When you started learning how to read I am sure you were introducing to a number of signs and symbols that represented different sounds. These symbols are what is known as alphabets. The schematics, as well as wiring diagrams, are the alphabets of the motor controls. Before you can determine the logic of a control circuit, first you have to learn how to read these schematics and wiring diagrams. The problem is that there is no actual standard that is used when it comes to motor control symbols. Different companies, as well as manufacturers, use their symbols when it comes to their in-house schematics. Besides, schematics that are used in other countries may totally mean a different set of control components. Although these symbols might vary from one manufacturer to another, or from one country to another country, when you have learned on how to interpret the circuit logic, you can easily identify what the different symbols stand for no matter how they are used within the schematic. The common standardized set of symbols that are used within the United States is offered by the National Electrical Manufacturer’s Association which is commonly referred to as NEMA. These symbols will be discussed in this chapter.
This one of the commonly used symbols in control schematics. The pushbuttons can be represented as normally open or normally closed. Most of the pushbuttons are momentary contact appliances that they work to connect or break a connection as long as there is a connection that is applied upon them. The pressure is normally supplied to them by someone’s finger that presses the button. As the pressure is released, the button returns to its normal position. The pushbuttons consist of both movable as well as stationary contacts. The stationary contacts are fitted to the terminal screws.
The normally open button is normally characterized drawing a movable contact at the top and not in contact with the stationary contacts. As the movable contact does not touch the stationary contacts, that means there is a formation of an open circuit and the circuit will not flow from the stationary contact to the other. The way in which the symbol is drawn it assumes that the pressure has to be applied to the movable contact. As the button is squeezed the movable contacts move below and connect the two stationary contacts completing the circuit. As the pressure is released from the button, a spring returns the movable contact to its former position.
When it comes to the normally closed push button symbol it is represented by drawing the movable contact under and touching the two stationary contacts. As the movable contact touches those two stationary contacts, a complete circuit is formed and the current can flow from one stationary contact to the other. In case pressure is applied to the button, the movable contact moves away from the two stationary contacts hence opening the circuit.
The double-acting pushing buttons
This is another most used push button that you can notice throughout the industry. The double-acting push buttons consist of both the normally open as well as the normally closed contacts. When you are connecting these buttons within a circuit you have to certain to join the wires to the proper set of contacts. The double-acting push-button contains four terminal screws. There are different ways of drawing the double push button. As the button is pressed the top movable contact moves away from the surface of the two stationary contacts and the bottom movable contact connects the bottom two contacts hence completing the circuit. When the components are drawn connected by using a dashed line within a schematic diagram, it identifies that the components are connected mechanically together. In case one component is pressed, all those components that are connected by the dashed line are pressed. This is the most used method of representing several sets of push-button contacts that are controlled with a single button.
The staked push buttons
This is a common connection that involves the use of multiple push buttons. It is normally referred to as an emergency button and might be used in stopping three motors whenever needed. The push button that consists of multiple contacts is normally referred to as stacked push buttons. The stacked pushbuttons are designed by connecting multiple contact systems together which are then monitored using a single push button.
The push-pull buttons
This is also another push button that is commonly used. Some of the pull-push buttons consist of both the normally open as well as the normally closed contacts identical to a double-acting push button, but the arrangement of the contacts is different. This push-pull button is normally made to offer both the start as well as the stop operations in a single push button thus removing the need of making a second push button. As the button is pulled, the normally closed contact remains in a closed mode and the normally open contact connects the two stationary contacts completing the circuit. As the button is released, the normally open contact retains its normal position and the normally closed sections remain closed. As the button is pushed, the normally closed section then opens to disconnect the circuit while the normally open section maintains its open state. The push-pull buttons have two normally open contacts which are used to offer a run jog control within the same button. When this is achieved, the run operation is normally accomplished with the application of the control relay.
Depending on the settings of the push-pull buttons or the mechanism in which they are used to control the circuit, they are normally used in providing the function of two different buttons within a single space. They are an ideal choice when you need to add controls within the existing control panel that might have no extra push buttons.
The lighted pushbuttons
The lighted pushbuttons are one of the examples that offer a second function within a single space. They are normally used to represent a running motor, stopped or tripped upon an overload. Most of the lighted push buttons are normally equipped with a tiny transformer that helps in reducing the control voltage to much minimal value. There are different lens caps of colors that are in existence.