Control circuits are divided into two basic types: the two-wire and the three-wire. Figure 19–1 shows a simple two-wire control circuit. In this circuit, a simple switch is used to control the power applied to the coil of a motor starter. If the switch is open, there is no complete path for current flow, and the motor will not operate. If the switch is closed, power is supplied to the motor starter and it closes the M contacts, connecting the three- phase motor to the power line.
Three-wire control circuits are used because they are more flexible than two-wire circuits. Three-wire circuits are characterized by the fact that they are operated by a magnetic relay or motor starter. These circuits are generally controlled by one or more pilot devices.
ELECTRICAL SYMBOLS FOR PILOT/CONTROL DEVICES
When people first begin to learn to read, they learn a set of symbols that are used to represent different sounds. This set of symbols is generally referred to as the alphabet. When learning to read electrical diagrams, it is necessary to learn the symbols used to rep- resent different devices and components. The symbols that follow are commonly used on control schematics and wiring diagrams. These are not all the symbols used. Unfortunately, there is no real set standard for the use of electrical symbols. Most of these symbols are approved by the National Electrical Manufacturers Association (NEMA). These symbols are as follows:
The contact symbols shown are standard and relatively simple to understand. There can be instances, however, in which symbols can be used to show something that is not at first apparent. For example, the symbol for a normally closed pressure switch is shown in Figure 19–2. Notice that this symbol shows not only the movable arm making contact with the stationary contact but also the movable arm drawn above the stationary contact. Figure 19–3 shows a normally closed held open pressure switch. This symbol is not a normally open contact symbol even though the contact arm is shown not making connection with the stationary contact because pressure is used to keep the contact open. If pressure decreases to a certain point, the switch contact will close.
Figure 19–4 shows a normally open pressure switch. Notice that the contact arm is drawn below the stationary contact. Figure 19–5 shows the same symbol except that the movable arm is making connection with the stationary contact. This symbol represents a normally open held closed pressure switch, that is, a pressure switch that is wired normally open but has its contact held closed by pressure. If the pressure decreases to a certain level, the switch will open and break connection to the rest of the circuit.
SCHEMATIC AND WIRING DIAGRAMS
Schematic diagrams show components in their electrical sequence without respect to physical location. Schematic diagrams are used to troubleshoot and install control circuits. Schematics are generally easier to read and understand than are wiring diagrams.
Wiring diagrams show components mounted in their general location with connect- ing wires. A wiring diagram is used to represent how the circuit generally appears. To help illustrate the differences between wiring diagrams and schematics, a basic control circuit will first be explained as a schematic and then shown as a wiring diagram.
Reading Schematic Diagrams
To read a schematic diagram, a few rules must first be learned. Commit the following rules to memory:
1. Reading a schematic diagram is similar to reading a book in English. It is read from left to right and from top to bottom.
2. Electrical symbols are always shown in their off or deenergized position.
3. Relay contact symbols are shown with the same numbers or letters that are used to designate the relay coil. All contact symbols that have the same number or letter as a coil are controlled by that coil regardless of where in the circuit they are located.
4. When a relay is energized, or turned on, all of its contacts change position. If a contact is shown as normally open, it will close when the coil is energized. If the contact is shown normally closed, it will open when the coil is turned on.
5. There must be a complete circuit before current can flow through a component.
6. Components used to provide a function of stop are generally wired normally closed and connected in series. Figure 19–6 illustrates this concept. Both switches A and B are normally closed and connected in series. If either switch is opened, connection to the lamp will be broken and current will stop flowing in the circuit.
7. Components used to provide the function of start are generally wired normally open and connected in parallel. In Figure 19–7, switches A and B are normally open and connected in parallel with each other. If either switch is closed, a current path will be provided for the lamp and it will turn on.
The circuit to be discussed is a basic control circuit used throughout industry. Figure 19–8 shows a start–stop push button circuit. This schematic shows both the control circuit and the motor circuit. Schematic diagrams do not always show both control and motor connections. Many schematic diagrams show only the control circuit.
Notice in this schematic that there is no complete circuit to M motor starter coil because of the open-start push button and open M auxiliary contacts. There is also no connection to
the motor because of the open M load contacts. The open M contacts connected in parallel with the start button are small contacts intended to be used as part of the control circuit. This set of contacts is generally referred to as the holding, sealing, or maintaining contacts. These contacts are used to provide a continued circuit to M coil when the start button is released.
The second set of M contacts is connected in series with the overload heater element and the motor, and the contacts are known as load contacts. These contacts are large and designed to carry the current needed to operate the load. Notice that these contacts are normally open and there is no current path to the motor.
When the start button is pushed, a path for current flow is provided to M motor starter coil. When M coil energizes, both M contacts close (Figure 19–9). The small auxiliary contact provides a continued current path to the motor starter coil when the start button is released and returns to its open position. The large M load contact closes and provides a complete circuit to the motor and the motor begins to run. The motor will continue to operate in this manner as long as M coil remains energized.
If the stop button is pushed (Figure 19–10), the current path to M coil is broken and the coil deenergized. This causes both M contacts to return to their normally open position. When M holding contacts open, there is no longer a complete circuit provided to the coil when the stop button is returned to its normal position. The circuit remains in the off position until the start button is again pushed.
Notice that the overload contact is connected in series with the motor starter coil. If the overload contact opens, it has the same effect as pressing the stop button. The fuse is connected in series with both the control circuit and the motor. If the fuse should open, it has the effect of disconnecting power from the line.
A wiring diagram for the start–stop push button circuit is shown in Figure 19–11. Although this diagram looks completely different, it is the same electrically as the schematic diagram. Notice that the push button symbols indicate double-acting push buttons. The stop
button, however, uses only the normally closed section, and the start button uses only the normally open section. The motor starter shows three load contacts and two auxiliary contacts. One auxiliary contact is open and one is closed, but only the open contact is used.
The overload relay shows two different sections. One section contains the thermal heater element connected in series with the motor. The other section contains the normally closed contact connected in series with the coil of M starter.