Industrial Motor Control: Relays, Contactors, and Motor Starters

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-- Discuss the operation of magnetic type relay devices.

-- Explain the differences among relays, contactors, and motor starters.

-- Connect a relay in a circuit.

-- Identify the pins of 8- and 11-pin relays.

-- Discuss the differences between dc and ac type relays and contactors.

-- Discuss the differences between NEMA- and IEC- rated starters.

Relays and contactors are electromechanical switches. They operate on the solenoid principle. A coil of wire is connected to an electric current. The magnetic field developed by the current is concentrated in an iron pole piece. The electromagnet at tracts a metal armature. Contacts are connected to the metal armature. When the coil is energized, the contacts open or close. There are two basic methods of constructing a relay or contactor. The clapper type uses one movable contact to make connection with a stationary contact. The bridge type uses a movable contact to make connection between two stationary contacts.


Relays are electromechanical switches that contain auxiliary contacts. Auxiliary contacts are small and are intended to be used for control applications.

As a general rule, they are not intended to control large amounts of current. Current ratings for most relays can vary from 1 to 10 amperes, depending on the manufacturer and type of relay. A clapper-type relay is illustrated in FIG. 1. When the coil is energized, the armature is attracted to the iron core inside the coil. This causes the movable contact to break away from one stationary contact and make connection with another. The common terminal is connected to the armature, which is the movable part of the relay. The movable contact is attached to the armature. The two stationary contacts form the normally closed and normally open contacts. A spring returns the armature to the normally closed position when power is removed from the coil. The shading coil is necessary to prevent the contacts from chattering. All solenoids that operate on alternating current must have a shading coil. Relays that operate on direct current do not require them.

A clapper-type relay is shown in FIG. 2.

FIG. 1 A magnetic relay is basically a solenoid with movable contacts attached.

FIG. 2 Clapper-type relay that contains one movable contact and two stationary contacts. This relay is single-pole, double throw.

Bridge Type Relay

A bridge-type relay operates by drawing a piece of metal or plunger inside a coil (FIG. 3). The plunger is connected to a bar that contains movable contacts. The movable contacts are mounted on springs and are insulated from the bar. The plunger and bar assembly is called the armature because it is the moving part of the relay. Bridge contacts receive their name because when the solenoid coil is energized and the plunger is drawn inside the coil, the movable contacts bridge across the two stationary contacts. Bridge contacts can control more voltage than the clapper-type because they break connection at two places instead of one.

When power is removed from the coil, the force of gravity or a spring returns the movable contacts to their original position. A relay with bridge-type contacts is shown in FIG. 4.

Electromagnet Construction

The construction of the electromagnetic part of a relay or contactor greatly depends on whether it is to be operated by direct or alternating current. Re lays and contactors that are operated by direct cur rent generally contain solid core materials, whereas those intended for use with alternating current contain laminated cores. The main reason for the laminated core is the core losses associated with alternating current caused by the continuous changing of the electromagnetic field.

FIG. 3 Bridge-type contacts use one movable and two stationary contacts. They can control higher voltages because they break connection at two places instead of one.

FIG. 4 A relay with bridge-type contacts.

FIG. 5 eddy currents are induced into the metal core and produce power loss in the form of heat.

FIG. 6 The molecules are in disarray in a piece of unmagnetized metal.

FIG. 7 The molecules are aligned in a piece of magnetized metal.

FIG. 8 When the magnetic polarity changes, all the molecules change position.

Core Losses

The continuous change of both amplitude and polarity of the magnetic field causes currents to be induced into the metal core material. These currents are called eddy currents because they are similar to eddies (swirling currents) found in rivers. Eddy currents tend to swirl around inside the core material, producing heat (FIG. 5). Laminated cores are constructed with thin sheets of metal stacked together. A thin layer of oxide forms between the laminations. This oxide is an insulator and helps reduce the formation of eddy currents.

Another type of core loss associated with alternating-current devices is called hysteresis loss.

Hysteresis loss is caused by the molecules inside magnetic materials changing direction. Magnetic materials such as iron or soft steel contain magnetic domains or magnetic molecules. In an unmagnetized piece of material, these magnetic domains are not aligned in any particular order (FIG. 6). If the metal becomes magnetized, the magnetic molecules or domains align themselves in an orderly fashion (FIG. 7). If the polarity of the magnetic field is reversed, the molecules re align themselves to the new polarity (FIG. 8). Although the domains realign to correspond to a change of polarity, they resist the realignment. The power required to cause them to change polarity is a power loss in the form of heat. Hysteresis loss is often referred to as molecular friction because the molecules are continually changing direction in an alternating-current field. Hysteresis loss is proportional to the frequency. At low frequencies such as 60 hertz, it is generally so small that it is of little concern.

FIG. 9 The current in an ac circuit continually changes amplitude and direction.

FIG. 10 As current begins to rise, a magnetic field is concentrated in the pole piece.

FIG. 11 The magnetic field of the shading coil causes the magnetic field of the pole piece to bend away and concentrate in the unshaded portion of the pole piece.

Shading Coils

As mentioned previously, all solenoid-type devices that operate on alternating current contain shading coils to prevent chatter. The current in an ac circuit is continually increasing from zero to a maximum value in one direction, returning to zero, and then increasing to a maximum value in the opposite direction (FIG. 9). Because the current is continually falling to zero, the solenoid spring or gravity continually tries to drop the armature out when the magnetic field collapses. Shading coils provide a time delay for the magnetic field to prevent this from happening. As current increases from zero, magnetic lines of flux concentrate in the metal pole piece (FIG. 10). This increasing magnetic field cuts the shading coil and induces a voltage into it.

Because the shading coil or loop is a piece of heavy copper, it has a very low resistance. A very small induced voltage can cause a large amount of current to flow in the loop. The current flow in the shading coil causes a magnetic field to be developed around the shading coil also. This magnetic field acts in opposition to the magnetic field in the pole piece and causes it to bend away from the shading coil (FIG. 11). As long as the ac current is changing in amplitude, a voltage is induced in the shading loop.

When the current reaches it maximum, or peak, value, the magnetic field is no longer changing and there is no voltage induced in the shading coil. Because the shading coil has no current flow, there is no magnetic field to oppose the magnetic field of the pole piece (FIG. 12).

When the current begins to decrease, the magnetic field of the pole piece begins to collapse. The collapsing magnetic field again induces a voltage into the shading coil. Because the collapsing magnetic field is moving in the opposite direction, the voltage induced into the shading coil causes current to flow in the opposite direction, producing a magnetic field of the opposite polarity around the shading coil. The magnetic field of the shading coil now tries to maintain the collapsing magnetic field of the pole piece (FIG. 13). This causes the magnetic flux lines of the pole piece to concentrate in the shaded part of the pole piece. The shading coil provides a continuous magnetic field to the pole piece, preventing the armature from dropping out.

A laminated pole piece with shading coils is shown in FIG. 14.

FIG. 12 When the current reaches its peak value, the magnetic field is no longer changing, and the shading coil offers no resistance to the magnetic field of the pole piece.

FIG. 13 As current decreases, the collapsing magnetic field again induces a voltage into the shading coil. The shading coil now aids the magnetic field of the pole piece and flux lines are concentrated in the shaded section of the pole piece.

FIG. 14 Laminated pole piece with shading coils.

Control Relay Types

Control relays can be obtained in a variety of styles and types (FIG. 15). Most have multiple sets of contacts, and some are constructed in such a manner that their contacts can be set as either normally open or normally closed. This flexibility can be a great advantage in many instances. When a control circuit is being constructed, one relay may require three normally open contacts and one normally closed, whereas another may need two normally open and two normally closed contacts.

Relays that are designed to plug into 8- or 11-pin tube sockets are popular for many applications (FIG. 16). These relays are relatively inexpensive, and replacement is fast and simple in the event of failure. Because the relays plug into a socket, the wiring is connected to the socket, not the relay. Replacement is a matter of removing the defective relay and plugging in a new one. An 11 pin tube socket is shown in FIG. 17. 8- and 11-pin relays can be obtained with different coil voltages. Coil voltages of 12 volts dc, 24 volts dc, 24 volts ac and 120 volts ac are common. Their contact ratings generally range from 5 to 10 amperes, depending on relay type and manufacturer.

The connection diagram for 8- and 11-pin relays is shown in FIG. 18. The pin numbers for 8- and 11-pin relays can be determined by holding the re lay with the bottom facing you. Hold the relay so that the key is facing down. The pins are numbered as shown in FIG. 18. The 11-pin relay contains three separate single-pole, double-throw contacts.

Pins 1 and 4, 6 and 5, and 11 and 8 are normally closed contacts. Pins 1 and 3, 6 and 7, and 11 and 9 are normally open contacts. The coil is connected to pins 2 and 10.

The eight-pin relay contains two separate single-pole, double-throw contacts. Pins 1 and 4, and 8 and 5 are normally closed. Pins 1 and 3, and 8 and 6 are normally open. The coil is connected across pins 2 and 7.

FIG. 15 Control relays can be obtained in a variety of case styles.

FIG. 16 relays designed to plug into 8- and 11-pin tube sockets.

FIG. 17 eleven-pin tube socket.

FIG. 18 Connection diagrams for 8- and 11-pin relays.

FIG. 19 Solid-state relay using a reed relay to control the action of a triac.

Solid-State Relays

Another type of relay that is found in many applications is the solid-state relay. Solid-state re lays employ the use of solid-state devices instead of mechanical contacts to connect the load to the line. Solid-state relays that are intended to connect alternating-current loads to the line use a device called a triac. The triac is a bidirectional de vice, which means that it permits current to flow through it in either direction. There are a couple of methods used to control when the triac turns on or off. One method employs a small relay device that controls the gate of the triac (FIG. 19). The relay can be controlled by a low-voltage source.

When energized, the relay contact closes, supplying power to the gate of the triac that connects the load to the line. Another common method for con trolling the operation of a solid state relay is called optoisolation, or optical isolation. This method is used by many PLCs to communicate with the out put device. Optoisolation is achieved by using the light from a light-emitting diode (LED) to energize a photo triac (FIG. 20). The arrows pointing away from the diode symbol indicate that it emits light when energized. The arrows pointing toward the triac symbol indicate that it must receive light to turn on. Optical isolation is very popular with electronic devices such as computers and PLCs be cause there are no moving contacts to wear and because the load side of the relay is electrically isolated from the control side. This isolation prevents any electrical noise generated on the load side from being transferred to the control side.

Solid-state relays are also available to control loads connected to direct-current circuits (FIG. 21). These relays use a transistor instead of a triac to connect the load to the line.

Solid-state relays can be obtained in a variety of case styles and ratings. Some have a voltage rating that ranges from about 3 to 30 volts and can control only a small amount of current, whereas others can control hundreds of volts and several amperes. The eight-pin IC (integrated circuit) shown in FIG. 22 contains two solid-state relays that are intended for low-power applications. The solid-state relay shown in FIG. 23 is rated to control a load of 8 amperes connected to a 240 volt AC circuit. For this solid-state relay to be capable of controlling that amount of power, it must be mounted on a heat sink to increase its ability to dissipate heat. Although this relay is rated 240 volts, it can also control devices at a lower voltage.

FIG. 20 Solid-state relay using optical isolation to control the action of a triac.

FIG. 21 A solid-state relay that controls a DC load uses a transistor instead of a triac to connect the load to the line.


Contactors are very similar to relays in that they are electromechanical devices. Contactors can be obtained with coils designed for use on higher volt ages than most relays. Most relay coils are intended to operate on voltages that range from 5 to 120 volts AC or DC. Contactors can be obtained with coils that have voltage ranges from 24 to 600 volts.

Although these higher voltage coils are available, most contactors operate on voltages that generally do not exceed 120 volts for safety reasons. Contactors can be made to operate on different control circuit voltages by changing the coil. Manufacturers make coils to interchange with specific types of contactors. Most contain many turns of wire and are mounted in some type of molded case that can be replaced by disassembling the contactor (FIG. 24).

FIG. 22 eight-pin integrated circuit containing two low-power solid-state relays.

FIG. 23 Solid-state relay that can control 8 amperes at 240 volts.

It should be noted that NEMA standards re quire the magnetic switch device to operate properly on voltages that range from 85% to 110% of the rated coil voltage. Voltages can vary from one part of the country to another, and variation of voltage often occurs inside a plant as well. If coil voltage is excessive, it draws too much current, causing the insulation to overheat and eventually burn out. Excessive voltage also causes the armature to slam into the stationary pole pieces with a force that can cause rapid wear of the pole pieces and shorten the life of the contactor. Another effect of too much voltage is the wear caused by the movable contacts slamming into the stationary contacts, causing excessive contact bounce. Con tact bounce can produce arcing, which creates more heat and more wear on the contacts.

Insufficient coil voltage can produce as much if not more damage than excessive voltage. If the coil voltage is too low, the coil has less current flow, causing the magnetic circuit to be weaker than normal. The armature may pick up, but not completely seal against the stationary pole pieces. This can cause an air gap between the pole pieces, pre venting the coil current from dropping to its sealed value. This causes excessive coil current, overheating, and coil burnout. A weak magnetic circuit can cause the movable contacts to touch the stationary contacts and provide a connection, but does not have the necessary force to permit the contact springs to provide proper contact pressure. This can cause arcing and possible welding of the contacts. Without proper contact pressure, high cur rents produce excessive heat and greatly shorten the life of the contacts.

FIG. 24 Magnetic coil cut away to show insulated copper wire wound on a spool and protected by a molding.

FIG. 25 Contactors contain load contacts designed to connect high-current loads to the power line.

Load Contacts

The greatest difference between relays and contactors is that contactors are equipped with large contacts that are intended to connect high-current loads to the power line (FIG. 25). These large contacts are called load contacts. Depending on size, load contacts can be rated to control several hundred amperes. Most contain some type of arcing chamber to help extinguish the arc that is produced when heavy current loads are disconnected from the power line.

Arcing chambers can be seen in FIG. 25.

Other contacts may contain arc chutes that lengthen the path of the arc to help extinguish it.

When the contacts open, the established arc rises because of the heat produced by the arc (FIG. 26). The arc is pulled farther and farther apart by the horns of the arc chute until it can no longer sustain itself. Another device that operates according to a similar principle is the blowout coil.

Blowout coils are generally used on contactors intended for use with direct current and are connected in series with the load (FIG. 27). When the contact opens, the arc is attracted to the magnetic field and rises at a rapid rate. This is the same basic action that causes the armature of a direct current motor to turn. Because the arc is actually a flow or current, a magnetic field exists around the arc. The arc's magnetic field is attracted to the magnetic field produced by the blowout coil, causing the arc to move upward. The arc is extinguished at a faster rate than is possible with an arc chute, which depends on heat to draw the arc upward. Blowout coils are sometimes used on contactors that control large amounts of alternating current, but they are most often employed with contactors that control direct-current loads. Alternating current turns off each half-cycle when the waveform passes through zero. This helps to extinguish arcs in alternating current circuits. Direct current, however, does not turn off at periodic intervals. Once a DC arc is established, it is much more difficult to extinguish.

Blowout coils are an effective means of extinguishing these arcs. A contactor with a blowout coil is shown in FIG. 28.

FIG. 26 The arc rises between the arc chutes because of heat.

FIG. 27 Magnetic blowout coils are connected in series with the load to establish a magnetic field.

FIG. 28 Clapper-type contactor with blowout coil.

Most contactors contain auxiliary contacts as well as load contacts. The auxiliary contacts can be used in the control circuit if required. The circuit shown in FIG. 29 uses a three-pole contactor to connect a bank of three-phase heaters to the power line. Note that a normally open auxiliary contact is used to control an amber pilot light that indicates that the heaters are turned on, and a normally closed contact controls a red pilot light that indicates that the heaters are turned off. A thermo stat controls the action of HR contactor coil. In the normal de-energized state, the normally closed HR auxiliary contact provides power to the red pilot light. When the thermostat contact closes, coil HR energizes and all HR contacts change position. The three load contacts close and connect the heaters to the line. The normally closed HR auxiliary con tact opens and turns off the red pilot light, and the normally open HR auxiliary contact closes and turns on the amber pilot light. A size 1 contactor with auxiliary contacts is shown in FIG. 30.

FIG. 29 The contactor contains both load and auxiliary contacts.

FIG. 30 Size 1 contactor with auxiliary contacts.

FIG. 31 Vacuum contacts are sealed inside a vacuum chamber.

Vacuum Contactors

Vacuum contactors enclose their load contacts in a sealed vacuum chamber. A metal bellows connected to the movable contact permits it to move without breaking the seal (FIG. 31). Sealing contacts inside a vacuum chamber permits them to switch higher voltages with a relative narrow space between the contacts without establishing an arc. Vacuum contactors are generally employed for controlling devices connected to medium voltage. Medium voltage is generally considered to be in a range from 1 kV to 35 kV.

An electric arc is established when the voltage is high enough to ionize the air molecules between stationary and movable contacts. Medium-volt age contactors are generally large because they must provide enough distance between the contacts to break the arc path. Some medium-voltage contactors use arc suppressers, arc shields, and oil immersion to quench or prevent an arc. Vacuum contactors operate on the principle that if there is no air surrounding the contact, there is no ionization path for the establishment of an arc. Vacuum contactors are generally smaller in size than other types of medium-voltage contactors. A three-phase motor starter with vacuum contacts is shown in FIG. 32. A reversing starter with vacuum contacts is shown in FIG. 33.

FIG. 32 Three-phase motor starter with vacuum contacts.

FIG. 33 reversing starter with vacuum contacts.

FIG. 34 Latching relay.

FIG. 35 Latching-type relays and contactors contain a latch and unlatch coil.

Mechanically Held Contactors and Relays

Mechanically held contactors and relays are often referred to as latching contactors or relays. They employ two electromagnets to operate. One coil is generally called the latch coil, and the other is called the unlatch coil (FIG. 34). The latch coil causes the contacts to change position and mechanically hold in position after power is removed from the latch coil. To return the contacts to their normal de-energized position, the unlatch coil must be energized. A circuit using a latching relay is shown in FIG. 35. Power to both coils is provided by momentary contact push buttons. The coils of most mechanically held contactors and relays are in tended for momentary use, and continuous power often cause burnout.

Unlike common magnetic contactors or re lays, the contacts of latching relays and contacts do not return to a normal position if power is interrupted. They should be used only where there is not a danger of harm to persons or equipment if power is suddenly restored after a power failure.

FIG. 36 diagram of a mercury relay.

Sequence of Operation

Many latching-type relays and contactors contain contacts that are used to prevent continuous power from being supplied to the coil after it has been energized. These contacts are generally called coil-clearing contacts. In FIG. 35, the L coil is the latching coil and the U coil is the unlatch coil.

When the ON push button is pressed, current can flow to the L coil, through normally closed the L contact to neutral. When the relay changes to the latch position, the normally closed the L contact, connected in series with the L coil, opens and disconnects power to the L coil. This prevents further power from being supplied to L coil. At the same time, the open the U contact, connected in series with the U coil, closes to permit operation of the U coil when the OFF push button is pressed. When the L coil energizes, it also closes the L load contacts, energizing a bank of lamps. The lamps can be turned off by pressing the off push button and energizing the U coil. This causes the relay to return to the normal position. Notice that the coil-clearing contacts prevent power from being supplied continuously to the coils of the mechanically held relay.

Mercury Relays

Mercury relays employ the used of mercury wetted contacts instead of mechanical contacts.

Mercury relays contain one stationary contact, called the electrode. The electrode is located in side the electrode chamber. When the coil is energized, a magnetic sleeve is pulled down inside a pool of liquid mercury, causing the mercury to rise in the chamber and make connection with the stationary electrode (FIG. 36). The ad vantage of mercury relays is that each time the relay is used, the contact is renewed, eliminating burning and pitting caused by an arc when connection is made or broken. The disadvantage of mercury relays is that they contain mercury.

Mercury is a toxic substance that has been shown to cause damage to the nervous system and kidneys. Mercury is banned in some European countries.

Mercury relays must be mounted vertically instead of horizontally. They are avail able in single-pole, double-pole, and three-pole configurations. A single-pole mercury relay is shown in FIG. 37.

Motor Starters

Motor starters are contactors with the addition of an overload relay (FIG. 38). Because they are intended to control the operation of motors, mo tor starters are rated in horsepower. Magnetic mo tor starters are available in different sizes. The size of starter required is determined by the horsepower and voltage of the motor it is intended to control.

There are two standards that are used to determine the size of starter needed: NEMA and IEC. FIG. 39 shows the NEMA-size starters needed for normal starting duty. The capacity of the starter is deter mined by the size of its load or power contacts and the wire cross-sectional area that can be connected to the starter. The size of the load contacts is reduced when the voltage is doubled, because the current is halved for the same power rating (P 5 E 3 I).

FIG. 37 Single-pole mercury relay.

FIG. 38 A motor starter is a contactor combined with an overload relay.

The number of poles refers to the load contacts and does not include the number of control or auxiliary contacts. Three-pole starters are used to control three-phase motors, and two-pole starters are used for single-phase motors.

FIG. 39 Motor starter sizes and ratings.


NEMA is the acronym for National Electrical Manufacturers Association. Likewise, IEC is the acronym for International Electrotechnical Commission. The IEC establishes standards and ratings for different types of equipment just as NEMA does. The IEC, however, is more widely used throughout Europe than in the United States. Many equipment manufacturers are now beginning to specify IEC standards for their products produced in the United States, also. The main reason is that much of the equipment produced in the United States is also marketed in Europe. Many European companies will not purchase equipment that is not designed with IEC standard equipment.

Although the IEC uses some of the same ratings as similar NEMA-rated equipment, there is often a vast difference in the physical characteristics of the two. Two sets of load contacts are shown in FIG. 40. The load contacts on the left are employed in a NEMA-rated 00 motor starter. The load contacts on the right are used in an equivalent IEC-rated 00 motor starter. Notice that the surface area of the NEMA-rated contacts is much larger than the IEC-rated contacts. This permits the NEMA-rated starter to control a much higher cur rent than the IEC starter. In fact, the IEC starter contacts rated equivalent to NEMA 00 contacts are smaller than the contacts of a small eight-pin control relay (FIG. 41). Due to the size difference in contacts between NEMA- and IEC-rated starters, many engineers and designers of control systems specify an increase of one to two sizes for IEC-rated equipment than would be necessary for NEMA-rated equipment. A table of the ratings for IEC starters is shown in FIG. 42.

Although motor starters basically consist of a contactor and overload relay mounted together, most contain auxiliary contacts. Many manufacturers make auxiliary contacts that can be added to a starter or contactor (FIG. 43). Adding auxiliary contacts can often reduce the need for control relays to perform part of the circuit logic. In the circuit shown in FIG. 44, mo tor 1 must be started before motors 2 or 3. This is accomplished by placing normally open contacts in series with starter coils M2 and M3. In the circuit shown in FIG. 44A, the coil of a control relay has been connected in parallel with motor starter coil M1. In this way, control relay CR operates in conjunction with motor starter coil M1. The two normally open CR contacts prevent motors 2 and 3 from starting until motor 1 is running. In the circuit shown in FIG. 44B, it is assumed that two auxiliary contacts have been added to mo tor starter M1. The two new auxiliary contacts can replace the two normally open CR contacts, eliminating the need for control relay CR. A motor starter with additional auxiliary contacts is shown in FIG. 45 below.

FIG. 40 The load contacts on the left are NEMA size 00. The load contacts on the right are IEC size 00.

FIG. 41 The load contacts of an IEC 00 starter shown on the left are smaller than the auxiliary contacts of an eight-pin control relay shown on the right.



By necessity, motor control centers have very low impedance and can produce extremely large fault currents. it is estimated that the typical MCC can deliver enough energy in an arc-fault condition to kill a person 30 feet away. For this reason, many industries now require electricians to wear full protection (flame-retardant clothing, face shield, ear plugs, and hard hat) when opening the door on a combination starter or energizing the unit. When energizing the starter, always stand to the side of the unit and not directly in front of it, in a direct short condition, it is possible for the door to be blown off or open.


Motor Control Centers

Motor starters are often grouped with other de vices such as circuit breakers, fuses, disconnects, and control transformers. This set of equipment is referred to as a combination starter. These components are often contained inside one enclosure (FIG. 46).

FIG. 42 IEC motor starters rated by size, horsepower, and voltage for 60 Hz circuits.

FIG. 43 Auxiliary contact sets can be added to motor starters and contractors.

FIG. 44 Control relays can sometimes be eliminated by adding auxiliary contacts to a motor starter.

FIG. 45 Motor starter with additional auxiliary contacts.

FIG. 46 A combination starter with fused disconnect, control transformer, push buttons, and motor starter.

FIG. 47 Combination starter with fused disconnect intended for use in a motor control center (MCC). Note that only two fuses are used in this module. delta-connected power systems with one phase grounded do not require a fuse in the grounded conductor.

FIG. 48 Motor control center.

Motor control centers employ the use of combination starters mounted in special enclosures designed to plug into central buss bars that supply power for several motors. The enclosure for this type of combination starter is often referred to as a module, cubicle, or can, FIG. 47.

They are designed to be inserted into a motor control center (MCC), as shown in FIG. 48. Connection to individual modules is generally made with terminal strips located inside the module. Most manufacturers provide some means of removing the entire terminal strip without having to remove each individual wire. If a starter should fail, this permits rapid installation of a new starter. The defective starter can then be serviced at a later time.

FIG. 49 The air gap determines the inductive reactance of the solenoid.

Current Requirements

When the coil of an alternating-current relay or contactor is energized, it requires more current to pull the armature in than to hold it in. The reason for this is the change of inductive reactance caused by the air gap (FIG. 49). When the relay is turned off, a large air gap exists between the metal of the stationary pole piece and the armature. This air gap causes a poor magnetic circuit, and the inductive reactance (XL) has a low ohmic value. Al though the wire used to make the coil does have some resistance, the main current-limiting factor of an inductor is inductive reactance. After the coil is energized and the armature makes contact with the stationary pole piece, there is a very small air gap between the armature and pole piece. This small air gap permits a better magnetic circuit, which increases the inductive reactance, causing the current to decrease. If dirt or some other foreign matter should prevent the armature from making a seal with the stationary pole piece, the coil current will remain higher than normal, which can cause overheating and eventual coil burnout.

Direct-current relays and contactors depend on the resistance of the wire used to construct the coil to limit current flow. For this reason, the coils of DC relays and contactors exhibit a higher resistance than coils of AC relays. Large direct-current contactors are often equipped with two coils instead of one (FIG. 50). When the contactor is energized, the coils are connected in parallel to produce a strong magnetic field in the pole piece. A strong field is required to provide the attraction needed to attract the armature. Once the armature has been attracted, a much weaker magnetic field can hold the armature in place.

When the armature closes, a switch disconnects one of the coils, reducing the current to the contactor.

FIG. 50 direct-current contactors often contain two coils.


1. Explain the difference between clapper-type contacts and bridge-type contacts.

2. What is the advantage of bridge-type contacts over clapper-type contacts?

3. Explain the difference between auxiliary contacts and load contacts.

4. What type of electronic device is used to connect the load to the line in a solid-state relay used to control an alternating-current load?

5. What is optoisolation, and what is its main advantage?

6. What pin numbers are connected to the coil of an eight-pin control relay?

7. An 11-pin control relay contains three sets of single-pole, double-throw contacts. List the pin numbers by pairs that can be used as normally open contacts.

8. What is the purpose of the shading coil?

9. Refer to the circuit shown in FIG. 29. Is the thermostat contact normally open; normally closed; normally closed, held open; or normally open, held closed?

10. What is the difference between a motor starter and a contactor?

11. A 150-horsepower motor is to be installed on a 480-volt, three-phase line. What is the minimum size NEMA starter that should be used for this installation?

12. What is the minimum size IEC starter rated for the motor described in question 11?

13. When energizing or de-energizing a combination starter, what safety precaution should always be taken?

14. What is the purpose of coil-clearing contacts?

15. Refer to the circuit shown in FIG. 29. In this circuit, contactor HR is equipped with five contacts. Three are load contacts and two are auxiliary contacts. From looking at the schematic diagram, how is it possible to identify which contacts are the load contacts and which are the auxiliary contacts?

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