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 input 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, etc, to know the conditions that exist in the circuit. These sensing devices are covered in greater detail later in the textbook. 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. The wedge represents the bumper arm
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. The flag symbol of the flow switch represents the paddle that senses movement. The flow switch symbol is used for both liquid and air flow 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. 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.
that do not have a standard symbol
Coils
The most common coil symbol used in schematic diagrams is the circle. The reason is that letters and/or numbers may be written in the circle to identify the coil. Contacts controlled by the coil are given the same number.
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. The two basic types of timers are 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.
Contact Symbols
Another very common symbol used on control schematics is the contact symbol. The symbol is two parallel lines connected by wires. 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.
Other Symbols
Not only does NEMA have standard symbols for coils and contacts, but there are also symbols for transformers, motors, capacitors, and special types of switches.
IEC Symbols
Many schematic diagrams provided by European companies employ the use of symbols adopted by the International Electrotechnical Commission (IEC). These Symbols can be confusing to electricians working in the United States and Canada.
