Electrical Symbols

Control circuits can be represented pictorially in various ways. One of the more common approaches is to use control logic diagrams which use common symbols to represent control components. Although control symbols vary throughout the world, the symbols used in this course are common in the United States and many other countries.

Contact Symbols                           Various devices incorporate contacts to control the flow of current to other control components. When in operation, a contact my be either open, a condition which blocks current flow, or closed, a condition which allows current flow. Control logic diagrams, however, cannot show the dynamic operation of contacts. Instead, these diagrams show contacts as either normally open (NO) or normally closed (NC).

                                                        

 

The standard method of showing contacts is to indicate the circuit condition produced when the actuating device is in the de-energized (off) state.

 

For example, in the following illustration, the contacts are part of a relay. The contacts are shown as normally open to indicate that, when there is no power applied to the relay’s coil, the contacts are open. With the contacts open, there is no current flow to light.

 

 

Symbols on a control logic diagram are usually not shown in their energized (on) state. However, in this course, contacts and switches are sometimes shown in their energized state for explanation purposes. In such cases, the symbol is highlighted.

 

Control methods

Manual Control                              A simple on-off lighting control circuit illustrates an example of manual control. Manual control requires someone to use a switch to turn something on or off. The device being turned on or off may be a light, as in the previous example. However, many other devices are also controlled manually. For example, a manual starter can be used to start and stop a motor.

 

                                                        

 

 

 

 

 

 

 

 

 

Automatic Operation                     While manual control of machines is still common practice, many machines are started and stopped automatically or by some combination of manual and automatic control. Automatic control occurs when circuits can turn something on and off without human interaction.

 

                                                        

Control Circuits

Control Circuits

Control  Control components are used in a wide variety of applications with varying degrees of complexity. One example of a simple control circuit is a circuit that turns a light on and off. In this circuit, the control component is often a single-pole switch.

 

 

Control circuits used in commercial and industrial applications tend to be more complex than this simple circuit and employ a broader variety of components. However, the function of these circuits is often the same, to turn something on and off. In some cases, manual control is used. More often, automatic control circuits or circuits that combine manual and automatic control are used.

 

 

Motor control

What is VFD

PLC basics

Contractor & relay basics

Variable speed drive basics

Electrical motor controls

Once the proper motor is selected, understanding the many various control devices available and
their uses and limitations becomes an important part related to reliable operation and protection of
the motor and the personnel using the motor.
Motor Control Topics
There are four major motor control topics or categories to consider. Each of these has several
subcategories and sometimes the subcategories overlap to some extent. Certain pieces of motor
control equipment can accomplish multiple functions from each of the topics or categories.
C The four categories include:

1) Starting the Motor
                      Disconnecting Means
                      Across the Line Starting
                      Reduced Voltage Starting
2) Motor Protection
                      Over-current Protection
                      Overload Protection
                      Other Protection (voltage, phase, etc)
                      Environment
3) Stopping the Motor
                      Coasting
                      Electrical Braking
                      Mechanical Braking
4) Motor Operational Control
                      Speed Control
                      Reversing
                      Jogging
                      Sequence Control
• An understanding of each of these areas is necessary to effectively apply motor control
principles and equipment to effectively operate and protect a motor.

Siemens PLC