Part 18(e)—Electrical Systems and Control, Part 5
By Ed Butts, PE, CPI
Up until now we have discussed virtually every aspect of a water works pumping plant for groundwater—including basic design criteria, hydraulics, pump selection and design procedures, piping and valving design, and electrical system requirements and planning.
This column will continue the seven-part subseries on electrical and control systems with a discussion on the basics of control systems for water system applications.
Please note while I have tried to remain true to the use of recognized terms, please note many of the following terms and definitions are simplified and those I have used for many years in control system design. In many cases I feel they describe the intended function and purpose of the device or system better; therefore, they may or may not coincide with established ISA (Instrumentation Society of America) guidelines.
Fundamental Control Methods
The basic definition of “control” as it applies to this subject is to manage or supervise an action, procedure, or process in an orderly fashion. Generally, there are two modes used for control of any system: manual and automatic.
An industrial control system (ICS) is a general term that encompasses several types of control systems, including supervisory control and data acquisition (SCADA) systems, distributed control systems (DCSs), and other smaller control system configurations such as skid-mounted programmable logic controllers (PLCs) often found and used in industrial sectors and critical infrastructures.
ICSs are typically used in industries such as electrical power generation and transmission; water and wastewater pumping, distribution, and treatment; oil and gas; chemical; transportation; pharmaceutical; pulp and paper; and food and beverage.
These control systems are critical to the continued operation of the U.S. critical infrastructures that are often highly interconnected and mutually dependent systems. It is important to note that almost 90% of the nation’s critical infrastructures are privately owned and operated. This exposes the control and controlled system to the potential of hacking or intrusion, particularly in systems that run on communication transmission means.
A manual control is the most basic and is simply one in which the process or system is externally controlled through manual operation. An automatic control comprises the vast majority of water system control and includes everything from a simple on-off pressure switch control for a domestic water system, an automatic preset timer on an irrigation system, to an elaborate and complex, multi-site, computer-controlled SCADA for a large municipal water system.
A control system is a device or set of devices that manages, commands, directs, or regulates the behavior of other devices or systems. Generally, a control system monitors a constant or varying system parameter to effect changes in operation. This parameter, called a process variable, can consist of a declining or rising water level, varying pressures, temperatures, using thermocouples, and other parameters.
Most control systems operate using analog or digital (discrete) signals, which are used to transmit information usually through electrical or electronic signals.
There is a difference between analog and digital. In analog technology, the information is translated into electrical or other types of short pulses of varying amplitude. In digital technology, the information is translated into a binary format (using a 0 or 1) where each bit is representative of two distinct amplitudes or states, such as with on and off signaling.
Analog signals are found in numerous ranges and values, such as electrical current often using 4-20 or 0-50 milliamp or voltage with common values of 0-10 volts or 0-5 volts DC and a commonly used span of 0-10 or 0-20 PSI for pneumatic or hydraulic signals.
Digital control systems also use electrical, pneumatic, or hydraulic imposed signals, but unlike analog signals, mechanically induced signals such as float or limit switches can be used in digital systems.
The two systems can also be used in concert, such as a wet well bubbler system in which a proportional analog value is used to reflect a constantly changing water column, that in turn operates digital snap-action pressure switches to activate the pump.
Control systems can use various means of transmission media, including electrical signals, pneumatic (air or gas), and hydraulic (water or other fluids) as the process media. Figure 1a illustrates a common water system control system using electrical analog signals, while Figure 1b displays the same system but with a pneumatic analog control loop.
The next basic distinction is whether the control system is an open or closed loop system. The primary difference between an open loop and closed loop system (Figure 2) is that an open loop system has no direct feedback.
Open loop control systems essentially have just an input signal. The input is processed by various integral components such as control valves, manual valves, etc., to effect a change to the final output. A closed loop control system, on the other hand, uses a set of mechanical or electronic interface devices that continually provide monitoring feedback to the controller, which automatically regulates the process variable to a desired state or setpoint without direct human interaction.
A control system in which the control action is totally independent of the output of the system, such as a manually operated system, is also classified as an open loop control system. It is important to understand the distinction between a control and a control system. A control is typically a single element of an assembly of components that becomes a part of and goes into the development of a control system.
Simply stated, you must always remember that any single control or component is always a part of the entire control system which includes the fuses, wiring, connectors, and installation and adjustment methods. Just as a pump cannot pump water without a motor, pipe, and wire, a control system cannot function without all the individual pieces working together in relative harmony.
This means the concern shown for selection of the controller must be the same as that displayed for the way it is installed. After all, I have seen as many water systems fail or malfunction due to a problem with a loose wire as I have with the device itself.
These rules also apply to pneumatic, hydraulic, or mechanical controls as much as they do to electrical controls. Although we will discuss electrical controls as the primary method of controlling water systems, all factors outlined here should also apply to the other classifications of controls.
The Three R’s of Controls
Most of us are old enough to remember school using the basic tenets of the three R’s—“reading, ’riting, and ’rithmetic.” Staying true to that adage, there are also three R’s associated with a good control and control system regardless of the application, method, or type. It must be reactive, repeatable, and reliable. I will attempt to explain each one of these concepts separately.
A good control or control system must first be reactive, meaning it must be capable of accurately and completely activating and deactivating the circuit at the proper setpoints and at the proper differential. This prevents troublesome motor and control chattering or short cycling.
For example, this means a float switch must fully close a motor start circuit at the proper water level and then reopen the same circuit at the preset higher or lower water level. A pressure switch must be capable of performing the same function at various preset points of pressure. To be fully reactive, there must not be any hesitation, incomplete open or closure of contacts, variation or drifting from the setpoints, or intermittent operation.
The device must fully close and open the circuit at the proper setpoints. In addition, for a device to be fully reactive, it must be capable of handling the voltage and amperage placed across the contacts.
Secondly, the control must be repeatable. This means it must be capable of performing its designed, intended, installed, and adjusted function over and over, maintaining and repeating the exact setpoints each and every time. In control system lingo, this is called repeatability.
Repeatability, for most industrial-rated controls, is often stated in cycles or operations to indicate the number of times the device is expected to reliably operate. A modern control device, particularly electrical switches or relays, will usually be represented to function in the millions of operations or cycles. As another example, a pressure switch that is designed to operate at a 40 PSI to 60 PSI pressure range must be able to repeat its operation each cycle by closing at 40 PSI and then reopening at 60 PSI, give or take a pound each and every time.
Finally, the control or control system must be reliable. This factor impacts both the device and its function. After all, what good is a control to you or your customer if you must return to babysit or readjust it each week?
Because a domestic water system is often expected to work under very extreme cycling conditions, hour after hour, day after day, year after year, it is incumbent upon the system designer to ensure that all primary controls are capable of up to 1000 daily reliable and repeatable cycles through 20 years of use.
If you do the math, you will discover this figure amounts to more than 7 million cycles! I realize that number is quite high, but in the real world you must always hope for the best and plan and design for the extreme or worst. Reliability is where the first cost of the device is often compromised.
Electrical Control Basics
In addition to the three R’s of controls, there are several other basic concepts of electrical controls that must be considered. One of the principal designations is defining the device as either input or output signals or devices.
Generally, input control devices are intended to transmit the process variable or other control signal to the primary PLC, controller, or device. Input devices are typically rated as pilot duty devices, as the current and voltage they handle is primarily low in value.
Input devices often include pressure and float switches; flowmeter analog and pulse signals; limit switches; and feedback signals from valves, motors, pumps, and other signal generating sources.
Outputs are typically signals that are sent outward from the PLC or main controller to control peripheral valves, motor controls, and other functional destinations. As opposed to most input load ratings, output loads are often rated up to 15 amps for resistive or inductive loads.
Electrical controls for motor applications are generally classified into one of two groups: industrially or commercially rated motor control or pilot duty.
An industrial or commercial motor control basically means the control is designed for and capable of directly operating the specific motor load it is applied to. For the most part, motor controls that apply to domestic water systems of 1 HP to 2 HP are classified as industrial motor controls.
To properly apply an industrial motor control, the designer must specify particular characteristics of the circuit and load. The primary and most important designation of an industrial motor control is the device’s rating, which is displayed in amperes or horsepower and voltage.
Above all other factors, it is critical that the rating of the device be at least equal to or greater than the motor’s (or load’s) nameplate amperage or horsepower rating and operating voltage. When amperage is used, it is also vital that the proper rating of amperage is also applied.
In many cases, particularly with IEC-rated components, a motor control will be rated in amperes, rather than horsepower common with NEMA components. If this is the case, remember that the device must be rated for an inductive load and not a resistive load. This is due to the higher inrush incurred by a motor during starting rather than the same level of amperage displayed by a resistive load with the same amperage, such as a lamp.
For example, a 3 HP, 230-volt, single-phase motor has an NEC full-load rated amperage of 17 amps and service factor rating of 19.55 amps. This means that any device that is intended to directly operate this load must be rated for no less than a 3 HP, 230-volt, AC, single-phase motor load, which equals 17-20 amps of inductive load at 230 volts, AC. This distinction is critical!
Industrial motor controls include devices such as motor starters, motor contactors, and overload relays, as well as some power monitors.
On the other hand, pilot duty devices are generally used as an intermediary device to operate the industrial motor controls. As per their name, pilot duty devices have the same function as an airline pilot who controls a much larger machine. They are typically applied and used as a process monitor and switching device to monitor, indicate, and transmit the level of the process medium, such as air or water pressure, water level, etc., and transmit a start or stop signal to the motor control for activation/deactivation.
When used with PLCs or SCADA systems, their function is simply to provide the process variable as an input to the control device, which then discriminates and interprets the signal for further action.
Pilot duty devices are typically rated for a much lower load (amperage) than the motor control, although the voltage rating may be equal. Pilot duty devices are generally classified as standard-duty or heavy-duty with how each type is applied, depending on the specific application.
Pilot devices often include pressure and float switches, selector switches, relays, and other similar devices. In many cases, particularly using fractional horsepower (less than 1 HP) motors, pilot duty devices can also be used to directly operate the motor with the same criteria for the proper rating that were applied earlier for industrial motor controls.
In addition to the rating of the device, there are other factors associated with controls that warrant mention. An expression used to designate the duty or permitted time operation of the device is often applied to control devices. There are two basic terms used to designate how long the device is permitted to operate without shutdown: continuous duty and intermittent duty.
For all intents and purposes, any control component intended to operate on a water system should always be classified and rated as a continuous duty component. This means the device is designed to operate continuously and without the need for rest at the rated load of the device.
The number of poles refers to the number of separate contacts contained within the device that are capable of individually passing an electrical current (Figure 3). For example, a two-pole device (common on pressure switches) means there are two separate sets of contacts in the device, each capable of routing a separate electric current to the motor or motor control.
Single-pole devices are commonly used for pilot duty applications, while two-pole devices are used for single-phase loads and motors, and three-pole motor controls are used for three-phase motors.
The position or state of the electrical contacts in the device is also important. Generally, a motor control or pilot duty device is equipped with contact arrangements that are said to be either normally open (N.O.) or normally closed (N.C.) when deenergized (Figure 4a). This arrangement is referred to as a single-throw set.
Both contact arrangements are sometimes found on the same device or set of contacts. In other words, a single or dual set of contacts may be equipped with a contact arrangement, including a common terminal (or line) that is connected to a normally closed contact when deenergized and a normally open contact when energized (Figure 4b). When this occurs, the contact is said to have a double-throw arrangement. This type of contact is common on relays and switches. The position of the contacts is always stated and based on the position of the contacts with the device deenergized or without any power applied.
The analogy I was taught years ago was to simply picture the device held in your hands, and that’s the stated position of the contacts. Typically, a motor control would be equipped with normally open contacts and a pressure switch with normally closed contacts, as examples. To summarize, when you put all these designations together, you will wind up with useful terms such as single-pole, double-throw (SPDT); double-pole, single-throw (DPST); or three-pole, double-throw (TPDT) contacts.
Finally, a basic discussion of controls would not be complete without mentioning a few words about relays. Relays are becoming more and more popular in the electrical field, especially for control.
They are available in general purpose, heavy duty, pilot duty, normally open, normally closed, time-delay on or off, one-, two-, three-, and even four-pole configurations. Suffice it to say they can be found in virtually any or every style you may need.
The primary reason I bring this topic up is to provide a general recommendation regarding the best type to use for most applications. Although there are certainly exceptions to every rule, for the most part, I highly recommend the use of ice-cube style, plug-in type, heavy-duty relays (often referred to as compact relays) for water system applications.
The advantages to an ice-cube type of relay are numerous. For one, the contacts are generally sealed within a translucent or clear plastic enclosure. Since most water system applications are inherently greasy, dirty, moist, or dusty, isolating the contacts and moving parts from this environment will prevent damage or coating to the contacts and other mechanical parts, while at the same time provide the ability to routinely inspect the contacts for wear or damage.
Also, by using a heavy-duty rating, the contacts will usually carry at least a 15-amp inductive rating. When used on a high inrush device, such as a motor starter or even a small motor, the heavier contact rating will ensure a much longer life.
Next, the relative short distance between the contact arm and coil creates less operational impact onto the coil or contact arm. This helps to drastically extend the predicted operating life and cycles, up to more than 20 million cycles in many cases.
For time-delay relays, the use of electronics for timing typically provides distinct advantages over bellows or pneumatic timers in accuracy and working life. The next distinct advantage is in cost, both initial cost and installation cost.
The advantage and cost savings afforded from simple and mass production of these devices, combined with their compact size that requires little enclosure space, and the ability to install them on a din-rail system, truly offer many advantages.
Finally, most industrial control relays are now constructed using a uniform base arrangement, either a round pin or square blade style with eight or 11 pins or blades. Imagine how fast your troubleshooting can be if you only have to simply pull out one relay and plug in another to test the relay or circuit without even needing to disconnect a single wire!
The one disadvantage to compact relays is their vulnerability to high voltage surges. The use of surge arrestors (commonly called R/C arrestors) on the device’s coil will often protect against many of these occurrences. Although these relays obviously cannot be used for every application—particularly those with 480-volt, three-phase, or high inductive loads—the above characteristics when combined with the low spare parts inventory requirement truly make these relays the relay of choice for many control functions.
This completes this year’s final installment of The Water Works. Until next time in January 2022, keep them pumping!
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.