Selection Of Pump And Motor Protection And Control Equipment
Electronic pump and motor protective relays offer low cost protection from equipment damage caused by electrical and/or mechanical problems. However, these relays are often not installed in an effort to save money or due to lack of understanding about how to properly apply the wide range of motor and pump protectors that are available. With a little bit of knowledge, the correct motor/pump protector can be chosen.
Advances in microcontroller technology have allowed many manufacturers to produce low cost smart controllers that compete with old-fashioned relay logic panels and Programmable Logic Controllers (PLCs). Programmable relay modules, application specific controllers and custom OEM control boards are gaining popularity. These products can help panel builders and others reduce costs, add functionality, simplify wiring and troubleshooting, and improve user-friendliness.
Protective relays tend to fall into two basic categories: voltage monitors and load monitors. The voltage monitors provide protection when the power source is inadequate for safe motor operation. In three phase applications, the voltage monitor will stop the pump for some or all of the following conditions depending on the model: undervoltage, overvoltage, voltage unbalance, single phase and reverse phase. Single phase voltage monitors provide protection from overvoltage and undervoltage faults. Load monitors will shut down the pump in case of underload, overload, current unbalance, reverse phase and/or single phase. Also, products, such as the SymCom Model 777 are available that combine voltage monitoring and load monitoring into one package.
Voltage monitors alone can provide a basic level of protection from electrical faults that may occur on the main power supply or generator. However, these products offer no protection from faults that may happen in the load or on the load side of the monitor. Typically, a load monitor of some sort should be installed in all pumping applications.
Underload protective relays are critical in many applications. If there is a chance that the well or tank may run dry or if the flow can be restricted (dead head), then underload relays must be installed. If underload relays are not used, submersible pumps can overheat, bearings can be damaged and/or pump seals can be ruined.
How does one select the correct load monitor / protective relay for the job? There are many manufacturers with a wide variety of products with different feature sets. Three general types of load monitors are available. They are current, power and power factor relays. In general, current relays are adequate for higher horsepower applications, and power factor relays are adequate in low horsepower applications. Power relays can be used equally in all applications.
The current in smaller pumps, especially fractional horsepower pumps, changes very little with load, but the power factor changes dramatically. Conversely, in larger motors the power factor stays the same, while current changes significantly with load. Since, power monitors incorporate both power factor and current into their measurements, they can be used in any application. Oftentimes, power monitors are used in high horsepower applications when the motor normally runs lightly loaded. The use of a power monitor in these cases maximizes sensitivity to underload conditions without increasing nuisance trips. Other specialty pumps and systems such as magdrive pumps, can pumps and low speed mixers used to agitate viscous substances before pumping also exhibit small current changes, requiring the use of power monitors to provide load side protection.
Some products integrate several timers, communications ports, and distributed I/O with the basic underload, overload, and phase relays. This allows one product to be customized for a wide range of applications and site specific requirements. Yet, the products remain low cost because of the high level of integration that can be obtained in one package.
Typically, one can standardize on using power monitors in all applications. There are low cost power monitors available for both single phase and three phase applications. Using power monitors allows one to stock a smaller number of parts because they can be used in a wide range of applications. However, current and power factor monitors may have a cost advantage over power monitors on a per piece basis.
Recent advances and cost reductions in electronic components have made economical, highly robust, feature rich protectors a reality. Low cost combination devices are available that combine the features of voltage monitors with load monitors. These devices often include communications capabilities and may also offer optional remote displays, distributed I/O and special control features. Various communications protocols are available such as Modbusâ, DeviceNetä, CANopen and Profibus.
Built-in communication ports allow these protective relays to be integrated into plant automation or SCADA systems. This allows the system operator a much higher level of control than is available with simpler devices. Communication capabilities allow for periodic logging of system parameters and remote reset or control of specific pumps or motors.
Even though PLCs (Programmable Logic Controllers) are very economical, they are often replaced by programmable relay modules, application specific controllers or custom control boards. Programming and maintaining PLC software can be difficult and/or expensive ordeals for some individuals and companies. Off-the-shelf application specific controllers or custom built control boards can reduce equipment costs and eliminate the difficulties associated with designing, building and maintaining PLC systems.
Programmable relay modules can be used to eliminate more expensive PLCs, timers and other components. Many times the best solution is to use an application specific controller or custom control board. Various manufacturers produce application specific controllers for the following applications: sewage lift stations, tank pump up/pump down, ground water remediation (pollutant recovery), coal bed or coal mine methane extraction, oil production and others. For higher volume OEMs, custom control boards can be the most cost effective way to control applications. Custom control boards can be custom designed with only the features needed and can be preconfigured to minimize production and assembly costs. When a custom board is built, the customer knows they will be getting all the features they need and will not be paying for things they can not use.
If you have not done so already, it will be well worth your time to research some of the recently released pump protection and control equipment that has become available. Many of these new products have improved levels of performance and functionality at a much lower cost than was previously available.
Source: SymCom, Inc.