How to Choose a Contactor?

Figure 1: A contactor in operation.

In industrial electricity, the contactor uses an energized coil to generate a magnetic field to close the contacts to control the load. It can be used to control the electrical loads of factory equipment, electric heaters, working mother machines and various power units. The contactor can not only make and cut the circuit, but also has the function of low voltage release protection.

The contactor is used as a device for switching on and off the load power supply, and its selection should meet the requirements of the controlled equipment. In addition to the rated voltage and the rated voltage of the controlled equipment, the load power, use category, control method, operating frequency, working life, installation method, installation size and economy of the controlled equipment are all the basis for selection.
The followings are factors to consider for contactor selection.

1. Voltage Level

The voltage level of the contactor should be the same as that of the load.

2. Rated Current

The calculated current of the load must conform to the capacity level of the contactor, that is, the calculated current is less than or equal to the rated current of the contactor. When the making current of the contactor is greater than the starting current of the load and the breaking current is greater than the load, the breaking current is required.
The actual working environment and working conditions should be considered for the calculation current of the load. For loads with a long start-up time, the peak current for half an hour cannot exceed the agreed thermal current.

3. The Coil

The rated voltage, current, number of auxiliary contacts and current capacity of the contactor attracting coil should meet the wiring requirements of the control circuit. The length of the line connected to the contactor control loop should be considered. If the line is too long, the contactor coil may not respond to the closing command due to too large a voltage drop; because the line capacitance is too large, it may not work for the trip command.

Figure 2: Technical specifications of a Schneider IEC magnetic contactor.

4. Operating Frequency

Check the allowable operating frequency of the contactor against the number of operations. If the operating frequency exceeds the specified value, the rated current should be doubled.

5. Protection Component

The parameters of the short circuit protection component should be matched with the parameters of the contactor. Please refer to the catalogue manual. The catalogue manual generally gives the matching table of contactors and fuses.

6. Distance between Devices

The installation distance of the contactor and other devices should comply with the relevant international standards and specifications, considering the maintenance and wiring distance.

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