The next generation solution for variable speed pumping applications are variable frequency drives (VFDs) with integrated advanced pumping software that solves various control applications without the need for a PLC or separate controller.
When properly applied, this innovative technology is capable of considerable energy savings, and reduces the pumping system Life Cycle Cost (LCC). The integrated software can be configured for the following applications:
- Pressure control for booster systems, submersible deep well, irrigation, etc.
- Flow control for metering pumps, closed loop circulating systems, etc.
- Level control for storage tanks, water tower, etc.
- Temperature control for heating/cooling systems, geothermal, etc.
In constant pressure control applications, a pressure transducer is mounted at the pump discharge and is wired directly to the drive (to monitor pressure feedback).
In automatic mode, the drive runs the pump and continuously makes speed adjustments until the pressure feedback equals the set pressure (or target pressure). The drive uses a built-in Proportional plus Integral (P.I.) controller to calculate the required speed corrections as the feedback pressure drops or rises above the set pressure.
In no-flow conditions, the drive reduces the pump speed to the minimum speed setting and eventually goes into a sleep mode (or pump stop). Upon a pressure drop below the start pressure, the drive will "wake up" and run the pump again.
Beyond the basic pressure control features, next generation VFDs are capable of advance control and protection features that include:
Pre-Charge Control
For pumping systems where water hammer is a problem, a pre-charge function runs the pump at a reduced speed until the piping is pressurized, then switches to automatic pressure control. This feature is useful for irrigation systems.
Thrust Bearing Control
The initial acceleration on submersible deep well pumps must be set to meet the manufacturer's specifications in order to protect the motor bearings. This function enables a quick ramp up on pump start, then switches to the normal pressure control acceleration.
Advanced No-Flow Detection
In addition to basic no-flow detection, the newest drives can be configured to detect low-flow conditions by monitoring changes in the motor RPM relative to sudden changes in pressure. This feature keeps the pump running during low-flow conditions, thus reducing the number of start-stop cycles on the pump.
A small pressure tank or a flow switch installed at the pump discharge can also be used in conjunction with no-flow detection features to reduce cycling during low-flow conditions.
Advanced Protection
Next generation drives have expanded their pump/system protection to include low pressure, high pressure, broken pipe, loss of prime, dry well, low water input (low suction pressure/low level), feedback loss alarm, and pump over cycling.
Motor protection for these new drives includes output phase loss, ground fault, motor overload, motor over temperature & broken shaft. Drive protection includes over voltage, input phase imbalance, under voltage, phase imbalance, and short circuit.
Automatic System Restart
Next generation drives can be configured to automatically restart in auto mode after power loss, power spikes, brown outs and dry well conditions.
Based on the system requirement supplied by the end user, the newest drives are factory configured and tested for each specific application to ensure they are properly sized and the pump operation and safety parameters are setup before shipping. This service greatly reduces time spent on-site programming and adjusting the drive.
For the pump end user, next generation drives use pump terminology in "quick pump start menus" on four-line LCD displays and drive keypads that simplify the visualization and configuration of the systems, giving non-VFD experts a great comfort level with drives.
The next generation drives are rated NEMA 1 (IP20 - environmental rating) and are suitable for indoor installations, but can be customized to suit specific requirements. Typical customization includes NEMA 12 painted steel enclosures with a main disconnect, terminal blocks for easy pump and transducer connections, and door mounted VFD keypads and HOA switches. Other options include line and load reactors, dv/dt filters, IEEE-519 compliance, outdoor rated enclosures with various compositions, remote monitoring and control (SCADA), etc.
The newest drives range from 5-hp to 150-hp at 200-V to 230-V and 5-hp to 500-hp at 380-V to 480-V. They can be set up for a single pump operation or for multiple pumps operation; one VFD + magnetic starters, or one VFD for each pump.
Next generation drives can be used in applications where three-phase power is not available. Single-phasing the drive is possible, provided it is de-rated by 50 percent. A three-phase motor must be used in all cases.
Pumps & Systems, October 2007
