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How PLCs Work With Solenoid Valves

Solenoid valves are an essential part of almost every fluid control system. These electromechanical valves automatically control the flow of liquids and gases. The solenoid valve may work with different circuits to achieve the desired control, ensuring control precision and versatility. It also eliminates the hassle for an engineer to operate the valve physically. That saves time and money while ensuring the safety of the personnel in some hazardous situations. 

There are numerous types of solenoid valves available, allowing the valve to be selected to fit the application in the issue. An increasing number of facilities are utilizing the solenoid valve.  

For precise and automatic control of these valves, controllers such as PLCs and Arduino are used. This article explains the use of a PLC to control a solenoid valve. 

What is a solenoid valve? How does a solenoid valve work? 

Figure 1: Typical Solenoid Valve

Electromagnetic solenoid valves, also known as solenoid valves, are current-controlled valves. A solenoid valve has two major components: a solenoid and a valve body (G). The standard components of a solenoid valve are shown below in Figure 1. The plunger (E) is an electromagnetically inductive coil (A) centered around an iron core. It will be either normally open (NO) or normally closed (NC) at rest. A normally open valve remains open throughout the de-energized phase. On the other hand, a normally closed valve remains closed during the de-energized phase.  

When electricity travels through the solenoid coil, it activates and generates a magnetic field that attracts the plunger. As a result, the plunger moves by defying the spring (D) force. The plunger (E) of a normally closed solenoid valve is pulled up (lifted), and the seal (F) opens the orifice that allows fluid to flow through the valve. In the case of a typical open solenoid valve, the plunger lowers, and the seal (F) closes the orifice, preventing media from passing through the valve. In AC coils, the shade ring, represented by (C) in Figure 1, eliminates vibration and humming. 

Solenoid valves are categorized into three types based on the number of pipe connections:

  • Two-way solenoid valve: It features one piping connection for the inlet and one for the outlet.
  • Three-way solenoid valves: It comes with two orifices and three pipe connections. When one orifice opens, the other closes, and vice versa.
  • Four-way solenoid valves: Solenoid valves of this sort contain four or five pipe connections: one pressure, two cylindrical, and one or two exhausts.

Solenoid valves can be either alternating current (AC) or direct current (DC). 12/24 volts DC solenoid valves are common in industrial automation. AC solenoid valves (120 volts AC /240 volts AC / 24 volts AC) are usually used only when higher power or the voltage drop (when the valve is hundreds of feet away from the controller) is a major factor. An AC valve is also preferred in situations where simplicity is required.

DC power is supplied to the valve by a battery, generator, or rectifier. While an alternating current (AC) is supplied to the valve from the mains voltage via a transformer.

Individual or banked solenoid valves are available. Direct action valves require more current (considerably larger solenoids), whereas piloted valves, which draw relatively little current, are far more common in factory automation. The direct drive solenoid requires 2 amperes to 4 amperes, and the piloted valve needs 10 milliamperes to 20 milliamperes.

What is a PLC? How does a PLC work? 

Figure 2: Programmable Logic Controller (PLC) Internal Configuration

A programmable logic controller (PLC) or programmable controller is a digital computer. This industrial computer control system (PLC) continually monitors the state of input devices and takes actions to control the state of output devices based on a custom program. It is used for the automation of electromechanical operations such as equipment control on industrial assembly lines, amusement attractions, or lighting fixtures. PLCs are utilized in a wide range of industrial applications and machines.

Unlike general-purpose computers, these programmable controllers are designed specifically for numerous input and output configurations, wide temperature ranges, electrical noise immunity, and vibration and impact tolerance. Programs that control machine functions are often stored in non-volatile or battery-backed memory. In these controllers, output results must be provided in response to input circumstances within a specified time. That’s why a PLC-based system is a representation of the real-time system.

The PLC comes with a central processing unit (CPU), memory locations, and suitable circuitry for sending or receiving data. As seen in Figure 2, PLC circuitry is subdivided into three major sections separated by isolation boundaries. Electrical isolation ensures that a malfunction in one region does not reflect changes in another region. Isolation boundaries shield the operator interface and the operator from faults such as power input faults, field wiring faults, and so on.

A typical PLC is made up of the following four distinct yet interconnected components. 

  • An input/output segment: It connects the PLC to the input and output devices. Input devices are usually switches, pushbuttons, limit switches, photoelectric sensors, encoders, and so on. Output devices are valves, motor starters, control relays, pumps, fans, solenoids, etc. 
  • A microprocessor-based central processing unit (CPU). 
  • A programming device: It can be a handheld programming console, a specific PLC desk-type programmer, a laptop computer, or a PC. 
  • A power source: It is usually 24 volts direct current power supply to power input sensors and output signals to the fluid power valves’ bulbs, motors, heaters, and solenoids. 

The power supply feeds power to all modules in the PLC unit. However, it does not deliver DC voltages to the PLC’s peripheral input /output (I/O) devices, which require high currents and voltages. 

How to Control a Solenoid Valve with a PLC? 

Figure 3: Wiring Diagram of PLC with Solenoid

As mentioned above, the solenoids are operatable with +12 volts DC, +48 volts DC, 110 volts AC, 120 volts AC, 230 volts AC, etc. The electrical connections and power supply units are adjusted accordingly.

However, this section will focus on the operation of 24 volts DC solenoid valve with PLC. The digital output module of the PLC is connected to the coil end of the valve in order for the PLC to control the solenoid valve. When the relevant point of the module has a voltage output, the solenoid valve coil is turned on, and the contacts are attracted. When there is no voltage output, the coil is turned off, and the contacts separate.

The wiring diagrams, as shown below in Figure 3, explain how to connect a PLC to a solenoid valve. The output modules serve as switches, but they rarely provide power. The output card is attached to an external power supply, and the card will turn the power off or on for each output channel.

As mentioned above, the solenoid valve is connected to the PLC’s digital output module. The solenoid is connected to the junction box in the field through a branch cable. The main cable connects the junction box in the control room to the marshaling cabinet. Then, it’s connected from the marshaling cabinet to the system cabinet’s corresponding digital output (DO) card channel.

Low-rated currents valves and their input /output modules can be directly powered from a PLC supply. However, some solenoids demand substantial currents to function. As a result, the PLC power supply, which is needed to power up PLC cards, cannot be used directly. A separate power source is available in the marshaling cabinets that are used to fix this problem. These are typically small positive and negative bus bar units with high-rated currents that can produce +24 volts of direct current.

Using intermediary relays, solenoid power will be drawn from this bus bar. The solenoid is powered by these bus bars (+24 VDC), while the relay is controlled by the digital output cards through relays. These relays are used to isolate PLC and field signals as well as to provide safety and amplify power or voltage signals.

When the Digital Output (DO) card receives a command from the PLC. The relay is energized by the DO card. The relay will be turned on. This relay’s NO contact is likewise looped with the +24 volts DC high-rated current. This high-rated power will be sent to the field via the junction box when the relay is energized. The solenoid will be switched on.

The DO (digital output) card de-energizes this relay when the PLC removes the command. The relay then disconnects the high-rated power from the solenoid, de-energizing it.

These DO cards typically contain eight to sixteen outputs of the same sort, each with a different current rating. Relays, transistors, and triacs are frequently used as DO (digital output) cards. The most versatile output devices are relays. Each digital output has its own dedicated relay. This enables AC and DC mixed voltages, as well as isolated outputs to safeguard other outputs and the PLC. However, they have a longer switching time. Response times are frequently larger than 10 milliseconds. Dry contacts are a term used to describe the outputs of relays. This approach is the least susceptible to voltage spikes and swings.

Transistors can only provide DC outputs, while triacs can only produce AC outputs. Switched outputs refer to the outputs of transistors and triacs. Triacs are ideal for AC devices that require less than 1 amperes. Transistor outputs typically use NPN or PNP transistors with current ratings of up to 1 ampere. Their response time is less than 1 millisecond.

Remarks: 

There are several ways to attach a solenoid valve. But first, double-check the technical specifications or determine the current required to activate the solenoid valve. You’ll know whether your digital output card can handle the valve or not. Each solenoid coil has its rated voltage. Lower voltages than recommended can cause it to fail to operate, while higher voltages shorten its life. Also, each solenoid can handle a specific amount of current. If the current exceeds the maximum current, then contacts and coil will be damaged. 

When choosing a relay or relay outputs on a PLC, the rated current is the most significant factor to consider, followed by voltage. The contacts will prematurely wear out if the rated voltage is exceeded. A fire might occur if the rated voltage is too high. Where to connect the wires is specified in the technical specification of your PLC card. When the current is supplied by the PLC, make sure not to overload it if you have multiple solenoids on that card.  

This entry was posted on June 22nd, 2022 and is filed under Education, Electrical, PLC, Uncategorized. Both comments and pings are currently closed.

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