“A contactor’s coil barely uses power — runtime is all about the main poles.” Myth or reality?

You hear it in every panel shop: “The coil draw is negligible — what matters is the contact rating.” That’s half true and half dangerous. The coil does consume trivial power at steady state (a few VA), but the question “runtime under real load” isn’t about the contactor lasting longer — it’s about whether it holds in during a voltage sag, whether the overload relay can be set correctly, and whether the combination can deliver rated current hour after hour without nuisance tripping. The real variable isn’t the main poles alone; it’s the coil technology and overload pairing that decide if the contactor stays closed when the motor pulls full load current and the bus voltage dips. Let’s funnel down to the single dimension that actually determines “runtime under real load”: coil ride-through vs. overload co-ordination.

1. Coil dropout threshold — the hidden runtime limit

The numbers: ABB AF contactors use an electronic wide-range coil rated 100–250 V AC/DC (one SKU covers both); the same family includes AF09 variants covering 24–500 V AC / 20–500 V DC. Siemens SIRIUS 3RT2 contactors (e.g. 3RT2016) use a conventional AC/DC coil with narrow tolerance; the datasheet lists a pick-up voltage of 0.8×Us and drop-out typically below 0.5×Us. Mechanism: The ABB contactor electronic coil contains a switched-mode power supply that maintains a regulated DC rail even when the line voltage drops to ~40 % of nominal. A conventional coil on the 3RT2 sees the full rectified waveform; if the RMS voltage dips below the magnetic hold threshold, the armature releases and the contactor drops out — even if the motor is still drawing locked-rotor current. Worked consequence: In a plant with a weak feeder (e.g., a rural sawmill where a 30 % voltage dip is common during a large motor start), an ABB AF contactor can ride through a 0.5 s sag and keep the motor online; a Siemens 3RT2 with a conventional coil would drop out at ~50 % voltage, killing the motor load. That lost runtime — minutes to hours of re-start sequencing — is a real availability cost. Reversal: If the site has a regulated, stiff bus (e.g., a data centre with UPS-backed 480 V), the dropout advantage of the electronic coil nearly disappears. For clean, stable grids the conventional coil is equally reliable and cheaper to replace.

2. Overload relay co-ordination — the pairing that makes or breaks runtime

The numbers: Siemens 3RT2 contactors are designed to pair exclusively with 3RU2 thermal or 3RB2 solid-state overload relays within the SIRIUS family. ABB AF contactors pair with the AF/Z overload range, but the electronic coil’s low control power allows the overload to be mounted directly without additional control transformer capacity. Mechanism: The runtime under real load is not just the contactor’s rated AC-3 current; it’s the trip curve of the overload relay matched to the contactor’s thermal memory. If an overload relay from a different family is forced onto a 3RT2 (a common field workaround), the bimetallic strip may not reset correctly at low load, or the trip class may drift because the contactor’s thermal mass differs from the overload’s calibration. Worked consequence: A mismatched overload on a Siemens 3RT2 can cause nuisance tripping at 90 % load, reducing effective runtime. With the correct 3RU2, the system holds rated current continuously. For the ABB AF, the electronic coil means the contactor can be paired with the AF overload without worrying about coil-induced voltage drop to the overload heater — a subtle but real reliability gain on long cable runs. Reversal: If the panel builder stocks only Siemens SIRIUS components and never substitutes, the co-ordination is guaranteed. The risk of mismatch is highest on retrofits or multi-brand panels. For new, brand-loyal installations, this advantage doesn’t separate the two.

3. Sustained AC-3 load — where the contactor lives or dies

The numbers: Both the Siemens 3RT2016 (size S00) and ABB AF09 are rated 9 A / 4 kW at 400 V AC-3. The mechanical life of AF09 is stated as ~1 million operations; Siemens contactor does not publish a fixed mechanical life for the 3RT2 series but typical S00 mechanical endurance is also in the 1–2 million range. Mechanism: “Runtime under real load” here means the contactor carrying rated motor current for hours without overheating the arc chamber or degrading the contacts. The contactor’s thermal dissipation is dominated by the main pole resistance, not the coil. With identical AC-3 ratings and similar dimensions (Siemens S00: 45 mm wide; ABB AF09: ~45 mm wide), the thermal headroom is comparable. Worked consequence: In a conveyor application that runs 8 hours at 3.5 kW (87 % of the 4 kW AC-3 rating), both contactors will reach similar steady-state temperatures (~65–75 °C above ambient, illustrative). Neither will drop out due to heat, and contact wear after 500 000 operations will be similar. Reversal: The difference re-appears if the load is actually AC-4 (frequent inching or plugging). ABB’s AF range does not list separate AC-4 ratings in the standard datasheet; Siemens 3RT2 datasheets list AC-4 for some models (e.g. 3RT2016 AC-4 2.5 kW at 400 V). For high-inertia reversing loads, the Siemens conventional arc chamber might have a slight edge due to tested AC-4 data. This is a niche but real reversal.