Motor control efficiency
Renewed industry focus on energy efficiency and energy costs is prompting plant managers and engineers to take a closer look at integrating efficient technologies such as variable frequency drives and soft starters into their plants. Successfully controlling peak current, which local utilities use to establish charges, can save energy and reduce operational costs.
Historically, VFDs were developed for process control, but energy conservation has become just as important. First-generation VFDs appeared in the 1970s and were only applicable to large horsepower motors. They used silicon-controlled rectifiers and were often unreliable. Second generation VFDs were introduced in the 1980s. They used power transistor technology and were more reliable. However, they were very expensive and had a relatively long switching transition time.
Today’s third-generation VFDs use pulse-width modulation technology enabled by insulated-gate bipolar transistors with high switching frequencies. When applied correctly, these drives can cut energy costs by 15% to 50% with partial loads.
Since VFDs allow ac motors to operate at different rotational speeds, they can be used in ventilation systems, pumps, conveyors and machine tool drives. Fan applications include HVAC, cooling towers, VAV, supply and return, exhaust fume hood, make-up air, induced and forced draft and furnace temperature control. Pump applications include chilled water, pressure boosting, cooling tower, wastewater, chiller, irrigation and hydro-storage.
Although VFD applications are extensive, they are not always the best fit for every situation. For example, if high starting current is an issue, consider using a soft starter. Instead of the traditional means of reduced-voltage starting using autotransformers, reactors or resistors, a soft starter uses reduced-voltage and advanced digital technology to protect and control motors.
Since the acceleration and deceleration of the motor to and from full speed are precisely controlled, electronic soft starters reduce mechanical wear and tear. This smooth performance reduces mechanical shock to the driving system, resulting in extended component and motor life along with lower peak energy costs.
New soft starter technology and sophisticated controls prevent pumps from reaching an overpressure condition at the end of the acceleration phase, and enable pumps to suppress water hammering conditions at the stopping phase. Its torque control produces results close to linear speed without tachometer feedback.
Making the choice
Both VFDs and soft starters help plant operators save energy, cut costs and extend the life of their motor-driven assets. When choosing which technology to apply to which application, asking these questions will guide your choice:
- What is the load of your application? If you intend to control variable torque loads such as pumps, consider using VFDs for potential energy savings
- Does the application have a high starting current? If starting current is high, consider using soft starters because they reduce mechanical wear and tear and lower peak energy charges associated with motor loads for pumps, fans, compressors, material handling, blowers, chillers, conveyors and machine tools
- Do you need to control speed during your entire process? If speed control is required instead of start-stop, use VFDs to enhance your process and increase energy savings.
- Increasing energy efficiency and reducing operational costs continue to be high priorities for managers of manufacturing and process plants. Using VFDs and soft starters to control motor-driven assets can contribute significantly to sustaining efficient plant operation.
|Daniel Nakano is the NEMA Control and Drives product manager for GE Consumer and Industrial. He has more than 20 years of experience with motor and drive applications.|