PROCEDIMIENTOS ESPECIALES

DC drives have come along way—from vacuum tube power structures of the 1940s to the SCR semiconductor technology of today. Design improve- ments have taken the drive from a simple analog “motor turner” to a sophisticated microprocessor design, which includes a minicomputer. The DC drive was once thought to be the “brawn” that was located in harsh industrial environments and caused an application to operate at a specific speed. Now the DC drive, with its microprocessor and PLC-type control, is looked upon as the “brains” and an important piece of automation equip- ment.

To make intelligent applications choices, it is necessary to review the improvements made in DC drive technology. DC drives have lost their foothold in the industrial world to the more popular AC drives and motors. In a certain sense, the control technology of the DC world

revolves around the performance of the DC motor, which requires routine maintenance procedures.

Though more applications are leaning toward AC, the DC drive and motor system has, and will continue to have, its place in certain industrial appli- cations. The DC drive is simple to maintain. When armed with the proper information, anyone who applies the DC drive can realize a cost-effective means of variable-speed control.

The following sections will outline, in general and specific terms, improve- ments in DC drive design and control. It is meant to generate ideas as to where DC drives and motors can be applied, with little additional equip- ment required.

All-in-One Package Design

The DC drive systems of today include all of the needed components to operate, troubleshoot, and maintain the system. Typically, the drive unit includes line circuit breaker (or fuse), main contactor, line inductor, SCR converter module, field exciter, line, control and field exciter fuses, auxil- iary transformer, heat sink fans, motor blower, motor starter and circuit breaker, communications connector, and control (i/o) wiring terminal block. The entire unit can be operational with three wires in (power input) and four wires out (two armature and two field wires). An additional set of wires would also be needed if an optional tach feedback unit is included in the motor assembly.

The “all-in-one” design is in contrast to the systems of the 1950s and 60s, where separate field exciters, converters, interconnections, and safety devices were found in various areas of the control cabinet. This type of installation required more space and more time to troubleshoot because of the nature of wiring, cable harnesses, and documentation. Designs of today are more compact and require little documentation to troubleshoot (many diagnostic features are visible through the software) and have less

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parts that require replacement. In short, DC drives of today are more reli- able. Figure 4-17 illustrates this type of package design.

Digital I/O (Inputs/Outputs)

Digital drives are programmable, which allows for a high degree of appli- cation 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 the software instead of rewiring the drive. The programming is typically done with a removable keypad or remote operator panel.

With older analog 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 a DC drive will contain analog inputs (AI’s for speed reference), digital inputs (DI’s for controls like start/ stop, reverse, etc.), analog outputs (connection for an auxiliary meter), digital outputs (DO’s for devices such as fault relays, at-speed relays, etc.), and provisions for tach feedback wiring (A+ and A-, B+ and B-, etc.). The digital inputs and outputs would typically operate on ±10 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 and stop speed-reference inputs, the drive would also include I/O for diagnostics such as blower motor acknowledge- ment, main contactor closed, field exciter operational, and emergency- stop energized—just to name a few. Figure 4-18 shows a typical I/O dia- gram for a package DC drive.

IGBT Technology

IGBT technology has been successfully applied to AC drives since the late 1980s. It has not been until recently that DC drives have included these devices in the power structure. SCRs are the standard conversion device for armature circuits because of the current capability and easy control scheme. However, IGBTs are now emerging as field exciter power stage semiconductors. IGBTs can be “turned on” and “turned 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 field exciter, which translates to less cost. In addition, IGBT technology is ideal for operating a DC motor in the field weakening range and does not require a voltage-matching trans- former (typically required to match SCRs to higher input voltage lines).

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.

Figure 4-18. Packaged DC drive with standard I/O (Courtesy of ABB Inc.)

1 2 3 4 5 6 7 8 9 Speed Ref +A +A A A A A I I I I 1 1 2 2 0 0 V 2 V V 0 0 0 1 1 1 + 1 2 3 4 5 6 7 8 9 I I I I 1 3 4 +24V 8 1 I I I I 2 5 6 7 D D D D D D D D 0V On/Off Run End Stop 0 X2 X4 Emg Stop

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With the latest advancements in E2PROM™s and flash PROMs, the pro- gramming panel can be removed from power. Values can now be stored for an extremely long 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 dam- age due to lightning.

Drive panels are, in many cases, “back-lit,” meaning that the LCD digits have an illuminated background that can be increased or decreased 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 on 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. Figure 4-19 shows a typical program- ming keypad.

Programming Macros and Software Function Blocks

Because of the digital design, many DC drives require a multitude of parameters for setup of the system. Armature and field supply parameters are required, along with start/stop, speed reference, current, voltage and feedback controls, and diagnostic parameters for troubleshooting. All totaled, many DC drives contain well over 150 parameters, which all need some type of value attached.

At first glance, it would be a daunting task to individually set each of the parameters. Fortunately, many manufacturers preassign “default” values to each of the parameters. 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 val- ues, 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 setting parameters, which could take more than an hour. Macros such as hand–auto, three-wire con- trol, torque control, and jog are available for easy configuration and setup of the drive.

Drive manufacturers try to make required parameter setup easy (i.e., armature and field voltage and current, field weakening point, etc.). One manufacturer includes a “commissioning template” or “wizard” that is much like a template wizard seen in office software programs. The tem- plate states the required parameter on the display and waits for the opera- tor’s input before advancing to the next screen display. In a matter of 8 to 10 keypad inputs, the operator can quickly set up the drive for initial oper- ation.

There may be cases where the standard parameter set will not effectively meet the application requirements. In those cases, several manufacturers allow the operator to reconfigure the internal drive software (typically called firmware). The firmware (parameter names and values) can be adjusted through function blocks. These blocks are actually the building blocks of software that, when connected inside the drive, allows for a spe- cific outcome (e.g., three-wire control with a jog and two constant

speeds). If a macro does not exist, the operator can create a macro to match a particular application. As expected, this type of programming would be reserved for the more advanced operator or engineer, schooled in drive technology. Serious safety concerns could result when software function blocks are connected and results not completely verified. Figure 4-20 shows a function block screen found in one manufacturer’s firm- ware.

Self-Tuning Armature, Field, and Current Loops

The DC drive system normally requires “tuning,” once the standard soft- ware parameters are set in the drive. The fine adjustments allow the drive to match the feedback loop with the speed reference circuit. Current loops

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also require adjustment if the operator is to obtain the response times when accelerating and changing directions.

In some older DC drives, this tuning procedure is a manual process. The operator must accelerate then 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 500 to 800 ms are possible with some sys- tems.

Serial and Fiber-Optic Communications

With today’s digital drives comes access to digital communications. Drives prior to the digital age required hardwiring to the control terminal block. This allowed remote operation only 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 DC drive can accept control and speed commands from process equipment several thou- sand feet away. This allows the DC drive to be integrated into the factory automation environment, where process control equipment is located in a clean, dry control room.

Serial links (3 wires plus a shield conductor) are more normal today, com- pared 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 a computer button. The communication speed of

a serial link makes it ideal for simple process lines and general coordina- tion of conveyors, where high-speed accuracy is not required.

Fiber-optic communication uses long plastic or glass fiber and an intense light source to transmit data. With optical fiber, thousands of bits of infor- mation can be transmitted at a rate of 4 mega baud (4 million bits per sec- ond). 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 very low in size.

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 com- puter. 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 trans- ferred to other computers or networks, or e-mailed to another factory within the same company. Hundreds, even thousands, of macros and file sets can be saved. The ultimate result is the ability to quickly respond to required changes in drive and application setup.

Serial and fiber-optic communications will be discussed in more detail in Chapter 6. Figure 4-21 shows a possible fiber-optic connection scheme.

Field Bus Communications (PLCs)

When factory automation systems are engineered, some type of commu- nication system is always specified. Data links to PLCs (programmable logic controllers) are common in many high-speed systems that process control and feedback information. PLCs provide the mathematical calcula- tions, timing circuits, and software “and/or” logic signals required to pro- cess drive, sensor, and switch status.

Figure 4-21. Fiber-optic communications

DC Drive

Adapter

(PCMCIA Card in Laptop) Converts connection port to Fiber Optic Transmit and Receives Connections

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Several manufacturers of PLCs offer a direct connection to many drive products. Because each PLC uses a specific programming language (usu- ally 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 manufac- turer 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. Field bus communications will also be address in Chapter 6.

AC Drives