Programmable logic controllers

A Programmable logic controller (PLC) is a type of industrial computer designed to control industrial equipment and processes such as motors, assembly lines and processing and handling machinery. Designed to be rugged to resist harsh conditions, PLCs are ideal for control applications in environments that experience high levels of dust or moisture, vibration, shock, and extreme temperatures.

Motors are widely used in industrial applications, and PLCs are an ideal solution to control them. They allow complex rules to be easily implemented: for example, when a start button is pressed, the motor should start only if sensors show that safety guards are present and there are no error conditions.

What is a PLC?

A PLC is an industrial controller based on a microprocessor. It has a programmable memory used to store programme instructions and various functions. A PLC collects data from sensors or other input devices, processes this data according to its set parameters and then generates appropriate outputs.

These outputs could be commands to start or stop a machine or set off alarms to warn that a process has strayed outside its limit values.

A PLC consists of five major components:

  • Processor unit – this interprets the inputs, executes the stored control programme and sends output signals
  • Power supply unit – converts AC voltage to DC
  • Memory unit – stores data received from the inputs as well as the programme executed by the processor
  • Input and output interface – used by the controller to receive and send data from and to the external devices
  • Communications interface – receives and transmits data on communication networks to and from remote PLCs

PLCs are easy to programme, even by people with little experience of programming languages. The most common method is a graphical programming language called ladder diagram. PLCs also offer high reliability and easy fault diagnosis for processes.

They replaced hardwired relay-based control systems, which were time consuming and difficult to design and troubleshoot. Despite the advent of more sophisticated control systems based on standard computer hardware, PLCs remain popular for their rugged construction and ease of use.

How does a PLC control a motor?

PLCs do not control motors directly. Rather, they provide an output signal to an intermediate device such as a relay or variable frequency drive (VFD), which commands the motor to switch on.

PLCs are in effect highly flexible industrial computers that can use a wide variety of input and output signals. These can take two forms, discrete or analogue signals. Discrete signals can take only an on or off value from devices such as limit switches, sensors and encoders.

Analogue signals can use voltage or current that is proportional to the variable being monitored and can take up any value within their scale. Input parameters using an analogue signal include pressure, flow rate and weight.

Discrete signals are the most suitable type for simple motor control. In most cases, operators will only need an on or off command to be outputted or to read the status of a sensor that determines if the motor is safe to operate.

Typical PLC motor control application

In a motor control application, the motor will be connected to a motor feeder which supplies the motor with power, typically a three-phase 415V supply. ladder diagram programming allows the PLC to be programmed to respond to inputs from switches and sensors. The PLC will therefore only provide the motor switch on signal if certain conditions are met.

A PLC will have several inputs including a starter button, a stop button and several interlock inputs – for example, motor vibration high, overload, motor temperature high and sensors on guard barriers. It will also have an input to indicate if the motor is under local control from the panel or remote control from the PLC.

Basic operation is to start the motor when the start button is pressed. If any of the interlock inputs are activated, the PLC programme will read this as a fault and prevent the motor from starting. The PLC must also stop the motor when the stop button is pressed or when the inputs from the interlocks go high, indicating a fault condition or safety issue.

PLCs may also use other inputs to control the motor during operation. Analogue signals such as motor speed, current and temperature can be used to ensure optimal performance of the task, while signals such as end stops will ensure the load being moved by the motor remains within a defined physical area.

PLC DC motor control

DC motors are most easily operated via relays. An electromechanical relay (EMR) is essentially a switch operated by an electromagnet. The relay turns a load circuit on or off by energising the electromagnet, which in turn opens or closes contacts connected in series with a load. Relays are generally used to control small loads of 15A or less.

A relay has two circuits, the coil input (also known as the control circuit) and the contact output (the load circuit). In motor circuits, EMRs are often employed to control the coils in motor contactors and starters.

A relay will usually have only one coil, but it could have many different contacts. EMRs have both stationary and moving contacts, with the moving contacts attached to the armature. Contacts are designated as normally open (NO) and normally closed (NC). When the coil is energised, an electromagnetic field is set up, causing the armature to move, closing the NO contacts and opening the NC contacts.

Coils are usually designated by a letter, with M used for a motor starter, while CR is used for control relays. Control relay contacts are small because they only need to handle the small currents used in control circuits, allowing them to contain numerous isolated contacts.

A similar device to an EMR is a contactor, with the main difference being the size and number of contacts. Contactors are intended for direct connection to high-current load devices. Devices switching more than 15A or in circuits rated at more than a few kilowatts are generally described as contactors.

How does a PLC control motor speed?

PLCs can be used to control the speed of AC motors via a Variable Speed Drive (VSD), also known as a variable frequency drive (VFD). In VFD motor control, the frequency of the AC supply to the motor is varied. As the speed of an induction motor depends on the supply frequency, the VFD can be used to vary its speed. They can also be used with synchronous motors.

A VFD is essentially a power converter that uses electronics to convert a fixed frequency and fixed voltage into a variable frequency and variable voltage. They will usually have a programmable user interface that allows easy monitoring of the speed of the electric motor.

As drives reduce the output of an application, such as a pump or a fan, by controlling the speed of the motor, this can often cut energy consumption by 50% and by as much as 90% in extreme cases.

While the VFD controls the speed of the motor, the VFD itself can be controlled remotely using a PLC. To achieve this, the PLC needs to provide the VFD with a set point for the motor speed. This can come either automatically from the PLC or can be set by the operator using the human-machine interface (HMI). In this arrangement, the PLC controls the drive speed through the set point and the VFD controls the motor speed by adjusting its frequency to achieve the set point.

A typical VFD application for controlling motor speed might take advantage of the PID control features that some PLCs offer. As the name suggests, a PID controller consists of main three coefficients – proportional, integral and derivative. As part of a closed loop control system, the PLC will use the PID function to assess the speed of the motor and generate an appropriate output. This will be sent to the VFD so it can command the motor to slow down or speed up to achieve the required set point.

Benefits of PLCs in motor drive applications

PLCs offer several benefits to motor drive applications. As motors in industrial settings are often found in dusty or damp atmospheres or where a high degree of mechanical vibration is present, control equipment needs to be rugged and robust – and PLCs provide this.

Good industrial PLCs are not usually affected by the electrical noise common in most industrial locations. As they have very few moving parts, the chances of defects and damage are also very much reduced. They are also compact and easy to site in many places where motor control might be needed.

Programming a PLC is also easy as they are programmed in relay ladder logic or other easily learned languages. Using a programming language built into its memory and with terminals for input and output field devices and communication ports, existing programmes can be modified easily at any time. This makes it much easier for engineers with little programming experience to write or adapt programmes to manage their motor applications.

Despite their simplicity, PLCs are based on solid state microprocessor technology and as such represent a great advance over purely electromechanical forms of motor control such as relays. Relays have the major drawback of needing to be hardwired to perform a specific function. This means that when the motor system requirements change, the relay wiring must also be changed or modified. With their programmability functions, PLCs have eliminated much of the hardwiring required with conventional relay-based motor control circuits.

Once a programme has been written and tested, it is easy to download it to other PLCs operating similar motor applications. As all the logic is contained in the PLC’s memory, the chance of logic wiring errors is eliminated.

When errors do occur, PLC based applications are easy to troubleshoot thanks to the PLC’s onboard diagnostics and override functions, which allow users to easily trace and correct problems in both software and hardware. As an example, users can display the control programme on a monitor and watch it in real-time as it executes. Troubleshooting is also simplified by fault indicators and messaging displayed on the programmer’s screen.

Due to their programmability, PLCs also offer more flexibility. With a PLC, relationships between the inputs and outputs are governed by the user programme rather than how they are interconnected. Original equipment manufacturers can update the system sending a new programme while end-users can modify the programme in the field. Users can also add extra function modules and sensors to enhance the flexibility and performance of a PLC based motor control system. For example, a temperature sensor could indicate that a process needs to be cooled or heated, and the PLC can switch the motor on to drive a cooling fan or transfer convection heat from a heat source.

PLCs also have sophisticated communication abilities and can communicate with other controllers or computer equipment. This allows them to perform functions such as supervisory control, gather data, monitor devices, and process parameters, and download and upload programmes. PLCs can also use a variety of standard communication protocols.

With all these advantages, PLCs are the ideal way to control motor applications. Rugged, robust, and easy to programme, offering high reliability and flexibility, PLCs bring advanced industrial motor control to facilities without the need to adopt a full automation system. Offering standalone capacity but with the flexibility to communicate with other units, as well as the ability to take in data from a wide range of sensors, PLCs can suit a wide range of motor control applications.


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