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GOALS:
-- Discuss symbols used in the drawing of schematic diagrams.
-- Determine the differences among switches that are drawn normally open, normally closed, normally open held closed, and normally closed held open.
-- Draw standard NEMA control symbols.
-- State rules that apply to schematic, or ladder, diagrams.
-- Interpret the logic of simple ladder diagrams.
When you learned to read, you were first taught a set of symbols that represented different sounds.
This set of symbols is called the alphabet. Schematics and wiring diagrams are the written language of motor controls. Before you can learn to properly determine the logic of a control circuit, you must first learn the written language. Unfortunately, there is no actual standard used for motor control symbols.
Different manufacturers and companies often use their own sets of symbols for their in-house schematics. Also, schematics drawn in other countries may use entirely different sets of symbols to represent different control components. Although symbols can vary from one manufacturer to another, or from one country to another, once you have learned to interpret circuit logic, it is generally possible to determine what the different symbols represent by the way they are used in the schematic. The most standardized set of symbols in the United States is provided by the National Electrical Manufacturer's Association, or NEMA. These are the symbols that we discuss in this SECTION.
FIG. 1 NEMA standard push button symbols.
FIG. 2 The movable contact bridges the stationary contacts
FIG. 3 The movable contact makes connection with the two stationary
contacts until the button is pressed.
Push Buttons
One of the most used symbols in control schematics is the push button. Push buttons can be shown as normally open or normally closed (FIG. 1). Most are momentary contact devices in that they make or break connection only as long as pressure is applied to them. The pressure is generally sup plied by someone's finger pressing on the button.
When the pressure is removed, the button returns to its normal position. Push buttons contain both movable and stationary contacts. The stationary contacts are connected to the terminal screws. The normally open push button is characterized by drawing the movable contact above and not touching the stationary contacts. Because the movable contact does not touch the stationary contacts, there is an open circuit and current cannot flow from one stationary contact to the other. The way the symbol is drawn assumes that pressure will be applied to the movable contact. When the button is pressed, the movable contact moves downward and bridges the two stationary contacts to complete a circuit (FIG. 2). When pressure is removed from the button, a spring returns the movable con tact to its original position.
The normally closed push button symbol is characterized by drawing the movable contact be low and touching the two stationary contacts, FIG. 3. Because the movable contact touches the two stationary contacts, a complete circuit exists, and current can flow from one stationary contact to the other. If pressure is applied to the button, the movable contact moves away from the two stationary contacts and open the circuit. When pressure is removed, a spring returns the movable contact to its normal position.
FIG. 4 A double-acting push button contains both normally open and normally
closed contacts.
FIG. 5 Double-acting push button.
FIG. 6 The double-acting push button has four terminal screws.
FIG. 7 Other symbols used to represent double-acting push buttons.
Double-Acting Push Buttons
Another very common push button found through out industry is the double-acting push button (FIG. 4). Double-acting push buttons contain both normally open and normally closed contacts.
When connecting these push buttons in a circuit, you must make certain to connect the wires to the correct set of contacts. The schematic symbol for a typical double-acting push button is shown in FIG. 5. Note that the double-acting push button has four terminal screws (FIG. 6). The symbol for a double-acting push button can be drawn in different ways (FIG. 7). The symbol on the left is drawn with two movable contacts connected by one common shaft. When the button is pressed, the top movable contact breaks away from the top two stationary contacts, and the bottom movable contact bridges the bottom two stationary contacts to complete the circuit. The symbol on the right is very similar in that it also shows two movable contacts. The right-hand symbol, however, connects the two push button symbols together with a dashed line. When components are shown connected by a dashed line in a schematic diagram, it indicates that the components are mechanically connected together. If one component is pressed, all those that are connected by the dashed line are pressed. This is a very common method of showing several sets of push button contacts that are actually controlled by one button.
Stacked Push Buttons
A very common connection employing the use of multiple push buttons is shown in FIG. 8. In this example, one stop button, referred to as an emergency stop button, can be used to stop three motors at one time. Push buttons that contain multiple contacts are often called stacked push buttons.
Stacked push buttons are made by connecting multiple contact units together that are controlled by a single push button (FIG. 9). In the example, shown in FIG. 9, the push button contains one normally open and two normally closed contacts.
Contact blocks with double-acting contacts are also available. The push button in this example is sup plied with colored discs that permit the color of the button to be selected.
FIG. 8 emergency stop button can stop all motors.
FIG. 9 Stacked push buttons are made by connecting multiple contacts
sets together.
FIG. 10 Push-pull button.
FIG. 11 This symbol represents a push-pull button.
Push-Pull Buttons
Another push button that has found wide use is the push-pull button (FIG. 10). Some push-pull buttons contain both normally open and normally closed contacts much like a double-acting push but ton, but the contact arrangement is different. This push-pull button is intended to provide both the start and stop functions in one push button, eliminating the space needed for a second push button.
The symbol for a push-pull button of this type is shown in FIG. 11. When the button is pulled, the normally closed contact remains closed, and the normally open contact bridges the two stationary contacts to complete the circuit. When the button is released, the normally open contact returns to its normal position, and the normally closed section remains closed. When the button is pushed, the normally closed section opens to break the circuit, and the normally open section remains open.
A schematic diagram showing a push-pull button being used as a start-stop is shown in FIG. 12.
Push-pull buttons that contain two normally open contacts are also available (FIG. 13). These buttons are often used to provide a run-jog control on the same button. When this is done, the run function is generally accomplished with the use of a control relay, as shown in FIG. 14. When the button is pressed downward, a circuit is complete to the M coil, causing all open M contacts to close and connect the motor to the power line.
When the button is released, the contact reopens and de-energizes the M coil, causing all M contacts to reopen and disconnect the motor from the power line. When the button is pulled upward, it completes a circuit to CR relay, causing both normally open CR contacts to close. One CR contact connected in parallel with the run section of the button maintains power to CR coil when the button is released. The CR contact connected in parallel with the jog section of the button closes and energizes the M coil, causing the motor to be connected to the power line. The motor continues to run until the stop button is pressed.
Push-pull buttons that contain two normally closed contacts can be obtained also (FIG. 15). These buttons are generally employed to provide stop for two different motors (FIG. 16). When the button is pulled upward, the connection to the two top stationary contacts is broken, causing coil M1 to de-energize. The bottom section of the button remains closed. When the button is pressed, the top section remains closed, and the bottom section opens and breaks the connection to coil M2.
Regardless of the configuration of the push-pull buttons or how they are employed in a control circuit, they are generally used to provide the function of two different buttons in a single space. They are a good choice if it becomes necessary to add controls to an existing control panel that may not have space for extra push buttons.
FIG. 12 Schematic using a push-pull button as a start-stop control.
FIG. 13 Some push-pull buttons contain two normally open contacts instead
of one normally open and one normally closed.
FIG. 14 run-Jog circuit using a push-pull button.
FIG. 15 Push-pull button with two normally closed contacts.
Lighted Push Buttons
Lighted push buttons are another example of pro viding a second function in a single space (FIG. 17). They are generally used to indicate that a motor is running, stopped, or tripped on overload.
Most lighted push buttons are equipped with a small transformer to reduce the control voltage to a much lower value (FIG. 18). Lens caps of different colors are available.
FIG. 16 A push-pull button with two normally closed contacts used to
provide a stop for two different motors.
FIG. 17 Lighted push button.
Switch Symbols
Switch symbols are employed to represent many common control sensing devices. There are four basic symbols: normally open (NO); normally closed (NC); normally open, held closed (NOHC); and normally closed, held open (NCHO). To understand how these switches are drawn, it is necessary to begin with how normally open and normally closed switches are drawn (FIG. 19). Normally open switches are drawn with the movable contact below and not touching the stationary contact. Normally closed switches are drawn with the movable contact above and touching the stationary contact.
The normally open held closed and normally closed held open switches are shown in FIG. 20. Note that the movable contact of the normally open held closed switch is drawn below the stationary contact. The fact that the movable con tact is drawn below the stationary contact indicates that the switch is normally open. Because the movable contact is touching the stationary contact, however, a complete circuit does exist because something is holding the contact closed.
A very good example of this type of switch is the low-pressure switch found in many air-conditioning circuits (FIG. 21). The low-pressure switch is being held closed by the refrigerant in the sealed system. If the refrigerant should leak out, the pressure would drop low enough to permit the contact to return to its normal open position.
This would open the circuit and de-energize coil C, causing both C contacts to open and disconnect the compressor from the power line. Although the schematic indicates that the switch is closed during normal operation, it would have to be connected as an open switch when it is wired into the circuit.
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FIG. 18 Lighted push buttons are generally equipped with a small transformer
to reduce the voltage to a much lower value. LOW VOLTAGE LAMP, LENS CAP,
TRANSFORMER TERMINALS
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FIG. 19 Symbols used to represent normally open (NO) and normally closed
(NC) switches.
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FIG. 20 Normally open, held closed (NOHC) and normally closed, held
open (NCHO) switch symbols.
FIG. 21 if system pressure should drop below a certain value, the normally open, held closed low-pressure switch opens and de-energizes coil C.
The normally closed, held open switch is shown open in FIG. 20. Although the switch is shown open, it is actually a normally closed switch because the movable contact is drawn above the stationary contact, indicating that something is holding the switch open. A good example of how this type of switch can be used is shown in FIG. 22. This circuit is a low water warning circuit for a steam boiler. The float switch is held open by the water in the boiler. If the water level should drop sufficiently, the contacts close and energize a buzzer and warning light.
Basic Schematics
To understand the operation of the circuit shown in FIG. 22, you must understand some basic rules concerning schematic, or ladder, diagrams:
1. Schematic, or ladder, diagrams show components in their electrical sequence without regard for physical location. In FIG. 22, a coil is labeled CR and one normally open and one normally closed contact are labeled CR. All of these components are physically located on control relay CR.
2. Schematics are always drawn to show components in their de-energized, or off, state.
3. Any contact that has the same label or number as a coil is controlled by that coil. In this example, both CR contacts are controlled by the CR coil.
4. When a coil energizes, all contacts controlled by it change position. Any normally open contacts close, and any normally closed contacts open. When the coil is de-energized, the contacts return to their normal state.
Referring to FIG. 22, if the water level should drop far enough, the float switch closes and completes a circuit through the normally closed contact to the buzzer and to the warning light connected in parallel with the buzzer. At this time, both the buzzer and warning light are turned on. If the silence push button is pressed, coil CR energizes, and both CR contacts change position.
FIG. 22 The normally closed float switch is held open by the level of
the water. if the water level should drop below a certain amount, the
switch returns to its normal closed position and completes the circuit.
The normally closed contact opens and turns off the buzzer. The warning light, however, remains on as long as the low water level exists. The normally open CR contact connected in parallel with the silence push button closes. This contact is generally referred to as a holding, sealing, or maintaining contact. Its function is to maintain a current path to the coil when the push button returns to its nor mal open position. The circuit remains in this state until the water level becomes high enough to re open the float switch. When the float switch opens, the warning light and CR coil turn off. The circuit is now back in it original de-energized state.
FIG. 23 Limit switch symbols.
Sensing Devices
Motor control circuits depend on sensing devices to determine what conditions are occurring. They act very much like the senses of the body. The brain is the control center of the body. It depends on in put information such as sight, touch, smell, and hearing to determine what is happening around it.
Control systems are very similar in that they depend on such devices as temperature switches, float switches, limit switches, flow switches, and so on, to know the conditions that exist in the circuit. These sensing devices are covered in greater detail later in the text. The four basic types of switches are used in conjunction with other symbols to represent some of these different kinds of sensing switches.
Limit Switches
Limit switches are drawn by adding a wedge to one of the four basic switches, FIG. 23. The wedge represents the bumper arm. Common industrial limit switches are shown in FIG. 24.
Float, Pressure, Flow, and Temperature Switches
The symbol for a float switch illustrates a ball float. It is drawn by adding a circle to a line, FIG. 25. The flag symbol of the flow switch represents the paddle that senses movement. The flow switch symbol is used for both liquid and airflow switches. The symbol for a pressure switch is a half-circle connected to a line. The flat part of the semicircle represents a diaphragm. The symbol for a temperature switch represents a bimetal helix.
The helix contracts and expands with a change of temperature. It should be noted that any of these symbols can be used with any of the four basic switches.
There are many other types of sensing switches that do not have a standard symbol. Some of these are photo switches, proximity switches, sonic switches, Hall effect switches, and others. Some manufacturers employ a special type of symbol and label the symbol to indicate the type of switch. An example of this is shown in FIG. 26.
FIG. 24 Typical industrial limit switches.
FIG. 25 Schematic symbols for sensing switches.
FIG. 26 Special symbols are often used for sensing devices that do not
have a standard symbol.
Coils
The most common coil symbol used in schematic diagrams is the circle. The reason for this is so that letters and/or numbers can be written in the circle to identify the coil. Contacts controlled by the coil are given the same label. Several standard coil symbols are shown in FIG. 27.
Timed Contacts
Timed contacts are either normally open or normally closed. They are not drawn as normally open, held closed or normally closed, held open. There are two basic types of timers, on delay and off delay.
Timed contact symbols use an arrow to point in the direction that the contact will move at the end of the time cycle. Timers are discussed in detail in a later SECTION. Standard timed contact symbols are shown in FIG. 28.
Contact Symbols
Another very common symbol used on control schematics is the contact symbol. The symbol is two parallel lines connected by wires (FIG. 29). The normally open contacts are drawn to represent an open connection. The normally closed contact symbol is the same as the normally open symbol, with the exception that a diagonal line is drawn through the contacts. The diagonal line indicates that a complete current path exists.
FIG. 27 Common coil symbols.
FIG. 28 Timed contact symbols.
FIG. 29 Normally open and normally closed contact symbols.
FIG. 30 Common control and electrical symbols.
Other Symbols
Not only are there NEMA standard symbols for coils and contacts; there are also symbols for trans formers, motors, capacitors, and special types of switches. A chart showing both common control and electrical symbols is shown in FIG. 30.
FIG. 31 A MAN-OFF-Auto switch is a single-pole, double-throw switch
with a center off position.
FIG. 32 The MAN-OFF-Auto switch is often drawn in this manner.
FIG. 33 A combination Start-STOP push button station with pilot lamp
and HAND-OFF Auto switch. FIG. 34 Selector switch with multiple poles.
FIG. 35 Symbol used to represent a three-pole, three-terminal selector
switch. The movable contacts are a common terminal for each of the three
poles.
FIG. 36 A selector switch with different sets of contacts.
FIG. 37 Control panel with selector switches, push buttons, indicating
lights, and meters mounted together.
Selector Switches
Selector switches are operated by turning a knob instead of pushing a button. A very common selector switch is the MAN-OFF-AUTO switch. MAN stands for Manual and AUTO stands for Automatic.
This is a single-pole, double-throw switch with a center off position, as shown in FIG. 31. When the switch is in the OFF position, as shown in FIG. 31A, neither indicator lamp is turned on.
If the switch is moved to the MAN position, as shown in FIG. 31B the red lamp is turned on.
If the switch is set in the AUTO position, FIG. 31C, the green lamp is turned on. Another symbol often used to represent this type of switch is shown in FIG. 32. A combination START-STOP push button station, pilot lamp, and HAND-OFF AUTO switch is shown in FIG. 33.
Selector switches often contain multiple contacts and multiple poles (FIG. 34). A symbol used to represent a selector switch with three poles, each having three terminals, is shown in FIG. 35. This selector switch contains a common terminal for each of the three poles. The common terminal is connected to the movable contact. A different type of selector switch is shown in FIG. 36. Switches of this type are often supplied with a chart or truth table indicating connections between contacts when the switch is set in different positions. In this example, there is no connection between any of the contacts when the switch is set in the OFF position. When the switch is set in position A there is connection between contacts 3 and 4, and 5 and 6. When the switch is set in position B, there is connection between contacts 1 and 2, 5 and 6, and 7 and 8. It is not uncommon to see a combination of selector switches, push buttons, and meters mounted on a single control panel (FIG. 37).
QUIZ
1. The symbol shown is a
a. polarized capacitor.
b. normally closed switch.
c. normally open, held closed switch.
d. normally open contact.
2. The symbol shown is a
a. normally closed float switch.
b. normally open, held closed float switch.
c. normally open float switch.
d. normally closed, held open float switch.
3. The symbol shown is a(n)
a. iron core transformer.
b. auto transformer.
c. current transformer.
d. air core transformer.
4. The symbol shown is a
a. normally open pressure switch.
b. normally open flow switch.
c. normally open float switch.
d. normally open temperature switch.
5. The symbol shown is a H O A
a. double-acting push button.
b. two-position selector switch.
c. three-position selector switch.
d. maintained contact push button.
6. If you were installing the circuit in FIG. 22, what type of push button would you use for the silence button?
a. Normally closed
b. Normally open
7. Referring to the circuit in FIG. 22, should the float switch be connected as a normally open or normally closed switch?
8. Referring to the circuit in FIG. 22, what circuit component controls the actions of the two CR contacts?
9. Why is a circle most often used to represent a coil in a motor control schematic?
10. When reading a schematic diagram, are the control components shown as they should be when the machine is turned off or de-energized, or are they shown as they should be when the machine is in operation?
11. Push-pull buttons are generally used because
a. they are smaller in size than standard push buttons.
b. they contain larger contacts that can with stand more current than standard push buttons.
c. they can perform more than one function while only requiring the space of one single-function push button.
d. they are larger in size than standard push buttons, making them more visible to an operator.
12. What device is generally used by lighted push buttons to reduce the voltage applied to the lamp?
a. Series resistor
b. Transformer
c. Series capacitor
d. Series inductor
13. How are components that are mechanically connected together generally identified on a schematic diagram?