Industry analysts predict that in the near future, most motors will be controlled by variable frequency drives. Here are the reasons why.
VFDs offer energy savings, increased functionality, integration with automated systems, and reduced maintenance, which are increasingly making them the product of choice for motor control. They have evolved significantly over the past 40 years in terms of physical size and weight, as well as in performance, reliability, functionality and cost.
As with other industrial automation equipment, the trend with VFDs continues to be that with each generation: the size, weight and cost decreases while the performance and functionality increases.
Three technologies have influenced the evolution of VFDs. First is the growth of power conversion components, such as transistors, diodes, silicon controlled rectifiers (SCRs), gate turn-offs (GTOs) and insulated gated bipolar transistors (IGBTs). The second technology is the progression of control electronics and performance control algorithms that migrated from analog circuits to digital control through the use of microprocessors. The third area of development is communication connectivity and advanced automation concepts using field customizable algorithms to meet specific application needs, and the integration of Web-based and Internet technologies.
1960s
VFDs came about during the 1960s to support the manufacturing of synthetic-fiber processes in the textile industry. The technology evolved from manufacturers that used DC motors and DC bus systems with simple electronics to control the speed of their motors.
The term "inverters" was used to describe the process of converting the existing DC bus systems to regulate motor speed. However, these early inverters were expensive, and reliability remained a major concern. Components typically had short life cycles that led to frequent and untimely replacement, sometimes resulting in unscheduled downtime and inefficiencies on the manufacturing floor.
1970s through the 1990s
In the 1970s and 1980s, early advancements in semiconductor technology, specifically for power electronic devices, improved semiconductor reliability and led to performance advancements in early VFDs.
The power electronics used in VFDs shifted from SCRs to Darlington transistors in the second power stage. The AC wave form reproduction was limited to a rough sinusoidal six-step output signal. This six-step output signal limited the operation of motors to 85 percent of their rated capacity or required a 1.15 service factor to prevent motor overheating. Low speed operation of the motor also was limited due to a cogging effect below 10 percent rated speed.
Some minor improvements were made with the introduction of the GTO. These power devices allowed a smoother output signal, but were difficult to control. The dilemma facing designers during this period was the trade-off between using semiconductor devices that had fast switching times but were less reliable, or using devices that were more reliable but had slower switching times. This made the use of drives in pumping operations less desirable since users were faced with drives that performed well, but weren't reliable, or more reliable drives that didn't perform as well.
Up to this point, the various algorithms to control the power conversions and AC form reproduction were accomplished using algorithms solved by extremely complex analog circuitry, which also added to the reliability concerns.
Then, in the late 1980s and early 1990s, VFDs shifted from analog to digital control with the implementation of microprocessors. The microprocessors dramatically improved their performance by enhancing the timing schemes of back-end devices such as GTOs. About the same time, improvements in semiconductor technology lead to the introduction of IGBTs.
The advancement in semiconductor manufacturing soon led to miniaturization and integration of multiple IGBTs into a single package. This dramatically reduced both the size and cost of the VFDs, while improving their thermal characteristics. With the use of microprocessors and IGBTS, the reliability concerns that limited the acceptance of drives in the past began to quickly dissipate.
During this same period, energy conservation became a hot topic to manufacturers as they sought ways to reduce energy costs. With reliability concerns being a thing of the past, the cost of VFDs being driven down, and the increasing interest in energy conservation, VFDs rapidly gained acceptance in a variety of new industries and applications outside of the traditional industrial process control applications. With pump applications consuming a large percentage of all electrical energy consumption, they became a target for variable torque loads drives installations and retrofits.
Further improvements to microprocessor design and computational speed performance soon allowed VFD designers to develop control algorithms that maximized the physical switching characteristics of IGBTs. This led to better control and performance of pulse width modulation (PWM) techniques.
New levels of microprocessor speeds then allowed VFDs to process additional feedback and paved the way for newer and improved techniques such as flux vector control with encoder feedback. This, in turn, led to modern methods such as sensor-less flux vector control and other advanced motor performance in torque generation and speed control. These developments reduced the concerns associated with harmonics and power factor correction, and opened the door for VFDs to even more applications.
By the mid-1990s, as VFDs became widely used, requests by users for network connectivity become more common. As a result, higher-end VFDs evolved to include the capability to connect to, and be controlled over most of the commonly used industrial and building automation networks. The ability to connect over various networks, and retrieve performance and diagnostic information increased VFD use exponentially. In addition, this greatly simplified installation, start-up and troubleshooting of VFDs.
The New Millennium
The popularity of VFDs grew in the early 2000s, along with advancements in microprocessor and Web-based technologies. Among the higher-end versions, VFDs today can be connected over an Intranet to serve Web pages up to a computer using a standard Internet browser for monitoring and configuring purposes. Other Web-based technologies have been implemented as well, including the ability to send out email and text messages in the event of an alarm or other fault.
VFDs continue to undergo transformations. More recently, they've seen increased microprocessor memory that allows modification of various control algorithms or generic functions, such as "macros," to meet the needs of a specific industry application. Previously, these macros were embedded into the VFD firmware and could not be altered in the field by users. This specialization trend led manufacturers to create and optimize VFDs for specific markets such as HVAC, pumping, and crane control.
Today, some of the leading-edge manufacturers have taken this concept to the next level by merging programmable logic controller (PLC) functionality into the VFD. In doing so, new and more complex control and applications are achievable, allowing OEMS to write applications, and local integrators and end users to address the exact needs of their applications.
PLC functionality also allows a VFD to control and monitor other devices including other VFDs, distributed input/output (I/O), PLCs, human machine interfaces (HMIs), and supervisory control and data acquisition (SCADA) systems over an industrial control or building automation network. This type of added functionality also can significantly simplify the integration and costs of distributed and remote systems such as a remote lift station or booster station. In addition, panel space is greatly reduced when a VFD with a controller (PLC) inside card is used, thereby reducing costs.
Using VFDs in Pump Applications
VFDs offer numerous advantages in pumping applications. For example, as an alternative to mechanical valves, they ensure more precise flow, as well as reduced maintenance requirements and the possibility of downtime.
In addition, VFDs offer controlled acceleration and deceleration ramps, providing a smooth transition during product duty cycle operation. This can eliminate maintenance costs associated with replacing the seals on check valves, often seen as "water hammer" (or "slamming") during stopping conditions. The costs of replacing seals can be quite expensive, and the elimination of water hammer alone can justify the installation of VFDs.
In addition, VFDs afford lower start-up and commissioning costs, as they provide users the flexibility to adapt to various manufacturers' pump characteristics and application parameters. These advantages, coupled with the potential for an aggressive payback period that easily can be in the 12 to 18 month range, make pumping applications an ideal choice for VFDs.
Today, pumping applications make up nearly 25 percent of all drives usage, including water/wastewater, commercial buildings, oil and gas, marine, turf management and even residential applications where constant pressure systems are gaining popularity. At the same time, before selecting a VFD over a soft start or other motor starter, special consideration must be given to harmonics, motor lead length, power source stability and motor characteristics.
The Future
Initially, VFDs were created to address speed control for industrial process applications. Over time they've come to address the critical need for energy conservation in a wide range of applications outside of the traditional industrial process market, including commercial applications such as pumping.
In August 2005, Congress signed into law the Energy Policy Act of 2005, reinforcing the country's great need to save energy. One section of the bill establishes annual energy reduction goals for federal buildings, and reduction incentives for the private sector.
In addition, according to the U.S. Department of Energy, electric motors consume 64 percent of the electricity used in industrial applications. Some estimate that more than half of these motors operate pumps and fans. As a result, pump users must become more efficient with their energy consumption. This means pumping system providers, OEMs and package system manufacturers, must do their part to bring new energy efficient technologies to the table.
Moving forward, the greatest advantage to using VFDs in the pump industry will likely be reduced energy consumption. VFDs also will continue to evolve, offering increased ability to control an application, better troubleshooting capability and feedback on system performance, smaller size and price, and more user friendly features.
Pumps & Systems, March 2007
