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Control Circuits

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

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