“My generator feed is so noisy the contactor hums itself to death.” — How to choose between Siemens SIRIUS and ABB AF when your AC wave looks like a sawtooth

Numbers: ABB AF contactors (AF09–AF range) use an electronic wide-range coil covering 100–250 V AC/DC, and the AF09 specifically offers four ranges: 24–500 V AC / 20–500 V DC. The electronic coil maintains sealed hold at about 80 % of nominal — but more importantly, it can tolerate deep sags down to roughly 40 % of the lower range bound for tens of milliseconds without dropping out (illustrative, based on electronic coil topology). Siemens SIRIUS 3RT2 contactors use a conventional tapped coil; the standard AC coil has a dropout voltage of about 70–75 % of rated control voltage (per IEC 60947‑4‑1, pick‑up ≤ 85 %, drop‑out ≥ 20 % but typical AC coil drop‑out ≈ 70 %).

Mechanism: A generator feed under load often produces harmonic distortion (5th, 7th) and short-duration dips when the AVR reacts to large block loads. The ABB AF’s electronic coil rectifies the input and uses a DC bus to hold the armature; it effectively decouples the coil from the instantaneous AC waveform. As long as the DC bus stays above the regulator threshold (~65 V for a 100 V range lower bound), the contactor remains sealed. A conventional tapped coil (Siemens 3RT) sees the RMS of the distorted wave; if the dip pushes the RMS below ~70 % for more than ½ cycle, the magnetic flux collapses and the contactor drops out — often causing an unwanted motor stop.

Worked consequence: If you are on a generator that regularly sees 30 % voltage sags for 3–5 cycles (e.g., a 100 kW genny hitting a 40 kW motor start), the Siemens SIRIUS with a standard coil will drop out. The ABB AF will stay in. The operator avoids a nuisance trip and the load stays online.

Numbers: The ABB AF electronic coil accepts AC 50/60 Hz without adjustment — its internal DC conversion is frequency‑agnostic within that range. The Siemens 3RT conventional coil is designed for a nominal frequency (50 Hz or 60 Hz); a 10 % frequency drop (e.g., 50 Hz → 45 Hz) increases the coil impedance and reduces current, which can lower the magnetic force by about 15–20 % (illustrative, derived from inductive reactance). At the same time, generator under‑frequency often coincides with voltage drop, compounding the effect.

Mechanism: For a conventional AC coil, X_L = 2πfL. If f drops 10 %, coil current drops 10 %, and the magnetic force (∝ I²) drops ~19 %. Add a 10 % voltage sag, and the combined force can fall below the spring‑return threshold. The ABB AF coil rectifies first — the DC bus is insensitive to frequency, so frequency wander alone does not reduce the holding force.

Worked consequence: On a portable gen set that might surge from 52 Hz down to 47 Hz during a welder strike, a Siemens SIRIUS with a 50 Hz coil will see its holding force drop by ~20 % from the frequency shift alone — if a voltage sag is superimposed, dropout is likely. The ABB AF will hold.

Numbers: Both contactors are designed to IEC 60947‑4‑1, which requires impulse withstand voltage (U_imp) of 6 kV for basic insulation (typical for 400 V systems). The ABB AF electronic coil includes internal varistor/suppressor circuitry — ABB contactor states the coil is protected against voltage surges up to 2.5 kV (illustrative, based on typical coil protection). Siemens SIRIUS conventional coils do not have active suppression; they rely on the external RC snubber or varistor that the panel builder must add. Without external protection, the coil insulation may break down under repeated 2 kV spikes.

Mechanism: Electronic coils have low‑power electronics that can be damaged by sustained overvoltage, but they also include clamping. Conventional coils are more robust against single spikes but can suffer turn‑to‑turn insulation fatigue from steep wavefronts. In a generator feed with poor surge suppression, the failure mode differs: ABB AF electronics may fail short or open (repair = replace contactor); Siemens contactor coil may short turn‑to‑turn causing humming and eventual burnout.

Worked consequence: If your generator is on a site with frequent lightning‑induced surges and no dedicated surge protector, the ABB AF’s internal clamping may self‑sacrifice after a few spikes. The Siemens 3RT + external MOV can be oversized (e.g., 320 V‑rms MOV) to survive thousands of surges. The choice depends on whether you prefer built‑in protection (ABB) or external ruggedisation (Siemens).