The real genius of control work is being able to combine the many different devices to get a very complex and difficult job done. This first lesson is a step in that direction.
Control work covers a lot of territory. The term itself conjures up different images in almost everyone's mind. When this writer thinks of the term "control work," he usually remembers the dreaded Duplex Pump circuit he had to learn in trade school. Sometimes an old electrician named Mr. Clark comes to mind: He's doing an emergency reroute of the controls for a sewage treatment plant, and stopping the several-million gallons of raw sewage that were spilling onto 34th St.
Some of you will think of assembly lines, limit switches, timing relays, motor starters, variable speed drives, and a hundred other applications of controls. We'll try to cover the more common control technologies here. But before we attempt to go through the technical explanations of equipment and control schemes, we should lay a good foundation as to why controls are important, who uses them, how they are used, and so on. You need a global understanding of how and why these systems work, not just how to connect "A" to "B."
Why we use controls. Electrical controls have been a big part of the electrical industry since it began:when the first electricians found out they could use circuitry and electromechanical devices to do things humans could do. Rather than employ a man to turn a machine on and off at appropriate times, electricians arranged a control circuit and a few special switches to do the same thing. The result: the same job done more reliably and at a much lower cost.
Since that time, we've expanded the use of controls continually, and for the same reason: They provide for reliable, inexpensive, automatic control of equipment, while allowing us to control machines in a variety of ways.
Types of control systems we use. For the purpose of clarity, we'll specify three basic types of control systems. Keep in mind, however, all of these systems can (and frequently do) operate together as a single control system. We're breaking them apart here for easier understanding.
- Electrical controls.
These operate by starting, stopping, directing, or regulating the flow of electricity in circuits. In other words, you're allowing electricity to turn a machine on, stop the machine by opening the circuit that controls it, or by redirecting the current in a different way.
- Electronic and computerized controls.
These operate by conveying complex messages from one place to another; changing, coding, or conditioning electrical currents; or performing complex and interactive routines.
- Pneumatic and hydraulic controls.
These allow for control of equipment in response to the pressures of air and fluids, respectively.
Types of control devices we use. The most basic control devices are:
Mechanical switches. Whether operated by hand or some other object pressing, pulling, or twisting it, hundreds of types of mechanical switches exist. They're used to stop, start, or redirect the electrical current passing through them.
Solenoids. These are coils of wire (acting as electromagnets) that pull a plunger (iron bar) into the core when energized. You can use the action of the solenoid (movement of the plunger) for anything. In most cases, it's used to operate some type of switch. Notice that a relay is a combination device, using a solenoid combined with mechanical switching, and building them into a single unit.
Signaling devices. Pilot lights, warning lights, bells, and the like are signaling devices. These alert someone to a pre-specified condition. LED and LCD displays are also signaling devices.
Sensors. There are hundreds of sensors. Some sense visible light, infrared, ultra sound, current level, and many others. All of them are designed to respond to a specific condition and pass along that information or respond in some way. The simplest and most common type of response is to throw a switch that's packaged in the same unit as the sensor.
Logic devices. These (usually microprocessors) can make some type of intelligent decision. The basic intelligent operation is an IF/THEN operation. For example, a logic device could operate as follows: If the occupancy sensors show a human presence, and if it is a weekday, and if it is between 6:30 and 7:00 am, then turn on the high bay lights.
This capability allows logic devices to do much more complex tasks than simply turning things on and off at preset times. This also requires the device listen, as it must keep track of which devices are being used. In other words, use the logical device with some type of sensor (whether it be internal or external to the logical device), so it can tell whether or not a specific control device is being used (the if function). These devices also require specific programming, so they can respond appropriately to a unique application.
Logic devices can be as simple as a single microchip in a cheap controller, or as complex as a mainframe computer with multiple peripheral devices.
Clocks and timers. These provide a time-reference for controlling a wide variety of devices.
Transformers. Use these to manipulate voltage in a variety of applications.
Hardware devices. These devices include lugs, harnesses, enclosures, and other supplementary items.
Valves. Use these devices in pneumatic and hydraulic controls as well as in process controls. They basically control some type of gaseous, liquid, or granular flow, usually combined with a solenoid-type of electric activation.
Communications media. This high-tech term refers to some method of getting information or current from one place to another in the newer electronic control systems. Perhaps the most common communications media is a twisted-pair cable, transferring binary signals between a computer and a programmable controller. But in other cases, it could be radio signals, infrared light, microwaves, or even power line carrier signaling. The Internet is a form of communications media, allowing for control of complex control processes from almost anywhere on the planet.
Transformative devices. These are devices that change one thing into another. An example is a thermocouple: It changes heat energy into electrical current. It generally serves as a sensor, but it's different in the physics of operation. A modem would be a computerized transformative device because it changes sound into electrical pulses and vice versa.
Electronic devices. By this we don't mean microchips and logic devices. Instead, we mean basic electronic devices: such as transistors, capacitors, choke coils, and other devices that act as electronic valves, amplifiers, or storage devices. They condition or change electrical currents in some way.
Wiring. This term includes conductors, raceways, fuses, connectors, etc. Really, this is supplementary equipment, but since it's absolutely necessary for control systems to function, we include it here.
Power sources. Again, this is usually supplementary equipment (as in the case of the power generating station down the road), but not always. Backup batteries or specific-use power sources are valuable and frequently used control devices. The above category of devices doesn't cleanly cover everything in the controls field. (For example, is a thermal overload a sensor or a mechanical switch?
Really, it's both.) However, this list gives you a good overview of control devices and their essential functions. Note: This list does not cover the things being controlled (such as motors and pumps), only the devices you would use to do the controlling.
How we combine devices and systems. The real genius of control work is being able to combine all these devices to get a complex and difficult job done. Someone who does this well (as in the case of Mr. Clark, the electrician) is both a skilled technician and an artist. Achieving this competence requires:
- A thorough understanding of all the items you want controlled and all of the control devices.
It's not enough to know how "A" connects to "B." You must understand the principles that make "A" and "B" work. If all you know about single-phase motors is that you connect 240V to Terminals 4 and 7, you won't be able to modify the motor for a custom use. You must understand the motor's primary principles of operation:rotating magnetic fields, current lead and lag, and so on.
- Coordination of specifications.
Some control devices operate on 12VAC; most computerized devices at 5VDC; and most motors at 240VAC or 480VAC, 3-phase. When combining devices, the most common problem is coordinating current and voltage requirements of each device. To combine a 480V motor and a 120V timer, you'll need an appropriately sized transformer to connect between them.
- Coordination of duty cycles.
Devices used together must be able to operate together over long periods. For example, some coils will burn out if you use them continuously. If you need a solenoid for continuous use, you probably need one with the mechanically held feature. In any event, be sure to properly match all control devices.
- Environmental factors.
Be sure all your devices function well in areas where they're installed. This can be as simple as using NEMA 3R enclosures on an outside wall, or as difficult as determining which types of contactors will operate correctly in the cargo bay of the space shuttle. The new motors you're going to control may vibrate so much that you need to isolate your control devices with rubber mounts.
- Effects upon existing devices and systems.
Don't forget the effects of the equipment you're installing upon existing equipment. Many control devices create vibrations, which may not be acceptable. Others produce magnetic fields, which can interfere with operation of sensitive equipment in the area.
Safety is an important issue. There's probably no way to avoid every possible injury where machinery is used, but you must get as close to that goal as possible. This requires the intelligent use of warning signs, overloads and resets, barriers and guards, interlocks, lockouts, clutches, and anything else you can use to avoid injuries.
When putting a control system together, consider ease of maintenance and accessibility of critical items for repair. A few changes in the beginning can save a lot of hassle over the next 30 years. In most cases, you'll have to combine items expected to last for 20 years with items reasonably expected to last only five or 10 years. As such, it's a good idea to make notes of such things for whoever will be maintaining the system.
You're installing control systems to serve a productive purpose. It follows that somebody will need to control the system from a distance, or at least determine the system's state of operation without difficulty.
In many instances, you'll build into your control system the ability to modify it for a different use at a later date. Again, a little forethought here can save money.
Consider the use of your equipment by real people. For example, you can mount a switch used only once a year at an inconvenient location. Think about using the equipment before you specify its location.
Coordinating this list isn't easy:it's a job for a professional. With some thought, this course will give you what you need to reach that goal.