In the U.S. industrial sector, motor-driven systems consume 70 percent of all electricity. Motor-driven pumps account for more than 30 percent of that amount-more than any other application. Considering energy and maintenance represent more than 80 percent of total motor life cycle costs, a growing number of system designers, specifying engineers, maintenance professionals and end users are turning to variable speed motor control systems that can save up to 60 percent in energy costs as well as significantly reduce maintenance and equipment costs, improve process control and enhance system reliability.
Rather than constantly run the motor at full speed, variable frequency drive (VFD) systems-also referred to as adjustable frequency drives, variable speed drives, AC drives or simply "drives"-monitor system characteristics like pressure and control the motor speed to match the system requirements only as needed, often at lower speeds. By modulating the power delivered to the motor (pulse width modulation or PWM), VFDs provide continuous control, smoothly adjusting motor speed to directly control pressure, flow and fluid levels.
In centrifugal pump applications with low head pressures, VFD controllers will typically save more than 50 percent of the energy used. While the greatest reduction in energy costs is realized with centrifugal pumps, most pumps will realize savings when less than full output is required. VFDs also improve electrical power factor and significantly reduce motor starting current typically by a factor of 4:1 to further reduce power demand from the local power utility. More advanced VFDs include a built‐in power meter and cost calculator to measure and record savings while eliminating the need for additional external monitoring devices.
The Science Behind the Savings
When a VFD starts a motor, it initially applies a low voltage at a low frequency to the motor. Starting at a low frequency and voltage avoids the high inrush current (typically 600 percent of its rated current) that occurs when a motor is started by turning on a switch or contactor to apply across-the-line voltage. The VFD then increases the applied frequency and voltage at a controlled rate to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop rated torque while drawing rated current. For smoothest starting, some VFDs incorporate S-ramp acceleration and deceleration functions that provide the least amount of mechanical shock loading on the pump, motor and system.
The key to maximizing energy savings is continuous control of the motor voltage and frequency commonly referred to as the Volts-to-Hertz ratio. Advanced VFDs provide selectable V/Hz control modes to provide the highest level of savings for single motor and multiple motor applications. For single motor control, dynamic V/Hz control uses the least amount of energy and a square law characteristic mode is best for multi-motor variable torque loads.
Matching VFDs to Motors
To optimize system savings, VFDs should be matched to the motor's performance characteristics. VFDs are available with voltage and current ratings to match the majority of three-phase motors manufactured for operation using across-the-line power. VFDs designed to operate at 115 VAC to 690 VAC are often classified as low voltage units. These units are typically designed for use with pumps rated to deliver fractional HP up to 1,000 hp, although they are commercially available above 2,000 hp.
The motor used in a VFD system is usually a three-phase induction motor. Various types of synchronous motors offer advantages in some situations, but induction motors are suitable for most purposes and are generally the most economical choice. Motors designed for fixed-speed, across-the-line voltage operation are often used, but certain enhancements to standard motor designs offer higher reliability and better VFD performance. Premium efficiency and inverter duty-rated motors are preferred for variable speed applications.
For successful variable speed installations, the following requirements should be considered when matching VFDs and motors:
Speed Range. Motors are rated for speed ranges stated as a ratio of rated speed to minimum speed with either a variable torque (i.e., centrifugal pump) or constant torque (i.e., positive displacement pump) characteristic. Typical values are 5:1 and 2:1, which mean that the motor can be operated down to 20 or 50 percent of the rated speed continuously. The motor should be suitably rated for the desired speed range or its thermal rating may be compromised. The VFD should be configured to provide the appropriate variable torque or constant torque V/Hz characteristic to optimize energy consumption and motor performance.
VFD to Motor Distance. The recommended maximum distance between the VFD and the motor is a combination of the VFD's carrier frequency and the motor's rated voltage. VFDs have the ability to vary the output carrier frequency (switching frequency) of the PWM waveform. Faster switching (e.g., higher carrier frequencies up to 18 kHz) results in lower audible motor noise but can reduce the maximum allowable cable distances. Lower carrier switching frequencies (e.g., 3 kHz) allow the motor and VFD to be installed farther apart. In general, shorter distances are recommended at higher carrier frequencies; however, premium efficiency motors can operate with longer motor cable lengths than standard or high-efficiency motors, and inverter duty-rated motors have the highest allowable cable distances.