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ELEMENTARY DIAGRAMS

Definitions

An elementary diagram (also called a schematic diagram) is defined as a diagram that shows in

straight line form the detail wiring of the circuit and device elements without regard to physical

relationships. These diagrams:

are a development of some portions of the one-line diagram data

provide supplemental wiring information that may not show on one-line

diagrams.

The straight line approach to connection of the circuit elements makes reading the diagram much

easier. A comparison of the elementary diagram with the one-line and complete wiring diagram

for a very simple circuit is shown in Fig. 4-1.

Figure 4-1 (a) shows the one line for control of a 500-W light by a single-pole

switch from a 120-V single-phase source.

Figure 4-1 (b) indicates the complete circuit wiring in a simple straight line form

called an elementary diagram. It is easy to see that the light is connected through

the switch to the ungrounded conductor and directly to the light from the

grounded conductor.

Figure 4-1 (c) develops the complete wiring and notes that both the grounded and

ungrounded conductors pass through the switch enclosure. This physical

relationship is disregarded in the elementary diagram.

Figure 4.1 Comparison of Diagram Types for a Simple Circuit

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Purpose and Use of Elementary Diagrams

Elementary diagrams are initially prepared in order to design the electrical circuit.

To design these circuits operate in a specific way they are intended to the designer

should develop his concepts by recording them in as simple a way as possible.

This makes design and understanding easy and uncomplicated.

The diagrams are used to relate understanding of system operations, to develop wiring data, and

to make a reference for circuit operation.

Such diagrams are often invaluable for trouble- shooting because they are much less

complicated than the complete wiring diagram.

Symbols

Graphic symbols are used on elementary diagrams as a method of showing the devices and other

elements of the circuit.

Standard symbols have been developed for this use and are published by engineering

societies and others for use in the engineering profession. Wherever. possible standard symbols

should be used. Some of the most common elementary diagram symbols for the electrical power

field are shown below.

3

4

5

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Many symbols require further clarification; this should be noted near the symbol. As an example

a pressure switch might require a note saying closes at 15 psi-opens at 30 psi. A float switch

might require the note closes at extreme high level-opens at low level. The inclusion of these

notes is important to reading the diagrams and should not be overlooked.

The study of elementary diagrams in this course will be limited to the preparation of motor

control circuits. Since the basic Direct-on-Line magnetic starter circuit has already been studied

in previous courses we will begin with a study of various control elements to be used in the

design of automatic control circuits.

Motor Control and Indicating Elements

The basic magnetic motor starter can be converter into a motor controller with endless functions

by the simply adding other devices capable of controlling the coil circuit. The controller can be

used in a manual, semi-automatic or fully automatic mode. A few of the more commonly used

devices are presented below.

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Pushbutton Control Stations

A pushbutton control station is an assembly of components that contain pushbuttons and may

contain other accessories.

A pushbutton is an assembly that contains a master switch, manually operable plunger or button

for actuating device. Pushbuttons can control a motor by the engaging the appropriate button.

Using pushbutton control station the motor can be controlled from several locations and used to

perform a variety of operations. The pushbutton consists of two basic components:

The contact block - the switches that make or break the electrical circuits

o Contacts are of the double break type

o Contacts are assembled in a number of contact arrangements

o Contact blocks are constructed for single and double circuit control

o Contain NC and NO contacts

o Several contact blocks may be stacked together and operated by one operator

The operator - the mechanism that operates the switches

Some of the basic building blocks of pushbutton stations are:

Start-Stop Station

o The most common pushbutton station

o used to manually start and stop a motor

o consists of two switches, one normally closed (NC) and one normally open (NO),

in one block. These switches may be electrically linked or may be separated.

o Each switch is operated separately

Multiple Start-Stop Station o used to control a motor starting and stopping from several remote locations

o contains a number of NC and NO switches

o to function

stop buttons are wired in series

start buttons are wired in parallel

Forward-Stop-Reverse o used to reverse the direction of rotation of a motor

o consists of three switches, one normally closed (NC) and two normally open

(NO), in one block

o each switch is operated separately

o switches can be mechanically linked (interlocked) so that only one operation can

occur at any given time

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Run-Stop-Jog

o used to control the motor running continuously once started (run) or intermittently

starting and stopping (jogging). Jogging operation can be performed manually or

automatically (inching).

o made with a number of different configurations

o typical switch consists of start, stop and ganged NC and NO jog switch. The NC

contact is wired in series with the start and auxiliary circuit. Pressing the start

button will close and latch in the control circuit. When the jog switch is pressed it

opens the start circuit so that the control circuit cannot latch in.

Selector Switches

o a master or manually operated switch with rotating motion for the actuating

device or assembly

o a mechanical switch made with a variety of poles and configurations and positions

o the switch selects only one connection at any given time

o the position of the switch may be maintained or spring returned

o may or may not have an off position

Illuminated Pushbutton Switches o pushbutton switches with a lamp that can be wired to indicate the condition of the

switch, control circuit and motor.

Pushbutton switches are used in a variety of situations in a variety of environments and under

a variety of conditions. Therefore switches must designed to function in the environment for

which it is intended. To accomplish this the National Electrical Manufacturers Association,

NEMA, specified several classes of switches according to:

o current carrying capabilities - eg. Standard or heavy duty

o enclosures - eg. General purpose, watertight

Pushbutton Accessories

Pushbuttons may be supplied with a variety of fittings to enhance their operating and

indicating functions such as:

o padlocking attachment – to lock the stop button in the depressed position

o pilot lights - lamps used to indicate the status of the switch, control

circuit and motor.

o legend plates - metal plates that indicate the function of the switch or the

- status of the switch in a particular position

- fit over the locknut or operator

Relays

A relay is an electrically controlled device that operates by being either energized or de-

energized to open and close electrical contacts to effect the operation of other devices in the

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same or other circuits. Relays are used primarily as switching devices in control circuits. Except

for small motors and solenoid, relays are not used in power circuits. Typical applications include:

switching starting coils

controlling other relays

turning on small devices such as

o pilot lights

o audible alarms

o signals

Classification

There are two major types of relays

electromechanical relays - electrical switches actuated by electromagnetic or

mechanical means

solid state relays - electrical switches actuated by electronic circuitry

Relays are also classified as light, medium and heavy-duty; Industrial, Commercial and

Military. Another classification is by Electrical Control, Performance, Mechanical Action,

Enclosure.

A Typical Electromagnetic General Purpose Relay

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The general purpose relay is a mechanical switch operated by a coil. Hence the relay has two

sections the relay power (energizing) circuit and the switching contacts. The power circuit must

be specified in terms of the circuit from which the signal will come i.e. in terms of supply

voltage. Typical information given is:

voltage system AC or DC

voltage value numerical value

The contacts are specified in terms of

type of circuit in which they should function AC or DC

power value usually given in terms of current

types of contacts provides information about possible

switching configurations

Relay contact operation

Three words are commonly used to describe relays:

Poles the number of completely isolated circuits that can pass

through the switch at one time

eg. Double pole switch carries current through two

simultaneously with each circuit isolated from each

other.

Throws the number of different closed contact positions per pole

that are available on the switch i.e. the number of

independent circuits that the switch can control.

Eg. Single throw = one circuit,

double throw = two circuits.

Break the number of separate contacts the switch uses to open or

close each individual circuit

Single break = electrical contacts broken in one

place

Double break = contacts broken in two places

NARM use number codes to specify each possible contact arrangement.

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The tables below illustrate the types of contact arrangements giving contact names and

designator.

The arrangement and types of relay contacts NARM Switching Identification

System

Solid State Relays

Solid state relays use electronic components such as TRIACS to perform switching. They are

faster and provide arcless switching.

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Relays are manufactured to be connected in one of two ways in a circuit:

o Contacts are wired to pins that protrude out of the casing and fit into separate

sockets called bases.

o Contacts are wired to terminals to which conductor are connected either by:

Soldering

Push on terminals

Screwed terminals

Whether electromechanical or solid state relays are used the specifications must be carefully

considered so that the correct relay is chosen for the given application.

Time Delay Relay

A time delay relay TDR is one in which the operation of the contact occur some time after the

relay has been energized. This is achieved by having some built-in device or mechanism to delay

the contact operation. TDRs have been designed to perform a number of switching functions.

The same relay may have different contacts that perform a variety of switching functions.

Typical examples are:

Contacts NC delay to open - TDO

Contacts NO delay to close - TDC

Some contacts operate delayed while others operate instantaneously

Some TDRs have fixed delay time while it is possible to adjust the delay time in others. One type

of TDR uses a motor and cam to produce a repeated cycle of operations.

Control Mechanisms for TDR

A typical Time Delay Relay Wiring Diagram

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A Typical Pneumatic TDR The Pneumatic Timing Unit

The Repeat Cycle Timer

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Special Control Switches

Switches designed to control motors base on predetermined stimuli. Some of these switches

convert mechanical stimuli into mechanical action that is used to operate contacts. Others

convert electrical stimuli into electrical signals, typically voltage and current. These are used to

turn on or off other control elements or vary existing circuit conditions. The diagram below

shows a typical float switch which is used to control a circuit based on rising or falling liquid

levels.

Mechanically Operated Switches

The symbols listed previously showed pressure switches, float switches, and temperature

switches that are made to operate through mechanical action.

Float switch An example is shown below.

The general idea behind a float switch is illustrated by the diagram below..

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Figure 4. Schematic Diagram of Float Switch Operation

As the liquid level rises in the tank, the ball float rises and the fixed stops on the rod

force the switch linkage up, which pulls the switch closed.

When the switch closes, it starts a pump that reduces the liquid level in the tank.

As the level is pumped down, the ball float settles, which causes the fixed stop on the

rod to force the switch linkage down and opens the switch, thereby stopping the

pump.

Limit switches are mechanically operated and as their name implies, they open or close upon

some limit of operation. For example, in the machine tool industry they are used to stop or start

particular machine operation once the tool or other device has reached its limit. A door can be

monitored by security personnel at a remote location through the application of limit switches. If

the door is opened, it triggers a limit switch, which could be wired to an alarm. There are many

types manufactured with a variety of operating mechanisms and contact arrangements.

These mechanically operated switches are used extensively and therefore often appear in

elementary diagrams. Their specific function is generally noted near the device symbol to clarify

its operation.

A solenoid is an electrical-mechanical device that operates to perform a mechanical function

when energized or de-energized. These devices are used extensively to facilitate operation of

mechanical devices such as valves, brakes, and clutches. They are similar in basic construction to

the relay except that the armature action drives the mechanical action directly. An example, is a

solenoid-operated valve is shown in Fig. 4..

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Selector Switches

Many control schemes require selective operations. For example, it is common to see motor

control that allows for selecting Hand-Off-Auto operation. A selector switch is used to provide

this feature. In the Hand position, control is in the hand of the operator; in the Auto position, the

control is by means of automatically operating devices such as pressure switches or temperature

switches. Typical selector switch operators for use in push-button station-type devices are shown

in Fig. 4.

Symbol for solenoid valve.

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Sources of Information for Preparing Elementary Diagrams

The principal source of information for preparing elementary diagrams is the one-line diagram.

Basic control data are indicated in abbreviated form on those diagrams and can readily be

developed into elementaries. For example, the Figure shows the one-line diagram for control of a

motor. The symbols indicate that the motor is protected by a combination fused disconnect and

an across- the-line motor starter that will have built-in overload relays. A start-stop push- button

station is to be used complete with a red indicating light (motor running) and a green indicating

light (motor shut down). An emergency stop push button has also been provided. Near the starter

is the letter T, which indicates that a control power transformer is provided for low-voltage

control power supply.

From the data indicated in the figure an experienced designer can easily prepare an elementary

diagram.

Figure 4. One-Line Diagram for Control of Motor

Other sources of information are usually furnished about the mechanical operations required, as

well as supplemental wiring requirements that developing design. For example, it may required

that the motor in the Figure above must be interlocked with another motor and prevented from

starting until the other motor is running. This information is provided as the mechanical systems

are engineered and the electrical designers are advised of how a system must function. Changes

are continually taking place and the alert designer must keep informed to be sure that the

electrical system will be properly designed.

Some firms use a logic diagram system, which is a diagrammatic tool for outlining the way in

which systems are to be designed to operate. It is a convenient method of recording what is

permitted to happen, not to happen, or only to happen if other events have occurred, etc. These

logic diagrams are also a source of basic data for preparing electrical elementaries.

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Preparing the Control Power Source

It is a generally accepted practice to start elementary diagrams by first laying out the source of

power that provides the energy to operate the devices in the circuit. The source of power for this

control bus may be either alternating or direct current, depending on the system involved.

In power plants, substations, and other locations where a reliable source of control power

is essential, the source is often a bank of batteries.

In less critical facilities, it is usually taken from the power supply for all other loads.

Usually this is an alternating. current supply.

It is good practice to keep the control power voltage at 120 V ac; direct-current supplies

are usually 125 V.

When the main alternating. current power is higher than 120 V ac, a small control power

transformer can be installed to provide the necessary voltage reduction. The control bus is

drawn either as two horizontal lines or as two vertical lines, depending on personal

preference

Figure 4. Typical Control Bus Power Supply Notations

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The source of power and the voltage, should be noted.

Any control or overcurrent protection of the control bus should be indicated.

Figure shows several control bus arrangements. Note that:

all the alternating-current supplies have their source through a control power transformer.

The transformer primary coil is shown near its supply and the secondary coil is shown as

the source for the control power. Actually the two coils are physically wound on the same

frame.

Developing an Elementary Diagram

First it is important to recognize that there are two separate circuits involved in the

operation of the motor.

o there is the power circuit, which supplies power to the motor

o there is the control circuit, which provides the means for controlling the motor.

o A complete elementary diagram will show both circuits so it can be readily seen

how the wiring of one relates to the wiring of the other.

Figure below shows the power circuit, including the connections to the primary winding of the

control power transformer.

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21

22

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Elementary Diagram Incorporating Automatic Control

The one-line diagram and the elementary diagram for automatic control of a pump through a

float switch is shown in Figure 4. below.

Two-Wire Control Circuits

The two-wire control circuit controls the pump motor is through a two-wire device e.g the

floatswitch. There are many variations of this circuit because of the many types of pilot devices

that form the two-wire control. Additional two-wire control elementary diagrams are shown in

below

.

The two-wire control device can be any device that operates automatically to close and open its

contacts. The high-pressure cutout switch is a safety device to shut down the circuit if an unsafe

condition develops. (Note that the high- pressure cutout operates for both hand and auto

positions.) The disadvantage to the circuit is that a no-voltage condition will cause the motor to

stop, but, then it will start again immediately when voltage is restored. There is a hazard in this

possibility because a maintenance worker might be investigating the shutdown and not expect

the system to start up again without warning.

Motor Starter with automatic Control

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To overcome this problem, some two-wire circuits are designed with a control relay and reset

push button as shown in Fig. Above.

To set up the automatic two-wire control for the circuit, the reset push button must be pressed,

energizing the CR relay coil, which in turn closes CR/I to hold the coil energized and CR/2 to

allow for automatic control through the pressure switch. Loss of voltage will de-energize the CR

coil and force use of the reset.

Sequence Control

The need for sequence control of motors and devices is common throughout industry. Conveyor

systems are an example of such a requirement. Another is when it is desired to have a second

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motor start automatically when the first motor has stopped. This might occur when the second

motor is needed to run a cooling fan or a pump. Figure below shows the elementary diagram for

a typical scheme.

When the start button is pressed both coils M1 and TR are energized.

The M1 motor starts and the M1 contact seals around the start button.

The TR/instantaneous contact, in the circuit to the M2 coil, opens and stays open.

The TR, time open contact closes, but because the instantaneous contact opened, the M2

coil is not energized and the M2 motor cannot start.

As soon as the stop button is pressed, both Ml and TR coils are de-energized.

The M 1 motor stops.

The TR/instantaneous contact returns to its normally closed position and because the time

opening contact of the TR relay is closed, the M2 coil is energized and the M2 motor

starts. The M2 motor will run until the time opening contact times out and opens.

Sequence control for a conveyor system is shown in Fig. below. The schematic arrangement of

conveyors shown with the elementary diagram illustrates that conveyor No.1 must be running

before No.2 is started; otherwise materials on No.2 would pile up at the base of No.1. The

control circuit is simple. Additional motors could be started in the same way. Automatic start-up

could be easily arranged through the use of time delay relays to replace the start buttons, which

follow in sequence the starting of the first conveyor by hand.

Sequence Control of Two Motors-One to Start and Run for a Short Time after

the other Stops

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Assigning Wire Numbers on Elementary Diagrams

To utilize the diagram in the practical sense of wiring between devices and in keeping track of

the individual wires, an identifying number should be assigned to each wire.

It is important that only one number is given to the same wire even though there may be

several branches to that wire in the circuit.

Wire numbers should change when the circuit passes through a device.

Standard wiring of motor starters by manufacturers generally assign wires numbered LI

and L2 to the control bus with wire No.1 as the line lead after the bus protection and

wires No.2 and 3 to the wires on either side of the seal-in contact.

There is no fixed standard for wire numbers beyond those generally used by manufacturers, as

described above, so consecutive numbers can be assigned to suit the designer's requirements. A

typical elementary with wire numbers is shown in Fig. below.

Starters Arranged for Sequence Control of Conveyor System

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The legend for devices is as follows:

T: Clutch coupling temperature cutout

LLS: Lower limit switch-closed except when door is fully closed

RLS: Raise limit switch-closed except when door is fully open

RBLS: Rollback limit switch-closed except when door is within 6 in. of the floor

SELS: Safety edge limit switch-closed except when door is within 6 in. of the floor

SES: Safety edge switch-momentary contact-closes when door hits object

SR: Safety edge relay

Close examination of the figure will show that:

wire numbers change only if the circuit passes through a device and the number is shown

only once.

Repeating numbers, even though on the same wire, can lead to possible numbering error

if a device were cut into the wire due to a design change.

Elementary Diagram for Overhead Door Operator Showing

Assignment of Wire Numbers

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Modifications to wire numbers such as postscript A, B, C, etc., identifications can be

used if it is felt they serve some useful purpose. As an example, assume that an additional

interlock contact were cut into the circuit right after the T interlock; it might be useful to

identify the wire between T and the new interlock as 2A rather than using No. 17, which

is the next consecutive unused number. This is a matter of judgment in maintaining

relationships between devices.

The elementary diagram of the figure is for use with motor-operated truck court doors.

The control system is designed so that the door may be manually stopped at any point

during the raising or lowering cycle.

A safety edge switch . is provided on the bottom of the door so that if the bottom of the

door strikes any object at a point more than 6 in. above the floor the door will

automatically rise to the fully open position.

Examination of the elementary will show that there is no seal-in circuit around the lower push

button so this button must be held closed to lower the door. This requirement forces the operator

to operate the door manually.

Determining Wire Requirements by Reading Elementary Diagrams

The elementary diagram is the principal source of information in counting up the wiring

requirements.

As an example, the safety relay above is to be mounted on the wall adjacent to the door. How

many wires go to it?

Review of the elementary will provide the answer.

The wires going to the SR relay are

L2 and 15 = 2 (coil leads)

7 and 8 = 2 (b contact in DL coil circuit)

3 and 6 = 2 (a contact seal-in around raise p.b.)

16 = 1 (lead between RLS-2 b contact and SR a contact in SR coil circuit) Total 7

This counting of wires can be done for all elements in the circuit so that physical wiring layouts

can be developed.