relay can have two throws, fixed contacts per pole. Figure 2.22 shows schematic symbols of a variety of relay contact configurations in ANSI standard and ASA standard, respectively.
Relays are often used in electrical circuits. Figure 2.23 shows a relay used in a simple electrical circuit. The relay consists of a relay coil (CR) and two contacts (CR-1 and CR-2) in this circuit. The relay coil directly controls contacts CR-1 and CR-2. When the relay coil is energized, both contacts will change their state from open to close. As the relay coil is de-energized, both contacts become open.
2.4.2Timers
The major function of a timer is to place information about elapsed time since an event is initiated into an electrical control circuit. A timer opens or closes electrical circuits to selected operations according to a timed program. The timing function of a timer can be achieved in various ways. There are three categories of timers that are applied frequently in industrial control circuits: delay timers, interval timers, and cycle timers. Only delay timers are discussed in this section.
Figure 2.22: Symbols for electromechanical relays
A delay timer provides a period of time delay between the time an event is initiated and the time the event is actually performed. It can be used in a hydraulic control system to delay any event, such as energizing a solenoid for extending a cylinder, or to set a dwell period before de-energizing the solenoid for cylinder retraction.
Figure 2.23: A circuit with a relay
Delay timers are divided into two basic classifications: on-delay and off-delay. When power is connected to the coil of an on-delay timer, the contacts delay changing their position for a specified period of time. Suppose the timer has been set for a delay of 10 seconds and the contact is normally open. When the power source is connected to the coil of the on-delay timer, the contacts will remain in the open position for 10 seconds and then close. When the power is removed, which in turn de-energizes the coil, the contact will immediately change back to its normally open position. The schematic symbols for relay coil and contacts of an on-delay timer are shown in Figure 2.24. Figure 2.25 provides a simple example of using an on-delay timer in a motor control circuit.
The operation of the off-delay timer is simply opposite to that of the on-delay timer. Assume that the timer has been set for a delay of 10 seconds and the contact is normally open. When power is applied to the coil of the off-delay timer, the contact will change immediately from open to closed. When the coil is de-energized, the contact will remain in the closed position for 10 seconds before it reopens. The symbols for relay coil and contacts of off-delay timers are given in Figure 2.26. Figure 2.27 illustrates an example of using an off-delay timer in a control circuit.
Figure 2.24: On-delay timer
Figure 2.25: An example using an on-delay timer
Figure 2.26: Off-delay timer
Figure 2.27: An example using an off-delay timer
2.4.3Counters
An electromechanical counter is used to open or close contacts after a pre-set number of events have been reached. The operation principle of electromechanical counters is similar to delay timers except that the counting is for the number of events instead of the duration of time in timers. There are two main types of counters: up counters and down counters. An up counter increments its accumulated value by one when the event is occurred. Its accumulated value normally starts from zero, with increments by one for each event until it reaches the pre-set value to trigger the actuation of contacts. A down counter decrements its accumulated value by one when the event is occurred. The accumulated value of down counters normally starts from a pre-set value, decrements by one for each event until it reduces to zero to trigger the actuation of contacts. Figure 2.28 shows the graphic symbols of an up counter and a down counter as well as their contacts.
Figure 2.28: Counters
Figure 2.29: Output state
2.5 Output Devices
Output devices are also referred to as action devices. They are energized or de-energized based on the state of continuity of their control rung. The output device is energized when its control rung has a continuity; otherwise, the output device will be de-energized (Figure 2.29).
A wide variety of output devices can be used in electrical control systems. They include commonly used output devices such as motors, motor starters, pilot lights, solenoids, alarms, and contactors.
2.6 A Simple Electrical Control Circuit
The ladder diagram shown in Figure 2.30 is a circuit for controlling a motor for forward start, reverse start, and stop. The circuit consists of four rungs with three push button switches, two control relays, four contacts, and two motor starters. Their legends are as follows:
| SF: | forward start switch |
| SR: | reverse start switch |
| SP: | stop switch |
| MF: | forward start relay |
| MR: | reverse start relay |
| MF-1, MF-2: | contacts controlled by the forward start relay |
| MR-1, MR-2: | contacts controlled by the reverse start relay |
| F: | forward motor starter |
| R: | reverse motor starter |
Figure 2.30: Forward and reverse start of a motor
The circuit can be started in two modes: forward start and reverse start. Pressing the forward start switch (SF) provides the continuity in the first rung to energize the forward start relay (MF). The change of state in two contacts (MF-1 and MF-2) accomplishes the following:
1.Establishes a power holding circuit in the first rung to continuously energize the relay (MF).
2.Breaks any continuity in the second rung to ensure the reverse start relay (MR) is de-energized.
3.Energizes the forward motor starter (F) in the third rung to cause the motor to run in forward direction.
The forward rotation can be stopped in one of two ways. Press the stop switch (SP) to break the continuity of the first rung to cause the forward motor relay (MF) to be de-energized. The other way is to depress the reverse start switch (SR) to energize the reverse start relay (MR). This causes the forward start relay (MF) in the first rung to be de-energized, which in turn de-energizes the forward motor starter (F) in the third rung. At the same time, the reverse motor starter (R) is energized in the fourth rung to cause