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Manual Contactor: Working Principle & Wiring Diagram

A manual contactor is a control device that uses pushbuttons to energize or de-energize the load connected to it. See Figure 1. A manual contactor manually opens and closes contacts in an electrical circuit.

Manual Contactor Working

Manual contactors cannot be used to start and stop motors because they have no overload protection built into them. Manual contactors are normally used with lighting circuits and resistive loads such as heaters or large lamp loads.

A fuse or circuit breaker is normally included in the same enclosure with a manual contactor.

A manual contactor uses pushbuttons to energize or de-energize the load connected to it.

Figure 1. A manual contactor uses pushbuttons to energize or de-energize the load connected to it.

Double-Break Contacts

Double-break contacts can act as a direct controller. Double-break contacts are contacts that break an electrical circuit in two places. Double-break contacts are used in pushbuttons. See Figure 2.

Double-break contacts allow devices to be designed with a higher contact rating (current rating) in a smaller space than devices designed with single- break contacts.

With double-break contacts, the movable contacts are forced against the two stationary contacts to complete the electrical circuit when a set of normally open (NO) double-break contacts are energized.

The movable contacts are pulled away from the stationary contacts and the circuit is opened when the manual contactor is de-energized. The procedure is reversed when normally closed (NC) double-break contacts are used.

Double-break contacts

Figure 2. Double-break contacts break an electrical circuit in two places.

A three-phase manual contactor has three sets of NO double- break contacts. One set of NO double-break contacts is used to open and close each phase in the circuit.

The movable contacts are located on an insulated T-frame and are provided with springs to soften their impact. The T-frame is activated by a pushbutton mechanism. Similar to a disconnect, the mechanical linkage consistently and quickly makes or breaks the circuits. See Figure 3.

three phase manual contactor

Figure 3. A three manual contactor has three sets of normally open (NO) double-break contacts.

The movable contacts have no physical connection to external electrical wires. The movable contacts move into arc hoods and bridge the gap between a set of fixed contacts to make or break the circuit. All physical electrical connections are made indirectly to the fixed contacts, normally through saddle clamps.

Manual Contactor Construction

In the past, a major problem with knife switches was that they were constructed from soft copper. Today, most contacts are made of a low-resistance silver alloy.

Silver is alloyed (mixed) with cadmium or cadmium oxide to make an arc-resistant material that has good conductivity (low resistance).

In addition, the silver alloy has good mechanical strength, enabling it to endure the continual wear encountered by many openings and closings.

Another advantage of silver alloy contacts is that the oxide (rust) that forms on the metal is an excellent conductor of electricity. Even when the contacts appear dull or tarnished, they are still capable of operating normally. See Figure 4.

contact oxidation

Figure 4. The oxide (rust) that forms on silver alloy contacts is an excellent conductor of electricity.

Manual Contactor Wiring Diagram

Manual contactors directly control power circuits. Power circuit wiring is shown on a wiring diagram. An understanding of wiring diagrams is required because an electrician may be required to make changes in power circuits as well as in control circuits. See Figure 5.

The wiring diagram for a double-pole manual contactor and pilot light shows the power contacts and their connection to the load.

As in a line diagram, the power circuit is indicated through heavy, dark lines and the control circuit is indicated by thin lines. In this circuit, current passes from L1 through the pilot light to L2, causing the pilot light to glow when the power contacts in L1 and L2 close.

At the same time, current passes from L1 through the heating element to L2, causing the heating element to be activated. The pilot light and heating element are connected in parallel with each other.

contactor power wiring diagram

Figure 5. A wiring diagram shows the connection of an installation or its component devices or parts.

Wiring diagrams may be complex. For example, the wiring diagram for a dual-element heater with pilot lights contains various circuit paths. In this circuit, the low-heat heating element is operated when the low contacts in L1 and L2 are closed so that a connection is made to the low and common terminals of the heater. This allows the low-heat heating element to be energized. See Figure 6.

To operate the high-heat heating element, the high contacts in L1 and L2 are closed so that a connection is made to the high and common terminals of the heater. This allows the high-heat heating element to be energized.

A low-heat pilot light and high-heat pilot light turn on to indicate each condition because each pilot light is in parallel with the appropriate heating element. See Figure 7.

wiring diagram for a dual-element heater

Figure 6. In the wiring diagram for a dual-element heater with pilot lights, the low-heat heating element is operated when the low contacts in L1 and L2 are closed so that a connection is made to the low and common terminals of the heater.

wiring diagram for a dual-element heater 2

Figure 7. In the wiring diagram for a dual-element heater with pilot lights, the high-heat heating element is operated when the high contacts in L1 and L2 are closed so that a connection is made to the high and common terminals of the heater.

One problem that may arise with a dual-element start is that someone may try to energize both sets of elements at the same time. This causes serious damage to the heater. To prevent this problem from occurring, most manual contactors are equipped with a mechanical interlock.

Mechanical Interlock

A mechanical interlock is the arrangement of contacts in such a way that both sets of contacts cannot be closed at the same time.

 Mechanical interlocking can be established by a mechanism that forces open one set of contacts while the other contacts are being closed.

Another method is to provide a blocking bar or holding mechanism that does not allow the first set of contacts to close until the second set of contacts opens.

An electrician can determine whether a device is mechanically interlocked by consulting the wiring diagram information provided by the manufacturer. This information is either normally packaged with the equipment when it is delivered or attached to the inside of the enclosure.

TECH FACT

An understanding of interlocking is required because a lack of interlocking can cause shorted power lines, injury to personnel, and damage to machines, motors, chains, and belts. An understanding of interlocking is also required to ensure it is being used correctly and can be tested for proper operation.

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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