Solenoid & Switches
In a country sense of the word, electrons flowing through a wire ainít doing nobody no good. However, they can create something that can be useful. This current can create heat, light, sound, and particularly important in our case, magnetism.
When electrons flow through a wire, a magnetic field is produced around that wire. In other words, the wire becomes a magnet. Basically, more current will produce more magnetism and, if we really want to get a great deal of magnetism in one small area, all we have to do is coil the wire so that the magnetism from each turn is concentrated in one area.
A metal plunger will not be moved much by the magnetism generated by a single wire but will when we coil the wire. When current flows through the wire, the coil becomes a very strong magnet. With this magnet we can move switches and, what is equally important, we can also turn rotors which are pieces of steel, iron or aluminum connected to shafts. The device containing the rotor is an electric motor.
For the purpose of identification, now and in the future, the electrical devices used in electric circuit wiring may be divided into two classes. These classes are either current consuming devices or switches. For our purpose, a current consuming device or "load" as it is sometimes called, is a device that uses the magnetism created by flowing electricity to do mechanical work. Any device that changes the electrical energy or current flowing through it into some other type of energy can be considered a current consuming device. For example, the magnetic action created by electricity can open or close valves, open or close switches, move motors, etc.
Flowing electricity can also create light in a light bulb or heat in a heater and sound. But in all cases, the flowing electricity is converted into another form of energy. A device that does this is known as a current consuming device or "load."
Electrically, all of our energy is directed toward getting the load devices to function at the proper time. Therefore, these devices will be covered first.
The electric motor is usually considered the most important load device in the electrical circuit. It can be represented symbolically in two basic ways.
The first symbol (figure 1) and the one that is used most frequently in our course, is the large circle. The type and purpose of the motor are identified by the letters within the circle. Two common examples might be "COMP.", indicating a Compressor motor or "MOTOR," representing fan motors or some other type of motor. Because motors are usually considered the most important devices in the electrical system, they are normally larger than the other diagram symbols.
The second symbol (figure 2) that may be used, actually shows the internal wiring of the Compressor motor. This is used frequently when the motor has unusual internal wiring such as wye delta or a part winding start.
Actually, there are many symbols for motors, always be sure that you are using the right symbol when troubleshooting for problems.
Because motors are usually the most important load devices of our electrical circuits, it is important that we be able to recognize them quickly and correctly. Remember also the very basic fact that when current flows through a motor, the motor will function.
The second current consuming device or load to be discussed is the solenoid. We said that flowing current created magnetism. We also said that we could coil a number of turns of wire together and concentrate this magnetism in one small area. This coil, in effect, becomes a magnet when current passes through it. The solenoid is a device designed to harness and use this magnetism. A solenoid or coil is most frequently used to open or close an electrical switch, open and close mechanical valves and use for many other purposes.
The word solenoid applies to the coil which becomes a magnet when current is passed through it. If this operates a valve, then the valve would be known as a solenoid valve. Because the solenoid is used for many purposes, it is important that we recognize the symbol for the solenoid alone. Its purpose is normally identified by the letters accompanying the solenoid symbol, but not always. Solenoids are very common and serve many purposes. Many of the devices described in the following paragraphs use the solenoid coil but donít use the word solenoid in a description of the device.
A very frequent use of the solenoid is the relay or "electrical switch (will be shown as contacts on the drawing)." In the relay (figure 4), the solenoid sets up a magnetic field which operates an electrical switch. A relay uses electricity in one circuit to open or close an electrical switch in a second circuit. The term "relay" is usually applied to the solenoid switch when the current involved is very small.
The terms normally open (N.O.) and normally closed (N.C.) refers to the position of the switch with no applied operating force. This is an extremely important point.
Most all wiring diagrams show electrical switches in the position that they will be in when the operating solenoid or mechanism is de-energized.
Most all electrically operated switches are shown in a diagram in the position that they will be in when no electrical energy is applied. And, when electricity is applied to the operating device, some switches will automatically change their position. Normally opened switches become closed and normally closed switches become open. In a relay, for example, the position of the switch will be shown as the position of the switch when no electricity is going through the relay coil. When the relay coil is energized, or current is flowing through the coil, then it will go to the opposite direction.
The switch symbols are normally used with a relay. There are other switch symbols that will be shown and discussed as we progress.
The moment current or operating force is applied to the coil of a normally open or "N.O." switch, it will close. Relays normally have only one coil but they may have many switches, both opened and closed, as it is physically and practically possible to build into the device. These may be identified as SPST, SPDT, DPDT, 3PDT or 4PDT and possibility others.
This is an important point. One solenoid is required for any relay. However, switches may, and frequently do, come in multiples. The letters identifying the solenoid will also identify every switch operated by that solenoid. Once the solenoid coil in the diagram is located, it is not difficult to find the switches which it activates. When reading wiring diagrams, it is important that you understand that this relay can operate more than one switch.
By identifying both the coil and the switch (contacts) with the same letter system, it is quite simple to locate which switches are operated by a particular coil.
Some of our drawings will show a coil with three different switches; two "N.O." and one "N.C." other drawings will show current passing through the coil, as indicated by the closed and/or open circuit. There may be two contacts close together that is operated by two different coils.
When current is applied to the relay coil, all contacts in that relay change position.
The greater the amount of current that is going to be turned on and off, the greater the size of the switch required. When the switches become heavier and capable of carrying more current, a small solenoid coil may not be able to create enough magnetism to move the switch. As a result, large solenoids with greater magnetic power are required. As the relay grows larger, its principle doesnít change but its name does. A relay of a larger size is frequently known as a contactor. The principle is exactly the same. The only difference is the size involved. The physical appearance will change but that is all.
The contactor has a coil symbol that looks identical to the relay coil. It also has switches which are identified by the same letter used in identifying the solenoid. It functions in exactly the same manner, that is, when current passes through the coil, the switch positions will change from the positions shown on the diagram.
As a country boy would say, the contactor is just a "grown-up" relay.
A third term which is also heard frequently when discussing magnetic switches is the word starter. A starter is nothing more than a contactor which has overload protection built-in as part of the starter. The wiring diagram symbols will not differentiate between a starter and a contactor except in the pictorial or line diagrams.
Another common load device is the resistance heater. Instead of converting electricity to magnetism to operate motors or solenoids, electricity can be converted directly into heat.
Where low resistance is encountered, little heat is produced. Where high resistance is encountered, more energy is expended overcoming the higher resistance, and more heat is produced.
Electric resistance heaters have special high resistance alloy wire which will give off high amounts of heat when current flows through them. Resistance heaters have many uses such as toasters, space heaters, etc., but our interests are mainly crankcase heaters for the refrigerated air dryer, supplemental strip heaters for the control cabinet and band heater for the Compressor sump tank. Some types may be called heat tracing tape and most commonly used on pipe or control line tubing.
On the refrigerated air dryer the crankcase heater is installed electrically in such a manner that electricity flows through the heater only when the Compressor is off. The heat from the heater keeps the Compressor crankcase temperature high enough to prevent liquid refrigerant from condensing within the crankcase.
Some crankcase heaters on smaller units are not turned off when the Compressor is started.
The final load device to be discussed will be the signal light. The signal light can be used to indicate almost any condition, good or bad, within the circuit. For example, the signal light can indicate that a load device is functioning or it can indicate that it is not functioning. It is frequently used to indicate the condition of an electrical device that is not easily seen or observed. Its uses are almost unlimited. The signal light symbol is easily identified by the radiating lines from the small circle. The letter is used to identify the color of the light.
Load devices can serve many purposes: they move switches, run fans, give heat, etc., but switches serve only one purpose, to start or stop the flow of electricity.
The only difference in switches is the type of action that opens or closes them. Electrically, all switches perform exactly the same function; they stop or start the flow of electricity to an electrical load device.
Switches are generally classified by the force that activates the switch itself. For example, a relay would be considered an electrical switch because a flow of electricity in the solenoid coil activates the switch. A high-pressure or low-pressure control is considered a mechanically operated switch because it is operated by physical pressures. Mechanically operated switches also include thermostats, manual switches, cam driven switches, etc.
It is important to know the difference between the switches because of the different ways in which they are represented in the wiring diagram. Unless you know the method or force which activates the switch, you cannot be sure of its normal position nor the purpose of the switch.