Automation Notes || 

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Knowing the truth about VFD operation can simplify the selection process for choose a variable frequency drive.

No matter how commonplace variable frequency drives (VFDs) have become to you, somewhere someone is using one or considering using one for the first time (see Figure 1). Think back to when you first thought about applying one of today's pulse-width modulation (PWM)-based VFDs to an ac motor. Chances are you probably had a few misconceptions about their abilities and designs. This article addresses five common VFD myths and corrects misconceptions about their proper usage.

Myth No. 1: The output of a VFD is sinusoidal

People tend to be more familiar with running their ac induction motors using motor starters. With a starter, starting the motor involves connecting the 3-phase leads of the motor to 3-phase power. Each phase is a sine wave with a frequency of 60 Hz and usually has a voltage amplitude of 230 V, 460 V, or 575 V in the U.S. This applied voltage creates a sine wave current waveform with the same frequency if checked at the motor leads. So far, running a motor is quite simple.

What happens at the output of a VFD is an entirely different story. A VFD typically rectifies the 3-phase input to a fixed dc voltage, which is filtered and stored using large dc bus capacitors. The dc bus voltage is then inverted to yield a variable voltage, variable frequency output. The inversion process is carried out using three insulated gate bipolar transistor (IGBT) pairs-one pair per output phase (see Figure 2a). Because the dc voltage is inverted into ac, the output stage is also called "the inverter." The duration for which each IGBT switch in a given pair is turned ON or held OFF can be controlled, which determines the RMS value of the output voltage. The ratio of the output RMS voltage to output frequency determines the flux developed in the ac motor. In general, there is a fixed relationship between the two. When the output frequency increases, the output voltage should also increase at the same rate to keep the ratio constant and thus the motor flux constant. Normally, the relationship between voltage and frequency is kept linear so that a constant torque can be produced. The resulting voltage waveform applied to the motor winding is not sinusoidal (see Figure 2b). Note that sometimes the voltage by frequency (V/f) ratio can be quite nonlinear, which is typical for fans, pumps, or centrifugal loads that do not require constant torque but instead favor energy savings.

What makes this work is that, as the name implies, an induction motor is a big inductor of sorts. A characteristic of induction is its resistance to current changes. Whether a current is increasing or decreasing, an inductor will oppose the change. What does this have to do with the PWM voltage waveform in Figure 2b? Instead of letting the current pulse rise on the same order as the applied voltage pulse, the current will start to rise slowly. When the voltage pulse has ended, the current doesn't disappear immediately, it slowly starts to ebb. In general terms, before the current has fallen back to zero, the next voltage pulse comes along, and the current starts to slowly rise again-even higher than before because the pulses are getting wider. Eventually, the current waveform becomes sinusoidal, albeit with some jagged up-and-down transitions as the voltage pulses start and end (see Figure 3).

Myth No. 2: All VFDs are the same

The common ac VFD of today is a fairly mature product. Most commercially available drives contain the same basic components: a bridge rectifier, a soft-charging circuit, a dc bus capacitor bank, and an output inverter section. Granted, there are differences in how the inverter section does its switching, the reliability of the components, and the efficiency of the thermal dissipation scheme. But the basic components remain the same.

There are exceptions to this "all-the-same" thinking. For example, some VFDs offer a three-level-output section. This output section allows the output pulses to vary from half-bus, voltage-level pulses and full-bus level pulses (see Figure 4).

To achieve the three-level output, the output section must have twice the number of output switches, plus clamping diodes (see Figure 5). The benefit gained by using a three-level output is reduction in voltage amplification at the motor due to reflected wave, lower common-mode voltage, shaft voltage, and bearing current.

The matrix-style inverter is an even more atypical type of VFD. VFDs with matrix-style inverters do not have a dc bus or a bridge rectifier. Instead, they use bidirectional switches that can connect any of the incoming phase voltages to any of the three output phases (see Figure 6). The benefit of this arrangement is that power is allowed to flow freely from line-to-motor or motor-to-line for fully regenerative four-quadrant operation. The drawback is that filtering is required on the input to the drive because extra inductance is necessary to filter the PWM waveform so that it does not affect the input ac lines.

In addition to VFDs with three-level outputs and matrix-style inverters, there are more examples that prove not all ac VFDs are the same.

Myth No. 3: VFDs cure power factor (PF) issues

It is not uncommon to see VFD manufacturers quote PF statistics like "0.98 displacement PF" or "near-unity PF." And it is true that input displacement PF improves after a VFD is installed ahead of an induction motor. The VFD uses its internal capacitor bus to supply any reactive current the motor requires, thereby protecting the ac line from being the source of the reactive current and lowering the displacement PF. However, displacement PF is not the full story.

The full story of the PF calculation is that it must include the reactive power demanded by harmonics that are created when ac voltage is rectified to dc. The diode bridge conducts the current from the ac line to the dc bus in a discontinuous way. It is important to remember that a diode conducts only when the voltage on the anode side is higher than the voltage on the cathode side (forward biased). This means that the diodes are only ON at the peak of each phase during both the positive and negative portions of the sine wave. This leads to a ripple-like voltage waveform. It also causes the input current to be distorted and discontinuous (see Figure 7).

Much can be said about how to calculate harmonics and how to mitigate them. Regardless, to calculate true PF, the effects of harmonics must be included. The following equation indicates how harmonics influence true PF:

Where THD = total harmonic distortion

For the discontinuous input current in the equation, THD would be in the neighborhood of 100% or more. Substituting that into the equation yields a true PF closer to 0.71, compared to a displacement PF of 0.98, which disregards harmonics.

Not to panic though, there are currently many ways to reduce THD. These techniques make use of passive and active methods of making the input current waveform much less distorted and the THD much lower. The aforementioned matrix-style inverter VFD is an example of an active method of THD reduction.

Myth No. 4: You can run a motor at any speed with a VFD

The beauty of using VFDs is that they can vary both their output voltage and output frequency. Their ability to run the motor slower and faster than its nameplate-rated speed is part of why they are specified so often. Take the motor out of the equation, and this myth is actually true. Without the limitations of the motor, the VFD can easily run any frequency within it speed range without limitation. But in reality, the motor is necessary to do real work, and its cooling and power needs start to limit the actual speed range of the motor/drive combination.

Limit No. 1: From a motor cooling perspective, turning some motors too slowly is not a good idea. In particular, totally enclosed fan cooled (TEFC) motors have limitations because the fan that blows air over the motor shell is attached to the motor shaft. The slower the motor is operated, the less cooling air that goes to the motor. Most motor manufacturers specify speed ranges for their motor designs that reflect how slow the motor can be run—especially while loaded. TEFC motors typically are not recommended for operation at full load below 15 Hz (a 4:1 speed range).

Limit No. 2: It is not always stated on the motor nameplate, but mechanically, motors have a speed range limitation. Commonly referred to as the maximum safe operating speed, this speed is tied to mechanical limitations, such as bearings and balance. Some motor data sheets specify the maximum speed.

Limit No. 3: Before the motor reaches its maximum operating speed, it could run out of torque. This speed limitation is not due to cooling or mechanics, but is due to power limitation, which is a product of speed and torque. To be exact, the VFD runs out of voltage. Note that the rotation of the motor also generates a voltage of its own, referred to as back-electromotive force (EMF), which increases with speed. The back-EMF is produced by the motor to oppose the supplied voltage from the VFD. At higher speeds, the VFD must supply more voltage to overcome the back-EMF so that current can still flow into the motor as current is instrumental in producing torque. After a certain point, the VFD cannot push any more current into the motor because the output voltage has reached maximum, and thus the motor torque reduces, which, in turn, reduces speed. This reduction in speed results in lower back-EMF, which, in turn, allows more current to flow into the motor again. There is an equilibrium point where the motor reaches the maximum speed for a given torque condition so that the product of torque and speed equals its power capability.

Let's take a step back. VFDs can produce constant torque from a motor by keeping the V/f constant (see Figure 8).

When the output frequency is increased, the voltage increases linearly. The problem arises when the frequency is raised beyond the base frequency of the motor, most commonly 60 Hz in the U.S. Beyond the base frequency, the output voltage cannot increase, which causes the V/f ratio to reduce. The V/f ratio is a measure of the magnetic field strength in the motor and reducing it reduces the torque capability of the motor. Hence, the ability to have the motor produce rated torque at higher-than-base speed must decline at a rate of 1/frequency, so that the product of torque and speed, which equals power, is constant. The region of operation above base speed is called the constant power range, while operation at speeds below the base speed is called the constant torque range (see Figure 9).

Myth No. 5: A VFD's input current should be higher than its output current

Perhaps this is not a myth but a misunderstanding. Some VFD users check their output and input currents with a current clamp meter or by using VFD display monitors and find that the input current is much lower than the output current. It doesn't seem to align with the idea that the VFD should have some losses due to its own thermal component losses, so input should always be slightly higher that output. The concept is correct, but it is power, not current that should be considered:

The voltage portion of the preceding power equation is straightforward. The input voltage is always at the ac line voltage. The output voltage varies with the speed per the V/f pattern. The current components of the equation are a bit more complex. The key to understanding current components is knowing that a typical induction motor has two current components: One is responsible for producing the magnetic field in the motor, which is necessary to rotate the motor; while the second component is the torque-producing current, which, as the name suggests, is responsible for producing torque.

The drive consumes input current proportional to the motor's active torque demand, or load. The current needed for producing the magnetic field typically does not vary with speed and is provided by the drive's main dc bus capacitors, which are charged during power up of the VFD. Under low torque conditions, the output current may seem to be much higher than the input current because the input current mirrors only the torque-producing current plus some harmonics but does not include the magnetizing current. The magnetizing current circulates between the dc bus capacitors and the motor. Even at full load conditions, the input current will typically be lower than the motor current because the input still does not have any magnetizing current component in it.

Remember, we are balancing input and output power. For example, consider a fully loaded motor at low speed. The input voltage is at the rated line while the output voltage will be low due to the low speed. Because of the full load on the motor, output current will be high. To balance the power equation, the input current must be lower than the output current.

Paul Avery is a senior product training engineer at Yaskawa America Inc. He has been a VFD technical trainer with the company since 2000 and has a degree in electrical engineering from the University of Michigan.

This article appears in the Applied Automation supplement for Control Engineering and Plant Engineering.

Paul Avery, Yaskawa America Inc.

http://www.controleng.com/single-article/the-truth-about-five-common-vfd-myths/f33fdd9d71db0f654ef0da3a290d97b0.html

Stick with your architecture and remember to reuse your code

Daniel Fenton

01/15/2014

Every PLC programmer will make a mistake when he or she is writing code. Whether it’s the result of pressure by the customer, lack of coffee in the morning, or simply getting distracted at the wrong time, here are the top five mistakes to avoid.

1. Not sticking to an organized architecture

When a program is first laid out, all of the code for each portion should fall into neat, orderly sections. Keeping each part of the code separate allows the person working with the program to easily visualize the overall execution of the program.

It also helps to debug the program if something goes wrong, since each piece of code is encapsulated into its own structure. Finally, it looks better, because there isn’t the mess of an entire program on the screen at one time.

Unfortunately, as the developer and post-deployment maintainers work on the code, sometimes the code begins to creep out of each section, and intertwine with the supposedly isolated functions around it. This leads to confusion as tags hop from one sheet to the next, or outputs are written twice during execution.

It also leads to disorganized code. Finally, this can be dangerous, as minor tweaks across the board cause little program issues to build up until something breaks. Maintaining code organization and encapsulation is critical to the longevity of a PLC program.

2. Not documenting the code

Documenting code as it is written and, further, as it is maintained is critical to keeping a PLC and its program operational during the long periods of time between updates and adjustments. A quick few sentences on each major portion can save a lot of time and headaches later on. It can also help the programmer put his or her thoughts on paper, which can be helpful in figuring out the next step.

Even though the code may make sense at the moment it is programmed, the five minutes spent explaining thoroughly why a particular technique was used can save hours when, months later, a user must decipher what is going on. All too often, code originally installed in a system will change due to fixes, updates, and feature additions. If the documentation is not updated with the code, this can lead to confusion and misinterpreted programming.

3. Creating redundant tags and variables

As logic statements and ladder rungs are written out, often a program can have multiple branches of logic that eventually lead to a tag that functions as a sort of “flag.” This flag will usually wind up activating other pieces of logic in the program. Sometimes, these pieces of logic are mutually exclusive, such as a Boolean variable, which requires the program to send a message to one remote station or another, depending on the status of certain inputs.

The IEC 61131 Structured Text statements IF and ELSE are purpose-built for this exact logic, so that only one tag needs to be tested. If the tag is true, one set of code is executed. If the tag is false, another set of logic is executed.

If a programmer is not paying attention, he or she can be put in the situation of creating a “flag_1” and a “flag_2,” with both of these tags performing the same duty. Now, both variables need to be tested, which leads to increased code complexity and greater memory utilization. Not only are both of these results poor programming practice, but repetitive use of redundant tags and variables will require that a program utilize a more powerful (and therefore expensive) processor than otherwise required.

4. Not reusing existing code

Aside from ease of maintenance, debugging, and readability, the reason it is so important to encapsulate and isolate code in a program is that code encapsulated inside of a function block can be reused over and over throughout the program. This removes the requirement to rewrite the code that already exists inside of a function block. Therefore, the programmer can generate a standard, all-use function block that can be debugged and modified once, and have that change reflected throughout the entire program.

Collections of these general use function blocks are called function block libraries, and many manufacturers offer application or industry-specific function block libraries for download on their websites. These function block libraries heavily cut down on the amount of development time required for a given application or project.

Sometimes, without realizing it, a programmer will find him- or herself writing code that’s already been written. Where five or six of the same pieces of equipment need to operate similarly, it is easy to create a function block that accepts the inputs of the sensors and outputs solutions for the actuators. On the other hand, it can be more difficult on seemingly one-off algorithms to find a reason to write generic code.

The effort needed to write a function block that is generic enough to be reused must be balanced against the time spent for development on the project. It is worthwhile, however, to bear in mind that a little extra time spent immediately can save a lot of time later on.

5. Not utilizing version control

Keeping your code organized is another major practice that is sometimes lost in the commotion of trying to complete a project for commissioning. Version control systems can be as simple as naming a project for a particular PLC once it has been deployed, or as powerful as a dedicated application used specifically to track what the changes are in each major revision.

The use of such versioning schemes mitigates situations where a particular fix or feature might have gone out to the field, and the PLC has a program that may or may not have those changes.

Programmers often forget to keep track of what changes are in the program on the PLC without a proper versioning strategy. Documentation, a good naming scheme, and possibly dedicated version control software can lessen these issues and increase the detail available to the programmer.

Dedicated version control software also allows a system to track the user that made every single change, documented or otherwise. This can be particularly useful if multiple programmers are working on the same program.

http://www.controleng.com/

IEC 61131-3: What’s the acceptance rate of this control programming standard? Ladder diagram remains the simplest and most popular approach for PLC programming, but may not be the most efficient way. Lack of interoperability may be a problem. To learn what languages PLC programmers prefer and to understand the level of awareness and use of the IEC 61131-3 standard for industrial control, Control Engineering surveyed readers as part of a custom research project on behalf of AutomationDirect.

Jeff Payne

12/23/2015

Although the IEC 61131-3 Programming Languages standard has been around for nearly 25 years, limited awareness of its scope and features has kept it from becoming a requirement in North America. A recent survey of Control Engineering readers showed that ladder logic remains the most popular programming method for programmable logic controllers (PLCs). Findings of the custom research project, on behalf of AutomationDirect, suggest that programmers could save time and money by using other standard programming languages more often. It also suggests that difficulty in code transportability among PLC brands may be an issue.

Overview: limited adoption

More than 586 responses were received from participants who met eligibility requirements, including relevant purchasing influence and authority, and also responsibility for hardware specifying or PLC programming. The results of the survey show low awareness and limited adoption of this standard in North America, indicating that situations where its application is required are rare.

The most common job functions of the respondents were system or product design; control or instrument engineering; or system integration or consulting. These functions accounted for more than 60% of the respondents. About one-third of the functions included process, production, or manufacturing engineering; operation or maintenance; or other engineering. Almost 10% of the participants were in general or corporate management, and this group was more likely to specify but not program PLCs.

The majority of the respondents, more than 60%, were employed at companies with more than 100 employees, some with 1,000 or more. However, the largest group of participants, at almost 40%, was from companies with fewer than 100 employees.

By company type, end users of PLCs were the largest group of respondents at almost 40%, and almost half of the respondents were system integrators, original equipment manufacturers (OEMs), or machine builders. The respondents were widely spread geographically throughout the United States.

What the experts say

The IEC 61131-3 standard has been around for nearly 25 years and includes a family of programming languages. IEC characterizes it as an international standard for programming PLCs. PLCopen, a nonprofit industrial trade organization, is mostly dedicated to IEC 61131-3 and contains significant information about the standard.

The PLCopen Website describes IEC 61131-3 as, "The only global standard for industrial control programming. It harmonizes the way people design and operate industrial controls by standardizing the programming interface." The organization says it is a standard programming interface with a common structure.

The standard includes a definition of the sequential function chart (SFC) language, used to structure the internal organization of a program. It adds four inter-related programming languages including two graphical ones, ladder diagram (LD) and function block diagram (FBD); and two text-based languages, instruction list (IL) and structured text (ST). Using logical elements, defined data types, task structure and scheduling, and execution control, each program can theoretically be structured to increase re-usability, reduce errors, and increase programming and user efficiency.

PLCopen has been working with technical committees to add extensions to the standard. There have been a number of functions added as a result of these activities including motion control, safety, OPC Unified Architecture communication (OPC UA from OPC Foundation), XML schema, reusability level definitions, and conformity level.

Awareness levels

So how familiar are PLC purchasers and programmers with IEC 61131-3? When Control Engineering polled its readers, a whopping 85% of the respondents said they are either not familiar with or only somewhat familiar with it (see Figure 1). While this standard may have great acceptance and use in Europe or other parts of the world, it has not had as much impact in North America. Implementing it does not appear to be a priority or a requirement for many respondents in the United States since, after more than 20 years, an overwhelming majority of programmers working in North America are, at best, only somewhat familiar with the standard.

More than 40% of the respondents reported no familiarity with the standard, and the highest concentration of these respondents was among those who say they are PLC programmers. Turning it around, among those who say they actually write programs, only 15% claim a high level of familiarity.

Why use PLCs supporting IEC 61131-3 programming?

Among the respondents who use or specify PLCs, and who say they are familiar with IEC 61131-3, the next question asked in the survey was why they use it. The answers (see Figure 2) suggest its use does not appear to be a requirement for most in North American industrial automation markets. The reason cited most often (39%) is simply because the PLC product came with the language. A quarter of the end users specified IEC 61131-3 programming language, and some of this can be attributed to U.S. companies shipping machines into Europe or Asia.

The fact that fewer than 10% of the applications demand the features of IEC 61131-3, while a larger percentage of respondents who don't program PLCs say it's because it's specified, hints that some of the totals are driven by hardware choice and selection.

http://www.controleng.com/

|| What Is A Servo?

Servo control, which is also referred to as "motion control" or "robotics" is used in industrial processes to move a specific load in a controlled fashion. These systems can use either pneumatic, hydraulic, or electromechanical actuation technology. The choice of the actuator type (i.e. the device that provides the energy to move the load) is based on power, speed, precision, and cost requirements. Electromechanical systems are typically used in high precision, low to medium power, and high-speed applications. These systems are flexible, efficient, and cost-effective. Motors are the actuators used in electromechanical systems. Through the interaction of electromagnetic fields, they generate power. These motors provide either rotary or linear motion. Here is a graphical representation of a typical servo system:

This type of system is a feedback system, which is used to control position, velocity, and/or acceleration. The controller contains the algorithms to close the desired loop (typically position or velocity) and also handle machine interfacing with inputs/outputs, terminals, etc. The drive or amplifier closes the inner loop(s) (typically velocity or current) and represents the electrical power converter that drives the motor according to the controller reference signals. The motor can be of the brushed or brushless type, rotary or linear. The motor is the actual electromagnetic actuator, which generates the forces required to move the load. Feedback elements such as tachometers, lvdts, encoders and resolvers, are mounted on the motor and/or load in order to close the various servo loops.

ADVANCED Motion Controls designs and manufacturers servo drives and amplifiers for use in servo systems. Servo drives and amplifiers are used extensively in motion control systems where precise control of position and/or velocity is required. The drive/amplifier simply translates the low-energy reference signals from the controller into high-energy signals to provide motor voltage and current. In some cases the use of a digital drive replaces the controller/drive or controller/amplifier control system. The command signals represent either a motor torque, velocity or position and can be either analog or digital in nature. Analog +/-10 VDC command is still the most common reference signal but it is quickly giving way to digital network commands.

ADVANCED MOTION CONTROL http://www.a-m-c.com/

When someone ask Why using AC Driives/Inverter/VFD/VSD ???

"AC Drive Efficient"

Process variables, including pressure and flow of gases and liquids, have long been regulated using mechanical clutches, throttles, and adjustable inlet guide vanes. These schemes waste energy, require frequent maintenance, and provide inaccurate control. Adjustable frequency drive control provides more efficient, maintenance-free performance, with more accurate control. They have become the preferred method of control for variable speed applications. These drives provide many benefits over traditional control methods. The benefits are both cost and performance related and include:
• use of the rugged, squirrel cage induction motors for reduced cost and easy application,
• advanced performance from digital microprocessor control and serial communications,
• competitive first costs by using standard off-the-shelf components.

Most adjustable frequency drives consist of four basic sections (see figure1):
• The converter section rectifies the AC line input power into the DC bus/filter.
• The DC bus/filter section smooths the DC ripple.
• The driver-regulator section consists of the control, measurement, logic, and command circuits necessary to integrate the drive elements into a system.
• The inverter section converts the filtered DC bus into an AC output.

The voltage level, power level and type of inverter technology determine the size and type of power semiconductors in the converter and inverter sections.

Most inverter sections consist of one of the following solid state switching devices:
• Bipolar transistors have a higher switching rate than silicon control rectifiers or gate turn-off thyristors. However, current ratings limit their application to low- power applications where high switching rates are required.
• Insulated gate bipolar transistors represent state- of-the-art technology in power semiconductor devices. They have very fast turn-off times, allowing them to switch at rates of up to 15-20 Khz. The current waveform is nearly sinusoidal, reducing peak currents by as much as 42% compared with bipolar transistors. The result is higher available torque throughout the speed range. This eliminates motor noise and reduces motor losses and heating.

The components of the driver-regulator section may use either analog or digital techniques. However, most adjustable frequency drive manufactures are turning to the flexibility of the digital microprocessor- driven regulators.

Microprocessors allow manufacturers to include optional control schemes via software modifications. They allow additional functions to be supplied at minimal incremental costs to the user. Furthermore, microprocessors provide enhanced fault diagnostics since most fault data can be stored and viewed at a later time. These features add to the performance and the ability of a single drive design to meet a wide variety of application requirements. As a result of microprocessor-based digital control, adjustable frequency drives are easy to start-up and operate, are resistant to damage and have simple troubleshooting procedures.

Another benefit of microprocessors that contributes to advanced system integration is serial communications protocol for control and monitoring. Many adjustable frequency drives use programmable logic controllers to manage data collection from peripheral controls. This allows the operator to customize the drives with software-executed features and programmable parameters.

A pulse width-modulation inverter controls both the width of, and the spacing between, DC pulses precisely. This allows the inverter to simulate a sinusoidal shaped output pattern. In turn, the output voltage has lower harmonic content. Pulse width- modulations have a wide speed range, smooth low speed operation, multi-motor operation ability, and a high-input power factor. Microprocessors provide improved modulation techniques. Higher-speed switching devices, such as insulated gate bipolar transistors, are making the pulse width-modulation inverter the standard for the 1-500 HP range.

The characteristics of the drive and motor must be considered as a system when applying adjustable frequency drives.

The duty cycle of the inverter-motor combination must be checked at all load conditions to ensure the combination is suitable for the application. It is also important to understand the load requirements. Depending on the application, the load can be classified as one, or a combination of three basic load profiles. (See figure 2).

Constant torque

Constant torque implies that any speed in the operating range requires the same amount of driving torque. Conveyors are an example of a constant torque application.

Adjustable frequency drives with many control features are ideal for conveyor applications. With the advanced factory automation, energy-savings during transportation has become essential. Inverters start up from low frequency and low voltage then increase both frequency and voltage. Current and torque are much smaller during acceleration compared to the cross-line start with commercial power supplies. Therefore, an inverter drive can eliminate the reduced voltage start unit.

Since current during acceleration is less, motor heating is reduced, allowing frequent run/stop operations. For example, feed conveyors running at constant speed consume more energy than a conveyor that runs only as fast as necessary. This happens even when there is no material to be fed. If a conveyor changes its speed rapidly, work pieces may be damaged. Such troubles can be avoided and product quality can be stabilized since inverters can change speed slowly by soft start/stop time adjustment. Additional benefits of adjustable frequency drives for conveyor applications include:

• Motors do not require space for speed change as do mechanical converters so that smaller size and lighter weight drives can be used.
• Since totally enclosed fan cooled motors can be used, they are suitable for conveyors under adverse conditions such as constant feeders with excessive dust, sive dust, paint lines with adhesive compositions, or bottling lines.
• Brush or commutators are not part of the induction motor design so maintenance is eliminated.
• Inverters can be used for existing motors under some conditions. By applying an inverter to existing circuits, conveyors which operated at a constant speed can be run at stepless variable speeds.
• A wide range of speed control, up to 40:1, can be controlled remotely by changing frequency reference for optimum speed.
• Since phase-rotation switching of inverter transistors performs the forward/ reverse operation, conventional main circuit contactors, are not required.

Variable torque

Often, variable torque loads require the driving torque to vary proportionally with the speed of the load. HVAC systems are an example of a variable torque application.

Centrifugal fans and pumps are sized to meet the maximum flow rate required by the system. In most applications however, maximum demand volume is required for only a small percentage of the total operating time. Most of the operating time is spent providing 40% to 70% capacity. For centrifugal devices, the torque varies by the square of speed and the horsepower varies by the cube of speed. Reduced fan speed achieves reduced air volume and reduces motor power consumption.

Most pumps are centrifugal and their operation is defined by two independent curves. One is the pump curve—a function of the pump geometry and motor characteristics. The other ¦s the system curve which depends on the geometry of the piping and valves connected to the pump. The intersection of these curves determines the natural operating point. If the system is part of a process that requires adjustable flow rates, then some method is needed to alter either the system characteristics or the pump parameters. These methods include valves or throttling to change the system curve or an adjustable frequency drive on the pump to modify the pump curve.

Constant Horsepower

Constant horsepower loads require high torques at low speeds and low torques at high speeds. Machine tool applications are perfect examples of such loads.

In gear-type speed changers, spindle speed can be selected only in steps so that delicate peripheral speed constant control is not available. Although dc spindles can make stepless speed changes, they are expensive, inefficient, and require regular brush maintenance. Using adjustable frequency drives for spindle drives eliminate these problems. Since a standard motor can achieve stepless spindle drive speed changes, the clutch mechanism can be eliminated. An adjustable frequency drive used in place of a gear-type or dc spindle provides the following benefits:

• higher accuracy in cutting of soft workpieces
• elimination of brush maintenance
• improved efficiency-field winding is not needed.
• increased machine output

Automated grinding is another constant horsepower application. Grinding speed requirements range from 20,000 to 180,000 rpm. At the beginning of a typical grinding cycle the wheel moves toward the work-piece at a fast feed rate. Then, at a safe distance, the feed rate slows to the normal grinding rate as it approaches the surface. However, due to variations in part dimensions, the distance traveled to reach the work piece at the slow rate can be significant.

To overcome this hurdle, some adjustable frequency drives offer a load sensing circuit to produce a signal that is a function of the current drawn by the grinding wheel drive motor. With this feature, the grinding wheel advances at a fast rate toward the work piece until it makes contact. Within 20 to 40 milliseconds after the wheel contacts the work piece, the adjustable frequency drive produces a signal that adjusts the feed rate to the normal lower value.

"Conclusion"

The use of adjustable frequency drives makes an important contribution to the efficient operation of industrial and commercial equipment.

The advances in the area of adjustable frequency drives have led to high-tech cost effective solutions that offer substantial advantages over both mechanical systems and dc drives.

The most effective adjustable frequency drives use state-of-the-art technology. Pulse width-modulation combined with microprocessor control provides optimum induction motor control and superior flexibility. The combination of insulated gate bipolar transistors and surface mounted device technology have allowed a more compact and less complex design with reduced costs compared to other variable speed drive technologies.

https://www.yaskawa.com

An ac variable frequency drive must simultaneously control output frequency and voltage to efficiently control the speed of a three phase induction motor.

Frequency controls the motor's speed Common 60Hz induction motors are typically offered at no load speeds of 3600rpm (2 pole), 1800rpm (4pole), and 1200rpm (6 pole). Applying 60Hz to a 4 pole motor will produce a motor speed of 1800rpm at no load. Actual speed at any applied frequency is influenced by motor load requirements. If frequency is cut in half (30Hz), then motor speed is cut in half.

Voltage is applied in proportion to frequency to achieve rated motor torque. If the motor is running half speed (30Hz), the voltage applied is also cut in half. Failure to reduce applied voltage with reduced speed will result in excessive current draw and motor overheating.

Pulse Width Modulation (PWM) is the present state of the art method used to control frequency and voltage. An AC power source is connected to the drive rectifier, converted to DC, and then "inverted" in a logic controlled output of DC pulses of varying width (voltage) and polarity (frequency). A motor is an inductive device constructed of coils of wire embedded in iron. The motor's inductance resists the rapid voltage changes, averaging (smoothing) the pulses and making them appear to the motor as a 3 phase sine wave.

There are three major elements in the PWM process:
Rectifier - converts AC power source to DC.
DC Bus - pulsating DC is smoothed by large capacitors. A measurement at the output of this section indicates a DC voltage equal to the AC peak value of approx. 1.4 times the AC input.
Inverter - receives instructions from control logic; converts DC to variable frequency variable voltage 3 phase PWM output.

The word “servo” comes from the Latin word “servus” which means the servant who acts according to his master’s instructions and works faithfully and quickly. To put it simply, servos are devices that conduct follow-up control to a target position and speed. They are good at moving quickly and repeatedly to a target position and are therefore adapted to a wide range of high-speed and high-performance equipments and automated machines used in plants. For example, servos are used in most manufacturing equipment for manufacturing semiconductors and LCD panels.

|What can servo do?

Torque control – Torque generated by a motor can be controlled by controlling a current of the motor.

Torque generated by a motor can be controlled by a torque reference input!

Speed control – Speed can be continuously changed in response to a speed reference voltage

Rotation speed of a motor can be controlled by a speed reference input! Position control – Rotation angle (position) and rotation speed (travel speed) of a motor can be controlled by a position reference. Rotation angle (position) and rotation speed of a motor are decided by a position reference. When a motor stops, an electrical brake (servo clamp) is applied in order to try to save the position of the motor.

Rotation position of a motor can be controlled by a position reference input!

• Ripple Compensation

Σ-7 SERVOPACKs can reduce speed ripples caused by motor cogging, even for machines for which speed loop gains cannot be set high. This ensures smooth operation.

Σ-7 Servomotors use an optimized magnetic circuit that improves motor efficiency and reduces heat generation.
(comparison with typical models.)

Below are the 111 finalists across 27 categories in the 2016 Engineers' Choice Awards program. Voting will close on Dec. 21, 2015; Winners and honorable mentions will be notified by Dec. 30, 2015.

Details and photos are available for each product. For program FAQs, please click here. Winners and honorable mentions will be featured in the February 2016 issue.

Finalists by category

Hardware — Handheld test, measurement, calibration

• TiX560 Infrared Camera, Fluke Corp., http://en-us.fluke.com
• TRAPtest VKP 40plus Ex testing, recording and evaluation system; Gestra AG; www.flowserve.com
• i-ALERT2 Equipment Health Monitor, ITT Goulds Pumps, www.ittproservices.com

Hardware — HMI, operator interface, thin-client 

• Series 82 anti-vandal pushbutton, EAO Corp., www.eao.com
• Series 84 Halo Compact multi-functional switch, EAO Corp., www.eao.com
• Allen-Bradley PanelView Plus 7 Performance Operator Interface, Rockwell Automation, www.rockwellautomation.com
• Wonderware Industrial Computers InTouch Panel PC, Schneider Electric, www.wonderware.com
• Simatic HMI Mobile Panel 2nd Generation, Siemens, http://usa.siemens.com/industry

Hardware — Industrial PCs 

• C6670 Industrial Server, Beckhoff Automation, www.beckhoffautomation.com
• EXPC-1519 industrial panel computer, Moxa Inc., www.moxa.com

Hardware — Integrated HMI controllers

• CP32xx Panel PCs, Beckhoff Automation, www.beckhoffautomation.com
• UniStream 15" PLC + HMI, Unitronics Inc., www.unitronics.com

Machine & Embedded Control — CNCs, board-level products

• IndraMotion MTX micro CNC, Bosch Rexroth Corp., www.boschrexroth-us.com
• DT7837 signal acquisition module, Data Translation Inc., www.datatranslation.com
• Sinumerik 808D Advanced panel-based CNC, Siemens, www.usa.siemens.com/808d

Machine & Embedded Control — Discrete sensors

• EZProx Inductive Proximity Sensors, EZAutomation, www.ezautomation.net
• PCH420 HART enabled 4-20 mA vibration sensors, Meggitt Sensing Systems, www.wilcoxon.com

Machine & Embedded Control — Machine vision, barcode readers

• DataMan 150/260 Series fixed-mount, image-based ID readers; Cognex; www.cognex.com
• In-Sight 8405 vision system with PatMax Redline, Cognex, www.cognex.com
• O3D Smart Sensor, a 3D sensor for machine vision with PMD technology; ifm efector inc.; www.ifm.com/us

Machine & Embedded Control — PACs, PLCs

• APAX-5580 DIN-Rail IPC, Advantech, www.advantech.com
• Productivity2000 micro-modular rack-based PLC, Facts Engineering, www.automationdirect.com
• PLC Logic pluggable smart relay system, Phoenix Contact, www.phoenixcontact.com/usa_home
• Simatic ET200SP Open Controller, Siemens, www.siemens.com/open-controller

Motion — Motors 

• AM8x1x Servomotors, Beckhoff Automation, www.beckhoffautomation.com
• Faulhaber 2057 BHS Brushless DC Servo Motor, Faulhaber, www.micromo.com
• Faulhaber 3274 BP4 Brushless DC Motor, Faulhaber, www.micromo.com
• Extreme Duck Ultra liquid-proof motor, Leeson Electric, www.leeson.com
• Sinamics G110M drive and gear motor system, Siemens, www.usa.siemens.com/sinamics-g110m

Motion Control

• IEH3-4096 Integrated encoder, Faulhaber, www.micromo.com
• UR3 table-top six-axis robot, Universal Robots, www.universal-robots.com

Motion Control — Drives, servo drives

• Ndrive QLe nanopositioning piezo drive, Aerotech Inc., www.aerotech.com
• i500 Inverter Series, Lenze, www.lenze.com
• ESCON module 24/2 servo controller, maxon precision motors inc., www.maxonmotorusa.com
• Allen-Bradley Kinetix 5700 Servo Drive, Rockwell Automation, www.rockwellautomation.com
• Allen-Bradley PowerFlex 527 AC Drive, Rockwell Automation, www.rockwellautomation.com
• Altivar Process Variable Speed Drive, Schneider Electric, www.schneider-electric.us
• U1000 Industrial Matrix Drive, Yaskawa America Inc., www.yaskawa.com

Network Integration — Ethernet hardware

• ProView Ethernet switch, Advantech, www.advantech.com
• LMP-1002G-SFP PoE+ Managed Ethernet Switch, Antaira Technologies, www.antaira.com
• DataTuff Cat 5e Profinet Type C trailing cable, Belden Inc., www.belden.com
• 829 Industrial Integrated Services Routers, Cisco Systems, www.cisco.com
• OCTOPUS wirespeed routing switch; Hirschmann Automation & Control, a Belden brand; www.hirschmann.com/en
• Modbus TCP/IP to Profinet Device Gateway (Two-Port), ProSoft Technology, www.prosoft-technology.com
• Allen-Bradley Stratix 5400 industrial Ethernet switch, Rockwell Automation, www.rockwellautomation.com

Network Integration — Network hardware 

• 800xA Networks Ethernet switch and firewall, ABB, www.abb.com/800xa
• STE-700 Serial Device Servers, Antaira Technologies, www.antaira.com
• Lion-Link EtherNet/IP Bus Coupler, Belden Inc., http://lumberg-automationusa.com
• MGate W5208 Ethernet gateway, Moxa Inc., www.moxa.com
• Network Integration — Wireless products
• APR-3100N network router and wireless access point, Antaira Technologies, www.antaira.com
• Industrial Wireless 3700 Series Wi-Fi access point, Cisco Systems, www.cisco.com
• Digi XLR Pro 900 MHz radio, Digi International, www.digi.com
• UC-8112-T-LX Linux-based programmable wireless gateway, Moxa Inc., www.moxa.com

Network Integration I/O — Ethernet, wireless

• WISE-4000 IoT wireless I/O modules, Advantech, www.advantech.com
• EJ Series EtherCAT I/O System, Beckhoff Automation, www.beckhoffautomation.com
• ER Series EtherCAT Box I/O, Beckhoff Automation, www.beckhoffautomation.com
• WaveContact wireless I/O, FreeWave Technologies Inc., http://freewave.com
• ioLogik 2542-HSPA-T I/O module, Moxa Inc., www.moxa.com
• Power — Energy, power protection
• Low Profile Compact Circuit Protector, Eaton, www.eaton.com/electrical
• Power Xpert gateway (PXG) 900, Eaton, www.eaton.com/electrical
• SC9000 encapsulated powerpole (EP) arc-resistant drive, Eaton, www.eaton.com/electrical
• EZPPS Din-Rail Mount Power Supply, EZAutomation, www.ezautomation.net
• PACT RCP Rogowski Coil Solution, Phoenix Contact, www.phoenixcontact.com/usa_home

Process Control — Flowmeters 

• FLUXUS XLF low-flow non-invasive metering system, Flexim Americas Corp., www.flexim.com
• Model 5700 Coriolis flow and density transmitter, Micro Motion Inc., www.micromotion.com
• Process Control Systems
• Analog Signal Splitter with 5-Way Isolation, Single 4-20 mA Input, Quad 4-20 mA Output; Automation Systems Interconnect; www.asi-ez.com
• Bedrock secure control system, Bedrock Automation, www.bedrockautomation.com
• Liquiline Systems CA80 Analyzer, Endress+Hauser, www.us.endress.com/en
• Platinum Series Temperature and Process Controllers, Omega Engineering, www.omega.com
• DCS Platform Migration, Weidmuller North America, www.weidmuller.com
• CENTUM VP R6 distributed control system, Yokogawa Electric Corp., www.yokogawa.com

Safety — Machine safety

• Allen-Bradley Guardmaster 440C-CR30 safety relay with EtherNet/IP module, Rockwell Automation, www.rockwellautomation.com
• Simatic S7-1200F Safety Controller, Siemens, www.usa.siemens.com/safety
• Safety — Process safety, intrinsic safety
• aeShield v4 Functional Safety Lifecycle software, aeSolutions, www.aesolns.com
• ProSys SLM Safety Lifecycle Management software, Mangan Software Solutions, www.mangansoftware.com
• STZ Functional Safety Dual Input Smart HART Temperature Transmitter, Moore Industries, www.miinet.com
• OptiSIS packaged safety instrumented system, Rockwell Automation, www.rockwellautomation.com

Software — Alarm management 

• DynAMo Alarm and Operations Management software, Honeywell Process Solutions, www.honeywellprocess.com
• Wonderware Alarm Adviser software, Schneider Electric, http://software.schneider-electric.com
• WIN-911 Enterprise alarm notification software, WIN-911 Software, www.win911.com

Software — Applications 

• AxiomView data visualization software, Canary Labs, www.canarylabs.com
• LOOP-Pro Tuner Premium, Control Station Inc., http://controlstation.com
• Fluke Connect Assets cloud-based wireless system of software and test tools, Fluke Corp., http://en-us.fluke.com
• Uniformance Asset Sentinel, process and equipment health surveillance software; Honeywell Process Solutions; www.honeywellprocess.com
• KEPServerEX 5.18 device connectivity software, Kepware Technologies, www.kepware.com
• Assembly Management System (AMS), Leidos, www.leidos.com/engineering
• MatrikonOPC UA Proxy for devices and application communications, MatrikonOPC, www.matrikonopc.com/opc-ua
• Dream Report v4.64 software to create reports and KPI dashboards, Ocean Data Systems, www.dreamreport.net

Software — Control design 

• Field Information Manager (FIM), ABB, www.abb.com/measurement
• TwinCAT 3 SOA-PLC, service-oriented architecture for programming; Beckhoff Automation; www.beckhoffautomation.com
• AssetWorX, software to create reusable templates (equipment classes); Iconics; www.iconics.com
• InduSoft IoTView data collection and visualization (HMI) software, InduSoft, www.indusoft.com
• Rockwell Software Studio 5000 Logix Designer Software, Rockwell Automation, www.rockwellautomation.com
• Sim Sci Logic Validator, Schneider Electric, www.simsci.com

Software — Diagnostics 

• PlantESP Loop Performance Monitoring, Control Station Inc., http://controlstation.com
• Power Xpert Insight Software to see real-time energy use, Eaton, www.eaton.com/electrical
• AMS Device Manager v13.0 intelligent field device software, Emerson Process Managment, www.emersonprocess.com
• Industrial Cyber Security Risk Manager, Honeywell Process Solutions, www.honeywellprocess.com
• PlantTriage MPC, model predictive control performance assessment software; Metso ExperTune; www.expertune.com
• FactoryTalk VantagePoint v6.0 Enterprise Manufacturing Intelligence software, Rockwell Automation, www.rockwellautomation.com
• Avantis PRISM predictive asset analytics software, Schneider Electric, www.instepsoftware.com

Software — HMI software

• InduSoft Web Studio 8.0, InduSoft, www.indusoft.com
• FactoryTalk View Site Edition v8.0 supervisory-level HMI software, Rockwell Automation, www.rockwellsoftware.com
• SCADA Expert Vijeo Citect 2015, Schneider Electric, http://software.schneider-electric.com
• UniLogic 1.14.33 PLC, HMI, and communications programming and configuration software; Unitronics Inc.; www.unitronics.com

Software — Mobile apps for controls, automation, instrumentation

• KPIWorX, app to create, save, and load self-service dashboards on tablets and smartphones; Iconics; www.iconics.com
• groov 3.0, to build mobile apps to securely monitor and control from smartphones and tablets; Opto 22; www.opto22.com

Software — Process applications

• Production Accounting & Reconciliation, Honeywell Process Solutions, www.honeywellprocess.com
• UniSim Competency Suite, training software for process industries; Honeywell Process Solutions; www.honeywellprocess.com
• FactoryTalk Batch v12.0 software, Rockwell Automation, www.rockwellautomation.com
• Wonderware Recipe Manager Plus, Schneider Electric, http://software.schneider-electric.com

Are you taking care of your motor? Are you avoiding failures? Are you practicing energy efficiency?

Don't suffer from total motion control failure. Check out our roundup of helpful motor articles to make sure your motor measures up.

1. Soft starters bring motors up to speed in a controlled fashion

The use of a soft starter can help save mechanical wear-and-tear and could save money in other ways. Read more.

2. How to raise motor efficiency

To paraphrase the Duchess of Windsor, a motor can never be too efficient. However, striving for more efficiency can entail examining everything from motors to entire systems. Read more.

3. When industrial steppers measure up

Selecting the right stepper motor—or any stepper motor at all—comes down to answering some questions about what's to be moved, how fast to move it and with what precision. After that, other factors such as cost and suitability to a given environment should be considered. Read more.

4. Profit from motor management best practices

Upgrading to more energy-efficient motors can be an important step to improve a facility's equipment reliability, increase productivity, and reduce downtime and repair costs. Read more.

5. Industrial motor efficiency and more

The gains might be small and the cost differential large if you change from a premium to a super-premium, or high-efficiency, motor. The extra expense can be worth it, though, given the duty cycle. However, other motor capabilities, such as being able to run efficiently enough at an appropriate and variable speed, might offer an even better payoff. Read more.

6. Three specifications, or so, for motor choice

When it comes to electric motors, there's a seemingly bewildering array of choices to the uninitiated eye. Machine builders can pick among brushless dc, cored and coreless dc, and stepper motors, and that can just be the offerings of a single vendor. Throw in the entire universe of motor makers and it's enough to lead to a specification nightmare. Read more.

7. Gain the upper hand with energy-smart machine design

Customers know the financial, operational and environmental advantages of energy efficiency. Machine builders are responding with smarter, more efficient technologies. Read more.