Variable Frequency Drives (VFDs) are used in many applications where motors drive fans and pumps. In one case, a city in South Carolina is using VFDs as a reliable alternative to obsolete technology for driving pumps in its wastewater treatment stations.
For example, one of the city’s Lift Stations, which was built in 1970, includes three centrifugal pumps which are driven by standard AC induction motors. The facility can handle 11 million gallons of water per day. In the course of its operations, the facility accomplishes a 143-foot lift of wastewater via a sequenced combination of pumps. In the original installation, pump flow was controlled by an eddy current clutch.
When one of the pump’s drive systems failed, the facility maintenance superintendent discovered that repair costs for the unit were exorbitant. He therefore decided to update the system instead of spending more money on the outdated equipment.
Technicians first removed the inefficient eddy current clutch and then coupled the existing 200 HP. 460 volt 447T-frame motor to the pump shaft. This operation just required a simple modification to the motor mount. An AC890PX variable frequency drive was powered up through the existing control panel and pump sequencing logic.
The bottom power entry configuration on the drive made for a clean installation with no visible wiring. In addition, the set up did not require an expensive external harmonic filter because the drive has a 3 percent line reactor built into its enclosure. The drive also includes line fuses and a local power disconnect switch with door interlock and lockable handle.
It turns out that replacing the existing speed control device with a solid state drive provides several tangible benefits:
Eddy current clutches rely on slip to adjust the speed of their load, and dissipate energy wastefully as heat. Their efficiency drops off significantly with speed. For example, at 60 percent speed, the system provides less than 60 percent efficiency. With a 200 HP motor, this adds up over time.
On the other hand, using an adjustable speed drive like the AC890PX to regulate the speed of the pump directly means no energy is lost to slippage, and the current draw of the motor drops dramatically with reduction of speed. With a centrifugal pump, simply reducing the speed to 80 percent results in nearly a 50 percent decrease in power consumption. After three months of operation, the annualized energy cost savings was estimated at $18,000 per year.
In addition to better energy efficiency from the drive as opposed to the heat-generating losses in the eddy current clutch, the 200 HP motor is now gently ramped up to speed with current limited by the drive. The old system started the motor across the line, resulting in a floor-shaking jolt and a high peak demand of current. This was rough on the motor and upped the power costs because utilities meter their customers’ peak demands and penalize them with surcharges based on these measurements.
Yet another benefit of the AC890PX VFD drive in a critical application is the device is reliable and therefore reduces downtime. The drive passes rigorous factory testing but on the remote chance that a component should fail, the converter module, phase modules and capacitor module are lightweight and can be quickly removed. Replacing a module does not require a factory service person or necessitate returning the drive to the factory for servicing. In a matter of minutes, any of the modules can be replaced, and the drive brought back on line.
For more information, please visit Parker's SSD Division
Article contributed by Lou Lambruschi, Marketing Services and E-Business Manager - Parker Hannifin Corporation SSD Drives Division/Energy Grid Tie Division
Other articles covering applications for VFDs: