Variable-frequency drives (VFDs) entered the motor-control lexicon just a
few years ago. Such drives use microprocessors to calculate and control the
torque-producing component of total motor current. Because of its precision, the
method has become the premier approach for matching motor torque and speed to
load demands, reducing energy consumption and improving motor life.
Good candidates for VFDs are motor operations that have a high degree of throttling with control devices, such as valves or dampers, or for systems that operate continuously for long periods of time. Typical applications include process and water pumps, ventilation and cooling fans, boiler fans, and cooling-tower drives.
Many VFD applications operate at an average capacity that could be efficiently handled at 88% of flow. With a drive able to reduce motor speed to 88% average, the motor draws only 68 to 70% of the power used if the motor ran at full speed and throttles were used to regulate system capacity.
Because VFDs employ microprocessors to monitor and adjust motor-control frequency, they've experienced a natural growth in computational and communications abilities. Today's drives can easily be integrated into larger control networks and offer human-language interfaces for simpler operation.
To illustrate the potential system improvements accruing to users of VFDs, consider the recent re-engineering of the heating, ventilation, and air-conditioning (HVAC) system at the 42-story Cityplace Center in Dallas, Texas.
The facility's managers needed to reduce electrical load in order to accommodate 1,000 new occupants. VFDs from ABB Industrial Systems provided the solution.
Retrofitting 86 air-handler units with ABB's ACH 500 series VFDs lowered fan power consumption approximately 50%. In addition, the computerized motor controllers contributed to an energy-saving strategy that increased chilled-water production by 40%, and reduced the temperature spread between the system's warm- and cold-water storage tanks by 16°F (9°C).
Completed in 1989, the 1.7-million sq-ft (160,000 sq-m) Cityplace Center East is one of Dallas' five largest buildings. Its central cooling plant has six 1,500-ton (1,360 Mt) centrifugal chillers for primary cooling. These supply ten 500,000-gallon (1,900 cu-m) thermal storage tanks. The chiller runs approximately nine hours at night with cooling provided from stored cold water the other 15 hours. Thermal storage reduces the facility's peak load by more than two megawatts, with the central plant taken off line before 6:00 a.m.
All the mechanical heating, ventilation, and air-conditioning (HVAC) equipment is located in four "cores," one in each corner of the building. It uses a Johnson Controls 8540 environmental control system with more than 4,000 points connected with shielded twisted-pair cable enabling RS485 communications.
In the old design, Cityplace's air-distribution system used constant-speed motors driving fans and variable-inlet guide vanes to control air volume. What prompted the redesign? "We couldn't reduce the number of lights," explains James Moore, chief engineer for Premisys Real Estate Services, the building's manager. "We couldn't shut off the computers or copiers, and we couldn't stop the elevators: Fan horsepower was the only place to reduce electrical load." To reduce total comsuption, Moore also wanted to shorten the old system's 12-hr chilled-water production time.
The new system works like this: Lowering the chilled-water temperature from 38°F to 34°F (3°C to 1°C) provides cooler air to the distribution system, in turn reducing the volume of air needed to cool the building. Since less air needs to be moved, the VFD can slow down the air-handler motor.
Colder water going into the heat exchangers also results in cooler water being returned to the chiller supply tank. The differential between the system's warm and cold tanks had been 22°F (12°C); it's now 16°F (9°C).
Instead of cycling the chiller with changes in cooling demand, decreasing demand enables increased chilled-water production to 4,000 gpm from 3,000 gpm (15,000 l /minute from 11,400 l /minute). Increasing the flow rate reduces off-peak chilled-water production time to less than nine hours. "We've increased chilled water production in the neighborhood of 40% and dropped fan power by 50%," explains Moore.
Drive selection hinged upon simplicity of integration into the building's existing control system. The ABB ACH 500 Drive communicates without ancillary integrator devices into third-party, higher- and lower-level controls over a shielded, twisted-pair cable. It accepts a 0-to-10V or 4-to-20 mA reference, compatible with the building's control. Further, standard serial communications allows easy integration into a higher-resolution control system to be installed early this year.
The completed retrofit design lets engineers monitor and modify individual drive performance. ABB remote-control keypads running DMT500PC Drive Monitoring Tool software control each of the building's four vertical air-handling passages. A building-wide network permits all drive operations to be monitored on a single PC.
The drive system's operator interface uses plain-language commands instead of numerical codes, thus simplifying their operation. "If you can read the display, you can program the drive," explains Moore.