Things You Should Know About Contactors

As we all know, contactor is a mechanical switching device controlled by an electromagnet with go-no go operation. This is the closing and opening of an electrical circuit under load and is most carried out by a contactor – first invented by Telemecanique (part of Schneider Electric) in 1924. The contactor has main poles that do the switching. These poles are opened and closed by energizing an electromagnet called a coil. The coil is typically designed for either ac or dc voltage and has a nominal control voltage.

The corresponding IEC standards for the contactors are IEC 60947-4-1and IEC 60947-5-1. There are several factors that should consider at the selection of contactors.

• Utilization Category
• Rated power (kW)/ current (A)/ Voltage (V)
• Electrical life & mechanical life
• Making & breaking capacities
• Coil voltages & Accessories
• Application & Environmental/ambient factors
• Are some of major factors.

What is Utilization category referred with contactors...?

This defines, with standard use of a contactor & depends on the load controlled & the conditions in which the making and breaking are applied. Mainly there are three types of electrical loads, i.e Resistive loads (Lighting loads, heating loads which the power factor close to 1.0), Inductive loads (Motors, solenoids, transformers etc.), Capacitive loads (Capacitor banks etc) Utilization categories define the use of contactors at different load types depending on the nature of control & applied conditions.

Utilization Categories AC

AC-1 Non-inductive or slightly inductive loads, resistance ovens.

AC-2 Slip-ring motors: Starting, disconnection.

AC-3 Cage motors: starting, disconnection of running motors.

AC-4 Cage motors: Starting, reversal of direction, inching.

AC-5a Control of discharge lamps

AC-5b Control of incandescent lamps

AC-6a Control of transformers

AC-6b Control of capacitor banks

AC-8a Control of sealed compressor motors for refrigeration with manual reset of overload trip.

AC-8b Control of sealed compressor motors for refrigeration with automatic reset of overload trip

Utilization Categories DC

DC-1 Non-inductive or slightly inductive loads, resistance ovens.

DC-3 Shunt motors: starting, current reversal, inching.

DC-5 Series motors: starting, current reversal, inching.

DC-6 Control of incandescent lamps.

AC -1 duty refers to the application of non-inductive or slightly inductive (resistive, power factor above 0.95) loads, which the starting, running & breaking current are almost same. Lighting circuits, heaters are good examples for AC-1 duty application. One of mostly utilized category is AC-3, i.e. starting & breaking during normal running of cage motors. On closing, the contactor makes the inrush current, which is about 5 to 7 times the rated full load current of the motor. On opening, the contactor breaks the rated full load current of the motor. AC-4 duty refers to start plugging, inching action of squirrel-cage motors. (rapid start stop action) Typically the contactor makes the inrush current which is 6-8 times the rated, and breaks the same. Eg; crane applications. AC-2 applies almost similar scenario but for slipring motors. Likewise, other duty categories also refer to different utilizations. Apart from the motor applications, if a contactor arrangement is used for a source changeover system, typically the capacity is considered with AC-1 duty.

Mechanical Life

This is the number of no-load switching operations that the contactor can perform without mechanical failure or maintenance.

Breaking capacity

Contactor’s rated breaking capacity is the value of current, on which the contacts of contactor can break the connections (i.e. Contactor can open their contacts) without any damage (arcing or melting). A.C. contactor’s breaking capacity is defined according to the Current R.M.S value. Typically, the breaking capacity of a contactor at AC-3 is =8 x operational current & 10 x operational current at AC-4. Importance of this breaking capacity comes into the picture when coordinating the starter components including protective devices.

Making capacity

This is the rms current value that the contactor can make accordance with the conditions imposed by the standards. (value of current, on which the contacts of contactor can make the connections safely) In practice, the making capacity is independent of the rated voltage. Typically making capacity at AC3 utilization =10 x operational current & AC4=12 x operational current

Coil supply voltages

Contactors available with different coil supply voltages and most of the time coils are changeable. When selecting a contactor, the external input circuit voltage should be known.

Environmental factors

It is essential to pay attention on surrounding environmental conditions which the contactor is going to install. For an example, ambient temperature, humidity etc. In such conditions the expected performance of the device can be deviated & relevant measures to be taken as per the manufacture’s recommendations.

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