Variable Frequency Drives

Active Front End (AFE) drives use two power sections. An active rectifier is used to minimize the amount of harmonic distortion reflected to the power grid by drawing nearly sinusoidal current from the power grid. Standard 6-pulse rectifiers used in most drives create harmonics (e.g.5th, 7th, etc.) that can generate anywhere from 30 to 100 percent current distortion. The active rectifier used in the AFE drive creates cancelling harmonics that effectively eliminate those that would be created in a standard 6-pulse drive.
Cost-effective solutions reduce harmonics and improve system reliability, efficiency.
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.
As oil prices and gasoline usage have increased, the demands on U.S. oil and gas wells have grown dramatically. In the past, there was a low cost supply from the Middle East, and U.S. oil wells were not used as frequently due to low reservoirs and the energy cost associated with extracting the oil. A large number of U.S. wells were shut down in reaction to high electric costs, low oil prices, the need for monitoring the oil reservoir and machine maintenance. As of 2004, there were just over 800,000 onshore oil wells in the world, with about two-thirds of them in the United States.
Variable frequency drive technology is more efficient and accurate, which leads to increased energy savings.
While driving a car with the gas pedal to the floor and then controlling your speed using the brake is obviously inefficient, many facilities use the approach for pump control. Flow control with throt­tling or restrictive devices sacrifices energy efficiency and results in unnecessary costs. However, with an understanding of basic principles, an analysis of the specific application, information about available control solutions and evaluation of technologically advanced equipment, facilities can make a quantum leap in improving the efficiency and econ­omy of pumping operations. pump efficiency
Production, distribution and refining applications in the oil and gas industry rely heavily on motor-driven pumps and pumping systems. Keeping electric motors driving critical operations at peak performance is vital to ensure maximum profitability.
There are several ways to control two identical, parallel pumps operating under variable frequency control in pumping applications.
When it comes to pressure and flow, variable speed pumping applications can be divided into three basic categories. Constant pressure-variable flow (Cp-Vf) attempts to keep pressure relatively constant over a range of flows. Constant flow-variable pressure (Cf-Vp) tries to achieve the opposite by using varying pressure to keep flow constant. Variable pressure-variable flow (Vp-Vf) applications can accommodate a change in both.
Last month we reviewed the pump selection criteria for a closed loop circulation system (Vp-Vf). This month we will take a look at a similar application that adds some elevation to the equation.

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