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1 Motor Control Functions

1.1 Types of Enclosures

1.1.1 NEMA 1 - General Purpose

1.1.2 NEMA 3 - Dust-Tight, Rain-Tight

1.1.3 NEMA 3R - Rainproof, Sleet-Resistant

1.1.4 NEMA 4 - Watertight

1.1.5 NEMA 4X - Watertight Corrosion Resistant

1.1.6 NEMA 7 - Hazardous Locations

1.1.7 NEMA 9 - Hazardous Locations - Class II

1.1.8 NEMA 12 - Industrial Use

1.1.9 NEMA 13 - Oil-tight, Dust-tight

1.2 Types of C ontrollers

1.2.1 Manual C ontroller

1.2.2 Semiautomatic C ontroller

1.2.3 Automatic C ontroller

1.3 Control Devices and S ymbols

1.3.1 Primary Control and Pilot Control Devices

1.3.2 Contacts

1.3.3 Pushbutton Switches

1.3.4 Toggle Switches

1.3.5 Indicating Lights

1.3.6 Coils, Relays, and Contactors

1.4 Magnetic Contactors1.4.1 Types of Magnetic Contactors

1.4.2 Inrush Current

1.4.3 Ratings

1.4.4 Voltage Variations

1.4.5 AC Hum

1.4.6 Magnetic Blowout

1.4.7 Magnetic Coil C ontrol Circuits

1.4.7.1 Magnetic Coil Data

1.4.7.2 Holding Circuit Interlocks

1.4.7.3 Interlocks

1.4.8 Overloads

1.4.9 Ambient Compensation

1.4.10 Rotary Switches

1.4.11 Automatic Switches

1.4.12 Float Switches

1.4.13 Pressure Switches

1.4.14 Timer

1.4.15 Limit Switches

1.4.16 Foot Switches

2 Control Circuits

2.1 Diagrams

2.2 Circuit Analysis

2.2.1 Three-Wire Control

2.2.2 Two-Wire Control

2.2.3 Common Control

2.3 Control Power Transformers

2.4 Hand-Off-Auto C ontrols2.5 Interlocking Methods for Reversing Control

2.5.1 Mechanical Interlocking

2.5.2 Pushbutton Interlocks

2.5.3 Auxiliary Contact Interlocking

2.6 Sequence C ontrol

2.7 Motor Control Center Power Supplies

2.8 MCC Single-Line Diagrams

2.9 Elementary Diagram Analysis

2.10 Standard Device Numbers

2.11 Developing a C ontrol Circuit

Motor Control FunctionsThe main functions of a motor controller are to start and stop the motor and to protect the motor, machine, and operator. The controller may also be calle

provide other functions that could include reversing, jogging or inching, plugging, operation at various speeds or at reduced current levels, and controlling

torque.

The purpose of controller enclosures is to provide protection of operating personnel by preventing accidental contact with energized components. In certai

applications, the controller is protected from a variety of environmental conditions including water, rain, snow, sleet, dirt, non-combustible dust, oils, coola


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lubricants. Motor control centers are designed to meet the requirements of the National Electrical Code (NEC), specifically Article 430 for motors and

control centers and Article 500 for electric equipment in hazardous locations.

Types of Enclosures

The National Electrical Manufacturers Association (NEMA) and other organizations have established standards of enclosure construction for control equipm

Common types of enclosures, per NEMA classification numbers are:

NEMA 1 - General PurposeThis type of enclosure is primarily intended to prevent accidental contact with the enclosed apparatus. It is suitable for general purpose applications indoor

not exposed to unusual service conditions. A NEMA 1 enclosure serves as protection against dust and light indirect splashing but is not dust-tight.

NEMA 3 - Dust-Tight, Rain-Tight

This enclosure type is designed to provide suitable protection against specified weather hazards. A NEMA 3 enclosure is suitable for application outdoors,

docks, canal locks, construction work, and for application in subways and tunnels. It is also sleet-resistant.

NEMA 3R - Rainproof, Sleet-ResistantThis type protects against interference in operation of the contained equipment due to rain and resists damage from exposure to sleet. It is designed with c

and external mounting as well as drainage provisions.

NEMA 4 - WatertightWatertight enclosures are designed to meet the following hosetest: "Enclosures shall be tested by subjection to a stream of water. A hose with a one-inch

be used and shall deliver at least 65 gallons per minute. The water shall be delivered on the enclosure from a distance of not less than 10 feet and for a per

minutes. During this period, it may be directed in any one or more directions as desired. There shall be no leakage of water into the enclosure."

NEMA 4X - Watertight Corrosion Resistant

These types of enclosures are generally constructed along the lines of NEMA 4 enclosures, except they are made of a material that is highly resistant to corr

this reason, they are ideal in applications, such as paper mills and chemical facilities, where contaminants could destroy an enclosure over a period of time.

NEMA 7 - Hazardous Locations

This type of enclosure is designed to meet the application requirements of the NEC for Class I hazardous locations. Class I hazardous locations are those in

flammable gases or vapors are, or may be, present in the air in quantities sufficient to produce explosive or ignitable mixtures.

NEMA 9 - Hazardous Locations - Class IIClass II locations are those that are hazardous because of the presence of combustible dust. The letter or letters following the type number indicate the par

group of hazardous locations as defined by the NEC for which the enclosure is designed. The designation is incomplete without a suffix letter or letter. Exa

9, Class II, Group F.

NEMA 12 - Industrial UseThis enclosure is designed for use in those industries where it is desired to exclude such materials as dust, lint, fibers, oil seepage, or coolant seepage. Ther

conduit openings or knockouts in the enclosure, and mounting is by means of flanges or mounting feet.

NEMA 13 - Oil-tight, Dust-tight

These generally are of cast construction, gasketed to permit use in the same environments as NEMA 12 devices. The basic difference is that, due to the cas

conduit entry is provided as an integral part of a NEMA 13 enclosure, and mounting is by means of blind holes rather than mounting brackets.

Types of Controllers

Manual ControllerA manual controller is one having its operations controlled or performed by hand at the location of the controller, as shown in Figure 1. Perhaps the most

single type in this category is the manual, full-voltage motor starter in the smaller sizes.


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Figure 1: Manual Control for a Motor

A manual starter is used frequently where the only control function needed is to start and stop the motor. The manual starter generally provides overload

for the apparatus being powered. Manual control, which provides the same functions as those achieved by the manual full-voltage motor starter, can be ha

use of a switch with fusing of the delayed-action type, which provides overload protection for the motor.

Examples of this type of control are very common in small metalworking and woodworking shops that use small drill presses, lathes, and pipe-threading m

Another good example is the exhaust fan generally found in machine shops and other industrial operations. In this installation, the operator or maintenanc

generally pushes the START button for the fan in the morning when the plant opens, and it continues to run throughout the day. In the evening, or when th

shut down, the operator then pushes the STOP button, and the fan shuts down until needed again.

A manual controller is easily identified because it has no automatic functions of control. This type of controller is characterized by the fact that the operator

a switch or push a button to initiate any change in the condition of operation of the apparatus being operated. A manual controller must, therefore, have tcomponents: a manually operated switch and a circuit protective device.

Semiautomatic ControllerA semiautomatic controller uses a magnetic starter (a switch operated by an electromagnet) and one or more manual devices such as pushbuttons and oth

equipment. Figure 2 shows a simple semiautomatic control scheme for a motor. Semiautomatic control provides flexibility of control by allowing remote a

control locations in installations where manual control would otherwise be impractical.

Figure 2: Semiautomatic Control for a Motor

The key to classification as a semiautomaticcontrol system lies in the fact that all the pilot devices are manually operated and that the motor starter is the m

type. There are probably more machines operated by semiautomatic control than by either manual or automatic. This type of control requires the operator

any change in the attitude or operating condition of the machine. Using the magnetic starter, however, this change may be initiated from any convenient lo

contrasted to the manual control requirement that the control point be at the starter.

Automatic ControllerAn automatic controller is a magnetic starter in which functions are controlled by one or more automatic control or pilot devices. Figure 3shows an autom

scheme for a motor. As shown, an automatic pilot device is some type of control device, such as a limit switch or float switch, that functions independent o

action to initiate a change in the operating condition of a motor or machine.


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Figure 3: Automatic Control for a Motor

Some systems may use a combination of manual and automatic devices in the control circuit. When a control system uses one or more automatic devices,

as an automaticcontroller.

Consider, for example, a tank that must be kept filled with water between definite limits and a pump to replace the water as it is needed. If we equip the pu

with a manual starter and station a person at the pump to turn it on and off as needed, we have manual control. Now, let us replace the manual starter wit

magnetic starter and put a pushbutton station at the foreman's desk. If we ring a bell to let them know when the water is low and again when it is high, the

other work and just push the proper button when the bell rings. This would be semiautomatic control. Now, suppose we install a float switch that will closewhen the water reaches a predetermined low level and open it when it reaches a predetermined high level. When the water gets low, the float switch will cl

circuit and start the motor. The motor will now run until the water reaches the high level, at which time the float will open the circuit and stop the motor.

Although the automatic system is more expensive to install, it requires less operator attention and functions more reliably and accurately.

Automatic control systems are found in many applications, such as large power plants, where they are used to control many mechanical systems in machi

where precision machines, such as drill presses and lathes, are automatically controlled for better accuracy and efficiency, and in the home, where automa

systems are used to control such common household machines as dishwashers and washing machines.

Control Devices and SymbolsUnderstanding, troubleshooting, and repairing control systems requires a knowledge and understanding of the physical devices that are used in control cir

the symbols and terminology that are used to designate those devices on wiring diagrams. Most symbols used have been standardized throughout the in

assure uniformity. Figure 4shows American National Standard Graphical Symbols for Electrical Diagrams. The chart shown in Figure 5shows standard sy

in motor control circuits.


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Figure 4: American National Standard Graphical Symbols for Electrical Diagrams


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Figure 5: Standard Wiring Diagram Symbols

Primary Control and Pilot Control DevicesAll components used in motor control circuits may be classed as either primary controldevices orpilot controldevices. Aprimarycontrol device is one that

the load to the line, such as a motor starter, whether it is manual or automatic. Pilotcontrol devices are those that control or modulate the primary control

Pilot devices are things such as pushbuttons, float switches, pressure switches, and thermostats.

An example (Figure 6) would be a magnetic contactor controlled by a toggle switch used to energize and de-energize the contactor, or M coil. To start the

toggle switch is switched ON, which energizes the contactor coil and closes the main line contacts, which energizes the motor. Switching the toggle switchenergizes the contactor coil, opens the main line contacts, and de-energizes the motor.

Figure 6: Basic Motor Control Circuit

In this example, the contactor, in that it connects the motor or load to the line, would be classed as a primary control device. The switch does not connect t

the line, but is used to energize and de-energize the coil of the starter. Therefore, it would be classed as a pilot control device.

For any given controller, there are generally two primary control devices used. These are the disconnecting means, or circuit breaker (usually a manual devi


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magnetic contactor. There may be many pilot devices used in parallel and series combinations to control the function of starting and stopping performed b

primary control device. The overload relays, for instance, which are included in the motor starter, are actually pilot devices used to control the primary devi

whenever the motor is overloaded.

Pilot devices vary greatly with their function and intended use. From manual switches to automatic control devices, pilot devices are what make a motor c

adaptable to fit a multitude of applications.

ContactsSymbols 1 and 2 represent electrical contact devices (see Figure 7). They may represent line contacts on a starter, contacts on a limit switch or relay, or an

of control device that has electrical contacts. Recall that circuit diagrams are shown in their de-energized condition. Therefore, Symbol 1 is a normally open

contact, and Symbol 2 is a normally closed (NC) contact.

Figure 7: Basic Symbols Used on Motor Control Circuits

The designations "a" or "b" associated with a set of contacts are used to identify the state of the contacts (open or closed) in reference to the main operatin

"a" contact will normally be closed when its associated coil is energized and its main contacts are closed. These same "a" contacts will open when the associ

de-energized. A "b" contact will normally be open when its associated coil is energized, thus operating the opposite of an "a" contact. Remember, however,

electrical drawings will indicate the state of contacts with this designator. If not, the drawing notes should annotate whether the circuit is in the energized o

energized state. This "a" and "b" notation holds true for auxiliary contactors and relays, as well as the main contactor. Note that all circuits are shown in the

energized, or shelf, condition unless stated otherwise.

Two other terms often used in conjunction with contacts, either relay contacts or switch contacts, are makeand break. When a contact goes closed, it is said

and when the contact opens, it is said to "break."

Pushbutton SwitchesSymbols 3 and 4, shown in Figure 7,represent manually operated pushbutton switches with normally open and normally closed contacts, respectively. Thi

returned type switch will return to its normal position when released by the operator. Because the switch returns to its original position, and its contacts arclosed or open for the moment (however long) the switch is pushed, these contacts are referred to as momentary.

Toggle SwitchesSymbols 5 and 6, shown in Figure 7, represent manual contacts of a toggle type of switch. Symbol 5 contacts are normally open, and Symbol 6 contacts ar

closed. This type of switch has maintaining contacts; that is, once switched to a different position, the switch will stay in that position. When found in a circ

the switch positions should be labeled as OFF or ON, FAST or SLOW, or other appropriate labeling. Symbol 7 is a toggle switch of the single-pole, double-th

type, where one contact is normally open and the other normally closed.

When more than one set of contacts are operated by moving one handle or pushbutton, they are generally connected by dotted lines, as in symbols 8 and

dotted lines represent any form of mechanical linkage that will make the two contacts operate together. One other method that is used frequently to show

pushbuttons that have two sets of contacts is shown in symbols 10 and 11. Symbol 10 has two normally open contacts, and symbol 11 has one normally

one normally closed contact.

Switches can be designed to operate in one of two ways. The first, and most common, is referred to as break-before-makecontact arrangement. In an arran

this type, one set of contacts opens before the next set of contacts closes.

The second arrangement is referred to as make-before-break. In this arrangement, when the switch is being switched from one position to the next, shortin

of the first and second set of contacts occurs, for a short period, during the contact transfer. This arrangement is used when it is necessary to ensure contin

power to a circuit during the switching evolution.

Indicating LightsSymbol 12, shown in Figure 7, is a pilot or indicating light that is indicated chiefly by the short lines radiating out from the center circle. Normally, the colo

is designated by the appropriate letter in the circle, such as RLfor red or GLfor green.

Coils, Relays, and ContactorsSymbols 13 and 14 shown in Figure 7represent a coil. It may be a relay coil or a main line contactor. Relays and contactors are electromagnetic devices in

that magnetic forces are produced when electric currents are passed through coils of wire; in response to such forces, contacts are closed or opened by the

plungers or pivoted armatures. Symbols 15 and 16 are discussed later in this text.

As defined by the National Electrical Manufacturers Association (NEMA),a relayis "a device that is operated by a variation in the conditions of one electric ci

effect the operation of other devices in the same or another electric circuit." A contactor, on the other hand, is "a device for repeatedly establishing and inte

electric power circuit." It is important to recognize the difference between the two, noting particularly that the relay, serving a secondary role, causes other

function, whereas the contactor is the primary unit, doing its work in the main power circuit.

A drawing showing the basic construction of a relay is shown in Figure 8. Note the relay coil and coil terminals.


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Figure 8: Relay Construction

This relay has one set of NO (C1) and one set of NC (C2) contacts. The path for current flow on each set of contacts is through the movable contacts to the c

terminal. In the de-energized state, contacts C2 are closed and C1 are open. When the coil is energized, the coil attracts the movable contacts to closed cont

open contacts C2.

Figure 9 shows a contactor assembly drawing. Note the solenoid assembly, moving armature and contact assembly, and the stationary contacts with termi

connections for line and load wiring. Also, note from the picture that the contacts are normally encased to protect the contact assemblies.

Figure 9: Contactor Construction

Contactor assemblies are frequently made with main contacts that serve to connect and disconnect the main power circuit and auxiliary contacts (both nor

and normally closed) that operate with the main contacts. The auxiliary contacts are then available for use in the control circuit of this or another machine.

Auxiliary contacts are frequently used to seal in a coil. "Sealing in" is when a parallel path for current flow is formed to keep a coil energized after the origin

current flow is interrupted. Auxiliary contacts of the coil being sealed in are commonly used to complete the parallel path for current, but this is not always

Figure 10shows a simple control circuit using a magnetic contactor to illustrate sealing in.

Figure 10: Simple Control Circuit Showing Seal-In Contact mA

Magnetic ContactorsMagnetic contactors are electromagnetically operated devices that serve to provide a safe, convenient way to connect and disconnect circuits. The magneti

this type of contactor consists of a magnet assembly, a coil, and an armature. The current flowing through the coil causes a magnetic flux to be set up in th

coil is physically wrapped around. The alternating magnetic flux (if it is an AC contactor) produces heat, which is reduced by the use of laminated cores.

The magnet assembly is simply the stationary part of the contactor. The coil is supported by, and surrounds part of, the magnet assembly to induce magne


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the iron when the coil is energized. Figure 11shows the essential parts, including the magnet, coil, and armature.

Figure 11: Magnetic Contactor Assembly

The armature is the moving part of the magnetic circuit. When energized, the coil induces a magnetic flux in the iron core and attracts the armature, which

toward it. When the armature has been attracted to its sealed position (closed), it completes part of the magnetic circuit.

When the armature has sealed in, it is held tightly against the magnet assembly. Notice in Figure 11that an air gap exists even when the armature is in the

position. This is because, when the coil is de-energized, residual magnetism is inherent in the magnet assembly. The air gap in the iron circuit prevents the

magnetism from being strong enough to keep the armature held in its sealed-in position.

Types of Magnetic ContactorsThere are four basic types of electromagnetic contactors (Figure 12):

Figure 12: Magnetic Contactors

Inrush CurrentWhen a magnetic controller is in its OPEN position, a large air gap exists between the armature and the magnet assembly. The impedance of the coil is low,

when the coil is energized due to the air gap, it will draw a high inrush current. As the armature moves closer to the magnet assembly, the air gap gets smal

smaller. The coil current drops off until the armature seals into its CLOSED position. This inrush current is typically 6 to 10 times the sealed-in value.

Magnet coils that are energized by AC voltage should never be connected in series. This is because, if one contactor seals in ahead of the second, the increa

impedance of the circuit will reduce the second coils current so that the second device will either not pick up or will pick up but not seal. AC magnetic coils

therefore, be connected in parallel.

Ratings

Clapper Type - It contains a hinged armature that pivots to seal in, thus closing the moveable contacts against the stationary contacts.

Horizontal Action - The armature and the contacts move horizontally in a straight line.

Vertical Action - The armature and contacts move in a straight vertical line.

Bell Crank - A bell crank converts the vertical movement of the armature into a horizontal motion. Longer contact life and reduced contact bounce result

lessened shock on armature pickup.


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The ratings of magnetic coils are usually given in volt-amperes, or VA. An example would be a coil rated at 600 VA inrush current and 60 VA sealed-in curre

inrush current would then be 600/120 =5 amps.

Voltage VariationsIf the applied voltage to a magnetic contactor is too high, the coil will draw more than its designed current. Excessive heatwill result, and this will cause ea

the insulation of the coil. In addition, the magnetic pull will be higher. This will cause the armature to close with excessive force. This, in turn, will result in a

the contact faces,contact bounce, and shortened contact life.

When the applied voltage is too low, similar effects occur. There will be a low coil current applied that will reduce the magnetic pull. On some types, especi

action, this may result in a contactor that picks up but does not seal, resulting in a continuous draw of inrush current. It will quickly burn up. Another effect

chattering as the coil strains to pick up and seal in its armature.

AC HumAC magnetic contactors have a certain hum associated with their operation. This noise is mainly because of the changing magnetic pull due to the alternati

the magnet. This humming, and changing magnetic fluxes, will cause small mechanical vibrations.

Excessive chattering and loud humming can result when:

Magnetic BlowoutSome larger magnetic contactors, especially older clapper models, have arc chutes installed. Inside these arc chutes are heavy copper coils, or blowout coil

mounted above the main contacts and are in series with them to provide arc suppression. Blowout coils are installed for contacts opening under AC and D

electric arcis similar to that found during the welding process.

Contacts that are subject to frequent interruption of large currents suffer a destructive burning if the arc is not suppressed or extinguished. Magnetic blowo

work on the principle of motor action; that is, the arc is lengthened and extinguished by the magnetic field setup due to a current-carrying conductor. Since

blowout coil is in series with the main line contacts, the strength of the magnetic field setup and the resultant extinguishing action will be in proportion to t

the arc.

Figure 13shows a section of a magnetic blowout coil with an arc conducting between the contacts. Figure 14shows the process of lengthening the arc. At

begins to deflect due to the blowout coil field. Next, as the contacts open further, the magnetic field lengthens the arc, and it moves near the tip of the horn

the arc is so lengthened that it is extinguished and unable to conduct.

Figure 13: Magnetic Blowout Coil

Figure 14: Lengthening the Arc

Magnetic Coil Control CircuitsAlthough the power circuit can be single-phase or three-phase, the control circuit to the magnet coil is always single-phase. The control circuit includes:

There is a broken shading coil. Shading coils are small copper turns placed near the magnet pole faces that have an induced magnetic flux 90 degrees ou

with the magnetic assembly itself. This serves to keep the armature sealed in as the alternating current falls through zero 120 times-per-second.

The operating voltage drops too low.

There is misalignment between the armature and the magnet assembly, causing the armature to be unable to properly seat.

The magnet coil

The contacts of the overload relay assembly

Momentary or maintained contact pilot device such as a pushbutton, pressure, temperature, liquid level, limit switch, or PLC signal

Relay contacts or timers taking the place of pilot devices


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Figure 15 shows a Size One starter control circuit.

Figure 15: Size One Starter Control Circuit

Magnetic Coil DataPublished charts list identification numbers, ratings, and operating characteristics of magnetic coils. These charts list the rated voltage and the coil volt-am

both inrush and sealed conditions. AC magnetic coils, in general, are designed to operate on line voltages fluctuating as much as 15% below, and 10% abov

rating. DC coils have corresponding limits of 20% below, and 10% above, nominal rating.

Holding Circuit InterlocksThe holding circuit interlock is a normally open (NO) auxiliary contact provided on standard magnetic starters and contactors. It closes when the coil is ener

form a holding circuit for the starter after the START button has been released. These are typically mounted on the upper-left portion of magnetic contactor

Auxiliary contacts are frequently used to seal in a coil. Sealing in creates a parallel path for current flow to keep a coil energized after the original path of cu

interrupted. Auxiliary contacts of the coil being sealed in are commonly used to complete the parallel path for current, but this is not always the case. Figur

a simple control circuit using a magnetic contactor to illustrate sealing in. The starting sequence for Figure 16is shown below.

Figure 16: Simple Control Circuit

Starting sequenceis a series of events that occurs to energize a machine once the sequence has been initiated by a pilot device, either manual or automatic.

START button is pushed, the M coil is energized, which will closethe M contacts, thus keeping the M coil energized when the START button is released. The

now sealed in. The M coil also closes the M contacts, energizing the motor. The "M" designation used here is frequently used in control circuits to designatecontactor that controls the switching of line power to the device being controlled. Multifunction controllers frequently do not use Mbut rather more specifi

designations such as For Rfor forward and reverse.

InterlocksControl circuits frequently control more than one contactor, such as in a two-speed motor control circuit or a control circuit for controlling the direction (fo

Auxiliary contact on the starter, designed as a holding circuit interlock; may be required in certain control schemes


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reverse) on a motor. In many situations such as this, equipment damage could result if both contactors were closed at the same time.

Two methods are used to provide an interlock to prevent this from occurring. First is an electrical interlock. A "b" contact from each contactor is in series wi

operating coil from the other contactor. Thus, if contactor A was energized, its open "b" contact would prevent energizing contactor B. The opposite would

contactor B was energized.

The second method employed is a mechanical interlock. To accomplish this, the two contactors are physically mounted side-by-side in the control box. A m

linkage that prevents both contactors from being closed at the same time connects them. If one contactor was closed and something occurred to energize t

contactor, the coil would be energized, but motion of the contact assembly would be physically blocked.

Overloads

Symbol 15, shown previously in Figure 7, represents the heating element of an overload relay. Overload relays are devices found on all motor controllers ior another. The current that a motor draws while running is directly proportional to the load on the motor. An overload condition, whether caused by mec

electrical fault, will result in increased current flow.

Overload protection is achieved in almost all controllers by placing heating elements in series with the motor leads on multiphase motors. These heater ele

activate electrical contacts, which open the coil circuit when used on magnetic controllers. When used on manual starters or controllers, the heating eleme

mechanical trip to drop out the line contacts. Older controllers use two overloads, while newer units are required to have three overloads in accordance wi

in the National Electrical Code.

The overload relay is sensitive to the percentage of overload; therefore, a small overload will take some time to trip the relay, whereas a heavy overload wil

almost instantaneous opening of the circuit. The overload relay does not give short-circuit protection, however. It is quite possible that, under short-circuit

the relay might hold long enough to allow considerable damage to the motor and other equipment.

Short-circuit protection is provided by installing either a fused disconnect or a circuit breaker ahead of the motor in the main feeder lines.

There are three types of overload relays in general use today. The first uses a low-melting-point metal that holds a ratchet assembly, as shown in Figure 17

Figure 17: Melting-Pot Relay for a Thermal Overload

When the metal is heated beyond the melting point, the ratchet releases, causing a set of contacts to open in the control circuit and open the main line cont

The second type of overload device, shown in Figure 18, uses a bimetallic element. The bimetallic element is made of two different metals bonded togethe

heated, the metals expand at different rates, and the element bends. The resultant motion releases a trip mechanism that opens contacts in the control circ

main line contactor trips open.

Figure 18: Bimetallic Type of Thermal Overload

Figure 19shows the third type of overload relay the magnetic type. A magnetic trip element uses an electromagnet in series with the circuit load. With nor

the electromagnet is not affected. As load current increases above the setpoint, the relay opens a set of contacts in the control circuit, and the main line con

open.


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Figure 19: Magnetic Overload

Overload relays must be reset after each tripping, either automatically or manually. The automatic reset type should not be used except on equipment that

designed. There can be no danger to life or equipment from the restarting of the motor. After the overload relay has been tripped, it requires a little time to

that there is some delay before resetting can be accomplished.

Factors that determine the overload relay thermal units or overload heaters are:

Motors with the same speed and horsepower do not necessarily have the same full-load current. Refer to the motor nameplate for full-load current and no

published charts. Charts tend to show averages of normal full-load currents. The full-load current of a specific motor may be different. Selection tables are

based on continuous duty motors with a service factorof 1.15 operating under normal conditions.

Ambient CompensationBimetallic overload relays that are ambient-compensated are designed for one particular situation: when the motor is at a constant temperature and the co

located somewhere else where the temperature varies. If standard overload relays are used, it may not trip consistently at the same level or motor current

temperature of the controller has changed. The surrounding temperature affects standard thermal overload relay.

To compensate for temperature variations that the controller may be subjected to, an ambient-compensated overload relay should be selected. Its trip poi

affected by temperature, and so it will consistently trip at the same value of current.

Rotary SwitchesThe last symbol in Figure 7, symbol 16, is a rotary selector switch. A rotary switch is a multicontact switch with the contacts arranged in a full or partial circ

of a pushbutton or toggle, the mechanism used to select the contact moves in a circular motion and must be turned.

Rotary switches can be manual or automatic switches. An automobile distributor and the ignition switch on a motor vehicle are rotary switches ( Figure 20).

rotary switches are made with several layers or levels. This arrangement makes possible the control of several circuits with a single switch.

Figure 20: Rotary Snap Switch

Switches can be either automatic or manual. A manual switch is a switch that is turned on or off by an operator. Examples of common manual switches, su

toggle, pushbutton, or rotary switches already covered, are a light switch, a dryer start button, and a TV channel selector switch. Each of these requires ope

to initiate a change in a control system.

Automatic SwitchesAn automatic switch is a switch that is controlled by a mechanical or electrical device, there is no need to turn an automatic switch on or off. Two examples

automatic switches are a thermostat and the distributor in a motor vehicle. The thermostat will turn a furnace or air conditioner on or off by responding to

temperature in a room. The distributor electrically turns on the spark plug circuit at the proper time by responding to the mechanical rotation of a shaft. Ev

switch that turns on the light in a refrigerator when the door is opened is an automatic switch.

Automatic switches are not always as simple as the examples given above. Limit switches, which sense some limit such as fluid level, mechanical moveme

or an electrical quantity, are automatic switches that are sometimes quite complicated.

Any switch that turns a circuit on or off without operator action is an automatic switch. Figure 21shows the symbols for various automatic switches comm

Motor full-load current

Type of motor

Possible difference in ambient temperature between motor and controller


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Figure 21: Symbols of Various Automatic Switches

Float SwitchesSymbols 1 and 2 shown in Figure 22represent normally open and normally closed liquid-level or float switches. Float switches take many forms in their p

mechanical construction. They consist of one or more sets of contacts, either normally open or normally closed, operated by a mechanical linkage. Many fl

units, as well as other pilot devices, use a mercury switch in place of metallic contacts. The simplest mechanical arrangement for a float switch, shown in Fi

would be a pivoted arm having the contacts fastened to one end and a float suspended from the other end.

Figure 22: Float Switch

As the water level rises, it would lift the float, thus moving the contact end of the level downward and either making or breaking the contact, depending on

stationary contact were mounted above or below the arm. If a single-pole, double-throw action of the contacts were desirable, then one stationary contactmounted above and one below the center of the arm. If the float were all the way up, it would make the lower set of contacts, and if the float were all the w

would make the upper set of contacts.

Pressure SwitchesSymbols 3 and 4, shown in Figure 21, represent normally open and normally closed vacuum or pressure switches. This mechanical motion is used to oper

more sets of contacts. A typical pressure switch design using a bellows as the pressure-sensing element is shown in Figure 23. Two other common sensing

used are the diaphragm and the bourdon tube. The type of detector is determined by the system requirements. Most devices of this type have a means to

setpoint of the sensing device.

Figure 23: Pressure Switch, Bellows Type

Symbols 5 and 6 shown in Figure 21represent temperature-activated switches, more commonly called thermostats. Many different types of thermostats a


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that employ different methods of sensing temperature. The two most common are bellows and bimetallic strips. As in the pressure switch, the mechanical

the sensing elements is used to operate a set of contacts. A typical thermostat is shown in Figure 24.

Figure 24: Thermostat, Bellows TypeSymbols 7 and 8 represent flow switches that are used to sense the flow of liquid, air, or other gas through a pipe or duct and to transform this flow or lac

into the opening or closing of a set of contacts.

One type of flow switch, shown in Figure 25, uses a pivoted arm that has contacts on one end and a paddle or flag on the other end. The end with the pad

inserted into the pipe so that the flow of liquid or gas causes a leverto move and open or close the contacts.

Figure 25: Flow Switch, Paddle Type

TimerSymbols 9, 10, 11, and 12 shown in Figure 21 represent timer contacts that are normally operated by a timing relay. This type of relay and contact arrange

provides two important advantages of automatically controlled circuits: sequencing and delaying events in a control system. Many types of timing relays ar

that can be adjusted to give time delays of as little as a fraction of a second to as much as several minutes. Moreover, extremely long time delays, up to se

are possible with timing relays that are motor-driven. Since most industrial control systems do not run through unattended repetitive cycles, the timing rel

installations are generally non-cyclical. Thus, timers are used to separate events in a control-starting sequence that occurs instantaneously from those that

Instantaneous events are those that occur as soon as a start circuit is initiated, the only delay being the time it takes coils to operate or contacts to open or

Delayed events are those that have some type of controlled delay provided by a pilot device.

Common designs are pneumatic, dashpot, and motor-controlled timers. Motor-controlled timers are generally used for operations that are repeatable, suc

signal controllers and sequentially operated, motor-starting circuits. A simple motor timer found in many homes is used to control the wash cycles of auto

washing machines.

A dashpot timer, shown in Figure 26, consists of a plunger which, when the coil of the timer is energized, moves slowly through a bath of oil and closes a c

end of its stroke. The dashpot is usually provided with a bypass near its upper limit of travel so that the contact is permitted to close with a snap action. Sn

allows quick-closing contacts to minimize arcing during the closing cycle. In addition, a valve is included in its construction to allow the oil to flow freely as

falls when the relay is de-energized. Many of these relays also have an adjustment that can be varied to change the time delay.


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Figure 26: Time Delay Relay, Dashpot Type

Another popular timer is the pneumatic timer. It uses the restricted airflow across a diaphragm to create the time delay. The airflow passes through an adju

orifice so the time delay is adjustable.

The time delay of a timing relay can be applied when the relay is energized or when it is de-energized. Symbols 9 and 10, shown in Figure 21, represent ti

that have timed closing after energization (TCAE) and timed opening after energization (TOAE), respectively. Symbols 11 and 12, also shown previously, rep

contacts that have timed opening after de-energization (TOAD) and timed closing after de-energization (TCAD), respectively. Some timers may be equipped

with contacts that are delayed but with contacts that operate instantaneously.

Limit SwitchesSymbols 13, 14, 15, and 16, presented in Figure 21, represent direct-actuated limit switches. Limit switches use an arm, lever, or roller protruding from th

that will be bumped or pushed by some piece of moving equipment (Figure 27). This movement is then used to operate a set of contacts. Limit switches va

size and design. There are large, rugged devices for heavy industrial use, such as that shown in Figure 27, and smaller, more accurate and precise units th

switches that can operate on very minute movements of the operating lever.

Figure 27: Limit Switch

Symbols 13 and 14 of Figure 21show limit switches in their normally open or closed condition, and symbol 15 represents a normally open limit switch, w

closed; symbol 16 represents the opposite.

Foot SwitchesSymbols 17 and 18 of Figure 21represent foot switches. Switches of this type are often used in applications that require the machine or process cycle to b

a time when the operators hands are both engaged in loading or handling the materials. Foot-operated switches are frequently employed for such purpose

examples of foot switches are punch presses, drill presses, and sewing machines. Foot switches are actually limit switches enclosed in a convenient and ru

for foot operation and are available in a variety of contact arrangements such as single-pole double-throw, two-pole double-throw, or other arrangements

specific need. Figure 28shows a typical foot switch.

Figure 28: Industrial Foot Switch


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Control CircuitsSimple control circuits are sometimes referred to as ladder diagramsin that they are drawn to resemble a ladder. Figure 29shows a simple control circui

components.

Figure 29: Simple Control Circuit and Components

Wiring diagramswill not have wires jumping one another. Wires are shown as crossing each other and, unless specified with a node or dot, are not conn

Mechanical connections, such as those found on double-pole switches and between mechanical interlocks, are shown as broken lines.

DiagramsElectrical circuits are generally shown by one of two types of diagrams: line diagramsand wiring diagrams. A wiring diagram includes all the component

circuit and shows the physical relationships between them. Wiring diagrams give the needed information for actually wiring the circuit and allow a troubles

physically trace the wires. However, wiring diagrams often look like an enormous maze of parallel and crossing lines that make it difficult, if not impossible

someone to recognize and understand the operation of the circuit.

Line diagrams simplify the circuit to a degree necessary to understand the operation of the circuit. Line diagrams, also known as elementary diagrams, do n

components in their actual physical locations. Control devices, such as relays, contacts, and pushbuttons, are shown on horizontal lines between two verticvertical lines always represent the power source. The connections of line diagrams are drawn such that both the function and sequence of operation can b

determined.

Circuit AnalysisThere are two basic types of control circuits: three-wire and two-wire. These designations stem from the fact that for three-wire circuit control, only three w

required from the ordinary across the line motor starter to the control components. In two-wire control, only two wires are required.

Three-Wire ControlA three-wire circuit uses momentary contact START-STOP buttons and a holding circuit interlock, or maintaining contact across the push-button START swit

the circuit energized after the push-button has been released. This type of scheme provides low-voltage protection. A low-voltage condition or loss of inco

will cause the starter to "drop out." Figure 30shows a three-wire LVP control circuit. When power is restored, the starter connected for three-wire control

up automatically since the maintaining contact around the start switch is now open. To restart the motor after a power failure, the pushbutton must be preway, a deliberate action must be performed, ensuring a measure of safety.

Figure 30: Three-Wire LVP Control Circuit

Two-Wire ControlA two-wire control circuit is, by its nature, a low-voltage release circuit. A reduction or loss of voltage stops the motor, but when power returns or comes b


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nominal value, the motor wil l restart. This type of restart can be a safety hazard to both personnel and machinery since power may return without warning

of circuit is shown in Figure 31.

Figure 31: Two-Wire LVR Control Circuit

Common ControlThe coil circuit of a magnetic starter or contactor is distinct from the power circuit. The coil circuit can be connected to any single-phase power source and t

controller would be operable, provided the coil voltage and frequency match the service to which it is connected.

When the control circuit is tied back to lines 1 and 2 of the starter, the voltage of the control circuit is always the same as the power circuit, and the term co

controlis used to describe this relationship. Other variations include separate control and control through a control power transformer.

Control Power TransformersIt is sometimes desirable to operate pushbuttons or other control circuit devices at some voltage lower than the motor voltage. For example, if the main se

volts, this voltage may need to be reduced to 120 volts. A fuse is often used to protect the X 1side of the transformer secondary while the other side is grou

NECSection 250 states the requirement for grounding the secondary of 120-volt control transformers. According to the rule, any 120-volt, two-wire circuit

normally have one of its conductors grounded. Other systems are also required to be grounded, although they have no bearing on this aspect of motor co

centers. This specific requirement has caused some difficulty when applied to control circuits derived from the secondary of a control transformer that sup

to the operating coils of motor starters, contactors, and relays. For example, there may be cases where a ground faulton the hot leg of a grounded contro

cause a hazard to personnel by blowing the protective fuse or operating a circuit breaker and thus shutting down the entire industrial process in a sudden,

way. This may result in excessive loss of production time and/or damage to equipment that is stopped abruptly. A sudden shutdown to a ground fault in th

a grounded control circuit would be objectionable in this instance.

NEC250.21(3) provides an exception to this rule. A 120-volt control circuit may be operated ungrounded provided ALL of the following conditions are met:

Hand-Off-Auto ControlsWhen it is desired to select the function of a motor controller either manually or automatically, a hand-off-automatic switch is used. Figure 32shows a typi

circuit with a standard duty, three-position selector switch.

Figure 32: Typical Control Circuit

When the switch is turned to the HAND position, the M coil is energized continuously and the motor runs. In the AUTOMATIC position, the motor will run

the contact in line with the M coil is closed. A timing relay, float switch, or any other type of control device can control this contact.

Figure 33shows a three-position, double-break selector switch. This is used for manual or automatic control in much the same way as the previous circuit.

Figure 33: Three-Position Double-Break Selector Switch

The system is used exclusively for the control circuit.

The circuit is derived from a transformer that has a primary rating less than 1,000 volts.

Whether in a commercial, institutional, or industrial facility, supervision will ensure that only persons qualified in electrical work will maintain and serve

circuits.

Continuity of control power is required.

Ground detectors are installed on the control system.


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Interlocking Methods for Reversing ControlSimply interchanging any two of the three incoming leads can reverse a three-phase motor. When magnetic starters are used, reversing starters reverse th

direction, as shown in Figure 34.

Figure 34: Reversing Starter

Reversing starters in conformity with NEMA standards interchange lines L1 and L3 or phases A and C. To accomplish this, two starters are needed, one for

direction and one for the reverse direction (Figure 35).

Figure 35: Reversing Contactor Line Connections

Interlocking is used to prevent both contactors from being energized simultaneously or closing at the same time. This would cause a short circuit. Three ba

of interlocking are:

Mechanical InterlockingMechanical interlocks are assembled at the factory and are physically located between the forward and reverse contactors. The interlock locks one contact

beginning of the stroke of either contactor to prevent both from closing simultaneously.

A broken or dotted line indicates a mechanical interlock. Often, the dotted line will be broken in the middle and angled with a solid bar at the middle juncti

Pushbutton InterlocksThis method is an electrical method of preventing both starter coils from energizing together. Figure 36shows an example of pushbutton interlocking.

Figure 36: Pushbutton Interlocking

When the forward pushbutton is pressed, the F coil is energized, and the normally open F auxiliary contact closes to maintain the circuit to operate the motforward direction. Pressing the reverse pushbutton automatically breaks the circuit in line with the F coil, dropping the forward coil out and energizing the

coil.

Reversing the direction of motor rotation on a repeated basis is not recommended, since this may cause the overload relays to overheat and disconnect th

from the circuit. NEMA specifications require a starter to be derated or to select the next larger size starter whenever it is going to be used for plugging or r

rate of more than five times-per-minute.

Mechanical interlocking

Pushbutton interlocking

Auxiliary contact interlocking


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Auxiliary Contact InterlockingThis method is also an electrical interlock. It consists of normally closed auxiliary contacts on the forward and reverse contactors, as shown in Figure 37.

Figure 37: Electrical Interlocking

In the forward direction, the normally closed contact (F) on the forward contactor opens to prevent the reverse contactor from being energized.

Sequence ControlA method by which starters are connected so that one cannot be started until another is energized is called sequence control.This is required whenever aux

equipment associated with a machine, such as a priming pump for a drain pump, must be operating to prevent damage to the main machine. Figure 38sh

standard starter wired for sequence control.

Figure 38: Sequence Control

The control circuit of the M2 coil is wired through the maintaining contacts of the M 1 coil. The result is the second starter is prevented from starting until a

coil is energized.

Many motors can be started automatically with one START-STOP button, as shown in Figure 39.

Figure 39: Automatic Sequence Control

Motor Control Center Power SuppliesThe power supplies to motor control centers are usually circuit breakers located in switchgear. Familiarity with the symbols and conventions of single-line

both switchgear and motor control centers is necessary to understanding the overall conception of the motor control center as a unit. Standard symbology


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discussed in a previous section. Figure 40shows some standard electrical symbols and conventions.

Figure 40: Standard Electrical Symbols and Conventions

The Rotating Apparatuscolumn of Figure 40shows one-line elementary and plan symbols for various motors. Note that the number inside the circle indica

horsepower, and the number at the lower right of the circle indicates speed in RPM. The absence of a number indicates 1,800 RPM.

Under the Switching and Protective Apparatuscolumn of Figure 40, notice the numbers located to the left of the circuit breakers. An example is:

The top number, in this instance, indicates the trip rating of the breaker, and the lower number indicates the frame size. This is not true in all one-line diagr

times, these two numbers will be interchanged, and the top number will indicate the frame size, whereas the bottom number indicates the trip rating of th

This annotation denotes that the trip rating is adjustable. Many modern breakers have this feature. It can consist of either an adjustable setting on the brea

faceplate or a rating plug that is inserted into a special socket. These rating plugs are often shipped separately from the breakers and must be checked upo

installation to ensure that they are installed according to the specifications and prints.

The static trip devices shown on the motor starters rated above 600 volts, as well as some circuit breakers, are denoted by the abbreviation ST.These are m

(digital) units that are adjustable over a wide range of available parameters including ground faultpickup, ground fault delay, instantaneous overcurrent, l

delay, long time pickup, motor-starting current, phase failure, and others.

Figure 41 shows other apparatus and devices associated with motor control centers, conduit and raceways, wire and terminal connection location symbol

input/output symbols.

Figure 43 is a one-line diagram of a 480-volt substation powering many motor control centers. This diagram indicates that this substation is a main-tie-mai

scheme. This means there are two main breakers and a tie breaker. The K symbol located in the box above the tie breaker and in the circle beside the main

indicates that these breakers are mechanically interlocked with a key system. The dashed lines connect both mains and the tie. This indicates that only two

can be shut at any one time. Other significant items to be seen:

Transformer ratings and connections

Breaker ratings and accessories

Current transformer ratios

Motor control center drawing numbers


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Figure 41: Standard Symbology

MCC Single-Line DiagramsFigure 42and Figure 43are one-line diagrams of 480-volt, three-phase motor control centers. Be certain to become familiar with all symbols on these dia

well as all abbreviations.

The upper left portions of both prints show the incoming power supply. MCC 3 is powered from 480-volt substation number 2. The motor control centers i

a 1,600-amp frame, 1,200-amp trip circuit breaker. An ammeterand a voltmetermonitor the voltage and current drawn by the motor control center. Cu

transformer and potential transformer ratios are indicated as well as the number of each one required.

Each starter is labeled by size. MCC 3 indicates this with "Size 5," for example, written next to the starter. MCC 12 simply places a number beside the starterThe number and letter combinations shown at the lower edge of the dotted lines indicate which position each starter bucket is located in the motor control

example, 2Dindicates that this starter is in section 2, position D. Manufacturers vary in regard to this labeling, so refer to the drawing that pictorially shows

control center and all bucket position labels.


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Figure 42: Three-Phase, 480V One-Line Diagram

Figure 43: Three-Phase, 480V One-Line Diagram

Elementary Diagram AnalysisRefer to Figure 44, an elementary diagram, for several starters located in motor control center 3. The first thing to notice is the absence of a start switch. In

start switch, an input from a programmable logic controller (PLC) is inserted in the control circuit. This symbol, for motor number 301M1, is at coordinate 0

Coordinate numbers are located vertically down each diagram, and the letters run horizontally across the top of the page. Notice how some devices listed a

sides of each diagram have a reference number. For example, for motor number 301M1, the M coil is described as a contactor and has the numbers 1, 2,

SP. These numbers indicate which lines (vertical numbers) have contacts or electrical connections to this coil. SPindicates a spare.

The PLC input at coordinate 08E has the number 0:072/01 above it.

0indicates a PLC output

072indicates the PLC rack and slot

01indicates the point


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PLC input signals, such as those shown at coordinates 11J and 12J, have a similar address number.

Figure 44: PLC Elementary Diagram

Standard Device Numbers

To simplify electrical diagrams, many switchgear devices are not labeled with reference to their function. Standard numbers are commonly used instead ofabbreviations. These standard numbers, like abbreviations, allow the designer to produce an uncluttered drawing by minimizing the amount of writing.

The following is a list of standard numbers for labeling switchgear devices. This list can be used for quick reference; memorizing the numbers is not necess

Numbers commonly used will become as familiar to you as common abbreviations.

Standard numbers for switchgear devices:

1. Master element 49. Thermal relay

2. Time-delay closing relay 50. Instantaneous overcurrent relay

3. Interlocking relay 51. AC time overcurrent relay

4. Master contactor 52. AC circuit breaker

5. Stopping device 53. Exciter or DC-generator relay

6 . Starting circuit breaker 54. High-speed DC circuit breaker

7. Anode circuit breaker 55. Power factor relay

8. Control power disconnect device 56. Field application relay

9. Reversing device 57. Short-circuiting or grounding device

10. Unit sequence switch 58. Rectifier failure relay

11. Reserved for future application 59. Overvoltage relay

12. Overspeed device 60. Voltage balance relay

13. Synchronous-speed device 61. Current balance relay

14. Underspeed device 62. Time-delay relay

15. Speed/frequency matching device 63. Pressure switch

16. Reserved for future application 64. Ground-protective relay

17. Shunting or discharge switch 65. Governor

18. Accelerating or decelerating device 66. Jogging device

19. Starting-to-running transition device67. AC directional overcurrent delay

20. Electrically operated valve 68. Blocking relay

21. Distance relay 69. Permissive-control device

22. Equalizer circuit breaker 70. Electrical ly operated rheostat

23. Temperature-control device 71. Level switch24. Reserved for future application 72. DC circuit breaker

25. Synchronizing check device 73. Load-resistor contactor

26. Apparatus thermal device 74. A larm relay

27. Undervoltage relay 75. Position-changing mechanism

28. Flame detector 76. DC overcurrent relay

29. Isolating contactor 77. Pulse transmitter


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30. Annunciator relay 78. Phase-angle relay

31. Separate excitation device 79. AC reclosing relay

32. Directional power relay 80. Flow switch

33. Position switch 81. Frequency relay

34. Sequence device 82. DC reclosing relay

35. Brush operating dev ice 83. Automatic transfer relay

36. Polarity device 84. Operating mechanism

37. Undercurrent/underpower device 85. Carrier receiver relay

38. Bearing protective device 86. Lockout relay

39. Mechanical condition monitor 87. Differential relay

40. Field relay 88. Auxiliary motor

41. Field circuit breaker 89. Line switch42. Running circuit breaker 90. Regulating device

43. Manual transfer device 91. Voltage directional relay

44. Unit sequence starting relay 92. Voltage/power directional relay

45. Atmospheric condition monitor 93. Field changing contactor

46. Reverse-phase relay 94. Tripping relay

47. Phase sequence voltage relay 95 to 99 Used for specific applications

48. Incomplete sequence relay

The following gives a brief description of the function of each of the switchgear devices in the previous list.

Device

NumberFunction and Description

1Master Element - Initiating device (control switch, voltage relay, float switch, etc), which places equipment into or out of operation; done either dir

through a permissive device, such as a protective or time-delay relay

2 Time-Delay Starting (or Closing) Relay - Device that provides a given amount of time delay before or after any operation in a switching sequence orrelay system; true except as specifically provided by devices 62 and 79, described later

3Checking (or Interlocking) Relay - Relay that operates in response to the position of other devices or to the predetermined conditions in equipment;

operating sequence to continue, stops the sequence, or provides a check of the position of the devices or the predetermined conditions for any pu

4Master Contractor - Device that makes or breaks the necessary control circuits to place equipment into or out of service when the required conditi

generally controlled by a master element (device 1), the necessary permissives, and protective devices

5 Stopping Device - Device in which the primary function is to place and hold equipment out of service

6 Starting Circuit Breaker - Device in which the primary function is to connect a machine to its source of starting voltage

7 Anode Circuit Breaker - Device that is used in the anode circuits of a power rectifier to interrupt the rectifier current if an arc-back occurs

8Control Power Disconnecting Device - Device (knife switch, circuit breaker, or pull-out fuse block) that is used to connect or disconnect the control

and from the control bus or equipment; control power includes auxiliary power that supplies small motors and heaters

9 Reversing Device - Device that is used to reverse a machines field or to perform any other reversing function

10 Unit Sequence Switch - Switch that changes the sequence in which units may be placed into or out of service in a multi-unit system

11 Reserved for future application

12 Overspeed Device - Direct-connected speed switch that functions when a machine overspeeds

13Synchronous-Speed Device - Device (centrifugal-speed switch, slip frequency relay, voltage relay, undercurrent relay, or any other type of device) t

at approximately the synchronous speed of a machine

14 Underspeed Device - Device that functions when the speed of a machine falls below a predetermined value

15Speed/Frequency Matching Device - Device that matches and holds the speed (frequency) of a machine or a system equal (approximately equal) to

another machine, source, or system


17

Shunting (Discharged) Switch - Switch that opens or closes a shunting circuit around any piece of apparatus (except a resistor) such as a machine fi

machine armature, a capacitor, or a reactor; excludes devices that perform the shunting operations that are necessary when a machine is started b

or 42; Device 73s function, switching of a load resistor, also excluded

18 Accelerating (Decelerating) Device - Device that closes circuits used to increase or decrease speed of a machine

19 Starting-to-Running Transition Contactor - Device that causes the automatic transfer of a machine from the starting to the running power connecti

20Electrically Operated Valve - Motor-operated valve that is used in vacuum, air, gas, oil, water, or similar lines; function of valve may be indicated b

of a descriptive word, such as Brake, in the function name (i.e., electrically operated brake valve)

21 Distance Relay - Relay that functions when the circuit impedance or reactance increases or decreases beyond predetermined limits

22 Equalizer Circuit Breaker - Breaker that controls the equalizer or current-balancing connections for a machine field or for regulating equipment in asystem

23

Temperature-Control Device - Device that raises or lowers the temperature of a machine or other apparatus (or of any medium) when its temperat

below or rises above a predetermined value; example is a thermostat that switches on a space heater in a switchgear assembly when the temperat

below the predetermined value; different from a device that provides automatic temperature regulation between close limits (90T device)


25Synchronizing (Synchronism-Check) Device - Device that permits or causes the paralleling of two AC sources when they are within the desired limit

frequency, phase angle, and voltage

26

Apparatus Thermal Device - Device that functions when the temperature of the field of a machine, a load-limiting or shifting resistor, a liquid, or a

medium exceeds a predetermined limit; also functions if the temperature of the protected apparatus, such as a power rectifier, decreases below a

predetermined limit

27 Undervoltage Relay - Relay that functions on a given value of undervoltage

28 Flame Detector - Device that monitors the presence of the pilot or main flame in apparatus, such as a gas turbine or steam boiler

29 Isolating Contactor - Contactor that is used expressly to disconnect one circuit from another to perform emergency operations, maintenance, or te

30 Annunciator Relay - Non-automatically reset device that gives a number of visual indications upon the functioning of a protective device; may alsoarranged to perform a lockout function

31Separate Excitation Device - Device that connects a circuit, such as the shunt field of a synchronous converter, to a source of separate excitation du

starting sequence; also energizes the excitation and ignition circuits of a power rectifier

32Directional Power Relay - Device that functions on a desired value of power flow in a given direction or upon reverse power resulting from arc bac

anode or cathode circuits of a power rectifier

33Position Switch - Switch that makes or breaks contact when the main device or piece of apparatus, which has no device function number, reaches


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position

34Motor-Operated Sequence Switch - Multi-contact switch that fixes the operating sequence of the major devices during starting, stopping, or other s

switching operations

35Brush-Operating (Slip-Ring Short-Circuiting) Device - Device that raises, lowers, or shifts the position of the brushes in a machine or that short-circ

rings; also engages or disengages the contacts of a mechanical rectifier

36 Polarity Device - Device that operates or permits the operation of another device on a predetermined polarity only

37 Undercurrent (Underpower) Relay - Relay that functions when the current, or power flow, decreases below a predetermined value

38Bearing Protective Device - Device that functions on excessive bearing temperature or on other abnormal mechanical conditions, such as undue w

may eventually result in excessive bearing temperature

39Mechanical Condition Monitor - Device that functions upon the occurrence of an abnormal mechanical condition (except that associated with beari

covered under Device 38); examples of abnormal conditions are excessive vibration, eccentricity, expansion, shock, tilting, or seal failure

40 Field Relay - Relay that functions on a given low value, or failure, of machine field current; also functions on an excessive value of the reactive comarmature current in an AC machine, which indicates abnormally low field excitation

41 Field Circuit Breaker - Device that applies or removes the field excitation of a machine

42Running Circuit Breaker - Device that connects a machine to its source of running voltage after the machine has been brought up to the desired sp

starting connection (motor starter)

43Manual Transfer (Selector) Device - Manually operated device that transfers the control circuits to modify the plan of operation of the switching eq

of some of the devices

44Unit Sequence Starting Relay - Relay that functions to start the next available unit, in a multi-unit system, on the failure or the non-availability of th

preceding unit

45Atmospheric Condition Monitor - Device that functions upon the occurrence of abnormal atmospheric conditions such as fumes, explosive mixtur

and fire

46Reverse-Phase (Phase-Balance) Current Relay - Relay that functions when the polyphase currents are reverse-phase-sequenced, unbalanced, or con

negative phase sequence components above a given amount

47 Phase-Sequence Voltage Relay - Relay that functions on a predetermined value of polyphase voltage in the desired phase sequence

48Incomplete Sequence Relay - Relay that returns the equipment to the normal, or off, position and locks it out; functions if the normal starting, oper

stopping sequences are not properly completed within a predetermined time

49Machine (Transformer) Thermal Relay - Relay that functions when the temperature of a machine armature or other load-carrying winding (element

rectifier, or a power transformer (including a power rectifier transformer) exceeds a predetermined value

50Instantaneous Overcurrent Rate-of-Rise Relay - Relay that functions instantaneously on an excessive value of current or on an excessive rate of cur

used to indicate a fault in the apparatus or circuit being protected

51AC Time Overcurrent Relay - Relay with either a definite or inverse time characteristic that functions when the current in an AC circuit exceeds a

predetermined value

52AC Circuit Breaker - Device that closes or interrupts an AC power circuit under normal conditions; also interrupts the circuit under fault or emerge

conditions

53Exciter (DC Generator) Relay - Relay that forces the DC machine's field excitation to build up during starting or that functions when the machines v

built up to a given value

54High-Speed DC Circuit Breaker - Circuit breaker that functions to reduce the current in the main circuit in 0.01 seconds or less; functions after the o

of a DC overcurrent or excessive rate-of-current rise

55 Power Factor Relay - Relay that operates when the power factor in an AC circuit rises above or falls below a predetermined value

56 Field Application Relay - Relay that automatically controls the application of field excitation to an AC motor at some predetermined point in the sli

57 Short-Circuiting (Grounding) Device - Power, or stored energy, device that short-circuits, or grounds, a circuit in response to automatic or manual

58 Power Rectifier Misfire Relay - Relay that functions if one or more of the power rectifier anodes fail to fire

59 Overvoltage Relay - Relay that functions on a given value of overvoltage

60 Voltage Balance Relay - Relay that operates on a given difference in voltage between two circuits

61 Current Balance Relay - Relay that operates on a given difference in the current input or output of two circuits

62Time-Delay Stopping (Opening) Relay - Time-delay relay that serves in conjunction with the device that initiates the shutdown, stopping, or openin

in an automatic sequence

63Liquid or Gas Pressure, Level, or Flow Relay - Relay that operates on given values of liquid pressure, gas pressure, flow, or level, or on a given rate

of these values; normally an auxiliary relay; see Devices 71 and 80

64

Ground Protective Relay - Relay that functions on the failure of the insulation of a machine, transformer, or any other apparatus to ground; also fu

flashover to ground in a DC machine; only assigned to a relay that detects current flow from the frame of a machine (enclosing case or the structur

apparatus) to ground; also detects grounds on normally ungrounded windings or circuits; not applied to a device connected in the secondary circui

of a current transformer, or current transformers connected in the power circuit of a normally grounded system

65 Governor - Equipment that controls the gate or valve opening of a prime mover

66

Notching (Jogging) Device - Device in which the function is to allow only a specified number of operations of a given device or equipment, or a spe

number of successive operations within a given time of each other; also a device that functions to energize a circuit periodically or that is used to p

intermittent acceleration (jogging) of a machine at low speed for mechanical positioning

67 AC Directional Overcurrent Relay - Relay that functions on a desired value of AC overcurrent flowing in a predetermined direction

68Blocking Relay - Relay that initiates a pilot signal for blocking a trip on external faults in a transmission line, or in any other apparatus, under prede

conditions; also cooperates with other devices to block trips or reclosures on an out-of-step condition

69Permissive Control Device - Generally, a two-position, manually operated switch that permits the closing of a circuit breaker, or the placing of an e

into operation, in one position, and prevents the circuit breaker or the equipment from being operated in the other position

70 Electrically Operated Rheostat - Rheostat that is used to vary the resistance of a circuit in response to some means of electrical control

71 Level Switch - Switch that operates on given values or on a given rate-of-change of level

72DC Circuit Breaker - Circuit breaker that closes or interrupts a DC circuit under normal conditions; also interrupts the circuit under fault or emerge

conditions

73Load Resistor Contactor - Contactor that is used to shunt or to insert a step of load limiting, shifting, or indicating resistance in a power circuit; also

switch a space heater in a circuit and to switch a light on the regenerative load resistor of a power rectifier or other machine into or out of a circuit74 Alarm Relay - Relay, other than an annunciator (Device 30), that is used to operate, or operate in conjunction with, a visual or audible alarm

75 Position-Changing Mechanism - Mechanism that is used to move a removable circuit breaker unit to and from the connected, disconnected, and te

76 DC Overcurrent Relay - Relay that functions when the current in a DC circuit exceeds a given value

77 Pulse Transmitter - Generates and transmits pulses over a telemetering or pilot-wire circuit to the remote indicating or receiving device

78Phase Angle Measuring (Out-Of-Step Protective Relay) - Relay that functions at a predetermined phase angle between two voltages, two currents, o

and current


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79 AC Reclosing Relay - Relay that controls the automatic reclosing and locking out of an AC circuit interrupter

80 Flow Switch - Switch that operates on a given value or on a given rate-of-change of flow

81Frequency Relay - Relay that functions on a predetermined value of frequency (either above or below normal system frequency) or rate-of-change

frequency

82 DC Reclosing Relay - Relay that controls the closing and reclosing of a DC circuit interrupter, generally in response to load circuit conditions

83Automatic Selective Control (Transfer) Relay - Relay that operates to automatically select between certain sources or conditions in an equipment; ca

perform automatic transfer operations

84Operating Mechanism - Complete electrical mechanism (or servomechanism), including the operating motor, solenoids, position switches, etc., for

changer or any piece of apparatus that has no device number

85Carrier (Pilot Wired Receiver) Relay - Relay that is operated or restrained by a signal used in conjunction with carrier-current or DC pilot-wire, fault-

relaying

86 Lock-Out Relay - Electrically operated (hand or electrically reset) device that functions to shut down and hold equipment out-of-service on the occuabnormal condition

87Differential Protective Relay - Protective relay that functions on a percentage, a phase angle, or other quantitative difference between two currents

other electrical quantities

88 Auxiliary Motor (Motor Generator) - Motor used to operate auxiliary equipment such as pumps, blowers, exciters, rotating magnetic amplifiers, etc

89Line Switch - Switch used as a disconnecting or isolating switch in an AC or DC power circuit when the device is electrically operated or has electric

accessories, such as an auxiliary switch, a magnetic lock, etc.

90Regulating Device - Device that controls a quantity (or quantities) such as voltage, current, power, speed, frequency, temperature, and load at a giv

between certain limits for machines, tie lines, or other apparatuses

91 Voltage Directional Relay - Relay that operates when the voltage across an open circuit breaker or contactor exceeds a given value in a given directi

92

Voltage and Power Directional Relay - Relay that permits or causes the connection of two circuits when the voltage difference between them excee

value in a predetermined direction and causes these two circuits to be disconnected from each other when the power flowing between them exce

value in the opposite direction

Motor Control Fundamentals - Industrial Wiki - Technology Transfer

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