SINAMICS V20: Sizing Output Filters for Legacy Motor Protection

Overview

Operating an older single-phase-rated induction motor from a modern PWM variable frequency drive (VFD) such as the SINAMICS V20 (catalog number 6SL3210-5BB23-0UV1) is electrically and mechanically feasible but it deliberately exposes the machine to phenomena the original insulation system was never designed to handle: high dv/dt common-mode voltages, reflected voltage peaks, high-frequency common-mode currents circulating through the bearings, and increased acoustic/thermal stress. The engineering question is therefore not whether the VFD will run the motor — it will — but what additional measures are required to keep peak winding-to-winding voltage, phase-to-ground stress, and shaft currents below the dielectric and mechanical limits of a rewind that may already be two repairs deep.

This reference quantifies the stress mechanisms, sizes the SINAMICS V20 against a 230 V single-phase nameplate that has been re-configured to U/V/W three-phase, and walks through the filter, reactor, and bearing-grounding decision matrix with cost envelopes suitable for residential or light-commercial pool-filtration duty. Cable runs are kept under 5 m to limit transmission-line reflections, and the drive is operated in fixed-speed, soft-ramp mode where reactivity is irrelevant.

VFD-Induced Stress Mechanisms

An IGBT-based PWM drive does not output a sinusoidal voltage. It switches the DC bus onto the motor terminals with a typical rise time of 0.1–1 µs and at a fundamental switching frequency between 2 kHz and 16 kHz. Three primary failure modes follow.

Peak Winding Voltage (Transmission-Line Reflection)

The motor cable and the motor windings form a transmission line. If the cable electrical length exceeds roughly half the rise-time period, the leading edge of the pulse reflects at the high-impedance motor terminals and approximately doubles in amplitude. The critical cable length is:

lc = v × tr / 2

where v is the propagation velocity in PVC/XLPE motor cable (≈ 150 m/µs) and tr is the IGBT rise time. With tr = 0.1 µs the critical length is ≈ 7.5 m; at 1 µs it is 75 m. A 5 m lead therefore sits at the borderline for shorter rise-time devices but is safely below the doubling threshold for typical V20 switching.

Reflected peak at the motor terminals is approximately:

Vp = VDC × (1 + Γ)

where Γ ≈ 1 for an open-circuit (motor) termination, giving Vp ≈ 2 × VDC. With a 230 V single-phase input the SINAMICS V20 builds a DC bus of:

VDC ≈ √2 × VAC ≈ 1.414 × 230 ≈ 325 V

so an open-circuit peak of ≈ 650 V at the motor terminals is realistic under worst-case reflection. A 230 V delta / 400 V Y winding — the most likely insulation class used by the rewind shop — sees a peak withstand of only:

Vpeak,400V = 400 × √2 ≈ 565 V

The 650 V reflection therefore exceeds the line-to-line insulation rating by ≈ 15 %. This is the technical reason filter mitigation matters.

dv/dt Stress

Even without reflection, the rate-of-change of the V20 output is 3–10 kV/µs. This non-uniform voltage distribution across the first turns of each phase (the proximity effect) drives inter-turn voltages well above the steady-state line-to-neutral value. NEMA MG1 Part 31 specifies that inverter-duty motors must withstand 1.6 kV peak at 0.1 µs rise time; standard random-wound machines of the type typically rewound locally are commonly rated to 1.0 kV / 1 µs, leaving a marginal safety margin.

Common-Mode Voltage and Bearing Currents

The PWM common-mode voltage with respect to ground alternates between ≈ +V_DC/2 and −V_DC/2 at the switching frequency. Because the rotor is capacitively coupled to the stator through the bearings and to ground through the coupled shaft, this common-mode voltage drives EDM (electric discharge machining) currents across the bearing race. Typical damage signatures are fluting, frosting, and grey/black race tracks appearing within 1 000–5 000 operating hours on non-inverter-duty machines.

Motor Specification Analysis

The nameplate of the legacy pump motor defines the operating envelope and the re-rating tolerance when driven from a three-phase VFD on a single-phase supply.

Three-Phase vs Single-Phase Reconnection

When a 230 V single-phase motor with U/V/W windings is reconnected in three-phase star, the line-to-line voltage is 230 V but each winding only sees 133 V (line-to-neutral). This is electrically equivalent to running the motor at ~58 % of its design flux (133 / 230). The V20 can compensate by raising the output voltage at low frequency, but at 50 Hz the motor will run with reduced torque and increased slip — acceptable for a centrifugal pump whose torque demand scales with the square of speed.

If the windings were originally 230 V delta (six leads) and are reconfigured 400 V star for the rewind, the VFD must be programmed to deliver 400 V line-to-line output. Verify with the rewind shop before commissioning; an over-voltage of √3 will saturate the magnetic circuit and destroy the motor within seconds.

SINAMICS V20 Sizing (6SL3210-5BB23-0UV1)

The SINAMICS V20 catalog 6SL3210-5BB23-0UV1 is the 230 V single-phase input / three-phase output variant in the 3 kW frame. Key ratings from the SINAMICS V20 operating instructions:

With motor rated current 11.5 A against drive rated 13.6 A, the V20 operates at 84.5 % of rated current — comfortably within continuous ratings and well below the 150 % overload envelope. The single-phase input draws:

Iin,1φ ≈ (Pout / η) / (V × cos φ) ≈ (3 000 / 0.95) / (230 × 0.85) ≈ 16.1 A

This must be accounted for in upstream MCB sizing (typically 20 A Type C or 25 A Type B for residential distribution).

Cable Length, Reflections, and dv/dt

The 5 m motor lead is short enough that transmission-line doubling is not a dominant failure mechanism provided the V20's switching rise time is held above ~0.4 µs. However, three practical issues remain.

Voltage Drop at Rated Current

For 2.5 mm² Cu cable (typical for 13.6 A service):

Vdrop = √3 × I × L × (R cos φ + X sin φ) ≈ √3 × 11.5 × 5 × (8.86 × 0.98 + 0.165 × 0.20) / 1000 ≈ 0.87 V

Less than 0.4 % — negligible.

High-Frequency Standing Waves

Even at 5 m the cable impedance mismatch causes ringing in the 30–80 MHz range at every switching edge. Without filtering, this ringing couples into adjacent control wiring and may disturb pool automation sensors. A reactor or filter raises cable-side impedance and damps the ring.

Capacitive Charging Current

Typical PVC motor cable capacitance is ≈ 130 pF/m. At 5 m and 4 kHz switching:

Ic = 2π × f × C × V ≈ 6.28 × 4 000 × 650 pF × 325 V ≈ 5.3 mA

This is small but cumulative through three phases and contributes to the bearing-current path.

V20 Switching Frequency and Ramp Parameters

The SINAMICS V20 exposes the switching-frequency selection through parameter P1800 (default 4 kHz on the 3 kW frame). For insulation-stress mitigation the recommended approach is:

Output Filter Topologies Compared

Four practical mitigation paths exist between the V20 output terminals and the motor leads. Each has a different electrical, mechanical, and economic profile.

Option 1 — Motor Reactor (Load Reactor)

A three-phase iron-core inductor inserted in series with each motor lead. Typical inductance 1.5–3 mH for the 3 kW frame. Effect:

• Lengthens voltage rise time from ≈ 0.1 µs to ≈ 50–100 µs
• Lowers peak voltage at motor terminals from ≈ 2 × V_DC to ≈ 1.1–1.3 × V_DC for cables shorter than critical length
• Reduces motor acoustic noise by ≈ 5 dB(A)
• Adds 2–3 % voltage drop at 50 Hz — negligible
• Typical cost: €60–€120 for the 3 kW frame
• No effect on common-mode voltage or bearing currents

Option 2 — dv/dt Filter with Voltage Peak Limiter (VPL)

A dv/dt reactor in series with the line, plus an RC snubber or MOV/thyristor-based peak limiter at the motor end. Effect:

• Limits dv/dt to ≈ 500 V/µs (NEMA MG1 Part 31 compliant)
• Clamps reflected peak to ≤ 1.5 × V_DC (≈ 488 V at 230 V input — comfortably below 565 V winding rating)
• Typical cost: €200–€350 for 3 kW
• Does not produce a true sine wave at the motor — switching ripple remains
• No common-mode attenuation

Option 3 — Sine-Wave Filter

A heavy LC low-pass filter, typically with L = 5–10 mH and C = 10–30 µF. Effect:

• Converts the PWM waveform to a near-sinusoidal output (THD < 5 %)
• Eliminates motor insulation stress entirely; motor sees the same voltage envelope as a mains-fed machine
• Reduces motor losses and acoustic noise to mains-equivalent levels
• Typical cost: €600–€1 200 for 3 kW — exceeds the V20 purchase price (€320)
• Introduces 5–8 % voltage drop; may require V20 boost compensation
• Reduces common-mode voltage by 50–70 %, partially mitigating bearing stress

Option 4 — Combined dv/dt + VPL Filter

Specifically engineered for short-cable (< 15 m), low-power installations. Provides 90 % of sine-wave filter benefit at 40 % of the cost.

Bearing Current and Grounding Kits

Even with perfect dv/dt filtering, common-mode voltage and its bearing-current consequence remain. For a motor that has already been rewound, bearing life is the dominant mechanical risk. Two practical mitigations exist.

Shaft Grounding Ring

A copper or silver-graphite brush rides the motor shaft at the non-drive end, shorting high-frequency shaft voltage to the motor frame before it discharges across a bearing. Typical installation cost €80–€150 plus 0.5 h labor. Effect: bearing life restored to manufacturer-rated 40 000–100 000 h.

Insulated Bearing (NDE or Both Ends)

Ceramic-coated rolling elements or aluminum-oxide-coated races interrupt the EDM current path. Effective only when combined with shaft grounding on the opposite end. Typical cost €120–€200 per bearing on a 3 kW frame.

Inverter-Duty Replacement Path (SIMOTICS)

If the existing motor has had two rewinds and bearing life is uncertain, replacement with an inverter-duty machine is the most cost-effective long-term option. Siemens offers two product lines:

The FL line is the practical choice for a pool-filtration duty. Both lines exceed NEMA MG1 Part 31 / IEC 60034-25 inverter-duty requirements (1.6 kV peak withstand, 0.1 µs rise time), eliminating the need for any output filter beyond a standard motor reactor for cable lengths exceeding 15 m.

Cross-reference is straightforward — the FL line covers the IE3/IE4 efficiency classes at 2-pole, 50 Hz, B3/B5/B14 mounting footprints that match the legacy pump bell housing.

Decision Matrix and Cost Envelope

For a 230 V single-phase-supplied pool filtration pump with a 5 m motor lead and a 2016 vintage rewound motor, the cost-benefit envelope resolves as follows.

The recommended path is D (V20 + dv/dt + VPL filter + shaft grounding ring) — total cost ≈ 1.9× the V20 alone, fitting comfortably within the user's stated 50–100 % budget envelope while protecting both the rewound windings and the bearings.

Commissioning Procedure

1. Verify motor nameplate — confirm whether windings are connected in 230 V delta or 400 V star. If delta, the V20 must be programmed for 230 V output; if star, program for 400 V output. Mismatching these will either under-flux or destroy the motor.
2. Pre-charge and parameter reset — on first power-up, set P0010 = 30, P0970 = 21 to factory-reset the V20, then perform the quick-commissioning wizard with the actual motor data.
3. Set switching frequency — P1800 = 2 (kHz) for softest edge. Verify no F0001 / F0002 (overcurrent / overvoltage) faults during the first ramp.
4. Configure ramps — P1120 = 25 s, P1121 = 25 s. For a centrifugal pump, the load curve is benign; do not use S-curve ramps (P1130, P1131) unless water-hammer risk demands it.
5. Motor identification — set P1900 = 1 to perform auto-tune; the V20 measures stator resistance and leakage inductance without rotating the shaft.
6. Install dv/dt + VPL filter — mount within 1 m of the V20 output, observe phase rotation (U2 → V2 → W2), and torque terminals to manufacturer spec (typically 2.5 Nm for 4 mm²).
7. Install shaft grounding ring — mount on the motor non-drive end, ensuring brush contact over the full shaft circumference. Verify < 2 Ω from brush to motor frame with a 4-wire ohmmeter.
8. First run unloaded — run the V20 uncoupled at 25 Hz for 5 min. Listen for any bearing growl; monitor motor frame temperature with an IR thermometer (should remain < 50 °C above ambient).
9. Couple to pump and ramp — re-couple, perform a full 0–50 Hz ramp, observe input current on the V20 display. Expect 11.5–12.5 A at full load.
10. Capture baseline — record input voltage, input current, output current, DC bus voltage, motor frame temperature, and bearing-shaft voltage with an oscilloscope (1 GHz, 100:1 probe). Save as reference for trending.

Verification and Field Monitoring

After commissioning, periodic monitoring confirms that filter performance has not degraded and that the motor remains within thermal limits.

FAQ