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Control

• Control, as applied to control circuits, is a broad term that means anything from a simple toggle switch to a complex system of components which may include relays, contactors, timers, switches, and indicating lights.
• Every electrical circuit for light or power has control elements. One example of a simple control circuit is a light switch used to turn lights on and off.
• Of course there are many other devices and equipment systems in industrial applications.
• Motor control, for example, can be used to start and stop a motor and protect the motor, associated machinery, and personnel.
• In addition, motor controllers might also be used for reversing, changing speed, jogging, sequencing, and pilot-light indication.
• Control circuits can be complex: accomplishing high degrees of automatic and precise machine operation.

Manual Control

• Control is considered to be manually operated when someonemust initiate an action for the circuit to operate.
• For example, someone might have to flip the switch of a manual starter to start and stop a motor.

Automatic Operation

• While manual operation of machines is still common practice, many machines are started and stopped automatically.

• Frequently there is a combination of manual and automatic control.
• A process may have to be started manually, but may be stopped automatically.

Control Elements

• The elements of a control circuit include all of the equipment and devices concerned with the circuit function.
• This includes enclosures, conductors, relays, contactors, pilot devices, and overcurrent-protection devices.
• The selection of control equipment for a specific application requires a thorough understanding of controller operating characteristics and wiring layout.
• The proper control devices must be selected and integrated into the overall plan.

Electrical Symbols

• Language has been developed in order to transfer ideas and information.
• In order to understand the ideas and information being communicated, an understanding of the language is necessary.
• The language of controls consists of a commonly used set of symbols which represents control components

Contact Symbols

• Contact symbols are used to indicate an open or closed path of current flow.
• Contacts are shown as normally open (NO) or normally closed (NC).

• Contacts shown by this symbol require another device to actuate them.
• The standard method of showing a contact is by indicating the circuit condition it produces when the actuating device is in the deenergized or nonoperated state.
• For example, in the following illustration a relay is used as the actuating device.
• The contacts are shown as normally open, meaning the contacts are open when the relay is deenergized.

• A complete path of current does not exist and the light is off.

Normally Open Contact Example 

• In a control diagram or schematic, symbols are usually notshown in the energized or operated state.
• For the purposes of explanation in this text, a contact or device shown in a state opposite of its normal state will be highlighted.
• For example, in the following illustration the circuit is first shown in the deenergized state. The contacts are shown in their normally open (NO) state.

• When the relay is energized, the contacts close completing the path of current and illuminating the light.
• The contacts have been highlighted to indicate they are now closed. This is not a legitimate symbol.
• It is used here for illustrative purposes only.

Normally Closed Contact Example

• In the following illustration the contacts are shown as normally closed (NC), meaning the contacts are closed when the relay is deenergized.

• A complete path of current exists and the light is on.
• When the relay is energized, the contacts open turning the light off.

Switch Symbols

• Switch symbols are also used to indicate an open or closed path of current flow.
• Variations of this symbol are used to represent limit switches, foot switches, pressure switches, level switches, temperature-actuated switches, flow switches, and selector switches.

• Switches, like contacts, require another device or action to change their state.
• In the case of a manual switch someone must manually change the position of the switch.

Normally Open Switch Example

• In the following illustration a battery is connected to one side of a normally open switch and a light to the other.

• Current is prevented from flowing to the light when the switch is open.
• When someone closes the switch, the path of current flow is completed and the light illuminates.

Normally Closed Switch Example

• In the following illustration a battery is connected to one side of a normally closed switch and a light to the other.

• Current is flowing to the light when the switch is closed.
• When someone opens the switch, the path of current flow is interrupted and the light turns off.

Pushbutton Symbols

• There are two basic types of pushbuttons: momentary and maintained.
• A normally open momentary pushbutton closes as long as the button is held down.
• A normally closed momentary pushbutton opens as long as the button is held down.

• A maintained pushbutton latches in place when the button is pressed

Normally Open Pushbutton Example

• In the following illustration a battery is connected to one side of a normally open pushbutton and a light is connected to the other side.

• When the pushbutton is depressed, a complete path of current flow exists through the pushbutton and the light is illuminated.

Normally ClosedPushbutton Example

• In the following example current will flow to the light as long as the pushbutton is not depressed.

• When the pushbutton is depressed, current flow is interrupted and the light turns off.

Coil Symbols

• Coils are used in electromagnetic starters, contactors, and relays.
• The purpose of contactors and relays is to open and close associated contacts.
• A letter is used to designate the coil; for example, “ M ” frequently indicates a motor starter and “ CR” indicates a control relay.
• The associated contacts have the same identifying letter.

• Contactors and relays use an electromagnetic action which will be described later to open and close these contacts.
• The associated contacts can be either normally open or normally closed.

Coil Example Using Normally Open Contacts 

• In the following example, the “ M ” contacts in series with the motor are controlled by the “ M ” contactor coil.

• When someone closes the switch, a complete path of current flow exists through the switch and “ M ” contactor coil.
• The “ M ” contactor coil actuates the “ M ” contacts which provide power to the motor.

Overload Relay Symbols

• Overload relays are used to protect motors from overheating due to an overload on the driven machinery, low-line voltage, or an open phase in a three-phase system.

• When excessive current is drawn for a predetermined amount of time, the relay opens and the motor is disconnected from its source of power.

Pilot Light Symbols      

• A pilot light is a small electric light used to indicate a specific condition of a circuit.
• For example, a red light might be used to indicate a motor is running.

• The letter in the center of the pilot light symbol indicates the color of the light.

AUTHORS
1.Bunty B. Bommera
2.Dakshata U. Kamble