Selecting a Main Contactor for SINAMICS G120: AC-1 vs AC-3

Selecting the main (line) contactor upstream of a SINAMICS G120 frequency converter is one of the most common specification errors in industrial drive panels. The confusion stems from a reasonable observation: a six-pulse diode rectifier presents a displacement power factor of roughly 0.98, so the input "looks" almost purely resistive. That observation is correct as far as it goes, but it does not justify selecting the contactor in utilization category AC-1. This reference walks through the IEC 60947-4-1 utilization categories, explains why the input of a G120 is not an AC-1 load despite its high power factor, and shows how to size and wire the correct Sirius 3RT contactor for each Power Module frame size (FSA through FSF).

1. The Engineering Question: Why Not AC-1 for a VFD Input?

The original question raised on the engineering forum was simple and reasonable:

"Before the SINAMICS G120, must the main contactor be selected in utilization category AC-3, or can I select it in AC-1? After all, ahead of the VFD there are no large starting currents and cos φ > 0.8. Why does Siemens recommend AC-3?"

The observation about the displacement power factor is technically correct. A modern 6-pulse diode rectifier feeding a stiff DC link draws current in narrow pulses near the line voltage peaks, and the displacement factor between the fundamental of the line current and the line voltage is close to unity. The Siemens SINAMICS G120 hardware documentation gives a power factor of approximately 0.98 for the line side of the converter. On that basis it is tempting to argue that the contactor sees a near-resistive load and should be selected in AC-1.

The error is to equate "high displacement power factor" with "non-inductive utilization category." The IEC utilization categories describe switching duty—how the contactor makes and breaks current—not the steady-state power factor of the downstream load. The contactor that sits ahead of the G120 does not switch the motor; it switches the rectifier. The rectifier, with its DC-link capacitors and the cable capacitance of the motor feeder, presents a make transient that an AC-1 contactor is not rated to handle, and a break transient that an AC-1 contactor is not required to handle because the AC-1 contactor is permitted to be selected on its thermal current (I_th) only. The make/break distinction is demonstrated in detail in the sections that follow.

2. IEC 60947-4-1 Utilization Categories: The Real Definitions

Low-voltage contactors are classified under IEC 60947-4-1 (Low-voltage switchgear and controlgear – Part 4-1: Contactors and motor-starters – Electromechanical contactors and motor-starters). The utilization category defines the type of load, the make and break conditions used to verify the contactor, and the conventional thermal current I_th that the device can carry in continuous service. The relevant categories for drive-input coordination are summarized below.

Three observations matter for the G120 case:

1. AC-1 is verified at 1.0 × I_e for both make and break. An AC-1 contactor is only certified to close on its own rated current with cos φ ≥ 0.95. It is not certified to close on the inrush of a capacitor bank.
2. AC-3 requires the contactor to make 6 × I_e. Even though a G120 input is not a 6 × I_e inrush, the AC-3 rating gives the contactor the contact gap, arc-chute design, and contact mass needed to interrupt the switching transients that do occur on a rectifier load—namely, the precharge inrush of the DC-link capacitors.
3. The category defines test duty, not steady-state. Selecting AC-1 means you are claiming the contactor will never be required to interrupt anything more onerous than a 1.0 × I_e resistive current. The G120 power-up transient violates that assumption.

3. Why the VFD Input "Looks Resistive" but Is Not

Three phenomena at the G120 input make AC-1 selection unsafe even when the displacement power factor reads 0.98 on a power-quality meter.

3.1 DC-link capacitor precharge inrush

When the line contactor closes on a discharged DC link, the input rectifier sees a near-short through the six-pulse diode bridge into the DC-link capacitor bank. The peak inrush current is limited only by the line impedance (transformer + cabling + any line reactor), and it typically reaches 5–20 × I_nominal for a duration of 0.5–5 line cycles. For a 22 kW G120 with a 47 A input rating, the inrush can easily reach 200–500 A peak for the first 5–10 ms. This is a make duty, not a break duty, and it is exactly the kind of stress that AC-3 verification is designed to cover.

3.2 Cable and motor-feeder capacitance

Long motor cables (VFDs are routinely installed with 50–150 m of shielded cable) add tens of nanofarads of cable capacitance. When the contactor closes, this capacitance charges through the rectifier diodes with a similar inrush profile to the DC link. The contactor is asked to make a highly capacitive load—a duty that the IEC 60947-4-1 AC-1 verification does not cover.

3.3 Harmonic current distortion (THD_i)

A six-pulse rectifier without a line reactor draws line current in narrow pulses. The total harmonic distortion (THD_i) typically lands in the 35–45 % range at full load. The RMS line current is therefore 5–15 % higher than the fundamental. AC-1 contactors are rated on I_th (the conventional thermal current in enclosure), and an undersized contactor selected only on the 0.98 PF will run hot under real THD_i heating. In installations that must comply with IEEE 519 or EN 61000-3-12 (typically 5 % THD_i or 8 % THD_i at the point of common coupling), a line reactor becomes mandatory; the reactor's voltage drop (2–4 %) further stresses the line contactor at make.

4. Power Factor 0.98 Is Misleading for Contactor Selection

The "0.98" quoted in Siemens marketing material is the displacement power factor of the fundamental component, defined as:

DPF = cos φ₁ = P₁ / (V₁ · I₁)

The true power factor accounts for all harmonics:

PF = (Σ V_h · I_h · cos φ_h) / (V_rms · I_rms) ≈ DPF / √(1 + THD_i²)

For a typical G120 PM240-2 with THD_i ≈ 40 %, the true power factor is closer to 0.93, not 0.98. And neither figure says anything about the contactor's switching duty. The selection of the contactor is governed by three independent quantities:

1. The thermal current it must carry continuously (related to true RMS, not PF).
2. The peak make current it must close against (related to DC-link precharge).
3. The prospective short-circuit current at the line terminals (for coordination with the branch protective device).

AC-1 is silent on items 2 and 3. AC-3 covers all three.

5. Does the SINAMICS G120 Even Need a Line Contactor?

Siemens is explicit on this point. Per the application note "Line Contactor Control using the ON/OFF1 Command":

"SINAMICS G120 converters are designed to be directly connected to the line supply; neither a line nor input contactor is required."

The PM240-2 Power Module includes an internal precharge circuit (a resistor and bypass contact in the smaller frame sizes, or an automatic precharge in the larger ones) that handles the DC-link capacitor inrush without external help. The G120 can be commanded on and off digitally with the ON/OFF1 command routed through a digital input or a fieldbus control word, and the internal precharge handles the switching transient. The full reference PDF is available at 62883732_MainContactorControl_v1_1_en.pdf.

A line contactor is still required in the following common scenarios:

When a line contactor is fitted, the Siemens application note recommends driving it from one of the G120's relay outputs (DO0, DO1 on the CU) or from a digital output of the controlling PLC, with the ON/OFF1 command gated on the line contactor's auxiliary contact. The precharge and run-up sequencing is then handled inside the converter, and the line contactor is only required to interrupt the steady-state line current (which is well within the AC-3 break rating of the contactor).

6. Sizing the Line Contactor: Calculations and Selection

The selection starts with the G120's input current, not the motor's full-load current. The input current is higher than the output current because the rectifier draws peaky current and the DC-link voltage is fixed. For a 400 V 3-phase supply and a PM240-2 Power Module, the input current is approximately:

I_in ≈ 1.05 · P_motor / (√3 · V_LL · η · cos φ)

with η ≈ 0.97 and cos φ ≈ 0.93. For 50 Hz, 400 V, this reduces to roughly I_in[A] ≈ 1.5 · P_motor[kW] for a first-pass estimate. Always read the exact value from the G120 Operating Instructions for the chosen Power Module and frame size.

Select the contactor on its AC-3 rating I_e at the ambient temperature of the panel, applying a derating factor for grouping, altitude, and side-by-side mounting. A typical derate is 0.8 for three or more contactors mounted side-by-side in a non-ventilated enclosure at 40 °C. With that derate:

I_{contactor,AC-3} ≥ I_in / 0.8

For an 11 kW G120 PM240-2 with I_in ≈ 25 A at 400 V, you need an AC-3 contactor rated at least 25/0.8 = 31.3 A. The next standard 3RT size is 32 A (3RT2027), so that is the minimum.

6.1 Worked selection table

7. Coordination with Branch Protection, Line Filter, and Reactor

The line contactor is one element of a coordinated feeder. To deliver a Type 1 or Type 2 coordination per IEC 60947-4-1, the following components must be matched:

1. Circuit breaker (or fused disconnect). Sized to the G120 input current, with the magnetic trip set above the DC-link precharge inrush (typically 8–12 × I_in for 100 ms). For PM240-2 frames FSA–FSC, Siemens recommends the 3RV20 series motor-protective circuit breakers. For larger frames, the 3VA or 3VL molded-case circuit breakers.
2. Line filter (optional but typical for CE). Required for compliance in residential and light-industrial environments per EN 61800-3, Category C2 or C3. The filter goes downstream of the line contactor, never upstream, so the contactor does not have to switch the filter's Y-capacitor charging current.
3. Line reactor (optional but recommended). A 3-phase line reactor (e.g., the 6SL3203-0CE series) reduces THD_i from ~40 % to ~30 % and limits the precharge inrush. The reactor's voltage drop (typically 2–4 % at full load) must be added to the line impedance when sizing the contactor's short-circuit withstand.
4. RFI filter. Same topology as the line filter. Do not place the line contactor between the RFI filter and the line; the filter must remain permanently connected to limit common-mode emissions.

The contactor's conditional short-circuit current I_q (typically 50 kA or 100 kA for Sirius 3RT) must be at least equal to the prospective short-circuit current at the panel busbars. The upstream protective device must clear the fault within the contactor's let-through I²t. For G120 frame sizes FSD and above, the standard 3RT202x/3RT203x contactors are 100 kA coordinated with the 3VA52/3VA62 breakers.

8. Control Wiring: ON/OFF1, STO, and Emergency Stop

When a line contactor is fitted, its coil must be controlled such that the contactor cannot be opened while the G120 is in a run state, and cannot be closed while the DC link is in a fault state. The recommended wiring per the Siemens application note 62883732 is:

The control sequence is:

1. PLC issues ON/OFF1 = 1 to the G120's digital input (DI 0 on the CU240E, for example) or to control-word bit 0 over PROFIBUS / PROFINET.
2. The G120's internal precharge runs; the line contactor's auxiliary NC contact is read by the G120 (or the PLC) as a permissive. A typical wiring is DI 3 = "line contactor closed" feedback.
3. After the DC link is charged (typically 0.5–3 s, monitored in parameter r0026), the inverter bridges are enabled and the motor runs. The precharge completion flag is r0899.2.
4. On OFF1 = 0, the motor ramps down along the configured deceleration ramp (p1121). Once the inverter detects zero speed (r0021 ≈ 0), the line contactor can be dropped by the PLC. If the contactor drops before zero speed, expect fault F30002 (DC-link overvoltage) as the regenerating motor back-feeds an unloaded DC link.

For STO (Safe Torque Off, SIL 2 / PL d), the line contactor is not the safety mechanism. The G120's STO inputs (terminals STO+ / STO− on the PM-IF interface) remove gate signals from the IGBT bridges; the line contactor can stay closed, and the STO function is faster, more reliable, and does not produce the inrush associated with re-closing on a charged DC link. The line contactor should drop only when the drive is at zero speed and the STO inputs are de-energized, or as part of a controlled emergency-stop category 1 stop.

For Emergency Stop category 0 (immediate removal of power), the line contactor is wired into the safety chain. The E-stop pushbutton breaks the contactor coil circuit and the STO inputs simultaneously. The line contactor can remain an AC-3 device because the G120's IGBTs stop switching on STO, and the contactor is then breaking only the line-side rectifier current at near-zero frequency—well within the AC-3 break rating.

9. Verification and Commissioning Checklist

1. Visual inspection. Confirm the contactor order code matches the schematic. Verify the coil voltage (24 V DC, 110 V AC, 230 V AC) matches the control supply.
2. Insulation test. Megger the contactor poles phase-to-phase and phase-to-ground at 500 V DC. Reading should be > 100 MΩ.
3. Coil test. Energize the coil; pull-in voltage should be 85–110 % U_c; drop-out 20–70 % U_c.
4. Auxiliary wiring check. Verify the NC auxiliary opens before the main contacts close (follow-through time < 5 ms typical for 3RT20).
5. No-load close test. With the G120 disconnected, close the line contactor and measure the line voltage at the G120 input terminals. Confirm phase rotation (L1-L2-L3).
6. Precharge test. Reconnect the G120. Issue ON/OFF1 = 1 and monitor the DC-link voltage (r0026) and the precharge current (r0027 in some firmware). The precharge should complete in < 3 s and the inrush should not trip the upstream breaker.
7. Run test. Issue a small speed setpoint (5 Hz), confirm smooth ramp, then issue OFF1 = 0 and verify the contactor drops only after zero speed (r0021 ≈ 0).
8. Coordination test. With the G120 in run, simulate a motor overload (mechanically load the motor). The G120's I²t protection should reduce output frequency before the line contactor or the upstream breaker trips.
9. Thermal check. After 1 hour at full load, measure the contactor terminal temperature with a thermal imager. Maximum 70 K rise above ambient on the terminals (class B insulation).
10. EMC verification. With the line filter installed, measure conducted emissions per EN 61800-3. The filter must remain connected; the contactor must not be in the filter's input path.

10. Common Mistakes and Field Pitfalls

10.1 Selecting the contactor in AC-1 because "the drive looks resistive"

AC-1 is not a thermal-current rating, it is a switching-duty rating. An AC-1 contactor is only certified to break its rated current with cos φ ≥ 0.95. The G120's precharge inrush on close, and the rectifier's recovery-current spikes on break, exceed that duty. Use AC-3.

10.2 Placing the line contactor between the RFI filter and the line

CE conformance under EN 61800-3 requires the RFI filter to remain permanently connected to the line. Putting the line contactor upstream of the filter means the filter is switched in and out with the drive, and the common-mode emissions on re-closing can exceed the C2 / C3 limits. Filter goes after the contactor, or the contactor is omitted entirely.

10.3 Forgetting the line reactor on long motor cables

Cable capacitance charges through the rectifier at every close of the line contactor. With 100 m of shielded motor cable, the peak charging current can exceed 100 A. A 3-phase line reactor (2–4 % impedance) limits the dI/dt and extends the contactor's electrical life by a factor of 3–5.

10.4 Using AC-1 because the panel builder is using AC-1-rated busbars

The panel's busbars are continuous-current devices and are correctly rated on their I_th. The contactor is a switching device and must be rated on its AC-3 (or AC-4) category. Do not mix up the two.

10.5 Dropping the line contactor on STO

STO removes gate drive from the IGBTs. The motor decelerates only by its own friction and load. If the line contactor also drops, the DC link discharges through the precharge resistor and the next start requires a fresh precharge cycle (3 s delay). Use STO alone for SIL 2/PL d; use the line contactor only for category 0 E-stop or for maintenance isolation.

10.6 Using the same contactor for multiple drives

A single AC-3 contactor can be used to switch several G120s in parallel only if the total inrush of all DC-link capacitors is below the contactor's 6 × I_e make rating, and only if a fault on one drive does not cause the contactor to break a fault current that exceeds its conditional short-circuit rating I_q. In practice, use one contactor per drive.

10.7 Ignoring ambient temperature derating

3RT contactors are rated at 40 °C ambient in a standard enclosure. Inside a non-ventilated cabinet in a hot plant, ambient can reach 55 °C. Derate the AC-3 current by 0.85 for 55 °C and 0.75 for 60 °C. Going up one frame size (e.g., 3RT2026 → 3RT2027) gives 30 % thermal headroom at the cost of a few euros.

11. Troubleshooting Matrix

12. Summary of Selection Rules

1. Select the line contactor in utilization category AC-3, not AC-1, regardless of the G120's high displacement power factor.
2. Size on the G120's input current, not the motor FLC.
3. Apply ambient and grouping derating (typically 0.8).
4. Choose the next standard 3RT frame up from the derated current.
5. Place the RFI filter downstream of the contactor; place any line reactor downstream of the filter.
6. Use STO for safe stop; use the line contactor only for E-stop category 0, maintenance isolation, or energy-saving shutdown.
7. Verify coordination with the upstream breaker (Type 1 or Type 2 per IEC 60947-4-1).
8. Read the Siemens application note 62883732 and the SINAMICS G120 Operating Instructions for the exact input current and any frame-specific precharge requirements.