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Contactor catalogues show two different current ratings for the same device: one for AC-1 and one for AC-3. The gap between them is often more than double, and choosing on the wrong one means the contactor lasts far less than expected.
What the utilisation category tells you
| Category | Load type | Switching condition |
|---|---|---|
| AC-1 | Resistive / slightly inductive | Heaters, resistors, power supplies |
| AC-3 | Squirrel-cage motor | Starting current, breaking while running |
| AC-4 | Motor, plugging and inching | Jogging, plugging, frequent starts |
| DC-1 / DC-3 | DC loads | Assessed from a separate table |
On a motor feeder, always take the AC-3 figure. “40 A” printed on a contactor does not mean it can drive a 40 A motor; that value is most likely AC-1, and the motor-side equivalent is a lower current corresponding to roughly 18 kW.
Switching frequency sets the life
Electrical life is measured in operations. A pump feeder that switches a few times an hour and a conveyor that starts and stops several times a minute do not need the same device. Where starts are frequent, moving up a frame size or controlling through a drive is cheaper in the long run.
Coil voltage: the most common ordering mistake
- 230 V AC — the classic panel control voltage and still the most common.
- 24 V DC — standard in modern panels driven directly from PLC outputs.
- 24 V AC / 110 V AC — seen in special specifications and older installations.
- Stating the coil voltage when ordering is faster than swapping coils later.
Frequently asked questions
When is an auxiliary contact block needed?
When the contactor's state must be reported to the PLC, an interlock is required, or a pilot light is driven. If the built-in contacts are not enough, a side- or front-mounted block is used; state the number of NO/NC contacts when ordering.
Is a contactor needed alongside a drive?
A contactor is not used to switch the motor on a drive output. Where safe stopping is required, the STO input on modern drives is preferred; on the supply side a separate load-break switch or contactor may be fitted for isolation during service.
The contactor coil is humming — why?
The most common causes are low coil voltage, dirt or corrosion on the pole faces, and foreign matter on the magnetic core. Measure the control voltage and increase the conductor size if it is dropping.
Does the overload relay go above or below the contactor?
The overload relay is fitted on the load side of the contactor so it measures the current going to the motor. Direct-mount types clip underneath; separately mounted types are wired in.