How a VFD works and when to use it

Did you know you could save on energy consumption and costs by using a VFD? You can also tighten your processes, increase production, reduce maintenance, and extend the life of your equipment.

But what is a VFD, you ask? Let’s take a look.

Defining VFD

VFD stands for variable frequency drive. It’s a motor controller for electric motors. VFDs are also known as adjustable speed drives, adjustable frequency drives, AC drives, microdrives, inverters, and variable speed drives.

The word “frequency” in the name relates to the frequency of the power delivered to the motor, which is measured in hertz. Changing the frequency changes the speed of the motor shaft.  If your electric motor doesn’t need to run at full speed for the entire process, you can save some juice and some wear and tear by installing a VFD to vary the speed of the motor.

A variable frequency drive can also get a motor started and ramp it up to speed at a controlled acceleration rate. This makes the start-up smooth, while also saving on electricity and motor life.

A VFD allows one motor to be used for processes that may require or allow different speeds.

Variable Speed Drive

How Does a VFD Operate?

A VFD converts fixed frequency AC line voltage to DC, then makes new AC at whatever voltage and frequency are needed to run the motor at the desired speed. The VFD consists of a converter section, a filter section, an inverter section and control section. 

• The Control section operates the entire VFD, monitors the VFD and motor for safe operation, and interact with the machine operator or automation control system. 
• The Converter uses diodes and/or SCRs to change AC utility power to DC
• The Filter ''Cleans'' the DC power with inductors and capacitors
• The Inverter makes new AC power for the motor using transistors as switches

Those switches are what allow the VFD to function at different speeds. The transistorized switches let the VFD adjust the frequency and voltage of the power supplied to the motor. As the frequency changes, so does the motor speed.

What’s It For?

Anytime you have a system run by an AC electric motor, you may have a need for a VFD. For example, a common use is controlling the speed of a water pump. If the pump is part of a water treatment process, a low demand for water can mean that the water doesn’t exit the plant at the same speed it enters for treatment.

To slow down the supply-side, a VFD is used to slow the water pump.

As mentioned before, a VFD can be used to get a motor started and smoothly accelerate it to operating speed. Energy usage is reduced if operating speed is below full speed.  There is less strain on the motor, less wear and tear on the machinery, and it doesn't just start with a jolt. Using VFDs on conveyors and belts eliminates those jerky starts and increases throughput without damaging equipment.

VFDs can be regulated with a PLC instead of manual adjustment. It’s an easy way to automate a repetitive task and reduce labor cost.

A VFD is a handy little gadget that can help you tighten your process controls, increase production, and minimize mistakes. Your maintenance and repair needs go down, and so does your electricity bill. At the end of the shift, your company has made a little more money than it did before.
You can also read: 

• Types of Signal Converters used in Instrumentation Control systems
• The Basics of Industrial Instrumentation Systems

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About the Author:

With over 25 years of experience in the industrial automation repair industry, Jeff Conner is the Dallas Service Manager for Control Concepts and serves on the Advisory Committee for the Electronics Technologies Department at Texas State Technical College.

Control Concepts helps design, fabricate, install, test, and program control systems. They service almost any brand of control found in automated systems and can send an experienced technician anywhere, wherever one is needed 24 hours a day, 7 days a week.

Types of Signal Converters Commonly used in Industrial Control

Signal converters change a signal from one form to another. In most cases, we have standard inputs and output ranges.

Most signal converters have two adjustments-zero and range.

Examples of Signal converters

Nozzle-flapper and differential pressure cells

The nozzle-flapper system is widely used in D.P. Cells. An example is shown below, that converts differential pressure (e.g. from a differential Pressure flow meter into a standard pneumatic signal). This is widely used in the control of air operated pipeline valves.

The bellows respond to the differential pressure and moves the lever. This moves the flapper towards or a way from the nozzle. The air supply passes through a restrictor and leaks out of the nozzle. The output pressure hence depends on how close the flapper is to the end of the nozzle. The range of the instrument is adjusted by moving the pivot and zero position is adjusted by moving the relative position of the flapper and nozzle. This system is used in a variety of forms. Instead of bellows, a bourdon tube might be used and this is operated by an expansion type temperature sensor to produce a temperature-pneumatic signal converter.

You can also read: Types of Proximity Sensors used in Industrial Control

Current/Pressure conversion 

The figures below show typical units for converting 4-20 mA into 0.2-1 bar or 3-15 psi. They contain adjustments for range and zero. They are widely used for converting the standard pneumatic and electric signals back and forth. They can also be adjusted to work with non standard inputs to convert them into a standard form.

Current to Pressure Converter

Pressure to Current Converter

Electric D.P. Cells

They provide the same functions as the pneumatic versions but they are given an output of 4-20 mA using electrical pressure transducers. They are typically used with D.P. Flow meters.

Differential Pressure Transmitter

Analogue -Digital Converters

Analogue to digital conversion is a process of turning an analogue voltage or current into a digital pattern which can read by a computer and processed, this is done by analogue -digital converters.  Lets look at the Binary Numbers, a number may be represented in digital form by simply setting a pattern of voltages on a line high or low. It is normal to use 4, 8, 16 or 32 lines. An 8 bit binary pattern is shown in the below figure:

The total pattern is called a word and the one above is an 8 bit word. The pattern may be stored in an 8 bit register. A register is a temporary store where the word may be manipulated. The Bit zero is called the least significant bit (LSB) and the bit with highest value is called the most significant bit (MSB). Each bit has a value of zero when off (LOW) or the value shown when on (HIGH). The maximum value for an 8 bit word is 255.

Digital to Analogue Converters

These are devices for converting a binary number into an analogue voltage. The change in the binary value from zero to a maximum corresponds with a change in the analogue value from 0 to maximum.

One of the manufacturers of signal converters is Omega.

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Programmable Logic Controllers (PLC)

Programmable logic controllers (PLC) play an important role in automation sector. Various industries like Food & Beverage, Chemical, Petrochemical, Power generation etc.  use PLC.

We have several types of PLC designs:

Compact PLC: This is built by several modules within a single case. The I/O capabilities are decided by the manufacturer and not the user.

Modular PLC: This is built with several components that are plugged into a common rack or bus with extended I/O capabilities. It contains power supply module, CPU and other I/O modules that are plugged together in the same rack, which are from the same manufacturers or from different manufacturers.

Soft PLC: This is an advanced PLC system that consists of compact, rack mounted components such as power supplies, I/O modules and a CPU which embeds a powerful PLC Control software.

Programming Languages of PLC

There are several programming languages used to write programs in a PLC. They include but not limited:

• Ladder Diagram
• Instruction List
• Functional Block Diagram
• Sequential Function Chart
• Structured Text

So what are some of the components that make up a Programmable Logic Controllers?

PLC  System

Functions of each component:

CPU – This the unit that contains microprocessors

Input and Output Sections – This is where the processor receives information from external devices and communicates information to external devices.

Power Supply Unit– It converts the Main AC voltage to low DC voltage.

Programming device – Used to enter the required program into the memory of the processor.

Memory Unit – This is where the program is stored that is used to control actions.

The Operation of a PLC

Check the input status: First the PLC takes a look at each I/O to determine if it is on or off.

Execute Program: Next the PLC executes the program one instruction at a time. 

Update output status: Finally the PLC updates the outputs. It updates the outputs based on which inputs were on during the first step. 

 

The Working of a PLC system

Advantages of PLC:

• More flexibility
• Lower cost
• Increased reliability
• Faster response
• Easier to troubleshoot
• Communication capability
• Remote control capability

Disadvantages:

• They can render some jobs redundant
• They have a high initial cost
• If a Programmable logic controller stops, then the production stops 

Industrial Applications of PLCs

• Food and Beverage industry
• Gas and Water Filling Stations
• Power Sector
• Bottling Plants

You can also read: 

• How to Integrate PLC into your Control System
• The Transition from Relays to PLC systems

Some of the Top PLC Brands in the world include:

• General Electric
• Toshiba
• Siemens
• Allen Bradley
• Omron

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The Basics of a Process Control System

Instrumentation is a science that deals with measurements and control of processes. Instrumentation is applied in almost any field from Medical, Manufacturing of pharmaceutical drugs, to simple processes like temperature control in homes using Thermostats. Note that if we can’t measure it then we have no need of controlling it.

Measurements can be in form of Fluid pressure, Fluid volume, Fluid Flow rate, Temperature, Electrical voltage or current, Chemical concentration etc. Once we have the quantity of the measured value,  we then transmit a signal  representing this quantity to an indicating or computing device where either human or automated action then takes place. If the controlling action is automated, the computer sends a signal to a final controlling device which then influences the quantity being measured. The final control device can be in the form of:

• Electric motor
• Control valve-for throttling the flow rate of a fluid
• Electric heater

The measurement device and the final control device connect to some physical system called the process.

 

Block Diagram of a Process Control System

• The measuring device senses
• The Controller decides
• The final control device influences the process
• The process reacts to the influence of the final control

You can also read: Basics of a Control Loop

So in a nutshell this is what we call a Process control system.

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Key differences between PLC and DCS systems

We have so many people with questions as to what are the similarities and differences if any between PLC and DCS systems. We will try to describe the working of these two systems and in the process help you understand the functions of each system.

Initially DCS was focused only on process control with analog signals that were used as main control system in process industries like Refining, Chemicals, and Petrochemicals etc. while PLC was focused on discrete automation with discrete on-off signals, that were used for example in Factory assembly lines and bottle lines but today DCS supports discrete I/O and some logic functions and PLCs support analog I/O with some control functions. In some instances Both PLC and DCS are used in the same plant i.e. PLCs are used on separate units on a plant floor which are then integrated with main plant-wide DCS for Control and Monitoring.

Let us now look at each system separately to help us understand more how they work.

PLC

Programmable Logic Controllers (PLCs) comes in different sizes which means various I/O and program capacities. Smallest sized PLCs are typically referred to as nano PLCs, micro PLCs and mini PLCs. They have fixed I/O and mainly used in stand-alone applications.

Large PLC support redundancy for CPU, power supply and possibly the control network, but typically not for I/O cards though there are large PLCs that support I/O redundancy by using duplicate I/O-subsystems with separate backplanes where the field instruments are wired in parallel to both I/O subsystems. The control network is typically a standard industrial Ethernet application protocol over Ethernet media and IP. The Field cabling comes directly onto the I/O card.

PLC usually support very fast scan times as required in discrete manufacturing but PID loops add to the CPU load, much more than discrete load thus making the scan time slower.

Loops are not handled individually in a PLC. Addition or change to loop requires a download of the entire program which affects other loops in the CPU as well.

PLCs are built around a given native protocol, this maybe: PROFIBUS, Modbus, DeviceNet etc. The PLC comes with its own native interface cards for native protocol supported by the PLC maker but relies on third-party interface cards for other Fieldbus protocols. The engineering software therefore automatically configures the communication interface card for the native protocol.

You can also read: 

• How Distributed Control Systems work
• The Journey from Relay Control Systems, to PLC Systems, how the Transition was done

Key Points to note on PLC

PLCs were designed to eliminate assembly-line relays during model changeovers. PLC is easier to change than relay panels; this has reduced the installation and operational cost of the control system compared with electromechanical relay systems.

BLOCK DIAGRAM OF A PLC SYSTEM

PLC offers the following advantages:

• Ease of programming and reprogramming in the plant
• Programming language is based on relay wiring symbols familiar to most plant electrical and instrumentation personnel
• High Reliability and minimal maintenance
• Small physical size
• Ability to communicate with computer systems in the plant
• Moderate to low initial investment cost
• Available in modular designs

DCS

Distributed Control System (DCS) supports redundancy for controllers, power supply and control network as well as redundant I/O cards including fieldbus interface cards in the same backplane. The control network supports peer-to-peer communication between controllers. The control network is typically a proprietary application protocol over Ethernet media and IP. The field cabling in DCS lands on a Field Terminal Assembly (FTA) where a special system cable with a connector takes the signals to the I/O card.

Loops in a DCS are executed individually. The scan time in a DCS is set individually for each loop. Most loops run at 1000ms although 250ms is common for pressure and flow loops in refining and petrochemicals and even 100ms is also possible. The scan time is constant, and does not change with task loading. This is important for PID control and time-based functions such as integration/totalizing and lead-lag dynamic compensation.

Loops in a DCS are managed individually. A change and download to one loop doesn’t affect the other loops.

A DCS has an integrated development environment where I/O control strategy and operator graphics are created together and stored in a single database. This means once a tag is created in the DCS it automatically becomes available everywhere in the system with the same human readable tag name for use in basic control, advanced control, graphics, faceplates, trending, alarming, and turning etc.  Without mapping data through registers or other tag names makes it easy to do changes or additions.

The Sensor & Actuator level “H1” Fieldbus network supported by DCS is basically FOUNDATION fieldbus for instrumentation and PROFIBUS-DP for motor controls.

The DCS comes with its own native Fieldbus interface cards. The engineering software therefore automatically configures the communication interface cards for the variables used in the control strategy and graphics.

Key Points to Note on DCS

DCS is miniaturized version of the multitasking, multivariable, multi-loop controller used for process control. It is functionally and geographically processing distributed system. Equipment making up a DCS is separated by function and is installed in two different work areas of a processing installation. Equipment for operator to monitor process condition and to manipulate the set point of the process operation is located in a central control room; from where the operator can view information transmitted from the process area and displayed on a video display unit and can change control condition from a keyboard. DCS systems are suitable for the following processes:

• Where a single centralized system is not adequate i.e. Power, Steel, Pulp & Paper plants, Fertilizer etc.
• Processes of different level of hierarchy
• Processes which can be divided into different and functionally independent sections, based on functional scope and geographical distribution

DCS offers the following Advantages:

• Compact to contain ON/OFF controllers
• Reduced complexity and easy expandability
• High Speed of the control processing
• Control Algorithms changes do not call for hardware changes
• Continuous trend data is available
• User friendly but higher data security
• Plant data are transparent on the network
• Sequential, batching and feedback control are possible

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Differences between PLC and DCS based Systems

PLC	DCS
Redundancy not possible or limited	Redundancy possible at every level
It is used for low loop count	It is used for any loop count
Performance drops with increasing loop count	No change in performance with increasing loop count
Purely free running mode	Highly efficient multitasking mode
Analog processing simulated through digital computer	Analog processing done in real frequency domain function
No interplant connectivity	Fully Functional Inter-plant connectivity
Individual database for every node	System-wide global database
Typical performance: 100 PID loops/sec	Typical Performance: More than 1000 PID loops/sec.

Do you have comments or questions on PLC or DCS? Feel free to post them in the comments section below. 

Components in a Controlled Automation System

Essential components in any controlled automation system include:

• The actuator (which does the work)
• The controller (which ”tells” the actuator to do the work)
• The sensor (which provides the feedback to the controller so that it knows the actuator is doing work)

An example of a simple controlled automation system is shown below:

A controlled system may either be analog controlled system or digital controlled system. Let's consider the following analog controlled system:

The actuator is a hydraulic servovalve and a fluid motor. The servovalve opens proportionally with the voltage it receives from the controller and the fluid motor rotates faster if it receives more hydraulic fluid. There is a speed sensor connected to the motor shaft, which outputs a voltage signal proportional to the shaft speed. The controller is programmed to move the output shaft at a given speed until a load is at given position. When the program requires the move to take place, the controller outputs an approximately correct voltage to the servovalve, then monitors the sensor’s feedback signal. If the speed sensor’s output is different from expected i.e. indicating wrong motor speed, the controller increases or decreases the voltage supplied to the servovalve until the correct feedback is achieved. The motor speed is controlled until the move finishes. As with any other control system, the program may include a function to notify a human operator if speed control isn’t working.

You can also read: 

• Introduction to Industrial Automation
• The Transition from Relays to PLCs

Having looked at analog controlled system above, let's now consider an example of a digital controlled system:

The above figure represents a simple digital controlled system in which the actuator consists of a pneumatic valve and a pneumatic cylinder that must be either fully extended or retracted. The controller is a PLC that has been programmed to extend the cylinder during some more complicated process and to go on to the next step in the process only after the cylinder extends. When it is time to extend the cylinder, the PLC supplies voltage to the valve, which should open to provide air to the cylinder, which then extend. If all goes well, after a short time the PLC will receive a change in voltage level from the limit switch, allowing it to execute the next step in the process. If the voltage from the switch does not change for any reason ( faulty valve or cylinder or switch, break in a wire, obstruction preventing full cylinder extension etc.), the PLC will not execute the next step. The PLC may even be programmed to turn on a “fault” light when such a delay occurs.

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How Triacs are used in Industrial Control Applications

The Triac is a three-terminal device that is similar to the SCR except that the Triacs can conduct current in both directions. Its primary use is to control power to AC loads such as turning AC motors on and off or varying the power for lighting and heating systems. The Triac is a solid state device that acts like two SCRs that have been connected in parallel with each other (inversely) so that one SCR will conduct the positive half-cycle and the other will conduct the negative half-cycle. Before the triac was designed as a single component, two SCRs were actually used for this purpose.

The figure 1 below shows the symbol for the triac, and its pn Structure. The terminals of the triac are identified as main terminal 1 (MT1), main terminal 2 (MT2), and gate. The multiple pn structure is actually a combination of two four-layer (pnpn) junctions.

Figure 1

As the name suggests, the load current passes through the main terminals, and the gate controls the flow. Figure 2 below shows the equivalent circuit of the triac, which consists of two back-to-back SCRs with a common gate.

Figure 2

Operation of the Triac

The operation of the triac can be explained by the two-SCR model in Figure 2.  From the figure, you can see the SCRs are connected in an inverse parallel configuration. One of the SCRs will conduct positive voltage and the other will conduct negative voltage.

When MT2 is more positive, the current flows through first SCR; when MT1 is more positive the current flows through Second SCR.

Unlike the two SCRs, the Triac is triggered by a single gate. This prevents problems of one SCR not firing at the correct time and overloading the other.

The operating characteristics of the Triac are best explained using the characteristic curve shown in Figure 3:

Figure 3

In the figure above, you can see that the triac can conduct both positive and negative current. The Voltage is shown along the horizontal x-axis, and current is shown along the vertical y-axis. This diagram also shows a second graph with four quadrants. These quadrants are used to explain the operation of the triac as polarity to its MT1 and MT2 and gate changes.

Notice that the right half of the graph (in quadrant 1) looks just like the SCR curve; no current flows until either the break over voltage is reached or the gate is triggered (indicated by dashed line).

This same pattern is repeated in quadrant 3 (for voltage and current of the opposite direction). Also, like the SCR, once the triac is triggered on, it will remain on by itself until the load current drops below the holding current value (IH)

A Single cycle of AC has a positive and a negative half-cycle. The triac requires a trigger pulse at the gate for each half-cycle and works best if the trigger is positive for the positive half-cycle and negative for the negative half-circle (Although in most cases the triac will also trigger if the gate goes negative in the positive half-cycle and if it goes positive in the negative half-cycle.

You can also read:

• Basics of Instrumentation and Control Systems
• How Silicon Controlled Rectifiers (SCRs) are used in Industrial Control

Applications of Triacs

The Triac is required in circuits where AC Voltage and Current need to be controlled like the SCR controls dc current. Another difference between the triac and SCR is that the Triac can be turned on either by a positive or negative gate pulse. The gate pulse need only be momentary and the Triac will remain in conduction until the conditions for commutation are satisfied.

A triac can be used as an-off solid-state switch for AC loads or to regulate power to an AC load, such as dimmer switch.

Triacs are available in various packages, some of which can handle currents up to 50 A (which is considerably less than the SCR).

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