The duty of electrical contractor is categorized by the types of electrical loads used such as inductive, or resistive or capacitive and duty cycle such as plugging or runtime breaking, short circuit breaking etc.
Let’s see the different types of contactor duties are used in electrical distribution system. Below mentioned duties are categorized by IEC (International Electrotechnical Commission). Generally, these rating are normally mentioned on the contactor Refer the picture.
AC duty (Alternating Current) for Contactors:
AC-1: Such duty contactors are used in resistive loads such as heaters and electrical furnace. Non-inductive or slightly inductive loads are included which means the power factor of the load lie between 0.95 to 1.
AC-2: These are used in Slip-ring motors starters such as Switching on and switching off the motor. They mostly prefer for high Torque current application.
AC-3: These type of contactors are generally preferred for starting of Squirrel-cage motors, and switches off motor during running time which means the contactor can withstand of high current continuously. Example. Lifts, elevators, fans etc.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.
Image above represents the Air Break Power Contactor from Siemens, with mentioning AC1 ratings of 30A while AC3 rating of 16A, Usually AC3 ratings are lesser than AC1 ratings because breaking the arc in resistive circuits(AC1) remains easy with minimal arcing and contact wear.
AC-4: Frequently on/off on Squirrel-cage motors such contactors are used. They have ability to break high starting current starting such as plugging and inching operation. Example: Cranes
AC-5a: These type of contactor is used in discharge lamps such as mercury vapor and sodium vapor lamps and Auxiliary Control circuit.
AC-5b: Switching of incandescent lamps
AC-6a: Transformers ON/OFF
AC-6b: These type of contactor is used in capacitor banks switching.
AC-7a: Small Inductive loads on house hold such as TV, mixers, drilling machine etc.
AC-7b: Rotating machines on households such as fans, central vacuum cleaners, washing machines etc.
AC-8a: Hermetic refrigerant compressor motor control with manual resetting on O/L.
AC-8b: Hermetic refrigerant compressor motor control with automatic resetting overloads
AC11: Auxiliary (control) circuits i.e. they don’t have power contacts such as NO (Normally open) and NC (Normally closed)
AC-12: Electronics switching using solid state devices on resistive loads
AC-13: Control of Resistive Load & Solid State Load with Transformer Isolation
AC-14: Control of small electromagnetic loads less than 72VA
AC-15: Control of A.C. electromagnetic loads greater than 72VA
AC-20: Connecting and disconnecting under no-load conditions
AC-21: Switching of resistive loads, including moderate overloads
AC-22: Switching of inductive loads as well resistive loads (Mixed)
AC-23: Switching of motor loads or other highly inductive loads
A: Protection of circuits, with no rated short-time withstand current
B: Protection of circuits, with a rated short-time withstand current
Direct Current Duties DC1, DC3, DC5:
DC-1: Non Inductive or slightly inductive loads, resistance furnaces, heaters
DC-3: Shunt-motors, starting, plugging (1), inching(2), dynamic braking of motors
DC-5: Series-motors, starting, plugging (1), inching(2), dynamic braking of motors
DC-6: Switching of incandescent lamps
DC-12: Control of resistive loads and solid state loads with opto-coupler isolation
DC-13: Control of D.C. electromagnetics
DC-14: Control of D.C. electromagnetic loads having economy resistors in the circuit
DC-20: Connecting and disconnecting under no-load conditions
DC-21: Switching of resistive loads, including moderate overloads
DC-22: Switching of mixed resistive and inductive loads, including moderate overloads (i.e. shunt motors)
DC-23: Switching of highly inductive loads (i.e. series motors)
However mostly in industries, AC1 and AC3 contactors are preferred
Difference between AC1 and AC3 contactors is AC1 contactor are used for less inductive loads such as resistive loads (the power factor of the load is near about 1) but AC3 duty contactors are used for high inductive, squirrel cage motors which will be off during the motor is running condition.
Example for AC1: Heaters, Electric Furnace
Example for AC3: All squirrel cage motors such as industrial fans, lifts, escalators, conveyors, bucket elevators, compressors, pumps, mixers, air conditioning units
