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Magnetic Contactor: Construction & Wiring | Two & Three Wire Control

Contactors may be operated manually or magnetically. Magnetic Contactors are devices for repeatedly establishing and interrupting an electrical power circuit. Contactors are used to make and break the electrical power circuit to loads such as lights, heaters, transformers, and capacitors. See Figure 1.

Contactors are used to make and break the electrical power circuit

Figure 1. Contactors are used to make and break the electrical power circuit to lights, heaters, transformers, and capacitors.

Magnetic Contactor Construction

Solenoid action is the principal operating mechanism for magnetic contactors. The linear action of a solenoid is used to open and close sets of contacts instead of pushing and pulling levers and valves. See Figure 2.

The use of solenoid action rather than manual input is an advantage of a magnetic contactor over a manual contactor. Remote control and automation, which are impossible with manual contactors, can be designed into a system using magnetic contactors.

Solenoid action is the principal operating mechanism for magnetic contactors

Figure 2. Solenoid action is the principal operating mechanism for magnetic contactors.

TECH FACT

To help understand the difference between a relay, contactor, and motor starter, it should be remembered that they are each designed to switch current using contacts operated by a coil.

Relays are used to switch low currents that are usually less than 15 A.

Contactors are basically the same as relays but are used to switch higher currents that are usually up to hundreds of amperes for large lighting or heating loads.

Motor starters are contactors that have an additional overload section to protect a running motor.

Magnetic Contactor Wiring

Control circuits are often referred to by the number of conductors used in the control circuit, such as two-wire and three-wire control.

Two-wire control involves two conductors to complete the circuit. Three-wire control involves three conductors to complete the circuit.

Two-Wire Control

 Two-wire control has two wires leading from the control device to the magnetic contactor or starter. See Figure 3.

The control device could be a thermostat, float switch, or another contact device. When the contacts of the control device close, they complete the coil circuit of the contactor, causing it to energize. This connects the load to the line through the power contacts.

The contactor coil is de-energized when the contacts of the control device open. This de-energizes coil C, which opens the contacts that control the load.

The contactor functions automatically in response to the condition of the control device without the attention of an operator.

two-wire control

Figure 3. In two-wire control, two wires lead from the control device to the contactor or starter.

A two-wire control circuit provides low-voltage release but not low-voltage protection. In the event of a power loss in the control circuit, the contactor de-energizes (low-voltage release), but it also re-energizes it if the control device remains closed when the circuit has power restored.

Low-voltage protection cannot be provided in this circuit because there is no way for the operator to be protected from the circuit once it has been re-energized.

Caution must be exercised in the use and service of two-wire control circuits because of the lack of low-voltage protection.

 Two-wire control is normally used for remote or inaccessible installations, such as pumping stations, water or sewage treatment, air conditioning or refrigeration systems, and process line pumps where an immediate return to service after a power failure is required.

Two-wire control circuits are used with motor loads and non-motor loads. Motor overload protection must be added to a contactor that is used to control a motor load. When motor overload protection is included as part of the contactor assembly, the unit is referred to as a motor starter.

 Magnetic Contactors are not used to control motors unless the motor is a small horsepower motor (normally fractional HP) that includes internal protection, or the contactor is used with a separate motor overload protection unit. With nonmotor loads, the contactor is used to directly control the power applied to the load.

Three-Wire Control

 Three-wire control has three wires leading from the control device to the starter or contactor. See Figure 4.

The circuit uses a momentary contact OFF pushbutton (NC) wired in series with a momentary contact ON pushbutton (NO) wired in parallel to a set of contacts that form a holding circuit interlock (memory).

When the normally open ON pushbutton is pressed, current flows through the normally closed OFF pushbutton, through the momentarily closed ON pushbutton, through magnetic coil C, and on to L2. This causes the magnetic coil to energize. When energized, the auxiliary holding circuit interlock contacts (memory) close, sealing the path through to the coil circuit even if the start pushbutton is released.

three-wire control

Figure 4. In three-wire control, three wires lead from the control device to the starter or contactor.

Pressing the OFF pushbutton (NC) opens the circuit to the magnetic coil, causing the contactor to de-energize. A power failure also de-energizes the contactor. The interlock contacts (memory) reopen when the contactor de-energizes. This opens both current paths to the coil through the ON pushbutton and the interlock.

The three-wire control provides low-voltage release and low-voltage protection. The coil drops out at low or no voltage and cannot be reset unless the voltage returns and the operator press the start pushbutton.

Control Circuit Voltage

Pushbuttons, limit switches, pressure switches, temperature switches, etc. are used to control the flow of power to the contactor (or motor starter) magnetic coil in the control circuit.

When the control circuit is connected to the same voltage level as the load (lamps, heating elements, or motors), the control circuit must be rated for the same voltage.

In most circuits in which the load is rated higher than 115 V (normally 208 V, 230 V, 240 V, 460 V, and 480 V), the control circuit is operated at a lower voltage level than the load.

A step-down control transformer is used to step down the voltage to the level required in the control circuit. Normally, the secondary of the transformer is rated for 12 V, 24 V, or 120 V.

The voltage of a control circuit can be any voltage (AC or DC), but it is commonly less than 120 V. See Figure 5.

step-down control transformer

Figure 5. A step-down control transformer is used to step down the voltage to the level required in the control circuit.

About Ahmed Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.

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