Motors and Drives Demystified -- AC and DC Drives [part 3]

<< cont. from part 2

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. FIG. 50 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. FIG. 51 indicates the inner race damage caused by bearing currents.

FIG. 50. Bearing currents discharge path

FIG. 51. Bearing "fluting" caused by bearing currents (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 system. FIG. 52 indicates cabling, shielding, and grounding that provides low impedance paths to avoid high frequency and bearing current dam age.

FIG. 52. Motor cabling, shielding, and grounding (ABB Inc.)

Recommended motor cable construction is described in FIG. 53. It consists of aluminum armor that provides a low-impedance, high-frequency return path to ground.

FIG. 53. Recommended motor cable construction (ABB Inc.)

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

FIG. 54. Recommended motor cable termination method (ABB Inc.)

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 is 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 is 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 is 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 out put). 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. FIG. 55 illustrates this type of package design.

FIG. 55. Standard AC drive package (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 DI is "high," it would register as an "I" 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. For example, 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 out put 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, for example, and then the language changed to Spanish before shipment to Mexico. FIG. 56 indicates a typical programming keypad.

FIG. 56. AC drive programming keypad (ABB, Inc.)

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 is 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 in FIG. 57.

FIG. 57. PID Control in a pumping application

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 $400-$500 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 is a soft ware program firmly imbedded, into the drive memory -- normally not user change able.

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 triplex 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. FIG. 58 indicates an example of a function block program.

FIG. 58. Function block programming

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 system, 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 in Section 6. FIG. 59 shows an operator interface scheme with fiber optic, serial, and hardwired connections.

FIG. 59. Operator interface scheme (communications)

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). Refer to FIG. 59 for an example. 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 trans former 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 is at fixed speed. FIG. 60 indicates a block diagram of the bypass drive.

FIG. 60. AC drive bypass unit

As seen in FIG. 60, 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. FIG. 61 indicates the effects of one such design.

FIG. 61. Power loss ride-through

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 is 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. FIG. 62 shows this type of circuit.

FIG. 62. 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.

Summary

Drives are at the heart of an automation system. They are the controlling element for DC and AC motors, which develop the torque to turn the wheels of industry.

In DC drive systems, an armature and field exciter are used to control the two separate elements in a shunt wound DC motor. SCRs are used to vary the DC voltage output, which has a direct bearing on the speed of the motor. The DC drive is the simplest of drive systems. The disadvantage of using SCRs for the drive power structure is the inherent nature of line notching. This notching is caused by the phasing on and off of the six SCRs located in the drive input section.

Digital DC drives are the latest entry into DC drive technology. With digital technology, precise control of speed and torque can be realized. Speed and current controller circuits use feedback to make small changes in armature supply and field exciter operation. Measuring and scaling circuits sample the actual speed and current output. Summing circuits take the error signal and translate those signals into corrective actions. Higher speed accuracy (<1% regulation) is obtained by the use of a tachometer generator or tach. When operated in the speed regulation mode, the drive closely monitors the speed-feedback signal. Armature voltage (or EMF) control would give a 1-2% regulation characteristic. IR compensation will improve the drooping speed due to load. When operated in the current regulation mode, the drive closely monitors the value of the current measuring circuit. The drive ignores the speed controls and calculates the current (torque) required by the load. The drive will automatically operate at the speed required to allow the motor to develop the desired torque.

Field exciters are constructed of SCRs or the newer IGBT power semiconductor technology. Some supplies are powered by single-phase or two phase power and have the ability to operate in field control mode. This mode is the weakening of the shunt field strength to allow above base speed operation. Form factor is the term used to compare the purity of the DC output from the armature or field exciters.

The operation of the DC drive can be compared with a relationship of quadrants. A four-quadrant drive would allow forward and reverse operation as well as regenerative capability (regenerating voltage back to AC line power). This type of armature supply includes 12 SCRs in a bridge configuration.

Braking methods include coast-to-stop and ramp-to-stop, which are typically the longest method of bringing a motor to a stop. Dynamic braking causes the back-fed voltage to be dissipated in heat through a high-watt age power resistor. The fastest electronic method of stopping a motor is through regenerative braking. This allows full voltage to be fed directly back to the AC line. Mechanical braking also allows fast braking of the motor armature through a brake pad assembly similar to that of an auto mobile.

A concern of DC drive use is the generation of harmonics, line distortion, and power factor. A line reactor is required ahead of the drive unit to reduce the distortion back to the utility system. To avoid the generation of RFI, shielded control and power cables are used. In addition, a shielded transformer is used ahead of the drive.

DC drives have been the object of technology improvements in recent years. An all-in-one package design allows users access to all the required circuitry within one location. Digital I/O and multi-function keypads make drive setup and adjustment easy, along with programming macros and setup routines called wizards by one manufacturer. The self-tuning of the armature circuit and the interface with PLCs make this new breed of DC drive much easier to commission compared with older analog designs.

There are three basic designs of AC VFDs: current source inverters, vari able voltage inverters (input inverters), and pulse width modulation. All AC drives operate under the same characteristic. They change a fixed incoming voltage and frequency to a variable voltage and frequency out put. Most of the AC VFDs built today are of the PWM variety. With PWM drives, the incoming voltage and frequency is rectified to a fixed DC volt age. That voltage is "inverted" back to AC, with the output, 0- 460 V and 0-60 Hz (or 0-230 V).

Braking methods for AC motors are similar to that of DC. In addition, the AC drive has the capability to "inject" DC voltage into the stator windings.

In doing so, the drive sets up a definite n and s polarity in the motor, causing high reverse torque and bringing the rotor to a fast stop.

Torque control AC drives basically fall into two categories: flux vector (feedback required) and sensorless flux vector (no feedback required). Vector control drives have the capability of generating full torque at zero speed. The drive forces the motor to develop the torque required to effectively handle the load. Special circuits (DSPs and ASICs) perform calculations every 25 µs. They allow the flux reference controller to respond to very small changes in torque requirements at the shaft.

Since the introduction of IGBTs into the power technology ranks, the size of AC drives has been reduced to less than half of its counterpart 10 years ago. With this technology improvement comes a challenge in AC motors- high-voltage spikes caused by voltage reflection. PWM drives produce an inherent oscillation between the drive output and motor input. The oscillating voltage actually amplifies itself into a value that is beyond the volt age insulation strength of the stator windings. The windings either suffer a short circuit between windings or an open phase. Precautions to be taken against this motor damage possibility include use of inverter duty motors, output reactors, or DV/DT output filters.

Harmonics are generated back to the AC line because of the technology of pulling voltages in bursts. The rectifier that accomplishes this is termed a switch mode power supply. All electronic devices with this type of supply causes harmonics back to the utility system. The local utility is very concerned about TDD (demand distortion) that affects other users on the power system. Line reactors, harmonic trap filters, and higher pulse drives are a few of the corrections to the harmonics issue. Improper shielding and grounding of AC drives can cause bearing current damage after prolonged use and can cause immediate RFI and EMI. By using proper installation methods, drastic reduction in conducted and radiated noise can result.

Package designs, digital I/O, IGBT technology, and multi-function keypads make AC drives easy to set up. Programming panels are removable and are able to store all drive values in flash memory. Macros and software pro grams like PID allow the user to perform more functions within the drive unit, rather than require a separate PLC program. Self-tuning, communications, and bypass capability make the VFDs of today a cost-effective choice in variable speed.

QUIZ

1. What are the two main circuits in a DC drive unit?

2. What are the main power components used in each circuit, and what are the characteristics of each?

3. How is line notching corrected in a DC drive system?

4. Describe what the following circuits are used for in a DC drive:

• Summing circuit
• Current controller
• Current measuring/scaling

5. Explain the difference between single-, two-, and four-quadrant systems.

6. What is dynamic braking and how is it accomplished?

7. What is RFI and how is it controlled in a DC drive system?

8. What is a macro? Describe its use.

9. What is the difference between a VVI, CSI, and PWM AC drive?

10. What is the carrier frequency or switch frequency?

11. What is motor cogging? What causes it?

12. What is injection braking and how is it accomplished?

13. What is the difference between scalar and vector drives?

14. How do sensorless flux vector drives differ from standard flux vector drives?

15. What is voltage reflection and how is it corrected?

16. What are harmonics and what are the corrective actions to reduce them?

17. What are the effective shielding and grounding methods used with AC drives?

18. What is PID and how is it used?

ANSWERS--Section 4

1. Armature supply and field exciter unit.

2. SCRs can be turned on with a small milliamp pulse, but must be forced off several amps. IGBTs operate on the same principle, with milliamps required to gate them on and off.

3. Line notching is corrected through use of a line reactor, connected ahead of the drive.

4. The summing circuit takes the speed feedback signal and matches it against the speed reference to obtain an error signal used for drive speed control. The current controller controls the firing angle of the SCRs and signals the firing unit how long and when to gate the SCRs on. Current measuring/scaling takes a sample of the current feedback signal and sends it to the summing junction error processing and correction of current out put.

5. Single-quadrant systems allow for motoring in the forward direction only.

Two-quadrant systems allow forward and reverse direction, with reverse torque available for braking; four-quadrant systems allow for forward and reverse direction operation, plus the capability of forward and reverse torque available for braking in either direction.

6. Inertia built up in the motor is transferred back to the drive by means of regenerative voltage. The voltage fed back to the drive is sent to a power resistor for dissipation as heat. A DB contactor is used to connect the motor voltage to the resistor. At the same time, the output contactor of the drive is opened so no voltage can be fed back into the drive output section.

7. RFI is radio frequency interference. This interference is caused by the control circuits, contactors, and oscillators used to control the drive. The best corrective action is to use shielded control cable, as well as shielded input and output power cable in extreme cases. The use of a shielded transformer will also reduce the conducted noise that could be transmitted back to the power line.

8. A macro is a predetermined list of parameter values. These values are designed to allow the user to match the drive parameters to the application. Macros such as three-wire, hand-auto, and PID make drive set-up quicker, since the default values closely approximate the needed programming.

9. VVI and CSI drives use a variable voltage DC bus circuit. The bus voltage is accomplished through an SCR bridge rectifier in the converter section of the drive. Displacement PF of the units drops as the speed drops. PWM drives use a fixed diode bridge rectifier in the converter section. This causes the DC bus to be a fixed voltage. The DC bus feeds the inverter section, where the variable output voltage and frequency is generated. PWM drives operate at a high displacement PF and could be considered power factor correction devices.

10. Carrier frequency is the speed at which the output IGBTs switch on and off. The higher the carrier frequency, the smoother the output waveform will be, and the closer it approximates that of sine wave power.

11. Cogging is the pulsations of the motor shaft at very low speeds. It is caused by a VVI or CSI drive output that sends out pulses in steps. The motor translates these steps into specific magnetic poles. The rotor flux searches for the next available stator pole, which causes the shaft to jump whenever it finds the next position.

12. Injection braking is the process of inducing a DC voltage into the AC stator winding. The amount of voltage will determine the amount of pole flux that will be set-up in the stator. The rotor is attracted to the definite polarity and stopping torque is the result.

13. Scalar is the term given to standard operation of a PWM voltage con trolled drive. With this drive mode, the motor must have several percent slip to rotate. The drive simply supplies the volts and hertz output, and the motor responds with rotation, per the designed slip characteristic.

Vector is the term given to a specialized drive that causes full motor torque at zero speed. Flux vector is another term used to describe the control of flux, which is also the control of torque.

14. Standard flux vector drives require feedback from the motor shaft to sup ply information to the controlling elements in the drive. The drive must know the position of the rotor at all times. The drive generates its control changes, based on the feedback from the shaft. Sensorless flux vector drives receive their name from the fact that no feedback device is used.

All the motor data (flux constants, hysteresis curves, temperature coefficients) is stored in a motor model in the drive. The drive responds to small amounts of information fed back by the DC bus, output current, and the actual IGBT switch positions.

15. Voltage reflection is the phenomenon of drive output voltages combining with voltage bouncing back from the motor. These combined peak voltages can cause damage to the stator windings if they are not rated to handle the voltage stress. Precautions would include using an inverter duty motor, adding drive output reactors and/or dv/dt filters. Inductors will reduce the high-voltage spikes that can occur, and keep the values within the range of the motor lacquer insulation.

16. Harmonics are described as distortion; they provide no usable work, yet are fed back to the AC line. This distortion is superimposed on the fundamental waveform of 60 Hz. For a standard six-pulse drive, the 5th, 7th, 11th, and 13th harmonic will be generated and will be the most destructive in value. Line reactors and isolation transformers help in the mitigation of voltage harmonics. Trap filters and 12-pulse drive units will reduce the harmonic content to a significant level.

17. Control wiring needs to be shielded, with the shield cut back and taped at the signal source end. In addition, control wiring must be kept away from power wiring or at a minimum of 12 inches away. Input and output power wiring also need to be separated. A continuous ground wire throughout the entire system is a requirement and will reduce the possibility of conducted noise.

18. PID stands for "proportional integral derivative." PID is the ability to automatically control temperature, pressure, level, humidity, or any other medium that can be supplied as an electrical feedback signal. The drive has the ability to make corrections in speed, due to the error given by the summing junction. This is considered a closed loop system. It operates at a very high performance rate, with small feedback errors translating to thousands of an inch rotation of the shaft.