Applying Motor Controls in a Water/Wastewater Pumping Application
This short paper will cover the basic building blocks of pump control. It is not intended to be the end-all for water and wastewater applications, but rather an introduction.
Water and waste water treatment requires moving water through the different stages of treatment. To do that, pump stations are used. Each part or station of the water treatment process can require a different type of pump. While the pumps are different, they all share the common architecture of an electric motor and a method of controlling those motors.
According to the US EPA, pump station capacities range from 76 lpm (20 gpm) to more than 378,500 lpm (100,000 gpm). Prefabricated pump stations generally have a capacity of up to 38,000 lpm (10,000 gpm). Usually, pump stations include at least two constant-speed pumps ranging in size from 38 to 75,660 lpm (10 to 20,000 gpm) each and have a basic wet-well level control system to sequence the pumps during normal operation. Source: EPA 832-F-00-069, September 2000
The process of moving water is extremely energy-intensive. In the US, electric motor-driven devices, including pumps, use almost 65%-70% of all electricity produced in the country. Water and wastewater systems are known to utilize almost 50% of the energy in any municipality, of which 90% of the energy is used by pumps.
Liquid level switches and sensors trigger when a desirable water level is attained. Trapped air column, or bubbler system that senses pressure and level, are commonly used for pump station controls. Other control alternatives are electrodes placed at cut-off levels and float switches. These sensors and switches signal the pump motor control systems to keep the water treatment process flowing and achieving optimum process efficiencies.
Municipal water systems use pumps to draw raw water from resources, such as lakes or rivers, for treatment to meet regulatory standards for potable water for human consumption or use in cooling towers, boilers, and other industrial applications.
Types of Pumps used in the Water and Wastewater Industry
Water and wastewater management has become a priority in industries such as chemical manufacturing, energy production, and food and pharmaceutical processing. The quality of water treatment entirely depends on the type of process employed. These treatment plants employ primary, secondary, and tertiary processes that vary depending on the level of contaminants in the water. The following are some popular pumps largely used in water and wastewater industry for water treatment.
- Positive Displacement Pumps
- Centrifugal Pumps
- Submersible Pumps
- Rotary Lobe Pumps
- Peristaltic Pumps
- Progressive Cavity Pumps
- Airlift Pumps
- Trash Pumps
- Water Pressure Booster Pumps
- Agitator Pumps
- Circulation Pumps
Some examples of where different types of pumps can be used are shown below in Table 1.
Proper pump, motor, and controls selection optimizes the performance of water treatment systems and can provide energy savings of 20% – 50%. Selecting a pump with the correct characteristics is achieved by studying pump performance curves. Below is an example of an ESP (Electric Submersible Pump) pump performance curve.
Horsepower motor load is the determining factor when selecting the correct motor controls.
Types of Motor Controls used with Water and Wastewater Pumps
- Contactor: Contactors are components designed to switch on and off heavy loads in pump motors. These components feature main contacts (poles), auxiliary contacts, and an operating coil. They energize the contactor to switch on and off the main contacts. Auxiliary contacts are designed for controlling and signaling various circuit applications, whereas main contacts are the current carrying parts of these contactors.
Typically these contactors feature 3-pole electrically operated switches, which take less space when installed inside electrical enclosures. The motors used in water treatment and wastewater treatment pumps are known to draw more energy at any voltage. The possibility of electric shock increases at high voltage and may cause heavy damage. However, AC and DC contactors are safe to use while starting the motor, as there is no current flow between the circuit powering a contactor and the circuit being switched.
The contactors are mounted so they do not touch the circuit that is being switched. Because these contactors use less power than the main switching circuit, they help reduce power consumption. Advanced motor contactors feature compact designs, which further help reduce the footprint of the device and its power consumption.
- Overload Relay: When a motor draws excess current, it is referred to as an overload. This may cause overheating of the motor and damage the windings of the motor. Because of this, it is important to protect the motor, motor branch circuit, and motor branch circuit components from overload conditions. Overload or overheating is one of the major reasons for pump failure. Overload Relays protect the pump’s motor from these conditions.
Designed as electromechanical devices, overload relays are distinguished as bimetallic, melting alloy, or solid-state electronic relays on the basis of their construction.
Bimetallic overload relays are one of the most common types of overload protection devices, and they feature adjustable trip points. Bimetallic overloads are engineered for automatic reclosing and compensate to prevent ambient temperature changes. In addition, these overload relays protect motors in extreme temperature environments.
Advanced bimetallic overload relays feature manual or automatic reset and test modes and a stop button that enables better device management. Many of these relays possess single phase sensitivity, which helps protect motors against phase loss conditions. These relays are provided in three trip class ratings:
- Class 10 is a quick trip rating, suitable for submersible pumps used in water and wastewater industries. This rating indicates that the bimetallic overload relay will trip automatically within 10 seconds of the overload condition.
- Class 20 is the standard and ideal for general motor applications.
- Class 30 is a slow trip rating, which is suited for motors that drive high inertia loads and require long starting periods.
These features help minimize energy consumption and increase motor efficiency.
- Motor Protection Circuit Breaker: Designed for protecting motors from short circuits, phase-loss, and overloads, motor protection circuit breakers (MPCB’s) are alternatives to thermal overload relays, and are equipped with several advanced features.
Commonly used in many water pumping systems, these circuit protection components are used as manual motor controllers or paired with contactors in several multi-motor applications. Motor protection circuit breakers are mainly distinguished as open or enclosed. The difference between these types is where the circuit breaker is secured, either inside an enclosure or open in the panel. Most advanced motor protection circuit breakers offer space savings, as they are designed without individual motor branch circuit fuses, overload relays, or circuit breakers.
- Direct-On-Line (DOL) Motor Starter: As the name suggests, these devices are used to start electric motors of pumps and other electronic devices such as compressors, conveyor belts, and fans. A motor starter features various electronic and electro-mechanical devices such as a contactor paired with a motor protection circuit breaker or an overload relay. DOL starters are used to start small water pumps because they provide several advantages such as 100% torque during starting, simplified control circuitry, easy installation and maintenance, and minimal wiring. Enclosed DOL Motor Starters are also an option, where the entire starter assembly is placed inside an enclosure.
- Programmable Logic Controller (PLC): The programmable logic controller or PLC is really an industrialized computer that operates without a keyboard or monitor. Originally, the PLC was a replacement for large panels of relays that switched on and off, controlling a machine operation. The programming language of the PLC mimicked the Relay Logic, making the transition from relays to PLC’s an easy to understand process. Today’s PLC’s offer much more complex operational capabilities and communications via Ethernet or proprietary networks. The ability to control multiple pumps in a coordinated fashion make PLC’s a common component of water management systems.
- Variable Frequency Drive (VFD): They’re used for running an AC motor at variable speeds or to ramp up speed for smoother start up. VFD’s control the frequency of the motor to adjust the pump motor RPM’s. VFD’s are widely used to regulate water flow at a water treatment plant, allowing more control over the flow of the pump.
- Soft Starter: Between the simplicity of a DOL motor starter and the complexity of a VFD sits the Soft Starter. Electric motors often require large amounts of electricity during their acceleration. A soft starter can be used to limit the surge of current torque of the electric motors, resulting in a smoother startup. Soft starters can protect an electric motor from possible damage and at the same time extend the lifespan of your electric motor by reducing the heat caused by frequent starting and stopping. Soft Starters limit the large inrush current demands on the electrical supply system. Soft Starters are used with pumps in a process which requires to bring them up slowly to reduce pressure surges in the water system.
Tips when Selecting Motor Controls for use with Water and Wastewater Pumps
These devices are often confused with relays, however, the main difference is contactors can easily switch higher currents and voltages, whereas the relay is used for lower current applications. Keep the following in mind when selecting contactors for your motors:
- Decide what amount of current, FLA – Full Load Amperage will be required to power your pump motor.
- Select the coil voltage for AC or DC operation based on your motor horsepower, input voltage, and single or 3-phase. Coils are mainly offered in control voltages such as 24VAC, 230VAC, 400VAC, 24VDC, and so on.
- Selecting a contactor with an IEC Utilization Category of AC-3 is typical for pump applications requiring starting and switching off motors during run time.
- Determine if your pump operation will require reversing of the direction, in which case a reversing contactor will be required.
- Choose the auxiliary contact based on normally open or normally closed configurations.
- In addition to the above considerations, it is important to concentrate on ambient and environment temperatures, necessity of latching, interlocking, enclosures, overload, timers or coil surge suppressors.
When used in water or wastewater industries, overload relays are governed by strict requirements. With so many designs available, choosing the right overload relay may become difficult. These factors will simplify the selection process:
- Choose the overload relay that provides the most thermal overload protection. Overload relays help protect the motor from dangerous overheating, which causes motor failure. This protection takes electricity consumption of the motor into consideration and applies it to an overload model to simulate the thermal energy inside the motor. Most overload relays are designed on either of the two overload models: Two-Body and I2T models. Most bimetallic relays use the I2T overload model, whereas many medium or large voltage electric motors use the Two-Body model.
- Phase loss protection is another important factor to consider, as phase loss is one of the major causes of motor failure. Phase loss occurs when the value of one phase equals to zero amps due to a blown phase. When the motor remains in this phase for a long time, it may be damaged permanently. Overload relays are designed to detect the phase loss condition, therefore it is important to understand the type of phase loss protection offered by overload relays.
- Other important protection factors to consider include underload, ground fault current, stall, jam, and power and voltage protection.
Motor Protection Circuit Breakers
Most pump systems in the market today include basic motor protection built into the motor or control box. This protection however is designed to safeguard against only current problems, therefore additional motor protection should be considered. There are many options available to choose from, each present slightly different performance characteristics under overload conditions. Factors to consider are:
- High fault short circuit current rating because it assures safety and reliability in extremely high fault applications.
- Trip indication helps determine the type of maintenance or service that may be required by identifying the cause of tripping – short circuit or overload.
- Self-protection of these devices assures excellent motor protection and helps eliminate the need for additional circuit breakers and upstream fuses.
Applications of Motor Controls in the Water and Wastewater Industry
Overload relays, contactors, and motor protection circuit breakers are largely used in the following applications.
- Influent and Effluent Pumps: The influent pumps are usually placed at the start of the wastewater treatment plant. The wastewater first enters into the influent pump, which pumps it to other parts of the treatment plant. Effluent pumps are used for treating water that may contain solids up to 3/4”.
- Booster Pumps: These pumps are used for boosting the water pressure for use in light industrial and commercial applications. The booster pumps are mainly used for potable water applications.
- Digested Sludge Pumps: Contaminated sludge is treated in wastewater treatment plants before being released into water bodies. The sludge contains solids which may block the pipeline, so special pumps are used for the purpose. Sometimes, sludge choppers are integrated into the pumps, or submersible centrifugal pumps are used for pumping fluids having high sludge content.
- Submersible Pumps: As their name suggests, these pumps are completely submerged in the liquid. These pumps are used to drain the slurry or sewage and are mostly placed under the sewage or wastewater treatment plants. Their operation is controlled using advanced motors.
Applicable Standards and Compliances
All motor controls referenced in this document are designed in accordance with standards published by NEMA (National Electrical Manufacturers Association) or IEC (International Electrotechnical Commission).
NEMA is primarily a North American Standard, whereas IEC is a global standard.
The NEMA ratings of a starter depend largely on the maximum horsepower ratings given in the National Electrical Manufacturers Association ISCS2 standard. The selection of NEMA starters is done on the basis of their NEMA size, which varies from Size 00 to Size 9.
The NEMA starter, at its stated rating, can be used for a wide range of applications, ranging from simple on and off applications to plugging and jogging applications, which are more demanding. It is necessary to know the voltage and horsepower of the motor when selecting the proper NEMA motor starter. In the case where there is a considerable amount of plugging and jogging involved, then derating a NEMA-rated device will be required.
The International Electrotechnical Commission (IEC) has specified the operational and performance characteristics for IEC devices in the publication IEC 60947. Standard sizes are not specified by the IEC. The typical duty cycle of IEC devices are defined by utilization categories. As far as general motor starting applications are concerned, AC3 and AC4 are the most common utilization categories.
Unlike NEMA sizes, they are typically rated by their maximum operating current, thermal current, HP and/or kW rating.
Pooling it all together
Below are three examples of pump control, using basic industrial motor controls. These are simple representations, and not intended to provide complete solutions.
We hope that this short paper has given you a good, basic understanding of pump control for water/wastewater. Look for other informative papers from c3controls, including our series on Industrial Control Basics, at c3controls.com/blog.
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