AC Motor Drives (part 2)

Drives (AC)-Technical Concerns

--- Voltage reflection/standing waves (made by ABB): IGBT Bi-Polar Transistor; Device Rise Time (dv/dt)

SCR and GTO control of AC drive power structures have been around since the 1960s. Forced commutated SCR PWM drives gained increased acceptance in the mid-1970s. This was followed by GTO and bipolar transistor-based PWM drives in the mid-1980s. In the late-1980s, IGBT PWM drives were emerging as the drive to take the variable-speed industry into the 21st century. By the early 1990s, several manufacturers were promoting a full-line of IGBT based AC drives for the industrial, as well as HVAC, marketplace. With these AC drive offerings came several advantages and some challenges. One of the advantages and challenges.

--- Voltage reflection characteristics: Drive M; PWM Pulse goes out Pulse is Reflected Back; Low HP; High Impedance Motor

As seen, as the technology era of power semiconductor devices changed, so did the number of circuit boards needed to support that technology. In the 1960s and 1970s, SCRs and GTOs needed more than a dozen circuit boards to support the gating of the power device.

Given the fact that each board had a retail value of $900-$1200, it’s easy to understand the high cost of AC drives in that era. Separate gate driver boards were needed for each SCR or GTO device to turn off the device and control timing circuitry. In relative terms, the device turn-on time was rather slow, compared with the other emerging technologies. With slower turn-on or switch times, the drives caused audible noise in the motor of 500-1000 Hz, quite a noticeable level. The laminations in the stator winding vibrated at the switch frequency, producing the noise much like an audio speaker.

With the advent of bipolar transistors came the requirement for fewer boards. Less sophisticated control circuitry was required since there was no need for separate gate driver boards. Fewer circuit boards meant less overall cost for the drive. In addition, the relative size of the drive was reduced, compared with SCR-based products. On the positive side, the bipolar transistor switched 3-6 times faster than SCRs or GTOs (in the 1- to 3-kHz range). This meant that the audible noise was also reduced to a more tolerable level.

When IGBT technology emerged in the early 1990s, it was considered the power technology of the future. The device switched over 10 times faster than bipolar transistors (3-12 kHz), which meant a drastic reduction in audible noise. The circuit board count was reduced to two. The control board contained all the circuits for timing and signal processing. The motor control board contained all the circuits to turn on and off the device. With only two circuit boards needed, the drives industry realized the lowest cost drive possible. There was also a reduction in size to about 1/3 that of bipolar transistor drives. With this technology advancement came an acute challenge for the device connected to the drive-the motor.

With the extremely fast switching times, came the rise of a phenomenon called voltage reflection. Voltage reflection is caused by the fast-rising voltage waveform versus unit of time. In essence, the IGBT turns on immediately compared with 30 times longer with other devices. When a drive switches at this high rate, a reflected wave back from the motor adds to the voltage leaving the output of the drive. The result is a voltage at the motor terminals greater than the original voltage output from the drive.

As seen, this situation is more of an issue when an impedance mismatch exists between the drive output/motor cables and the motor terminals. The phenomena is similar to the standing wave ratio (SWR) that exists in citizens band (CB) radio antenna setups. The coil of the CB antenna must be installed and tuned correctly, so that there are no waves reflected back to the transmitter, which could cause damage. If tuned properly, the antenna absorbs all the energy the transmitter can deliver.

The amount of increased voltage at the motor terminals is a function of the drive output voltage, length of motor cable, and the amount of mis match. This situation is a possibility more often in smaller motors, which have a higher impedance compared with motors in the 150-HP range or more. In some cases, it’s not uncommon to see more than twice the drive output voltage at the motor terminals. Many 460-V motor insulation systems are not designed to handle that type of spike voltage. The motor volt age spike issue can be seen below.

--- Motor terminal voltage ( Trans-Coil, Inc.) Voltage Reflection Spikes; DC Link Voltage

The spike voltages created in this particular case are close to 1500 V (460 V drive). Because AC, IGBT drives are installed at significant distances away from the motor, the impedance mismatch can be present for various types and brand names of motors. For example, it has been determined that a typical IGBT drive would cause twice the output voltage at the motor terminals. This would be true if the motor is installed greater than 75 feet away from the drive.

There are several possibilities in protecting motors against damage or coping with the issue. On new drive installations, verify that motors installed a significant distance away from the drive meet NEMA MG1, part 31.4.4.2 standards. These motors are designed with insulation systems that are able to handle the over-voltage stress. ---45 indicates the construction of a random wound versus a form wound motor. The concentric wound or form wound motor is designed to handle spike voltages generated.

--- Random wound vs. concentric wound motors: First Turn, Last Turn, Concentric Winding Random Winding

--- Effects of a dv/dt filter on voltage reflection. Voltage Reflection Spikes (75% reduction)

Motors that meet the MG1 standard are concentric wound and are termed inverter duty motors. These motors contain stator windings that are care fully formed around the stator slots, so that the first winding turn is not next to the last winding turn. Voltage spikes poke minute holes in the insulation. When that occurs in a random wound motor, the likelihood is that the first and last turn are next to each other. A voltage spike hole would therefore short out the winding and make the motor useless until rewound. Inverter duty motors also have extra slot paper insulation separating the windings of different phases. In addition, these motors are typically dipped in lacquer insulation after the windings are complete to add to the insulation strength and cover insulation holes that may have occurred.

Some inverter duty motors are actually dipped a second time to improve the dielectric (insulation) strength.

Another means of protecting the motor against possible damage is to install output reactors, (similar to line reactors) at the output of the drive.

The drive manufacturer can make recommendations. Usually 1.5-3% impedance will protect existing motors to about 500 feet. If distances greater than 500 feet are encountered, dv/dt filters can be installed at the output of the drive. These filters are usually effective up to distances of about 2000 feet. (Note: dv/dt means change of voltage vs. change in time.) This is a special resistor-inductor-capacitor filter designed to drastically reduce the over-voltage spikes at the drive output.

Additional precautions include installing a sine filter at the output of the drive, which is not limited to motor distance. In addition, a snubber circuit installed at the motor will have over-voltage reduction similar to dv/dt filters. Snubber circuits don’t usually have any distance limitation. A reduction in spike voltage generation with the installation of an output dv/dt filter.

At long motor cable lengths (e.g., 250 to 300 ft or more), another phenomenon can occur - that of capacitive coupling. Conductors separated by an insulator make up a capacitor. With additional capacitance at the VFD output, higher current is calculated by the VFD. To the VFD, the motor will appear to consume increased amounts of current, which can cause nuisance "overcurrent" trips. This condition can be exaggerated by higher IGBT switch frequencies and the lack of output snubber circuits. In most cases, the motor is consuming appropriate current, but the motor cabling causes inappropriate readings by the VFD. Typically, an output reactor, as previously indicated, can serve a dual purpose-protect motor insulation from damage, and improve the VFD current calculations. This ultimately results in more stable VFD performance with less nuisance faults. Some manufacturers include hidden parameters to assist in tuning up the VFD current sensing circuit.

--- Distortion caused by the 5th and 7th harmonic (Trans-Coil, Inc.)

Harmonics Generation

Harmonics are basically a distortion of the original waveform. In the case of AC drives, harmonics are a distortion of the three-phase waveform, with the harmonic components fed back onto the AC power line. Harmonics are caused by the fact that AC drives draw current from the supply line in "bursts" rather than in a "linear" fashion. Because of this characteristic, AC drives are considered nonlinear loads. As a matter of fact, any electronic device that draws nonlinear currents causes a certain amount of harmonics.

VCRs, big screen TVs, stereos, and laptop and desktop computers all fall into the nonlinear load category. They include a switch mode power supply that changes AC to DC-a rectifier. Because rectifiers draw current in bursts, they create harmonic currents, which are fed back to the power source.

A six diode bridge AC drive (called a 6 pulse drive) produces the 5th, 7th, 11th, and 13th harmonic. The values of these harmonics is enough to distort the AC supply waveform. An example of distortion created by the 5th and 7th harmonic.

Note: The fundamental frequency is 60 Hz. Harmonic frequencies are multiples of the 60-Hz waveform (e.g., 5th = 300 Hz, 7th = 420 Hz, and so on). Users of AC drives need to be concerned about harmonics, which are generated back to the line supply. Harmonic currents don’t provide any useful work. Harmonic current distortion generates additional heating in transformers and cables, reducing the available capacity of the equipment.

Current distortion can also create resonance conditions between the line supply reactance and power factor correction capacitors (if used). In addition, the high frequencies of harmonics can cause electronic interference with telephone and telecommunications equipment.

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--- PCC and harmonic distortion ( ABB Inc.): To other utility customers To other utility customers PCC1 (Harmonic Current Distortion) 13.6 KV, Substation Transformer 4.16 KV PCC2 (Harmonic Voltage Distortion) MV PWM 480 V LV

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Harmonic voltage distortion causes increased heating in motors. Voltage distortion can also cause malfunctions in sensitive communications and computer equipment. In many areas, local electrical codes or drive-installation specifications require compliance with IEEE 519-1992. The locations where voltage and current harmonics can be an issue are termed the point of common coupling or simply, PCC.

The local power utility deals with the PCC, where the customer's building connects directly with utility power. This is the current harmonic distortion concern and is termed the total demand distortion or simply, TDD. The utility customer is faced with the voltage harmonic distortion concern, where non linear loads meet other loads, such as linear loads (inductors, line operated motors). ---48 indicates the locations of concern for both harmonic current and voltage distortion.

Overall, harmonics are a system issue. Harmonics that are produced by an individual drive are only important when they represent a significant portion of the total system. For example, if the drive load on a transformer represents over 1/4th of the kVA load, then harmonics could be an issue and requires further investigation. If the total drive load is 5 HP on a 1000 kVA transformer, harmonics would not be an issue. It’s worth noting that the addition of linear loads, such as line-operated motors, tend to reduce the overall system harmonic levels.

IEEE 519-1992 indicates limits of THD (voltage distortion) as 5% for general systems (e.g., factories and general office buildings, not including hospitals, airports, and power systems dedicated to drive loads). Current distortion limits (TDD) are based on a ratio. The ratio is the short-circuit current available at the PCC divided by the maximum fundamental load current. Therefore the limits will vary on the basis of the amount of the electrical current tank available. If the current capacity ratio is high, the allowable TDD will be high, compared with a low ratio. (Example: If a 10 lb rock is dropped in a bathtub full of water, the waves created represent the current harmonics generated-quite a significant amount. If that same rock is dropped off the Golden Gate bridge in San Francisco, the amount of waves hitting the shoreline would be almost non-existent-no significant current harmonics generated.) It’s important to note that an 80% THD nonlinear load will result in only a 8% TDD if the non-linear load is 10% of the total system load. With that in mind, there are several ways of reducing (mitigating) harmonics.

Using the above 80% THD example, the following comparisons could be drawn. Line reactors could be added to the input of the drive. A 5% line reactor (or equivalent DC bus inductor) could drop the THD from 80% down to 28%. Adding a 5th harmonic trap filter to the line reactor could drop the THD down to 13%. The cost of a line reactor may be 15-25% the cost of the drive (depending on drive horsepower). A harmonic trap filter could add 25-50% the cost of the drive (depending on drive horsepower).

Beyond these techniques, more serious mitigation could be realized, including higher associated costs. A 12-pulse drive input rectifier could be installed, along with a 5% impedance transformer. (Note: A 12-pulse drive is two six-diode bridge rectifiers, with a special delta-delta-wye input transformer, which could be 1/2 the cost of the drive unit itself. A 12-pulse drive effectively reduces the 5th and 7th harmonic. The total cost of the drive and transformer could be about double that of a six-pulse drive, depending on horsepower.) With this configuration, the THD could be reduced to 8%. With a 12-pulse drive, a 5% impedance transformer and an 11th harmonic trap filter, a reduction of THD down to 4% could be realized. Installing an active harmonic filter would reduce the THD down to 3%.

An active harmonic filter is essentially a regenerative drive. As stated above, this type of drive would yield the highest amount of harmonic mitigation of all the techniques. It could also be over twice the cost of a standard 6-pulse drive.

When dealing with harmonics, it’s helpful to work with the drive's manufacturer or a company specializing in harmonic mitigation techniques.

Some drive manufacturers offer a "no-cost" analysis service-submitting a harmonics report on the basis of the installation of their drive in a specific system. Harmonics will be even more of an issue in the future, with VFD's being applied in applications traditionally deemed fixed speed. The positive side to harmonics is that cost-effective techniques are available for a wide variety of installations.

Power Factor

When discussing electronic power conversion equipment, there are two ways to identify power factor: displacement and total or true power factor.

Displacement PF is the power factor of the fundamental components of the input line voltage and current. Total PF indicates the effects of harmonic distortion in the current waveform. No matter how PF is viewed, the power utility imposes penalties for customers that use equipment with a poor PF.

Displacement PF for an AC drive is relatively constant. It’s approximately

0.96-0.97. This value is primarily independent of the speed of the motor and its output power. The harmonic current distortion is determined by the total values of inductance, capacitance, and resistance, from power source to the load. The power distribution system has impedance, which also enters into the calculations for total PF. A general indication of displacement and total PF for AC as well as DC drives.

The curves shown are for drives at full load and constant torque operation. The shaded area indicates a variation of total PF for typical drive installations. For an AC drive, the total (true) PF varies from roughly 0.94 at rated load, down to below 0.75 when under a light load.

As you may recall, a lagging PF is seen for an AC induction motor used in a power system. The AC drive does an effective job in isolating the input power source from the lagging PF at which the motor operates. In a certain sense, AC drives could be considered PF correctors by means of its isolation from the AC line. Because of this fact, PF correction is not normally applied to AC, PWM drives. If existing PF capacitors are connected of the motor, they must be removed when an AC drive is installed in place of a full-voltage starter. PF capacitors between the drive output and the motor can cause physical damage to the drive output IGBTs.

--- Total and displacement PF (DC and AC drives). Power Factor; DC Drive Displacement pt AFD Total pt DC Drive Total pt AF Drive Displacement pt; Speed: 0% 25% 50% 75% 100%

Shielding and Grounding

Nearly all of the procedures related to DC drives shielding and grounding apply to AC drives. However, there are some additional guidelines that must be followed regarding AC installations.

AC, PWM drives tend to expose AC induction motors to high levels of common mode voltages. Common mode voltages are created at the neutral zero point of the output of a three-phase AC drive. This is due to the creation of three-phase AC, generated from a DC bus. Also at issue is the fact that a high level of dv/dt is also possible because of the IGBT power structure. (Note: dv/dt is defined as delta voltage per delta time, that is, change of voltage per change of time.) Common mode voltages can become apparent in several ways.

Damaging high-frequency-bearing currents can occur, causing damage inside the bearing inner race. The phenomena of bearing currents have been around for years. The incidence of motor damage, however, has increased during recent years, after the introduction of IGBTs. This is due to the rising voltage pulses and high switching frequencies that are created by the IGBT power structure. These voltage pulses and frequencies can cause repeated discharging through the bearings and result in a gradual erosion of the bearing inner race. --- indicates this discharge path through the bearings.

High-frequency current pulses are generated through the motor bearings.

If the energy of the pulses is high enough, metal transfers from the ball bearing and the races to the lubricant. This process is known as electrical discharge machining (EDM). Because of the high-frequency pulses, thou sands of these machining pits are created, which translate to metal erosion that can accumulate quickly. --- notes the inner race damage caused by bearing currents.

--- Bearing currents discharge path: Ball Bearing, Bearing Race, Motor Frame, Motor Base; Shaft, Stator, Rotor, Discharge, Current

--- Bearing "fluting" caused by bearing currents (by ABB Inc.)

In addition to bearing erosion, high-frequency ground currents can lead to malfunctions in sensitive sensor and instrumentation equipment.

To avoid these damaging bearing currents, a high-frequency, low-impedance path to ground must be provided between the drive and the motor.

This is accomplished by installing continuous corrugated aluminum armored cable, shielded power cable, and a properly installed conduit sys tem. --- indicates cabling, shielding, and grounding that provides low impedance paths to avoid high frequency and bearing current dam age.

---. Motor cabling, shielding, and grounding ( ABB Inc.) Incoming Feed; NEC Ground Input Transformer PWM Inverter ASD PE Ground Bus Building Floor Ground Grid Building Steel Column (1) - Armor, Shield, or Conduit (2) - NEC Ground Wires (4) - Auxiliary Motor Ground Concrete Pad (3) - Baseplate; Ground Wire 3 0 Induction

Motor

Recommended motor cable construction is described. It consists of aluminum armor that provides a low-impedance, high-frequency return path to ground.

--- Recommended motor cable construction: Insulating/Protective Outer PVC jacket Continuous Corrugated Aluminum Armor/Shield Bare Copper Ground Conductors (3) Insulated Phase Conductors (3) Sized per NEC for the Application

Once the motor cable is installed in the proper location, the termination of the cable is vitally important. ---54 indicates the recommended termination method for AC drives.

---54. Recommended motor cable termination method: Power Cables (3) Ground Wires (3) Locknut Continuous Corrugated Aluminum Armor Cable with PVC Jacket Ground Connection to PE Bus Ground Bushing Mounting Surface Plane Cable Fitting Body

The proper termination method includes 360º contact with the corrugated armor and grounding bushings for the connection of safety grounds.

Metal-to-metal contact with the mounting surface is extremely important in the installation.

Following these guidelines will reduce the effects of frequency generation in typical AC drive installations.

EMI and RFI

AC variable frequency drives generate a certain amount of electromagnetic interference (EMI). The same installation procedures for DC drives apply to AC, with a few additional guidelines. The following would be considered general wiring practices related to AC drives. Many of these items have been presented before, but deserve a review.

Never install motor cables (e.g., 460 VAC) and control wiring (e.g., 4-20 mA or 0-10 VDC) in the same conduit or cable tray. It’s also recommended that control wiring be shielded cable (for signals less than 24 V). Even though shielded control cable is used, the PWM output can have adverse effects on the low-power control signals. Unstable operating conditions can develop. It’s further recommended that the control wiring be installed in its own individual conduit. In essence, the best installation procedure would be three separate, grounded, metallic conduits: one conduit for input power to the drive, one conduit for output power to the motor, and one conduit for control wiring. If EMI could be a serious issue with the installation, the use of ferrous metallic conduit rather than aluminum can help. The traditional steel conduit contains a certain amount of iron that can provide additional shielding against stray EMI signals.

If control wiring and power wiring are not in separate conduits, then the control wiring must be kept a minimum of 12 inches away from power wiring. The crossing of control and power wiring at 90º angles will reduce the EMI effects if the sets of cables must be close to each other. The previously indicated grounding techniques are also required to reduce EMI. Process control sensors and equipment must also be connected using shielded cable. The shield should be grounded only at the drive end, with the shield at the signal end cut back and taped to avoid contact with ground. This will avoid the possibility of ground loops that could cause EMI.

On any installation that is to conform to EMC compliance, the manufacturer's recommendations must be strictly followed. Their documentation indicates that the required research was conducted. If their instructions are followed, the complete installation will be EMC-compliant. This includes the use of shielded cable and CE-rated equipment, as well as specific cable-termination techniques. Any deviations from their guidelines would void the compliance.

Radio frequency interference (RFI ) can be an irritating problem, just like EMI. RFI could be considered electrical noise and would take the form of conducted or radiated.

Conducted noise is noise that is conducted or reflected back onto the line supply. Since the AC VFD generates a carrier frequency, it could be considered a radio station, with the power input cable being the transmitter antenna. The simple procedure of installing power input and output cable in separate conduits will reduce the possibility of RFI. RFI can have serious effects on other control equipment connected to the same line input, especially if the equipment is frequency controlled (e.g., carrier current lighting, theft control, or security screening systems).

If RFI is not below a federally mandated level, the FCC has the authority to shut down the installation until compliance is proven. In cases where the interference is not reduced by separate input and output power conduit, the installation of RFI filters would be required. RFI filters are designed to reduce a specific frequency that is causing the disturbance to other equipment. The manufacturer of the drive can assist in identifying the proper filter, if required.

Radiated electrical noise is just as implied-the radiation of radio frequencies, much like the conducted noise that eventually radiates from the power input cables. Radiated noise is typically described as the radio frequencies emitting directly from the drive itself, without external connections. Any drive manufacturer that complies with CE ratings will have already designed the drive with radiated noise reduction in mind. The CE compliance standards contain stricter guidelines compared with those of the United States, related to AC, VFD design. The point to keep in mind is that if the drive is designed to meet certain RFI standards, it must be installed, grounded, and shielded per the manufacturer's requirements.

Otherwise, it would be in violation of the standards and would be subject to penalties or shut down until remedies are installed.

Drives (AC)-Innovations and Technology Improvements

AC drives have been sold in increasing numbers during recent years. Up until the 1950s and early 1960s, the reliability of some AC drives was questionable, at best. The failure rate of some AC drives, out of the box, was 20% or more. Many drive service technicians always came with a package of spare parts. It was assumed that something would go wrong during startup. Fortunately, those days are behind us as industrial and HVAC drive consumers. The reliability and intelligence of AC drives from all major manufacturers has increased dramatically over the past 30 years.

AC drives are fast becoming more and more a commodity item for simple applications like pumps, fans, and conveyors. The more complex AC drive applications are now accomplished with AC drives, but with modifications in drive software-which some manufacturers call "firmware." The AC drive, with its high-speed microprocessor and PLC compatibility, is viewed as a critical piece of the automation system. The ability to connect to a variety of industrial networks makes the AC drive a viable variable speed choice for years to come. The sizes and shapes of AC drives have been reduced over recent years. On the other hand, the power density (per square inch of chassis space) has increased dramatically. IGBT technology and high-speed application chips and processors have made the AC drive a true competitor to that of the traditional DC drive system. In this age of efficiency and network control, AC drives have emerged as a prominent choice. The emphasis is now on less motor maintenance and increased energy savings, with the ability to communicate to a variety of network systems. With these benefits in mind, the AC drive will continue to gain acceptance in applications that have been traditionally fixed speed or DC variable speed.

To make intelligent applications choices, it’s necessary to review the improvements made in AC drive technology. The following sections will outline, in general and specific terms, improvements in AC drive design and control. In certain cases, AC drive improvements have paralleled that of DC drives. This section is meant to generate ideas as to where AC drives and motors can be applied, with little additional equipment required.

Compact Package Design

The AC drive systems of today include all of the needed components to operate, troubleshoot, and maintain the system. Because of the use of microprocessors and IGBTs, a 1-HP drive of today is about 1/3 the size of a 1-HP drive 10 years ago. This size reduction is also attributed to the surface mount technology used to assemble components to circuit boards. Twice as many components are possible on one board because of the placement of components on both sides of the board. The entire unit can be operational with three wires in (power input) and three wires out (power output). An additional set of wires would also be needed if optional external control is used.

AC drive designs of today are more compact with little documentation required to troubleshoot (many diagnostic features are visible in software), and have less parts that require replacement. In most cases, packaged AC drives of approximately 50 HP or less use only two circuit boards-control board and motor control board. In short, AC drives of today are more reliable than their ancestors of 30 years ago. --- notes this type of package design.

--- Standard AC drive package (made by Rockwell or ABB)

Digital I/O (Inputs/Outputs)

AC digital drives allow for simple programming and a high degree of application flexibility. The idea is to connect all control and power wiring and set up the drive for the application, through software programming. If the application is altered, the drive functions can quickly be reprogrammed through software, instead of rewiring the drive. The programming is typically done with a removable keypad or remote operator panel. Both AC and DC drives of today share in this technology improvement.

In earlier versions of AC drives, the drive had to be shut down and the control terminal block rewired for the new application. Downtime is costly. The less time a system is shut down, the more productivity is obtained.

Typically, the control wiring section of an AC drive will contain analog inputs (AI's for speed reference), digital inputs (DI's for controls such as start/stop, reverse, etc.), AO's (analog outputs) (connection for an auxiliary meter), and relay outputs (RO's for devices such as fault relays, at speed relays, etc.). The digital inputs and outputs would typically operate on ±10 or 12 VDC or ±24 VDC logic. A software function would operate if the assigned terminal voltage is high (meaning 8 V or higher on a 10-VDC control). Any voltage less than that would indicate a digital logic "low" and the function would not operate.

In addition to the standard start/stop speed reference inputs, the drive would also include I/O for diagnostics such as auxiliary fault, motor over load, and communications status. Many of the drive manufacturers include a section in software called I/O status. This section of software is dedicated to the monitoring and viewing of drive inputs and outputs.

When a D1 is "high," it would register as an "1" on the LCD display. If "low," it would register as a "0." Note: High and low are relative terms in digital technology. A "high" would mean that a high control voltage is applied to a circuit. A "low" would indicate that a low control voltage is applied to a circuit. E.g., in a 24-V control system, a "high" value might mean 20-24 V. On the other hand, a "low" value might mean 5 V or less.

The same type of display would be seen for analog signals, with a true readout appearing. By viewing the I/O status section of software, it can quickly be determined if the drive has a problem or some interconnection device in the system, outside of the drive.

IGBT Technology

IGBT technology has been successfully applied to AC drives since the late 1980s. Before IGBTs, bipolar transistors and SCRs were the standard output power conversion devices. IGBTs can be turned on and off with a small milliamp signal. Smaller control driver circuits are required because of the smaller control signals needed compared with SCRs. Smaller control circuitry also means a smaller sized control circuit boards, which translates to less cost.

Multi-Language Programming Panel

Many of the programming panels (touch keypads) are removable and may or may not include a panel extension cord. Up until the last decade and a half, programming panels required continuous attachment to the drive control board. Storage of parameter values was a function of the control board and associated memory circuits.

With the latest advancements in E2 PROMs and flash PROMs, the programming panel can be removed from power. Values can now be stored for an indefinite period of time with no batteries required for backup. This type of capability allows for a backup plan in case any or all parameter values are lost because of a drive malfunction or electrical damage due to lightning.

Drive panels are in many cases back-lit, meaning that the LCD digits have an illuminated background that can increase or decrease in intensity. This is helpful when the drive is installed in a brightly or dimly lit room. Many programming panels allow for individual display of several different languages. This is very helpful when the drive is mounted onto a machine and shipped to another country. The programming setup can be accomplished in English, e.g., and then the language changed to Spanish before shipment to Mexico. A typical programming keypad.

--- AC drive programming keypad -- Advanced Control Panel

Additional functions of the modern-day drive panels include "soft keys" similar to that of a cell phone. The function of these keys change depending on the mode of the keypad (e.g., operating, programming, local/ remote, menu, etc.).

The ability of the keypad to guide the user through a multitude of situations is also the trend of current drives. Inherent programs such as a

"Start-up Assistant," guide the user through the required steps to start up a drive for the first time. The "Diagnostic Assistant" aids the user in providing suggestions as to where to correct a fault situation. The "Maintenance Assistant" can be programmed to alert the user when routine maintenance is suggested (e.g., checking or replacing the heatsink fan).

Programming Macros

Because of the digital design, many of the AC drive parameters are preprogrammed in software before shipment from the factory. During drive startup, the operator need only load motor data values and values to customize the drive to the application. In most cases, an operator can install parameters and start up a drive in a matter of minutes, compared with an hour or two for analog AC drives.

Many AC drive manufacturers use pre-assigned values to each of the parameters, in what would be known as default values. The values would not exactly match the motor and application, but would allow the drive to operate a motor. In addition to defaults, several manufacturers include preprogrammed sets of values, known as macros. Individual macros would allow the operator to match the drive parameters and diagnostics to the motor and application that it’s connected to. In many cases, all of the parameters can be set in a matter of seconds, rather than individually set parameters, which could take more than an hour. Macros such as hand- auto, three-wire control, torque control, and PID are available for easy configuration and set up of the drive.

Several drive manufacturers offer a macro or default setup for proportional integral derivative (PID) control. Proportional integral derivative is essentially the automatic control of drive speed by receiving a controlling input such as temperature, pressure, humidity, or tank level. Because of the microprocessor power in today's AC drives, much of the mathematical functions are now standard features of the drive's control board. A water treatment application would be a prime candidate for PID control and is illustrated.

A simple principle of PID is to keep the error (difference between set point and feedback) at zero. AI1 would be the set point or desired value-in this case 120 ft, which would be changed to voltage or current signal. The drive takes that signal and matches it with the transducer feedback signal (AI2) from inside the tank. This actual level would also be converted to a feedback signal current (milliamp). The PID controller takes the resulting error signal and increases the speed of the well pump to match the demand. When the level in the water reservoir goes down, the drive speeds up.

In the past, PID controllers were separate units, which added $800-$900 to the installation costs. With the latest AC drives, the PID function resides in the software.

Note: Some companies refer to software as "firmware" indicating that it’s a soft ware program firmly imbedded, into the drive memory - normally not user changeable.

--- PID Control in a pumping application: Feedback to drive (Ai2) (Actual Level) Water Reservoir 120 Ft., 90 Ft.

250 Mg Capacity Set Point (Ai1)

AC Drive

Well Pump Supply Water In Supply Water Out

--- Function block programming

Several manufacturers include the software logic to engage fixed-speed lag pumps. Alarm circuits wired into the drive would warn an operator or automated system control when the level reached the danger low-level of 90 ft. If that would occur, another fixed-speed lag pump could be engaged to keep up with demand, not handled by the regulated drive pump.

Duplex and tri-plex pumping systems can easily be accomplished with software logic inherent to the drive.

Enhanced Programmability

Even though "pre-programmed" macros are a tremendous time-saver when setting up a drive to match an application, there are instances where a "customized" macro is required. Many drive manufacturers provide a "firmware customization" service for their customers. The innovative manufacturers provide the customer with a means to construct their own specialized macro. The capability of "function block programming" allows the user to reassign many I/O points of the software blocks that create the original firmware. Software blocks, such as the "AND" or "TIMER" block, can be found in PLC programs and high-performance DC drives. Now they have been brought into the programmable functions of the VFD. An example of a function block program.

Sensorless Vector as a Standard Industrial Drive

The operation of V/Hz drives has been discussed, as well as the benefits of sensorless vector drives. The use of the sensorless vector drive has increased in industrial applications due to ease of set-up and the requirement to handle high-starting torques. V/Hz drives require the motor to "slip" in order for torque to be developed. With sensorless vector drives, full-rated motor torque is available at zero speed, with no slip required in the process.

Self-Tuning Speed and Torque Loops

A high-performance AC drive system normally requires tuning, once the standard software parameters are set in the drive. This would especially hold true for vector, flux vector, and DTC drives. The fine adjustments allow the drive to match the feedback loop with the speed and torque reference circuits. Through the tuning process, the operator is able to obtain efficient response times when accelerating, decelerating, and changing directions.

Some older AC drives require a manual tuning procedure. The operator must accelerate and decelerate the load, observe the behavior of the sys tem, and make manual adjustments. In some drives, however, this tuning process is done automatically by the drive control circuitry. The response gains and recovery times are preloaded at the factory. When the drive sees dynamic adjustments occurring during commissioning, it adjusts the tuning parameters to match the outcome of a pre-assigned value set. Drive response times as low as 1 to 5 ms are possible with some systems.

Several manufacturers have reduced the guesswork during the process of dynamic speed and torque loop tuning. After the motor data is entered and when prompted by the drive, the operator conducts an identification run (ID). During this process, the motor is disconnected from the application. This is required so the drive can obtain a complete mental image of the motor characteristics (magnetic properties, hysteresis, thermal time constants, etc.). The drive conducts a 30-60 second program of fast accelerations and decelerations and full energizing of the stator windings to develop the complete mental image. Once the ID is done, the operator can install any standard parameter values required by the application (accel/decel times, digital and analog inputs and relay outputs).

Serial and Fiber-Optic Communications

Access to digital communications is a must in today's automated facilities.

Drives used before the digital age required hard wiring to the control terminal block. This allowed only remote operation from a distance where control voltage or current loss could be kept to a minimum (maybe 25 feet or less). With today's serial and parallel mode of information transfer, the AC drive can accept control and speed commands from process equipment several thousand feet away. This allows the AC drive to be integrated into the factory automation environment, where process control equipment is located in a clean, dry control room.

Serial links (three wires plus a shield conductor) are more of the norm today, compared with a decade ago. Control and diagnostic data can be transferred to the upper level control system, at a rate of 100 ms. With only three wires for control connections, the drive "health" and operating statistics can be available at the touch of computer button. The communication speed of a serial link makes it ideal for simple process lines and general coordination of conveyors, where high-speed accuracy is not required.

Fiber-optic communications use long plastic or silica (glass fiber) and an intense light source to transmit data. With optical fiber, thousands of bits of information can be transmitted at a rate of 4 megabaud (4 million bits per second). An entire factory can be wired with high-speed fiber optics with very little, if any, electrical interference. This is due to the high frequency of light waves, as opposed to the lower frequency of a wire conductor serial link. With fiber-optic communications, steel processing, coating lines, and high-speed cut-to-length applications are possible. With small error signals fed back to the speed controller, the drive can immediately respond with a correction. This keeps the quality of the product very high, and the deviation in size very low.

Several drive manufacturers offer serial and fiber-optic software that installs directly to a laptop or desktop computer. With this software installed, all drive parameters are accessible from the stand-alone computer. This makes parameter changes simple and fast. Parameters can be changed in the computer, downloaded to the drive for verification, and saved in the computer as a file or macro. The file can then be easily transferred to other computers or a network or e-mailed to another factory with the same company. Hundreds, even thousands, of macros and file sets can be saved. The ultimate results are the ability to quickly respond to required changes in drive and application setup.

Serial and fiber-optic communications will be discussed in more detail later. An operator interface scheme with fiber optic, serial, and hardwired connections.

--- Operator interface scheme (communications): Field Bus Modules Fiber Optic Bus Overriding Controller Control Panel RS-232/485 Converter; Panel Bus - 9600 baud PC Tools; RS - 485 Standard IO 5 digital 2 analog 1 analog 2 relay 24 V; Fiber Optic Option Module

Field Bus Communications (PLCs)

Some types of communication systems are almost always specified with AC drives sold today. Data links to PLCs (programmable logic controllers) are common in many high-speed systems that process control and feed back information. PLCs provide the mathematical calculations, timing circuits, and software "and/or" logic signals required to process drive, sensor, and switch status.

Several manufacturers of PLCs offer a direct connection to many drive products. Because each PLC uses a specific programming language (usually ladder-logic programming), drive manufacturers are required to build an adapter box. This adapter (sometimes called a field bus module) is used to translate one language to another (called a protocol). The drive manufacturer installs one internal protocol, and the PLC installs another. Field bus modules allow for a smooth transfer of data to the PLC, and vice versa, with little loss of communication speed.

Drive Configurations

Many drive manufacturers offer out-of-the-box configurations. Several manufacturers offer sensorless vector drives that include the automatic tuning described above. In addition, some manufacturers offer a vector-ready configuration. The circuitry for a vector drive is included in the package, but the customer must add a feedback option to make the performance a reality. Because these products are packaged products, the drive vendor can offer very competitive pricing. Little to no additional optional devices are required to be installed at the job site.

Several manufacturers offer a variation of the standard six-pulse drive. An AC drive that is termed 12-pulse ready offers the optional feature of converting a standard six-pulse drive to a 12-pulse drive. As previously discussed, the 12-pulse drive does an impressive job of reducing the 5th and 7th harmonic content back to the power line. This type of drive includes 12 diodes in the converter section as standard. A delta-delta-wye transformer must be connected to the drive input to make the required phase shift possible. Depending on horsepower, this approach may require a slightly smaller initial investment compared with packaged 12-pulse drives. Active front-end drives that include two sets of IGBTs also make it possible for a packaged approach to harmonic mitigation.

The fact is harmonic reduction is a requirement in more applications today. Twelve-pulse or active front-end drives will increase in their importance, as EMI, RFI, and harmonics issues become more acute in the future.

Many AC drives are seen in centrifugal fan and pump applications. The energy savings realized is of major importance in this age of energy conservation. One of the features of AC drive technology is the ability to bypass the drive, if the drive stopped operating for any reason. This configuration, known as bypass, is used in many applications where the fan or pump must continue operating, even though it’s at fixed speed. A block diagram of the bypass drive.

--- AC drive bypass unit: Service Switch Fuse Line Reactor Bypass Contactor Output Contactor Overload

As seen, many bypass AC drives include all the features required for operation in drive or bypass mode. Many manufacturers include a service switch to allow for drive troubleshooting while the bypass contactor is closed-running the motor at full speed. The line reactor is also standard on many packaged units as is line input fusing.

One manufacturer in particular offers electronic bypass circuitry. An electronic circuit board operates all the diagnostics and logic required for an automatic transfer to bypass. Because of the electronic means of bypass control, information can be fed back to the drive and to a building automation system control. This would not be possible if the overload element was just a mechanical device. Light-emitting diodes are also a part of the bypass control board, allowing instant and clear indication of drive operation, bypass operation, and the run status. Building safety indications and run enable signals are also indicated with this type of bypass system.

Features/Software Enhancements

Additional features and innovations include a wide voltage input tolerance. Many manufacturers specify their drive as a 460-V drive, ±10% input voltage. However, several manufacturers offer a low and high value as part of the range of operation. The range of a 460-V drive may have input parameter values of 440, 460, 480, and 500 V, ±10%. This means that the drive input voltage could drop as low as 396 V or increase as high as 550 V. With this wide range of operation, the drive will continue to run and not trip offline because of a slight power dip or short-duration brown out condition.

Along with this circuit, many manufacturers offer the capability of "power loss ride through." This circuit is standard from almost all drive manufacturers, but efficient handling of the power loss is not. One manufacturer, in particular, uses the regenerated voltage from the motor inertia to back feed the DC bus voltage. --- indicates the effects of one such design.

---Power loss ride-through: Supply Voltage, DC Link Voltage, Output Frequency, Motor Torque

The DC link (bus) voltage will drop slightly in response to the loss of sup ply voltage. When supply voltage is cut off, the drive control automatically reduces the speed reference command. The motor acts as a generator since, for a short period of time, it’s rotating faster than the speed reference. The following example may illustrate this type of circuit.

A drive is set for a 60-Hz speed reference. The motor spins at the 60-Hz commanded speed. The building suffers a power outage. The drive immediately reduces the speed reference to 59 Hz. For a short period, the motor continues to spin at 60 Hz, which now causes regenerative voltage (the motor acts as a generator). The excess energy is fed back to the drive DC bus, and the microprocessor continues to operate. Eventually, the motor will coast down to 59 Hz.

Power has not returned to the building. The drive then responds with a speed reference of 58 Hz. The motor is spinning at 59 Hz, which causes regenerative voltage to be pumped back to the DC bus. The motor eventually coasts down to 58 Hz. If input power doesn't return, this same scenario is repeated until the drive DC bus drops below a minimum level (typically 65% of nominal value). At that point, the drive would shut down due to lack of DC bus voltage.

If the amount of motor inertia is high (as with a fan or flywheel), the "ride-through" time may be several seconds to over several minutes. This circuit has an advantage where short-duration power outages are common (and the application can tolerate automatic speed reduction while the drive stays operational).

Critical frequency or skip frequency is another circuit offered by many manufacturers. A critical frequency is a frequency that can cause severe mechanical vibration if the drive operates the application continuously at that speed. HVAC system cooling towers, some pumping applications, and certain machines have critical frequencies. This type of circuit: Critical frequency circuit.

As an example, a critical frequency may appear at 32-34 Hz. When programmed, the drive would continue to output 32 Hz until there was enough speed reference to cause the drive to output 34 Hz. The drive would pass through that range, but the operator could not unknowingly set the drive to continuously operate at 33 Hz. When the operating speed was above 34 Hz, the critical frequency circuit would operate the same, only in reverse. The drive output would stay at 34 Hz until the speed reference was decreased below 32 Hz. This circuit causes less stress for mechanical equipment and is easily programmed in drives on the market today.