicking the MM440 in HW Config and selecting the PPO type from the catalog.

PKW allows the S7-300 to read/write any MM440 parameter. PZD allows 12 ms cycle-time control of the speed setpoint and a fast read of the actual speed. The reference speed that corresponds to NSOLL = 4000 hex is set in P2000.

USS Protocol as an Alternative to PROFIBUS

If the PROFIBUS option-module cost is a concern, the CPU 315-2DP can drive up to 31 MM440 units over a shielded twisted-pair RS-485 bus running the Siemens USS protocol on the second interface (PtP, RS-485 half-duplex). The Mobile Panel 170 cannot drop on this same RS-485 segment because it has only a PROFIBUS port, so PROFIBUS is the more practical choice for this project.

USS implementation on the S7-300 uses the SIMATIC MICROMASTER USS library (STEP 7 → Drive ES SIMATIC). The two key blocks are FB 60 / FC 60 (USS SEND/RECEIVE) and FB 61 / FC 61 (USS read/write parameters). The baud rate is fixed at 9.6 or 19.2 kbps, the cycle time is 30–60 ms per drive, and only one master is allowed.

Parameterization Software Selection: STARTER vs. Drive ES Basic vs. SIZER

Siemens sells three different tools that overlap in scope. The right choice depends on what stage of the project you are in:

For this project, install both:

1. SIZER during quotation to size the MM440, the line reactor, and the brake resistor. SIZER will give you a 22 kW MM440 part number, the matching 6SL3000 line filter, and the recommended cable cross-section.
2. STARTER for offline parameter set creation, then online commissioning. STARTER is the only tool that supports trace recording on the MM4 series (analog scope of speed, current, torque over time).
3. Drive ES Basic only if you want the MM440 to appear in the STEP 7 hardware catalog and to expose its parameters as DB tags to the S7-300 program.

Current Measurement: True-RMS Clamp Meter Behavior on VFD Output

The original poster uses a clamp meter on the line side of the motor to measure load current. Once the VFD is installed, the current waveform on the line side of the motor (between the MM440 and the motor) is no longer a 50/60 Hz sine wave — it is a PWM waveform with a fundamental frequency equal to the motor shaft speed and a carrier (switching) frequency of 4 kHz by default in the MM440 (P1800).

A True RMS clamp meter with a bandwidth of at least 1 kHz and a frequency derating curve that extends to 100 kHz will give a reading within ±2 % of the actual RMS current. An average-responding (cheap) clamp meter will read 10–30 % low. The MM440 also exposes r0027 (output current, A RMS) on its BOP, AOP, or the fieldbus, which is the most convenient reading for a PLC program or a panel trend.

For trend logging, prefer r0027 (output current) read via PROFIBUS or through the MP170 tag from the PLC. This avoids the bandwidth question entirely and gives the operator a live bar on the panel.

Commissioning Sequence

1. Mechanical pre-checks: verify coupling alignment, free rotation, gearbox oil level, motor earthing.
2. Electrical pre-checks: insulation test (1000 V Megger, ≥ 100 MΩ phase-to-phase and phase-to-ground), shield continuity from VFD to motor, PE loop impedance ≤ 0.3 Ω, PROFIBUS termination enabled only at the two physical ends.
3. MM440 quick commissioning (P0010 = 1): enter motor nameplate data P0304–P0311, P0335, then run P3900 = 1 to compute the motor model from the nameplate.
4. Motor identification (P1910 = 1): with the motor decoupled from the gearbox, run the static motor ID. This measures the stator resistance, leakage inductance, and dead-time compensation.
5. PROFIBUS check: from STEP 7 → Accessible Nodes, verify that the MM440 (address 3) and MP170 (address 4) come online. Read P0700 / P1000 and confirm P0700 = 6 (fieldbus command source).
6. First run on the fieldbus: from the S7 program, write control word 0x047E (off, ready to run), then 0x047F (run, ramp enable) and set the speed setpoint to 10 %. Verify that r0021 (DC bus voltage) is ~540 V DC and r0027 (output current) is ≤ 5 A at no load.
7. Ramp tuning: set P1120 (ramp-up) and P1121 (ramp-down) to the elevator bucket's safe limit, typically 10–20 s for a 22 kW bucket.
8. Load test: run the elevator empty, then with a half load, then full load. Record r0027 and the gearbox output speed. Adjust the V/Hz boost (P1310, P1311) if the motor stalls under 50 % load.
9. Safety chain test: verify that the emergency stop, the overspeed trip, and the motor PTC (if fitted) all stop the drive and assert the STW1 bit 7 (acknowledged fault).

Troubleshooting Matrix

Specifications Summary

FAQ