Mismatched AC Contactor Sizing? These 4 Typical Issues You Must Not Ignore – Click to View Solutions

AC contactors (with the electrical schematic code KM) are core electrical devices used to control the connection/disconnection between power supplies and loads in circuits, and they are also common equipment that electricians need to work with regularly. In practice, it is not uncommon to find that some peers make mistakes in selecting AC contactors, leading to improper sizing and subsequent persistent issues. Here, four of the most typical common mistakes are highlighted for reference.

I. Over-Reliance on Rated Current for Sizing

When selecting a suitable AC contactor based on the load, some electricians often only refer to the load’s rated current. This results in frequent burnout or melting of the AC contactor’s main contacts during actual operation.

The root cause of this fault is that the method of selecting an AC contactor (based on main contact capacity) using only the rated current is only applicable to purely resistive loads such as electric heating wires. For inductive loads like three-phase asynchronous motors, the starting current—affected by factors such as the starting method, type of driven load, and starting frequency—typically ranges from 4 to 7 times the rated current during the start-up phase (before stable operation). Therefore, it is crucial and necessary to take the load’s starting current into account when selecting an AC contactor.

II. Neglecting Coil Voltage Selection (Priority for Safe Voltage)

With the growing awareness of safe electricity use and compliance with safe operation standards, and to reduce unnecessary electric shock accidents, it has become a general trend to prioritize safe voltage levels (AC36V) for AC contactor coil voltages.

Thus, during the design, selection, and assembly of AC contactors, priority should be given to products with a coil voltage rating of AC36V. Every effort should be made to avoid situations where multiple coil voltage levels (such as AC380V and AC220V) coexist in the circuit.

III. Ignoring Auxiliary Contact Requirements

To reduce the number of other auxiliary devices (e.g., intermediate relays) and minimize the size of the electrical control system, the type of AC contactor should also be comprehensively determined based on the number of auxiliary contacts required for the contactor in the circuit.

For example, if the circuit requires a large number of auxiliary contacts for the AC contactor, it is more sensible to select CJX series AC contactors (which can be equipped with 2 or 4 additional auxiliary contacts) rather than CJT series AC contactors.

IV. Improper Control Connection with PLCs

Beyond the three factors mentioned above, a supplementary note concerns the control method of the AC contactor (coil). Currently, industrial control devices such as PLCs (Programmable Logic Controllers)—which facilitate centralized control—are increasingly widely used. However, many peers have directly connected AC contactor coils to PLC output terminals, leading to damage to internal PLC output components (relays, transistors, thyristors).

The cause of this fault is simply that the current during the contactor coil’s pull-in process exceeds the current-carrying capacity of the PLC’s output components. Therefore, when using a PLC to control an AC contactor, a relay should be used as an intermediate control link between the two.