FACILITY ELECTRICAL CONTROL CIRCUITS
In this article, we review control circuits and diagrams, including wiring and ladder line diagrams. In the final section of the article, we review control circuit logic.
A connection diagram is a "picture" or a physical representation of a circuit. A ladder diagram is a schematic representation of a circuit. Diagrams that show every electrical part in a circuit can become complex. When circuits involve many components, showing each device clearly and realistically is impossible. For this reason, maintenance technicians have developed a set of symbols and abbreviations that represent components in electrical diagrams. Although these symbols are not universal, they are accepted as the standard throughout industry.
Symbols used in schematic diagrams show where electrical parts are located in a circuit and how they are related. As a technician, you need to understand the symbols used in these diagrams to install, troubleshoot, and repair circuits and machinery.
Electrical symbols are a type of electrical shorthand. Each symbol represents a single part of an electrical circuit, such as a limit switch or a relay. Since many motor control circuits are complex, symbols provide a convenient way to represent many electrical devices in a small amount of space.
A universal set of symbols does not exist, but the Joint Industrial Council (JIC) symbols in this module are a standard throughout industry.
As you may have noticed earlier, some JIC symbols appear in two or more positions. These positions tell us the contact condition or the conductive status of a device, or the position of the part that "connects" the circuit.
When the device contacts touch (contact) each other, current flows through the device and energizes the circuit. When the contacts do not touch, current cannot pass through the device and the circuit is de-energized. There are four contact conditions:
Figure 1: Normally Open and Normally Closed Relays
In Figure 2 (upper left side), the symbol for a normally closed relay contact looks identical to the thermal contact overload symbol (upper right). If you had to identify either one on a diagram, you could easily confuse the two. However, the abbreviations on the right eliminate the possibility of mixing up the two devices. If more than one relay appeared in the diagram, each relay would have a number added to the abbreviation (lower right).
Figure 2 - Symbols for Relays and Thermal Overloads Look Similar in Appearance
A wiring diagram (also called "connection diagram") shows the physical relationship between electrical devices in a circuit. This type of diagram provides all of the information needed to make wire connections and trace a circuit. A technician would find this type of diagram useful when searching for specific wires in a circuit. A wiring diagram shows the exact size, type, and location of circuit components. It is a physical representation.
A line (or "ladder") diagram shows the sequence of operations in a circuit. It is also called an elementary diagram. A ladder diagram shows what must occur in the circuit before a specific action can take place. Instead of showing the exact location of a component, a schematic shows the logical path of current flow through the circuit. This type of diagram is easy to follow, but it does not show as much detail as the wiring diagram. It is a schematic representation.
Figure 3A shows the wiring diagram for a typical magnetic starter. Figure 3B shows the elementary diagram for the same circuit. Study the two diagrams carefully. Note that it is easier to follow the logical sequence of events on the elementary diagram. However, a maintenance technician installing the starter would find the wiring diagram more helpful.
A control circuit is any circuit that has as its load, the operating coil of a magnetic motor starter, a magnetic contactor, a relay, a pilot light, or any other control device that exercises control of another circuit. In other words, a control circuit exercises control over one or more other circuits.
The elements of a control circuit include all the equipment and devices that are involved with the function of the circuit itself. These elements include conductors, power supplies and sources, overcurrent-protection devices, relay coils, contactors, starters, as well as all of the switching devices.
Control circuits are represented in electrical drawings or diagrams that essentially provide you the complete story and sequence of the process or machine control.
Ladder diagrams got their name because they show the sequence of operations in separate lines that resemble the look of the rungs of a ladder. One step from the top leads to the next step down and so on (Figure 4).
The ladder diagram provides two important pieces of information:
A ladder diagram is intended to give you the basic operation of the control system (see Figure 5). A layout and connection diagram gives the physical placement or location of all the components in a control panel.
Figure 5 - Control System Ladder Diagram
A wiring diagram, shown in Figure 6 for a magnetic starter, is intended to show you the actual physical relationship of all the various devices that form the control system. Note that the heavy lines are the power circuits.
Figure 6 - Wiring Diagram
Let us look at a very simple ladder circuit in pictorial form in which we have a normally-open pushbutton switch connected to a pilot light. One terminal of the pushbutton switch is connected to the hot leg (L1), while the other terminal is connected to the load (PL1). The pilot light (PL1), in order to complete the circuit, is also connected to the L2 neutral line.
The one load per line rule simply states that only one load may be connected between L1 and L2 for each line. For instance, if we have a simple switch controlling a light and a heater, we would not connect the switch to the light and to the heater as shown in Figure 7. Two loads, the light and the heater, are both in the same ladder line between L1 and L2. The desired function, of course, is that both loads turn on when the switch is closed.
Figure 7 - Improper Configuration
To properly apply the one load per line rule, we would have to redraw the circuit as shown in Figure 8. Here, there is only one load per line between L1 and L2. When the switch is closed, both loads are energized.
Figure 8 - One Load Per Line
In the configuration shown in Figure 7, when the switch closes, neither of the two loads will receive the entire 120 volts that is required for proper operation and, therefore, will result in a fault or failure.
In the correct configuration, as shown in Figure 8, both loads receive the required full voltage between L1 and L2. If we were to place a voltmeter across each of the loads, we would read 120 volts AC.
As shown so far, all loads (control relays, pilot lights, and solenoids) are connected to L2 to complete the circuit from L1 through the control and switching devices (limit switches, pushbuttons, and relay contacts) (see Figure 9).
Figure 9 - Loads Connect to L2 to Complete Circuit
Safety reasons dictate why loads are always connected to L2. We always want to switch the hot leg (L1) to the loads, not the neutral or common as shown in Figure 10. In this situation, both terminals of the solenoid are always "hot", waiting for a switch connection to the common (L2).
Figure 10 - Improper Load Connection
There is one exception to this rule. What about the arrangement of the magnetic starter coil and the overload contacts Here is an example in Figure 11. This is one instance where we want to, in fact, must be able to switch the common-disconnect the common from the coil. However, it's not actually "we" (us), its the motor and its current condition or status and that of the overload relays that control the opening and closing of the overload contacts.
Figure 11 - Overload Contacts Connected Between Load and L2
The schematic representation of the motor starter coil is slightly different from a regular control relay simply because of the series overload contacts. We know that magnetic motor starters contain the normally-closed overload contacts that are indirectly connected to the coil through wiring installed by the manufacturer.
As we will see later, the "decision" that governs the response of a load is represented on the left side of the load in the ladder diagram. The overload contacts do not decide on whether to open or closed the starter load circuit, they merely respond in a protective manner. The motor and the overload sensing elements are the ones that control (decide) the status of the contacts. That is another reason why we represent the overload contacts on the right of the motor starter load.
The number of overload contacts between the starter coil and L2 depends on the type of starter and the number of phases used in the circuit (see Figure 12). A three-phase starter represented in a ladder diagram may have all three sets of normally-closed overload contacts illustrated in Figure 13. To avoid confusion and simplify the ladder diagram, we can represent all three sets of contacts with the labeling All OLs or All Overloads, as shown in Figure 14.
Figure 12 - Starter Overload Contacts - Three-Phase
Figure 13 - Starter Overload Contacts - Three-Phase
Figure 14 - Starter Overload Contacts - Three-Phase (Simplified)
Logic, as used in control circuits, can be defined as "the necessary arrangements of input signal conditions that need to take place to cause a control output." The switches, relay contacts, and interconnecting wires of the circuit implement this arrangement (see Figure 15).
Figure 15 - Implementation of Logic
These are three basic types of logic functions:
These functions cover nearly all of the possibilities you may encounter in a control circuit.
The AND logic function simply states that for an output to occur, all of the input signals must provide power continuity from all of the input signal devices must be conducting current (see Figure 16).
Figure 16 - AND Logic Function
An AND function must be used if we want the light to turn on only if both PB1 and PB2 are pushed at the same time. The AND logic simply dictates that, if both pushbuttons are providing power, the light will turn on (Figure 17).
Figure 17 - AND Logic
The OR logic function describes the result of an output if at least one of its input signal conditions provide a path from L1 to the load to L2 (see Figure 18).
Figure 18 - OR Logic Function
For example, the OR logic in Figure 19 simply states that, if either PB1 or PB2 is pushed, the light will turn on. The wiring configuration described by the OR function in this ladder diagram is a parallel connection of the two pushbuttons; PB1 and PB2 are in parallel. Note that the connection from PB2 in line 2 to PB1 in line 1 is shown by a node to indicate a junction or wiring connection.
Figure 19 - OR Logic Function
Most control circuits use a combination of AND and OR logic to control an output. The logic shown in Figure 20 drives a pump motor starter. This combination logic and design are based on a certain need or requirement.
Figure 20 - AND - OR Combination
The logic of this circuit can be read simply by saying that, "the pump will turn on if the temperature reaches a certain level, thus activating the temperature switch TS1. AND if the flow switch is activated, AND if PB1 OR FTS is activated the motor starter will be energized."
Interlocking can be defined as "the means by which a device is actuated by the operation of some other device to which it is associated." Interlocking is used in a variety of circuits, but it is most frequently used in motor control circuits to prevent, for instance, a command for a motor to go forward and reverse at the same time.
There are three types of commonly used interlocking techniques:
Mechanical interlocks are generally found in magnetic reversing starters that allow the control circuit to direct the motor in a forward or reverse direction. This mechanical interlock is usually factory installed by the starter manufacturer and is represented in the ladder diagram by dashed lines between contactors (see Figure 21). The interlock is made up of mechanical linkages placed in such manner and position that makes it impossible to close both starter controls in the forward and reverse state at the same time.
Figure 21 - Mechanical Interlock
The pushbutton interlock is another common technique of protecting two circuits from being on at the same time (see Figure 22). This interlock is provided by a double circuit pushbutton, having one set of contacts normally-open and one set normally-closed.
Figure 22 - Pushbutton Interlock
In a forward/reverse motor control circuit, these pushbuttons are connected as shown in Figure 23. In line 1, the normally-closed reverse pushbutton is in series with the normally-open forward pushbutton. This allows the forward starter to turn on when the forward pushbutton is pressed. If the reverse pushbutton is pushed, then the forward circuit will break and the reverse starter will be turned on.
Figure 23 - Forward/Reverse Motor Control Circuit
Notice that this pushbutton interlocked motor control circuit provides instantaneous change from forward to reverse and vice versa without having to stop the motor. However, the mechanical interlock circuit shown in Figure 24 requires the operator to first stop the ongoing direction of the motor before reversing it.
Figure 24 -Mechanical Interlocking
A word of caution regarding the rapid reversal of a motor under full load should be mentioned here. In many instances, motors and the equipment that provides power, cannot withstand rapid or sudden reversals of direction; therefore, you must exercise care to determine under what load conditions you may execute a rapid motor reversal. You must also consider the braking methods that are provided to slow the machine to a safe speed before reversal.
Another method of interlocking is provided by auxiliary contacts in control relays. This interlocking method is also referred to as electrical interlocking. These auxiliary contacts are also available in most motor starters to provide secondary interlock backup, even if mechanical interlock is already provided.
In a motor control circuit as shown in Figure 25, without mechanical interlock, we would add in series, auxiliary contacts R2 (reverse starter) in line 1; auxiliary contacts F2 (forward starter) in line 3 (see Figure 26). These contacts will prevent the forward and reverse starters from being on at the same time.
Figure 25 - Motor Control Circuit Before Adding Auxiliary Contacts
Figure 26 - Auxiliary Contacts Interlocking
When neither pushbutton has been pressed, both of the auxiliary contacts, R2 and F2 will remain in their normal statenormally-closed. If the operator pushes the forward (FWD) pushbutton, there will be power continuity from L1 to the forward starter, and its sealing contacts F1 will seal around the forward pushbutton. The auxiliary normally-closed F2 contacts will open because the starter coil (F) is now energized. These F2 contacts, now open, will block any reverse pushbutton activation of the reverse starter.
If we wanted to reverse the motor, the operator would have to stop the motor so that the F1 seal is broken and the open F2 auxiliary contacts return to their normally-closed state. Then, the operator can press the reverse (REV) pushbutton to reverse the motor direction.
As you can see, interlocking forms an important part of control circuit design and troubleshooting. Interlocking circuits are used extensively, not only in motor control circuits but also in many other machine control circuits where one event must prevent another one from happening at the same time. Conversely, interlocks can be used to allow an event to happen only when another one occurs.
Remember ladder logic diagrams are a schematic of a system's logic and control circuitry. Power components such as motors, fuses, circuit breakers, main disconnects, and power-switching contacts are not shown on ladder diagrams (see Figure 27).
Figure 27 - Ladder Logic Diagram
A ladder logic diagram is so named because of its basic shape. The two power lines, L1 and L2, shown vertically on the left and right, are the rails of the ladder. The horizontal lines running in parallel between them resemble the rungs. Each rung is labeled with a number in the left margin. The function of each rung is often noted in the right margin.
Sometimes individual wires are numbered on ladder logic diagrams so that the diagram can be used for tracing and locating wires on the actual equipment. Components are labeled and numbered to identify them. For example, Pushbutton switches are labeled PB1, PB2, ...; pressure switches may be labeled PS1, PS2, ...; temperature switches TS1, TS2, ...; and so on. Relay coiled are usually shown with a letter-number label, such as CR1, for "Control Relay 1," M1 for "Motor Starter Contactor 1," or M2 for "Motor Starter Contactor 2."
Numbers in the right margin refer to other rungs containing contact sets that are operated by a coil on that rung. For example, at the right end of rung 1, the number "2" refers to the contact set (M1) in rung 2 which is operated by the coil (M1) in rung 1. At the right end of rung 2 is a "3" which refers to a contact set (M2) operated by the coil (M2) in rung 2. Underlined numbers in the right margin refer to contact sets in other rungs that are normally-closed. Numbers without underlines refer to normally-open contacts.
Contacts M1 and M2 are auxiliary contacts on the motor contactors. The main power contacts operated by M1 and M2 do not appear anywhere on the ladder, since they are on the power circuit diagram.
Each rung is a series string of components, each of which must pass current to activate the device at the end of the rung. Sometimes there are connections between rungs that put components in parallel or some combination of series and parallel.
Reading ladder logic diagrams involves tracing voltage to see what component should be on or off, active or not active, at each stage in the system operation.
On each rung, voltage from L1 passes or does not pass through one or more switches. A switch such as limit switches sense conditions within the system; others like stop or start switches, provide input from outside the system.
Many rungs will also include relays, timers, or other contacts. These will pass or interrupt current, depending upon whether their coil is active or not active on another rung of the ladder.
On the right end of each rung is the output device, usually a relay coil, solenoid, or indicator of some kind. Which either acts or does not act depending on whether voltage reaches it. There will never be more than one output device on each rung.
Overload breaker contacts are usually shown between contactor coils and L2, even though they are operated by thermal sensors in the lines to the motor, rather than by anything on the ladder logic diagram. Three-phase loads are likely to have three overload breakers, to protect each line. The symbol shown in line 1 clumps them together.
Ladder logic diagrams are normally read from left to right and from top to bottom, just like a printed page. In general, this will correspond to the sequence of events in the system operation.
Usually, the best way to trace the operation of a circuit on a ladder logic diagram is to begin at the start switch. Push it, follow current through it, and see what happens next. Usually, one event will cause something else to happen which will result in still another event, and so on.
Sometimes, events occur at exactly the same time, and there is no sequential order between them. Different contact sets on the same relay, for example, are activated at the same time by the coil. Often, however, events that occur at virtually the same time do have a cause and effect order that will become clear when tracing through the circuit.
The first two rungs of the circuit shown in Figure 28 represent a common magnetic motor starter.
Pushing Start button PB2 sends power to coil M1. It pulls in, closing all M1 contacts, including the power line contacts (which are only shown on the power circuits diagram), and starts the motor.
Switch PB2 and contacts M1 are in parallel. Normally-open switches or contacts in parallel make up what is called an OR circuit; the coil will be pulled in and the motor will run when either PB2 OR contacts M1 are closed.
Figure 28 -Magnetic Motor Starter
As soon as M1 has pulled in, power can go to the coil through contacts M1. In addition, as long as power goes to coil M1, contacts M1 will stay closed. So contactor M1 is sending power to itself and will stay active after PB2 is released. PB2 normally-open contacts return to their not active state, but the motor will keep running since contacts M1 stay closed. This is often called a seal-in circuit.
The Stop switch PB1 is wired normally-closed, just like all emergency stop devices. When activated, it cuts control voltage to everything in the line, and lets coil M1 "drop out." All M1 contacts open, including the seal-in contacts, stopping the motor. When PB1 is released, the motor will not restart until PB2 is pushed again.
A motor can be started and stopped with a simple switch, but a magnetic motor starter circuit has several advantages: