Fluid Power System Electrical Control Chapter 13 Fluid Power System Electrical Control Electrical Quantities • Basic Electrical Circuits • Electrical Control Circuits • Electrical Diagrams and Applications

Direct current flows in one direction only, while alternating current reverses its direction of flow at regular intervals. Current flows through an electrical circuit when a power source is connected to a component that uses electricity. Current (I) is the amount of electrons flowing through an electrical circuit. Direct current (DC) is current that flows in one direction only. Alternating current (AC) is current that reverses its direction of flow at regular intervals. See Figure 13-1.

The voltage needed to move electrons in an electrical circuit is comparable to the fluid pressure needed to cause hydraulic fluid to flow through a pipe in a hydraulic system. Voltage (V) is the electrical pressure, or electromotive force, that causes electrons to move in an electrical circuit. Voltage is measured in volts. A difference in electrical polarity is required for electricity to flow in a circuit. Polarity is the positive (+) or negative (–) state of an object. The voltage needed to move electrons in an electrical circuit is comparable to the fluid pressure needed to cause hydraulic fluid to flow through a pipe in a hydraulic system. See Figure 13-2.

The higher the resistance, the lower the current, and the lower the resistance, the higher the current. Resistance (R) is the opposition to the flow of electrons. Resistance is measured in ohms (Ω). A resistance of 100 ohms is written as R = 100 Ω. Resistance limits the flow of current in an electrical circuit. The higher the resistance, the lower the current, and the lower the resistance, the higher the current. See Figure 13-3.

Insulation commonly used in fluid power systems are plastic coverings over electrical wires, cables, and other conductors. If a material has a high amount of resistance, it is considered an insulator. An insulator is a material that has a high resistance and resists the flow of electrons. Insulation commonly used to cover fluid power electrical wiring is typically made of rubber, vinyl, or plastic. See Figure 13-4.

Electrical devices used in fluid power systems are typically connected in parallel so that they can have the same amount of voltage. Electrical devices (loads) used in fluid power systems are typically connected in parallel so that they can receive the same amount of voltage. However, they are connected in parallel when they can be controlled by the same formation of switches. For example, if three cylinders with three separate directional control valves need to be extended at the same time, their solenoids are connected in parallel. When the switches are closed, current flows to all three solenoids and actuates them. See Figure 13-5.

A series circuit has two or more components connected in a manner so that there is only one path for current to flow. A series circuit is an electrical circuit with two or more switches connected so that there is only one path for current to flow. When switches are connected in series, the current enters the first switch and then flows to the next switch if the first switch is closed. Once the current has flowed through all of the switches connected in series, it reaches the load. When two or more switches are connected in series, all of the switches must be closed for current to flow to the load. See Figure 13-6.

A series-parallel circuit has at least one load connected in series with two or more loads connected in parallel. A series-parallel circuit is a circuit with a combination of series- and parallel-connected components. A series-parallel circuit has at least one switch connected in series and two or more switches connected in parallel. A series-parallel circuit can have many combinations, which are always created with at least three switches or relay contacts. A series-parallel circuit is typically used in industrial applications that use a lamp to indicate when a solenoid is on. See Figure 13-7.

A holding circuit allows a load to remain on after the switch that controls it is released. Series-parallel circuits are common in fluid power systems because they offer a wide range of options for control. The most common type of series-parallel circuit is a holding circuit. A holding circuit is an elec- trical circuit that allows a load to remain on after the switch that controls it is released. See Figure 13-8. A holding circuit consists of an NC momentary pushbutton connected in series with an NO momentary pushbutton that is in parallel with a set of NO relay contacts.

An electrical control circuit is an electrical circuit that determines when the output component is energized or de-energized, while a power circuit accomplishes work in a fluid power system. Specific combinations of electrical components are used in electrical control circuits to operate a fluid power system. An electrical control circuit is an electrical circuit that determines when the output component is energized or de-energized. A power circuit is a circuit controlled by the electrical control circuit to accomplish work in a fluid power system. See Figure 13-9.

All electrical circuits are comprised of an electrical power source, conductors, load, and a control switch or a relay. All electrical control circuits are comprised of an electrical power source, conductors, and a load. Without these three basic components, an electrical control circuit cannot operate. Along with these three basic components, most electrical control circuits also have a control switch. See Figure 13-10. The components that comprise a common electrical control circuit come in many different forms and accomplish different types of work.

When using the AWG identification system, the larger the number, the smaller the diameter of the wire. Wire conductor size is indicated by a wire gauge number and is standardized by the American wire gauge (AWG) identification system. The American wire gauge (AWG) is a standardized wire sizing system used in the United States for the diameters of round, solid, nonferrous, electrically conducting wire. The diameter of the cross-sectional area of a wire is the factor used for determining its capacity for carrying current. When using AWG, the larger the number, the smaller the diameter of the wire. See Figure 13-11. Common AWG sizes used with electrical control circuits are 14 AWG, 16 AWG and 18 AWG. Size 14 AWG is the largest, while size 18 AWG is the smallest.

When a switch is actuated, current flows through conductors and actuates a load (solenoid). A load is an electrical device with a specific amount of resistance that allows current to flow through it to accomplish work. The load that is typically used for the electrical control of a fluid power system is a solenoid. A solenoid controls the spool in a directional control valve. For example, when a pushbutton is actuated, current flows through the conductor to the solenoid. The solenoid uses the current to create a magnetic field, which moves the directional control valve spool. See Figure 13-12.

There are many types of switches used for the electric control of fluid power systems. Switches are available in many shapes and with manual or automatic operation. Once actuated, a switch changes the position of contacts. The contacts are used to start and stop the flow of current in an electrical circuit. See Figure 13-13. The type of switch that is used for the electrical control of a fluid power system depends on the application. For example, a pushbutton is a switch that is used to extend the rod of a cylinder. When the rod is completely extended, it actuates a limit switch, which shifts a directional control valve and causes the rod to retract.

A general-purpose relay is a mechanical switch operated by a magnetic coil. A general-purpose relay is an electrical switch operated by a magnetic coil. General-purpose relays typically include two, three, or four sets of non- replaceable NO and NC contacts. See Figure 13-14. The contacts are normally rated at 5 A to 15 A. Special attention must be given to the contact current rating when using general-purpose relays because the rating for switching DC is lower than the rating for switching AC. For example, a 15 A AC contact is normally rated for only 8 A to 10 A DC.

The difference between a PLC programming diagram and a ladder diagram is that the inputs and outputs have the same symbol in a PLC programming diagram. The difference between a PLC programming diagram and a ladder diagram is that a ladder diagram uses different symbols for each switch, while a programming diagram uses almost the same symbol for all inputs and almost the same symbol for all outputs. See Figure 13-15. Each input and output terminal on a PLC is labeled with a number and connected to a device with the same number. That number is used to identify the device in the program.

The electrical control of a single-acting cylinder is usually accomplished with a two-position, four-way, spring-offset, solenoid-actuated directional control valve. The electrical control of a single-acting cylinder is usually accomplished with a two-position, three-way, spring-offset, solenoid-actuated directional control valve for a pneumatic system and a two-position, four-way, spring-offset, solenoid-actuated directional con- trol valve with actuator port B plugged for a hydraulic system. Electrical control is typically used for single-acting cylinders in hydraulic systems, such as those operating industrial cardboard compactors. The single-acting cylinder in an industrial cardboard compactor has a crush plate attached to its rod that extends into a hopper when actuated. See Figure 13-16.

The control of the single-acting cylinder in the industrial cardboard compactor can be improved by connecting an NC pressure switch in series with the pushbutton and solenoid. The control of the single-acting cylinder in the industrial cardboard compactor can be improved by connecting an NC pressure switch in series with the pushbutton and solenoid. The NC pressure switch senses pressure at the inlet of the cylinder. If the pressure reaches a point that is too high because there is either too much material or incompressible material in the hopper, the cylinder cannot extend any further and retracts. See Figure 13-17.

When the rod has fully extended, an NC limit switch has the same effect on the electrical control circuit as an emergency stop pushbutton. To retract the rod when it is fully extended, an NC limit switch is installed. When the rod has fully extended, the NC limit switch has the same effect on the electrical control circuit as an emergency stop pushbutton. See Figure 13-18. An industrial cardboard compactor with a start-stop station and an NC limit switch operates through the following procedure: 1. When the start pushbutton is actuated, current flows to the solenoid and the control relay. 2. When current flows through the solenoid and control relay, magnetic fields are created around both. …Complete procedural list on page 416.

The electrical control of a double-acting pneumatic cylinder is commonly performed a two-position, four-way, five ported, spring-centered, NC, solenoid-actuated directional control valve. The control of a double-acting pneumatic cylinder is commonly performed by a two-position, four-way, five-ported, springcentered, NC, solenoid-actuated directional control valve. This often requires electrical control with separate pushbuttons to allow the directional control valve to control the extension and retraction of the rod. See Figure 13-19.

In an interlocking circuit, one branch can be energized at a time with a limit switch that has both NO and NC contacts. An interlocking circuit is a circuit in which one branch can be energized at a time with a limit switch that has both NO and NC contacts. The NC switching capability can be used to disengage a load that must be turned off before another load can be turned on. See Figure 13-20.

The electrical control of a double-acting cylinder can be used in a thermoforming press that requires a specific amount of force and heat to be applied to a plastic workpiece for a specific amount of time. Electrical control of a double-acting cylinder can also be used in a pneumatic thermoforming press. The pneumatic thermoforming press requires a specific amount of force and heat to be applied to a plastic workpiece for six seconds. The operator actuates an NO momentary pushbutton and the rod extends. Upon reaching the workpiece, the rod forms it and holds it in position for six seconds while heat is applied. After six seconds, the rod retracts. An on-delay timer properly controls the time needed for heating and forming the workpiece. See Figure 13-21.

In a metal shearing press, a holding circuit must be used to extend the rod and place the directional control valve spool into position. In a metal shearing press, the electrical control circuit must use a holding circuit to extend the rod and lock the directional control valve spool into position. An NC limit switch is used to disengage the relay and allow the spring to return the spool to its normal position, retracting the rod. See Figure 13-22. A metal shearing press operates through the following procedure: 1. An NO pushbutton is actuated and current flows to the solenoid and control relay. …Complete procedural list on page 419.

Electrical control circuits can sequence two cylinders without the use of a sequencing valve. The sequencing of clamping and pressing cylinders can be performed electrically with two two-position, four-way, spring-offset, NO, solenoid-actuated, directional control valves, three control relays, a limit switch, a proximity sensor, a magnetic reed switch to sense the position of the cylinder piston, and a control circuit that allows the proper logic of the hydraulic system to be followed. See Figure 13-23. A sequenced hydraulic press operates through the following procedure: 1. When the workpiece is in place, a proximity sensor activates solenoid 1, and control relay 1 locks solenoid 1 into place. 2. The clamping cylinder rod extends, clamping the workpiece and activating the magnetic reed switch. …Complete procedural list on page 420.

Three double-acting cylinders connected in series can operate in sequence. A master start-stop station is used to start the operation or stop it in an emergency. There are usually other safety features as well that do not allow the operation to start unless all three rods are fully retracted. For example, the metal bending machine will not start until the magnetic reed switches sense the correct position of the rods, the proximity sensor activates, and a second start pushbutton is actuated. See Figure 13-24.