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Applied Automation April 2014 • A3

A4 Electric motor power measurement and analysis Over the next three issues of AppliedAutomation, we will discuss a

three-step process for making precision electrical and mechanical power measurements on a variety of motors and variable speed drive systems. We will also show how these measurements are used to calculate the energy efficiency for motor and drive systems.

A8 Selecting the right control chart Knowing right way to look at collected manufacturing or process data turns

numbers into valuable information; here’s how to choose the right control chart to make real-time control monitoring more valuable.

A12 Linear position sensors gain acceptance Today’s industrial process control applications increasingly use automated

systems to optimize operations and ensure a safer, more productive process. Linear position sensors used in these automated systems provide highly accurate feedback on product parameters, control states, and outputs to machine controllers.

Contents

A12

COMMENT

System integrators represent a significant demographic of the AppliedAutomation readership. Regardless of whether automation end users are discrete,

process, batch, or hybrid manufacturers, or whether they are utilities or municipalities, chances are your organization has had some contact with automation system integrators.

Automation system integration is among the many information channels on the Control Engineering and Plant Engineering Websites. Content specific to this topic can be located by searching the archives maintained by both of these CFE Media publications.

Several types of media are included in this content. For example, Control Engineeringmagazine publishes the Automation Integrator Guide annually. Each issue features articles about automation system integration best practices, industry outlooks, and an industry directory with profiles of automation system integrators. The online version of this directory

is a tool for identifying automation system integrator talent. This searchable guide provides information about company size, industries supported, engineering specialties, product experience, professional affiliations, and other important search criteria.

Webcasts and training videos are available online by accessing the Education & Training and People and Training tabs on the Control Engineering and Plant Engineering Websites, respectively.

Each year, a panel of Control Engineeringeditors and industry expert judges select System Integrator of the Year Award winners by evaluating business skills, technical competence, and customer satisfaction. Winners are then inducted into the Control Engineering System Integrator Hall of Fame.

Control Engineering also names System Integrator Giants, the 100 largest automation system integration firms, according to revenue, that respond to the magazine’s annual survey.

Finding system integration resources

Jack SmithEditor

A4

A4 • April 2014 Applied Automation

cover story

Electric motor powermeasurement and analysis

Understand the basics to drive greater efficiency.

E nergy is one of the high-est cost items in a plant or facility, and motors often consume the lion’s share of plant power, so making sure motors are operating optimally is vital. Accurate power

measurements can help to reduce energy consumption, as measurement is always the first step toward better perfor-mance and can also help extend the life of a motor. Small misalignment or other issues are often invisible to the naked eye, and the slightest wobble in a shaft can nega-tively affect productivity and quality, and even shorten the life of the motor.

Over the next three issues of AppliedAutomation, we will discuss a three-step process for making precision electrical and mechanical power measurements on a variety of motors and variable speed drive (VSD) sys-tems. We will also show how these measurements are used to calculate the energy efficiency for motor and drive systems.

In addition, we will provide an understanding of how to make precision power measurements on complex distorted waveforms, as well as what instruments to use for different applications.

Basic electrical power measurements Electric motors are electromechanical machines that

convert electric energy into mechanical energy. Despite differences in size and type, all electric motors work in much the same way: an electric current flowing through a wire coil in a magnetic field creates a force that rotates the coil, thus creating torque.

Understanding power generation, power loss, and the different types of power measured can be intimidating, so let’s start with an overview of basic electric and mechani-cal power measurements.

What is power? In the most basic form, power is work performed over a specific amount of time. In a motor, power is delivered to the load by converting electrical energy per the following laws of science.

In electrical systems, voltage is the force required to move electrons. Current is the rate of the flow of charge per second through a material to which a specific voltage is applied. By taking the voltage and multiplying it by the associated current, the power can

be determined.

P = V x I where power (P) is in watts, voltage (V) is in volts, and current (I) is in amperes.

A watt (W) is a unit of power defined as one Joule per second. For a dc source the calculation is simply the voltage times the current: W = V x A. However, determining the power in watts for an ac source must include the power factor (PF), so W = V x A x PF for ac systems.

The power factor is a unitless ratio ranging from -1 to 1, and represents the amount of real power performing work at a load. For power factors less than unity, which is almost always the case, there will be losses in real power. This is because the voltage and current of an ac circuit are sinusoidal in nature, with the amplitude of the current and voltage of an ac circuit constantly shifting and not typically in perfect alignment.

Since power is voltage times current (P = V x I), power is highest when the voltage and current are lined up together so that the peaks and zero points on the voltage and current waveforms occur at the same time. This would be typical of a simple resistive load. In this situation, the two waveforms are “in phase” with one another and the power factor would be 1. This is a rare case, as almost all loads aren’t simply and perfectly resistive.

Two waveforms are said to be “out of phase” or “phase shifted” when the two signals do not correlate from point to point. This can be caused by inductive or nonlinear loads. In this situation, the power factor would be less than 1, and less real power would be realized.

Due to the possible fluctuations in the current and the voltage in ac circuits, power is measured is a few differ-ent ways.

Real or true power is the actual amount of power being

By Bill Gatheridge Yokogawa

FIRST OF THREE PARTS APRIl: Electric motor power measure-

ment and analysis

JunE: Selecting the right instruments

AuguST: Electrical power measurements for a 3-phase ac motor.


Figure 1: The slightest wobble in a shaft can negatively affect productivity and quality. All graphics courtesy: Yokogawa

used in a circuit, and it’s measured in watts. Digital power analyzers use techniques to digitize the incoming voltage and current waveforms to calculate true power, following the method in Figure 2:

Figure 2: True power calculation.

In this example the instantaneous voltage is multiplied by the instantaneous current (I) and then integrated over a specific time period (t). A true power calculation will work on any type of waveform regardless of the power factor (Figure 3).

Figure 3: These equations are used to calculate a true power mea-surement and true RMS measurements.

Harmonics create an additional complication. Even though the power grid nominally operates at a frequency of 60 Hz, there are many other frequencies or harmonics that potentially exist in a circuit, and there can also be a

dc or dc component. Total power is calculated by consider-ing and summing all content, including harmonics.

The calculation methods in Figure 3 are used to pro-vide a true power measurement and true root mean square (RMS) measurements on any type of waveform, including all harmonic content, up to the bandwidth of the instrument.

Power measurementWe’ll next look at how to actually measure watts in a

given circuit. A wattmeter is an instrument that uses volt-age and current to determine power in watts. The Blondel Theory states that total power is measured with a mini-mum of one fewer wattmeter than the number of wires. For example, a single-phase two-wire circuit will use one watt-meter with one voltage and one current measurement.

A single-phase three-wire split-phase system is often found in common housing wiring. These systems require two wattmeters for power measurement.

Most industrial motors use three-phase three-wire circuits that are measured using two wattmeters. In the same fashion, three wattmeters would be necessary for a three-phase four-wire circuit, with the fourth wire being the neutral.

Figure 4 shows a three-phase three-wire system with load attached using the two-wattmeter method for mea-surement. Two line-to-line voltages and two associated phase currents are measured (using wattmeters Wa and


Wc). The four measurements (line-to-line and phase current and voltage) are utilized to achieve the total measurement.

Figure 4: Measuring power in a three-phase three-wire system with two wattmeters.

Since this method requires monitoring only two current and two potential transformers instead of three, installation and wiring configuration are simplified. It can also measure power accurately on a balanced or an unbalanced system. Its flexibility and low-cost installation make it a good fit for production testing in which only the power or a few other parameters need measurement.

For engineering and research and development work, the three-phase three-wire with three-wattmeter method is best as it provides additional information that can be used to balance loading and determine true power factor. This method uses all three voltages and all three-currents. All three voltages are measured (a to b, b to c, c to a), and all three-currents are monitored.

Figure 5: When designing motors and drives, seeing all three volt-ages and currents is key, making the three-wattmeter method in the figure above the best choice.

Power factor measurementIn determining the power factor for sine waves, the

power factor is equal to the cosine of the angle between

the voltage and current (Cos Ø). This is defined as the “displacement” power factor, and is correct for sine waves only. For all other waveforms (non-sine waves), the power factor is defined as real power in watts divided by appar-ent power in voltage-amperes. This is called the “true” power factor and can be used for all waveforms, both sinu-soidal and non-sinusoidal.

Figure 6: Total power factor is determined by summing the total watts divided by the total VA measurement.

Figure 7: Using the two-wattmeter method, the sum of the total watts (W1 + W2) is divided by the VA measurements.

However, if the load is unbalanced (the phase currents are different), this could introduce an error in calculat-ing the power factor because only two VA measurements are used in the calculation. The two VAs are averaged because it’s assumed they’re equal; however, if they’re not, a faulty result is obtained.

Therefore, it’s best to use the three-wattmeter method for unbalanced loads because it will provide a correct power factor calculation for either balanced or unbal-anced loads.

Figure 8: With the three-wattmeter method, all three VA measure-ments are used in the above power factor calculation.

Power analyzers use the method above, which is called the 3V-3A (three-voltage three-current) wiring method. This is the best method for engineering and design work because it will provide a correct total power factor and VA measurements for a balanced or unbalanced three-wire system.

Basic mechanical power measurementsIn an electric motor, the mechanical power is defined as

the speed times the torque. Mechanical power is typically defined as kilowatts or horsepower, with 1 W equaling 1 Joule/sec or 1 Nm/sec.

cover story

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Figure 9: Mechanical power measurements in watts are defined as 2π times the rotating speed (rpm) divided by 60 times the torque (Nm).

Horsepower is the work done per unit of time. One hp equals 33,000 lb-ft/min. Converting hp to watts is achieved using this relationship: 1 hp = 745.69987 W. However, the conversion is often simplified by using 746 W/hp (Figure 10).

Figure 10: Mechanical power measurement equations for horse-power often use a rounded figure of 1 hp = 746 W.

For ac induction motors, the actual or rotor speed is the speed at which the shaft (rotor) rotates, typically measured using a tachometer. The synchronous speed is the speed of the stator’s magnetic field rotation, calculat-ed as 120 times the line frequency divided by the number of poles in the motor. Synchronous speed is the motor’s theoretical maximum speed, but the rotor will always turn at a slightly slower rate than the synchronous speed due to losses, and this speed difference is defined as slip.

Slip is the difference in the speed of the rotor and the synchronous speed. To determine the percentage of slip, a simple percentage calculation of the synchronous speed minus the rotor speed divided by the synchronous speed is used.

Efficiency can be expressed in simplest form as the ratio of the output power to the total input power or efficiency = output power/input power. For an electri-cally driven motor, the output power is mechanical while the input power is electrical, so the efficiency equation becomes efficiency = mechanical power/electrical input power.

Bill Gatheridge is a product manager at Yokogawa. He is a member and vice chairman of the ASME PTC19.6 com-mittee on electrical power measurements for utility power plant performance testing.


DATA CAPTURE

Selecting the right control chartFor real-time monitoring, a control chart is a statistical tool to analyze the past and

predict the future. Choosing the wrong one from among hundreds increases the riskof errors. Advice follows on how to choose the right control chart.

Knowing right way to look at collected manu-facturing or process data turns numbers into valuable information; here’s how to choose the right control chart to make real-time con-trol monitoring more valuable.

Would a manufacturer knowingly embark on a fixed-cost job without first understanding the risks of losing money, shipping defective product, missing the delivery schedule, running on incapable equipment, or using unqualified employees? While all these risks

are understood because the price quoted for the job includes an allowance for their associated costs, many of these risk items are actually either unknown or not fully defined. Thus, decisions to pursue a job are usually based on history, opinion, and faith alone.

Luckily, the chance of a catastrophic financial hit due to these unknowns is relatively small as long as the profit margins remain high enough after negotia-tions. However, as margins are squeezed and demands increase, manufacturers must understand these uncer-tainties better to ensure they avoid the financial break-ing point. The good news is that understanding risk and making better business decisions is as simple as apply-ing statistical monitoring and analytics.

Real-time monitoring,control charts

Statistics is the science of pre-dicting the future. Industrial statis-tical methods are the application of statistical methods where the population of “things to measure” is produced in real time. For real-time monitoring, the prescribed statistical tool is a control chart. Academic training introduces stu-dents to three types of variables charts (Xbar-R, Xbar-s, and IX-MR) and four types of attribute charts (p, np, u, and c). There are hun-dreds of control charts from which to choose. Regardless of statistical background, not having the right control chart increases the risk of encountering Type I (false positive) and Type II (false negative) errors. The purpose of a control chart is to describe a process’s personality in terms of normal versus abnor-mal levels of variation. When using control charts for real-time deci-sion making, corrective actions are recommended only when variation levels or patterns exceed the statis-

1) What is the sample size? 2) Will multiple parts be combined on the same chart?3) Will test characteristics with different target values be combined on the same chart? For example, if the sample size is 1, multiple parts will be combined, but all the targets are the same, so the perfect control chart to use is the Group IX-MR. Alternately, if the sample size is 5 with multiple parts and different targets, the chart to use is the Group Target Xbar-R. Courtesy: InfinityQS International

Variable control chart decision tree

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DATA CAPTURE

tically defined levels of what’s nor-mal. When inferior sampling strate-gies are implemented or the wrong control chart is deployed, the risk of making unwise adjustments (Type I error) or missing a signal that war-rants attention (Type II error) is elevated.

Why invest time and effort in col-lecting and analyzing data just to make wrong decisions? Taking the extra step to learn how to pick the right chart could mean the difference between failure and success.

Ask these questions to choose a control chartFortunately, selecting just the right control chart

requires answering only a handful of questions that will pinpoint the perfect chart to use from a pool of 12 potential, standard variables charts.

Basic questions for variables data are: 1. What is the sample size? 2. Will multiple parts be combined on the same

chart? 3. Will test characteristics with different target values

be combined on the same chart?

To answer these questions properly and ultimately select the correct control chart, a thoughtful sampling strategy is key. In some cases, simple strategies will suffice where a machine is set up to run the same part for weeks or months, and only one or two characteristics are measured to monitor the health of that process. For example, a machine that makes 0.07 mm pencil lead will be busy as long as 0.07 mm mechanical pencils are being used and this par-ticular product is being sold. Of course, there are many contributing factors that will cause a lead machine to misbehave, but as far as a statistical sampling strat-egy, diameter and length may be all that’s monitored. Depending on the historical adjustment frequencies, five leads may need to be collected only once an hour. Though this may be a common case for textbooks, it reflects the real world for only a few industries.

For most manufacturers, machines are used to run many different shapes, sizes, weights, materials, col-ors, and features. To accomplish this, one machine is designed to accept different programs, tooling, fixtures, speeds, feeds, pressures, temperatures, flow rates, and others. The uncertainties and combinations of things that could go wrong multiply with every added

level of machine flexibility. In these cases, one must create customized sampling strategies and pick the best statistical monitoring tool(s) unique to each machine’s input and product output complexities.

Items to consider in a sampling strategy include sampling fre-quency, sample size, test charac-teristics, measurement devices, and

methodologies. These decisions help define the best way to illustrate and update the visual output as new data is captured. Essentially, the data describes the process’s personality so it is easier to understand what normal variation one can expect and what constitutes a significant deviation from the norm.

Variation, different units With a strategic sampling strategy in place, it is

much easier to answer the questions necessary to use the variable control chart decision tree (see graphic). In addition to a sampling strategy, more complicated scenarios require only two more questions:

1. Will within-piece and piece-to-piece variation be monitored?

2. Will different types of tests with different units of measure be combined on the same chart?

Adding these two questions expands the list of potential control charts to 48. With each of those 48 charts, one could apply even more refinements, taking the potential number of charts into the hundreds.

Above all, remember that a control chart is the vehicle that will help those involved to remain

engaged with the data collected. By engaging with the right data and using the right control chart, no fortune-teller is needed to predict risks and make better busi-ness decisions.

Steve Wise is vice president of statistical methods, InfinityQS International Inc.

Go onlinewww.controleng.com/archivesMarch, with this article, link to process details in an InfinityQS International whitepaper, “A Practical Guide to Selecting the Right Control Chart.”www.infinityqs.com

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Today’s industrial process control applications increasingly use automated systems to optimize operations and ensure a safer, more productive process. Linear position sensors used in these automated systems provide highly accurate feedback on product parameters, control states,

and outputs to machine controllers. Whether implemented as a stand-alone component or

as part of a control or safety system, the linear variable dif-ferential transformer (LVDT) is capable of providing linear displacement measurements from micro inches to several feet, under various operating and environmental conditions with high accuracy and reliability. Essentially, the LVDT plays an important role in machine control by providing feedback about product location. To some extent, it is the LVDT that ensures proper machine operation.

Mechanics of a LVDTIn basic terms, a LVDT

is an electromechanical device that converts linear position or motion to a proportional electrical out-put (see Figure 1). More specifically, the LVDT pro-duces an electrical output signal directly proportional to the displacement of a separate movable core. Typically, the ferrous core within the LVDT is attached to the moving element on the piece of equipment requiring posi-tion feedback.

The basic LVDT design consists of three elements:

1. One primary winding2. Two identical second-

ary windings

3. A movable magnetic armature or “core.”

The primary winding is excited with an ac supply generating a magnetic field which, when the core is placed in the central or “null” position, includes equal voltages in both of the secondaries. The secondaries are wired series opposed so that their combined output represents the difference in voltage indicated in them, which in this case is zero. As the core is moved left or right, the difference in inducted voltages produces an output that is linearly proportional in magnitude to the displacement of the core. Its phase changes 180-deg from one side of the null position to the other.

In the oil and gas industry, compact LVDTs are used in the position feedback control of down-hole drilling equipment such as bore scopes that measure the ID of the drilled hole. The sensor coil assembly and sepa-rable core inherent to the technology can withstand extremely high pressures of the environment as the mechanical configuration of the coil assembly is vented

(pressure balanced) to the pressure of the nonconductive medi-ums. As the sensor coil assembly can withstand a combination of high pressure, elevated tem-peratures, shock, and vibration, the LVDT is able to make measure-ments in down-hole drilling equipment pos-sible where space is at a premium and the environment is hostile.

In operation, the LVDT’s primary winding is energized by alter-nating current of appro-priate amplitude and frequency, known as the primary excitation. The LVDTs’ electrical output signal is the dif-


SENSORS

Linear position sensorsgain acceptance

Linear variable differential transformers can deliver better machine operation.

Figure 1: The basic LVDT design. All images courtesy: Macro Sensors

By Eileen OttoMacro Sensors

ferential ac voltage between two secondary windings, which varies with the axial position of the core within the LVDT coil. Usually this ac output voltage is con-verted by suitable electronic circuitry to high-level dc voltage or current for convenient use by a computer or other digital output device.

Because there is normally no contact between the LVDT’s core and coil structure, no parts can rub together or wear out. This means that a LVDT features unlimited mechanical life. This factor is highly desirable in many industrial process control and factory automa-tion systems.

Enhanced use in process controlRecent innovations in construction materials, manu-

facturing techniques, and low-cost microelectronics have revolutionized the LVDTs into a more reliable and cost-effective technology for process control applications. In the past, electronics necessary to operate LVDTs prop-erly were complicated and expensive, prohibiting their wide use in process control applications for displacement measurement.

Modern ASIC and microprocessors give LVDT tech-nology more complex processing functions and enable signal conditioning within the sensor housing so LVDTs generate digital outputs directly compatible with comput-er-based systems and standardized digital buses. As a result, today’s linear position sensors can provide more accurate and precise measurement of dimensions in a wider variety of quality control, inspection equipment, and industrial metrology applications including online parts inspection, servo-loop positioning systems, and manufac-turing process control.

For applications where sensors must operate in extreme environments, the sensing element can be seg-regated from the electronic circuitry, unlike capacitive, magnetostrictive, and other high-frequency technologies. Connected by long cables up to 31 m (100 ft), ac-oper-ated LVDTs can work with remotely located electronics that power the sensors, and amplify and demodulate their output. Output is, then, displayed on a suitable readout and/or inputted into a computer-based data acquisition system for statistical process control. This ability to trans-mit data to a remote computer has made linear position sensors popular in quality assurance schemes.

Smaller diameters, new materialsWhile linear position sensors were once considered

too long for applications with limited space, new wind-ing techniques and computer-based winding machines allow the linear position sensor body to be reduced while maintaining or increasing stroke length. With the improved stroke-to-length ratio (now up to 80%), the LVDT becomes a viable position measurement device for

machine tool positioning, hydraulic cylinder positioning, and valve position sensing.

Smaller, contactless linear position sensors also fea-ture a lightweight low mass core that is ideal (see Figure 2) for process control applications having high-dynamic response requirements, such as plastic injection molding machines, automatic inspection equipment, and different robotic applications requiring displacement feedback to ensure proper machinery operation.

LVDTs are also configurable in a variety of mechani-cal and electrical designs to meet the measurement and environmental requirements of various process control applications. New corrosion-resistant/high-temperature materials such as Monel or Inconel enable the LVDT to operate in more hostile environments, including those with high and low temperature extremes, radiation expo-sure, or vacuum pressure conditions. For applications where sensors must withstand exposure to flammable or corrosive vapors and liquids, or operate in pressur-ized fluid, its case and coil assembly can be hermetically sealed using a variety of welding processes.

For example, in power generation applications (see Figure 3), linear position sensors designed for high temperature and mild radiation resistance can perform in power plants to provide feedback on the position of nuclear steam and gas turbine control valves for increased plant efficiency and reduced operating costs.

In a typical power plant, steam turbines contain a number of control valves—a reheat stop value, an inter-ceptor valve, a governor valve, and a throttle valve. Typically, plants have very precise control schemes for valve position to increase operating efficiency and save fuel. Operating within the harsh environment of a power or steam plant, linear position sensors can determine if valves are fully opened or closed to within a thousandth of an inch, providing output to remote electronics that can


Figure 2: In the oil and gas industry, compact LVDTs are used in the position feedback control of down-hole drilling equipment such as bore scopes that measure the ID of the drilled hole.

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be monitored by operators if something is not working properly. The combination of LVDTs with modern comput-erized turbine control systems saves power companies mil-lions of dollars per year.

Sensors also play an important role in the predic-tive maintenance of gas tur-bines as part of process con-trol systems used to monitor shell expansion and bearing vibration. When installed on turbine shells, hermetically sealed LVDTs measure shell expansion, providing linear output that operators can utilize to determine proper ther-mal growth of a turbine shell during start-up, operation, and shutdown.

LVDTs designed to withstand shocks and heavy pounding are used in the press and dye industry for the mechanical control of machine operations as improper operation can lead to broken dyes that result in downed machines, while the ambiguous force of presses can lead to misshapen and out-of-spec parts. Spring-loaded

LVDTs are installed on presses so that the plunger of the sensor is compressed as the punch press comes in contact with the metal being shaped. The output of the LVDT is fed back into the machine’s control system, providing feedback on how far a press has moved and when to stop.

For more than six decades, LVDTs have served as part of measurement and control systems, providing

essential information without which many process con-trol systems couldn’t function. From its limited use as a laboratory tool more than three decades ago, the LVDT has evolved into a highly reliable and cost-effective linear feedback device, making it the preferred technology for critical and reliable linear displacement measurements in an array of industrial process control applications.

Eileen Otto is the sales and marketing manager at Macro Sensors.

SENSORS

Figure 3: Hermetically sealed linear position sensors offer a highly accurate and long life solution for the position mea-surement of steam control valves in power generation plants.

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process applications

control engineering

integral sensors

sensors transmitters

control states

right control chart

electronic pressure

continuous level measurement