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AC and DC Contactor Working Principle

AC contactor assemblies may have several sets of contacts. DC contactor assemblies typically have only one set of contacts. See Figure 1.

In three AC contactors, all three power lines must be broken. This creates the need for several sets of contacts. For multiple contact control, a T-bar assembly allows several sets of contacts to be activated simultaneously. In a DC contactor, it is necessary to break only one power line.

AC or DC contactors

Figure 1. Contactors have either an AC coil or a DC coil, but they may have either AC or DC contacts.

AC Vs DC Contactors

AC contactor assemblies are made of laminated steel, while DC assemblies are solid. Laminations are unnecessary in a DC coil because the current travels in one direction at a continuous rate and does not create eddy current problems.

The other major differences between AC and DC contactors are the electrical and mechanical requirements necessary for suppressing the arcs created in opening and closing contacts under load.

Arc Suppression

Arc suppression is required on contactors and motor starters. An arc suppressor is a device that dissipates the energy present across opening contacts.

Without arc suppression, contactors and motors may require premature maintenance that results in excessive downtime.

Opening Contact Arc

 A short period of time (a few thousandths of a second) exists when a set of contacts is opened under load. During this time, the contacts are neither fully in touch with each other nor completely separated. See Figure 2.

An electrical arc is created between contacts as they are opened

Figure 2. An electrical arc is created between contacts as they are opened. Prolonged arcing may result in damage to contact surfaces.

As the contacts continue to separate, the contact surface area decreases, which increases the electrical resistance. With full-load current passing through the increasing resistance, a substantial temperature rise is created on the surface of the contacts.

This temperature rise is often high enough to cause the contact surfaces to become molten and emit ions of vaporized metal into the gap between the contacts. This hot ionized vapor permits the current to continue to flow in the form of an arc, even though the contacts are completely separated.

The arcs produce additional heat, which, if continued, can damage the contact surfaces. The sooner the arc is extinguished, the longer the life expectancy of the contacts.

DC Arc Suppression

 DC arcs are considered the most difficult to extinguish because the continuous DC supply causes current to flow constantly and with great stability across a much wider gap than does an AC supply of equal voltage.

To reduce arcing in DC circuits, the switching mechanism must be such that the contacts separate rapidly and with enough of an air gap to extinguish the arc as soon as possible on opening.

 DC contactors are larger than AC contactors to allow for the additional air gap. In addition, the operating characteristics of DC contactors are faster than AC contactors.

When closing DC contacts, it is necessary to move the contacts together as quickly as possible to prevent some of the same problems encountered in opening them.

 One disadvantage to rapidly closing DC contactors is that the contacts must be buffered to eliminate contact bounce due to excessive closing force.

Contact bounce may be minimized through the use of certain types of solenoid action and springs attached under the contacts to absorb some of the shocks.

AC Arc Suppression

 An AC arc is self-extinguishing when a set of contacts is opened. In contrast to a DC supply of constant voltage, an AC supply has a voltage that reverses its polarity 70 times a second when operated on a 5 hertz (Hz) line frequency.

The alternation allows the arc to have a maximum duration of no more than a half-cycle. During any half-cycle, the maximum arcing current is reached only once in that half-cycle. See Figure 3.

The contacts can be separated more slowly and the gap length may be shortened because an AC arc is self- extinguishing. This short gap keeps the voltage across the gap and the arc energy low. With low gap energy, ionizing gases cool more rapidly, extinguishing the arc and making it difficult to restart.

AC contactors need less room to operate and run cooler, which increases contact life.

maximum arcing current

Figure 3. The maximum arcing current is reached only once during any half-cycle of AC voltage.

Arcs at Closing

 Arcing may also occur on AC and DC contactors when they are closing. The most common arcing occurs when the contacts come close enough that an arc is able to bridge the open space between the contacts.

Arcing also occurs if a whisker or rough edge of the contact touch first and melts, causing an ionized path that allows current to flow.

In either case, the arc lasts until the contact surfaces are fully closed. Contactor design is quite similar for both AC and DC devices.

The contactor should be designed so that the contacts close as rapidly as possible, without bouncing, to minimize the arc at each closing.

Arc Chutes

 An arc chute is a device that confines, divides, and extinguishes arcs drawn between contacts opened under load. See Figure 4.

Arc chutes are used to contain large arcs and the gases created by them. Arc chutes employ the de-ion principle, which confines, divides, and extinguishes the arc for each set of contacts.

Arcs may also be extinguished by using special arc traps and arc-quenching compounds. This method of extinguishing arcs is a circuit breaker technique that attracts, splits, and quickly cools arcs as well as vents ionized gases.

Vertical barriers between each set of contacts, as well as arc covers, confine arcs to separate chambers and quickly quench them.

Arc chutes and arc traps

Figure 4. Arc chutes and arc traps are used to confine, divide, and extinguish arcs drawn between contacts opened under load.

DC Magnetic Blowout Coils

 When a DC circuit carrying large amounts of current is interrupted, the collapsing magnetic field of the circuit current may induce a voltage that helps sustain the arc.

Action must be taken to quickly limit the damaging effect of the heavy current arcs because a sustained electrical arc may melt the contacts, weld them together, or severely damage them.

One way to stop the arc quickly is to move the contacts some distance from each other as quickly as possible. The problem is that the contactor has to be large enough to accommodate such a large air gap.

Magnetic blowout coils are used to reduce the distance required and yet quench arcs quickly. Magnetic blowout coils provide a magnetic field that blows out the arc similarly to blowing out a match.

A magnetic field is created around the current flow whenever current flows through a conductive medium (in this case ionized air). The direction of the magnetic field around the conductor is determined by wrapping the right or left hand around the conductor.

When the thumb on the right-hand points in the direction of conventional current flow, the wrapping fingers point in the direction of the resulting magnetic field. When the thumb on the left-hand points in the direction of electron current flow, the wrapping fingers point in the direction of the resulting magnetic field. See Figure 5.

magnetic field around the conductor

Figure 5. The direction of the magnetic field around the conductor is determined by wrapping the right or left hand around the conductor. The electron flow motor rule indicates the motion of an arc cutting through magnetic lines of force.

The electron flow motor rule states that:

 When a current-carrying conductor (represented by the middle finger) is placed in a parallel magnetic field (represented by the index finger), the resulting force or movement is in the direction of the thumb. This action occurs because the magnetic field around the current flow opposes the parallel magnetic field above the current flow. This makes the magnetic field above the current flow weaker, while aiding the magnetic field below the current flow, making the magnetic field stronger. The net result is an upward push that quickly elongates the arc current so that it breaks (blows out). An electromagnetic blowout coil is often referred to as a puffer because of its blowout ability. See Figure 6.

Electromagnetic blowout coils rapidly extinguish DC arcs

Figure 6. Electromagnetic blowout coils rapidly extinguish DC arcs.

Contact Construction

Contact design and materials depend on the size, current rating, and application of the contactor. Double-break contacts are normally made of a silver-cadmium alloy. Single-break contacts in large contactors are frequently made of copper because of the low cost.

Single-break copper contacts are designed with a wiping action to remove the copper oxide film that forms on the copper tips of the contacts.

The wiping action is necessary because copper oxide formed on the contacts when not in use is an insulator and must be eliminated for good circuit conductivity.

In most cases, the slight rubbing action and burning that occur during normal operation keep the contact surfaces clean for proper operation. Copper contacts that seldom open or close, or those being replaced, should be cleaned to reduce contact resistance. High contact resistance often causes serious heating of the contacts.

AC/DC Contactor Sizes and Ratings

Magnetic contactors, like manual contactors, are rated according to the size and type of load by the National Electrical Manufacturers Association (NEMA).

Tables are used to indicate the number/size designations and establish the current load carried by each contact in a contactor. See Figure 7.

The rating is for each contact individually, not for the entire contactor. For example, a size 0, three- pole contactor rated at 8 A is capable of, and rated for, switching three separates 8 A loads simultaneously.

Tables indicate the number/size designations and establish the current load carried by each contact in a contactor

Figure 7. Tables indicate the number/size designations and establish the current load carried by each contact in a contactor.

Contactor dimensions vary greatly, ranging from inches to several feet in length. Contactors are selected based on type, size, and voltage available. See Figure 8.

Contactors are also available in a variety of enclosures. The enclosures offer protection ranging from the most basic protection to high levels of protection required in hazardous locations where any spark caused by the closing or opening of the contact could cause an explosion.

Contactor dimensions

Figure 8. Contactor dimensions vary from inches to several feet in length.

TECH FACT:

NEMA contactors and motor starters are relatively large devices and are designed for a broad range of applications.

When selecting a NEMA contactor or motor starter for an application, only the load voltage and power requirements must be known.

IEC contactors and motor starters are relatively small devices and are designed for specific applications.

When selecting an IEC contactor or motor starter for an application, the load type, full-load current, duty cycle, voltage, and current must be known.

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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