Pumps & Systems , March 2008
Many items must be considered when designing pump station control systems with power requirements, level control method and control panel location often among the most important.
Power Requirements
The pump horsepower and power available at the site must be known to prepare a comprehensive control package. The pump horsepower-in combination with the line power amplitude-determines the motor-starting means and branch circuit protection requirements. For medium-to-small horsepower applications, "across the line" starters are used most often because of their simplicity, low cost and reliability.
Starters and contactors are available in several different styles: NEMA (National Electrical Manufacturers Association), IEC (International Electrical Commission) and definite purpose type. NEMA starters typically provide longer performance than their IEC and definite purpose counterparts, but can be initially more expensive. There are also several versions of manual starters which may be used with a contactor to provide automatic motor control.
Starters differ from contactors in that starters have some type of overload relay between the contactor and the load for overcurrent protection. For fractional horsepower single-motor applications, an overload relay may not be required because a thermal switch, which opens when the motor is overloaded, will likely be installed in the motor assembly for protection.
For larger pump motors, some type of reduced voltage starter may be necessary. Many state and local codes require reduced voltage starters for ratings above 20-hp at 208/230VAC or 40-hp at 460VAC to reduce line power voltage sags when a large inductive load is connected. Reduced voltage starters are either electromechanical or solid state.
Electromechanical reduced voltage starters are available in several different versions. The most common are auto-transformer, wye-delta (star-delta), part winding and primary resistor. These starters are reliable and some are designed to accommodate a specially-wound motor. However, they tend to be large and expensive, especially when compared to a solid state reduced voltage starter.
A solid state reduced voltage starter (SSRVS) is just that-a starter. An "up to speed" bypass contactor is typically used in conjunction with the SSRVS to divert the motor current so that the solid state components do not have to handle the full load for extended run times, therefore prolonging the device life. Many manufacturers integrate the bypass contactor on small horsepower solid state starters.
A SSRVS has advantages over the electromechanical type in that the ramp-up is more controlled to provide a smoother transition from stopped to full speed. Other advantages include controlled ramp-down or "pump stop" features-which can reduce or eliminate water-hammer-a much smaller footprint and it is often less expensive. However, electromechanical reduced voltage starters are far more resilient when considering power fluctuations and temperature extremes and they tend to be easier to repair.
VFDs (variable frequency drives) are often used for constant level applications to provide a discharge to equal a varying inflow, reducing pump starts and stops. VFDs also have inherent "soft start/stop" capabilities via adjustable acceleration and deceleration times. Variable frequency drives may also be used to provide "phase conversion" in some applications. If only single phase power is available on site and the pumping requirements call for a motor horsepower that is impractical in a single phase configuration, a three phase motor may be used with the VFD providing phase conversion through the inverter circuit.
There are some important advantages when using VFDs instead of static or rotary phase converters, including controlled load ramp up/ramp down capability, simplified installation and smaller overall control system size. Some disadvantages of VFDs when compared to other phase conversion methods are the possibility of harmonics introduced on the power line and somewhat higher initial costs when dealing with lower horsepower applications.
Branch Circuit Protection
Depending on the installation, branch circuit protection for the load (motor) may be provided in the control panel, MCC (motor control center) or in a separate load center. Branch circuit protection will be in the form of a circuit breaker or fuses. Circuit breakers (for motor loads) are available in two types: thermal magnetic or motor circuit protectors.
Motor circuit protectors (also referred to as "instantaneous trip" or "magnetic only" circuit breakers) do not have the thermal characteristic and provide short circuit branch circuit protection only. Their adjustable magnet trip threshold reduces nuisance tripping when subjected to high inrush currents, and they often reduce costs by allowing the system integrator to stay with a smaller breaker frame size. MCPs must be used in conjunction with a NEMA rated starter (or other starter with a listed overload accepted by the MCP manufacturer) to maintain a UL listing.
Fuses provide equivalent, and in some cases, superior branch circuit protection when compared to circuit breakers in motor circuit applications. Fuses typically have a higher interrupt rating (often equal to or exceeding 200KAIC) and on average are more cost effective in situations where the voltage exceeds 250V. Fuses are not as well received by many operators in that they cannot be "reset" like a circuit breaker when tripped. They must be replaced, which may be inconvenient
A common misconception associated with branch circuit protection is that the fuse or breaker should protect a designated device. Circuit breakers or fuses do not protect individual components. They protect the other portions of the system and wiring by isolating the problem load. A circuit breaker for home kitchen outlets will not protect the toaster oven from shorting when a child tries to cook a Play-Doh pizza. The breaker will trip or the fuse will open and keep the short circuit situation from overloading the wiring and possibly causing a fire.
Level Control Method
There are many ways to sense the liquid level of a storage vessel. Choosing the best level control method for a specific water/wastewater application requires a comprehensive understanding of the system characteristics. The capacity and dimensions of the well or reservoir, how quickly it fills or empties, what type of liquid is being pumped and the location and type of pumping equipment installed are all determining factors for which method of level control is best
