Introduction to Variable Frequency Drive (VFD)

In the previous chapter, we learned about the various types of electric motors. Among the various types, the asynchronous (induction) motor is the most commonly used motor in the industry. However, machines in the industry, such as conveyors, mixers, agitators, pumps, compressors, and others, that utilize an induction motor require a variable speed, which an induction motor alone cannot provide because induction motors are designed to run at a constant speed (that is, their speed remains almost the same for various load range).

A variable speed drive is an electronic device used to achieve the varying speed needed by either an asynchronous or synchronous motor for various applications.

This chapter looks at a Variable Frequency Drive (VFD), its working principle, benefit, wiring, and programming. We will also look at some practical applications, including forward, reverse, and speed control of an induction motor.

In this chapter...

An overview of a VFD

A VFD is a device used to vary the speed of Asynchronous or Synchronous (AC) electric motors by varying the frequency and voltage supplied. A VFD can also be referred to as an Adjustable Speed Drive (ASD) or inverter. It is the most effective way of controlling the speed of an induction motor. The principle of operation of a VFD is based on the fact that the speed of an induction motor is directly proportional to the frequency of the voltage supplied to the motor. When frequency increases, speed also increases, and when frequency reduces, the speed will also reduce. A synchronous motor runs at synchronous speed (the speed of the rotating magnetic field of the stator) while an asynchronous (induction) motor’s speed is slightly less than synchronous speed. In the previous chapter, it was mentioned that synchronous speed is determined by the frequency of the voltage source and the number of poles, as shown by the following formula:

Synchronous speed = 120f...

8 Understanding the Programmable Logic Controller – Part 2

In the previous chapter, we learned about the hardware part of the Programmable Logic Controller (PLC), which includes wiring the PLC to sensors and actuators. A PLC wired to switches, sensors, and actuators will do nothing unless there is a program written into it. A program (a set of instructions written using a programming language) is required for the CPU of the PLC to make decisions based on the inputs and carry out the required control function. It is important for an industrial automation engineer to have knowledge of both the hardware part of the PLC, including the wiring, and the software part, which includes programming.

This chapter focuses on the software aspect of the PLC. You will learn the basic things you need to start programming PLCs with the most common PLC programming language, Ladder Diagram (LD). You will learn how to download, install, and use Totally Integrated Automation Portal (TIA Portal), powerful...

Technical requirements

While every part of this book is valuable, Chapter 2, Switches and Sensors – Working Principles, Applications, and Wiring, Chapter 3, Actuators and Their Applications in Industrial Automation, and Chapter 7, Understanding the Programmable Logic Controller – Part 1, are very important to have a better understanding of this chapter.

Understanding software/program in PLCs

Before looking into PLC programming languages, which this chapter is all about, let’s get some knowledge of the software/program in PLCs.

In computing, generally, software is a set of instructions written in a language the computer understands to perform a task. Software can also be reffered to as program.

A PLC requires two types of software that must be executed by the CPU for the PLC to function. They are as follows:

• The operating system (firmware)
• User program

The operating system is the program written by the PLC manufacturers and designed to run automatically in the PLC once powered. It is responsible for executing the user program, establishing communication between devices, memory management, and updating the output.

A PLC user program is a set of instructions in either textual or graphical form that represents the control function that will be carried out for a specific industrial task or application. It is the program that the...

Introducing PLC programming languages

According to International Electrotechnical Commission (IEC) standards, there are five programming languages, stated in part 3 of IEC 61131, for writing a program for PLCs to control and automate a task. These programming languages are the following:

• Ladder Diagram (LD)
• Function Block Diagram (FBD)
• Sequential Function Chart (SFC)
• Instruction List (IL)
• Structured Text (ST)

IEC 61131-3 can be purchased at their web store at the following link. However, there is an abstract you can read and also a preview that can be downloaded for free:

https://webstore.iec.ch/publication/4552

Note

IEC is an international standards organization responsible for preparing and publishing international standards for all electrical, electronics, and other related technologies.

Let’s discuss each of these programming languages in detail.

Ladder Diagram (LD)

This is most common among several PLC programming languages. It is a graphical programming language that...

Introducing PLC programming devices

These are devices or tools used to write, edit, and dump programs to the PLC. They can also help in troubleshooting PLCs. There are two common programming devices:

• Handheld devices
• Personal Computers (PCs)

Handheld devices are usually connected to the PLC via a cable. They consist of keys for entering a program, editing a program, and dumping or delivering a program to the PLC. They also consist of a small display for viewing instructions. They are small in size and have limited capability.

A PC is the most popular programming device. A programming software from the PLC manufacturer can be installed on a PC for writing a program, editing a program, and dumping or delivering a program to the PLC. A (communication) cable is also required between the PC and PLC. We will focus on using a PC as a programming device in this book.

Let’s proceed to learn about PLC programming software.

Looking at the different PLC programming software

In Chapter 7...

Understanding the basics of LD

LD is an easy-to-learn programing language for creating control programs in PLCs to automate or control machines in industries.

In Chapter 1, Introduction to Industrial Automation, it was mentioned that prior to the introduction of PLCs, there was automation but through the use of relays, timers, contactors, and so on. These were hardwired together to perform the required control or automation task. Relay logic circuits were used when designing such control and were also used for troubleshooting the system.

A relay logic circuit is an electrical circuit or drawing showing symbols of components, such as relays, switches, timers, and contactors, and their connections for the desired control or automation task. The circuit usually consists of two vertical lines (rails), one on the extreme left (supply voltage potential, live or positive) and the other on the right (zero potential, neutral or negative). One or several horizontal lines in a relay logic circuit...

Downloading and installing TIA Portal V13 Professional and PLCSIM

Siemens offers a 21-day free trial of their programming software (TIA Portal). There is also a simulator that allows you to simulate your program. Hence, you can write a program and see how it runs even when you don’t have a live PLC by using the simulator (PLCSIM). You can download both TIA Portal V13 Professional and PLCSIM free online and use it to learn PLC programming.

Downloading TIA Portal V13 and PLCSIM

The following steps should be followed to download TIA Portal V13 and PLCSIM:

1. The first step is to visit the download page. The following link should take you to the download page:

   https://support.industry.siemens.com/cs/document/109745155/simatic-step-7-including-plcsim-v13-sp2-trial-download?dti=0&lc=en-WW

   The following screenshot shows the page of the preceding link:

   

   Figure 8.18 – Download page of Siemens TIA Portal V13 Professional and PLCSIM (1)

2. Scroll to the STEP 7 Professional V13 SP2...