Contactors are most commonly used in applications concerning control of electric motors. Contactors are used to start, stop, reverse, jog and plug the motors depending upon the application requirement. Contactors along with thermal overload relays also provide protection to the motor against overloads.
The most basic data required for contactor selection is the motor HP rating and it’s rated current. However this data is alone not sufficient. The type of load, duty cycle of the load, switching frequency are some of the factors that influence contactor selection. The switching capability of contactors is majorly dependent on the type of application, and hence international standards (IEC 60947-4-1) specify utilization categories which cover a broad range of applications. These utilization categories and the data associated with them are used by manufacturers to establish contactor ratings.
The utilization categories as per IEC 60947-4-1 are as follow:
AC-1 : Non-inductive or slightly inductive loads, resistance furnaces
AC-2 : Slip-ring motors : starting, switching off
AC-3 : Squirrel-cage motors : starting, switching off motors during running 1)
AC-4 : Squirrel-cage motors : starting, plugging, inching
AC-5a : Switching of electric discharge lamp controls
AC-5b : Switching of incandescent lamps
AC-6a : Switching of transformers
AC-6b : Switching of capacitor banks
AC-7a : Slightly inductive loads in household appliances and similar applications
AC-7b : Motor loads for household applications
AC-8a : Hermetic refrigerant compressor motor to control with manual resetting of overload release
AC-8b : Hermetic refrigerant compressor motor to control with automatic resetting of overload release
AC-15 : Control of a.c electromagnetic lods
DC-1 : Non-inductive or slightly inductive loads, resistance furnace
DC-3 : Shunt-motors : Starting, Plugging, Inching
Dynamic braking of dc motors
DC-5 : Series-motors : Starting, Plugging, Inching
Dynamic braking of dc motors
DC-6 : Switching of incandescent lamps
1) AC-3 category may be used for occasional inching (jogging) or plugging for limited time periods such as machine set-up: during limited time periods, the number of such operations should not exceed five per minute or more than 10 in a ten minute period.
2) A hermetic refrigerant compressor motor is a combination consisting of a compressor and a motor, both of which are enclosed in the same housing, with no external shaft or shaft seals, the motor operating in the refrigerant.
The utilization categories most commonly encountered in contactor applications are AC-3 & AC-4
Applications under utilization category AC-3 (Normal Switching) are: Compressors, Pumps, Fans, Conveyors, Mixers, Agitators, Air conditioners, Elevators etc
Applications under utilization category AC-4 (Plugging, inching) are: Printing presses, Wire drawing machines, Centrifuges etc
The making and breaking capacities of contactors are dependent on the utilization categories and the standard specifies that the contactors or starters shall be capable of making and breaking currents without failure under the conditions stated.
1) AC-3 category may be used for occasional inching (jogging) or plugging for limited time periods such as machine set-up: during limited time periods, the number of such operations should not exceed five per minute or more than 10 in a ten minute period.
2) A hermetic refrigerant compressor motor is a combination consisting of a compressor and a motor, both of which are enclosed in the same housing, with no external shaft or shaft seals, the motor operating in the refrigerant.
The utilization categories most commonly encountered in contactor applications are AC-3 & AC-4
Applications under utilization category AC-3 (Normal Switching) are: Compressors, Pumps, Fans, Conveyors, Mixers, Agitators, Air conditioners, Elevators etc
Applications under utilization category AC-4 (Plugging, inching) are: Printing presses, Wire drawing machines, Centrifuges etc
The making and breaking capacities of contactors are dependent on the utilization categories and the standard specifies that the contactors or starters shall be capable of making and breaking currents without failure under the conditions stated.