Rising energy prices are motivating industry to explore new methods – such as energy-efficient motor control solutions – for lowering operating costs. Engineers and consultants are tasked with selecting the most reliable motor control solution with the lowest total cost of ownership, which must take into consideration lifetime costs such as installation, operating efficiency, maintenance and energy use, explains Jonathan Smith, field business leader for power control at Rockwell Automation
- Since over 80% of pump and fan applications require control methods to reduce flow to meet demand, those applications are crucial to savings. Process engineers commonly use fixed-speed controllers and throttling devices such as dampers and valves, but these are not very energy efficient.
Variable-frequency drives (also known as adjustable speed drives) offer an alternative that will both vary the motor speed and greatly reduce energy losses. Advancements in drive topology, careful selection of the hardware and power system configuration and intelligent motor control strategies will produce better overall operating performance, control capability and energy savings.
Things to consider when choosing a motor control solution include peak-demand charges, operating at optimised efficiency, power factor, isolation transformer cost and losses, regeneration capabilities, synchronous transfer options and specialised intelligent motor control energy-saving features.
Beat peak-demand charges
It’s important to be aware utility companies charge higher peak-demand electricity prices when companies exceed a preset limit or base load of electricity. Peak demand charges often occur when industrial motors draw large peaks of current when started across-the-line. Variable frequency drives (VFDs) help reduce the peaks by supplying the power needed by the specific application, and gradually ramping the motor up to speed to reduce the current drawn. The VFD also automatically controls the motor frequency (speed), enabling it to run at full horsepower only when necessary. Running at lower speeds and power levels during peak times contributes to a reduction in energy costs and increased operating efficiency.
Kraftwerke Zervreila, a hydroelectric power generation plant in Switzerland, was causing a 20 percent under-voltage condition and line flicker on the electrical grid every time it started its 3.5 MW synchronous water pump motors that drew 1,600A in full-voltage starting conditions. In 2000, Zervreila retrofitted its 40-year-old motors with Allen-Bradley PowerFlex 7000 medium-voltage drives, which limited their starting current to 200A, greatly reducing its peak energy demand.
Optimise power usage
In addition to starting the motor, also consider how energy-efficiently the pump or motor operates. In applications where motors are unloaded or lightly loaded, VFDs can deliver additional energy savings and performance capabilities. Centrifugal loads, such as pumps and fans, offer the greatest potential for energy savings when applications require less than 100 percent flow or pressure. For example, significant energy savings can be gained by using VFDs to lower speed or flow by just 20%. If this reduction doesn’t impact the process, it can reduce energy use by up to 50%, which in many operations, can equate to substantial energy savings.
Energy consumption in centrifugal fan and pump applications follows the affinity laws, which means flow is proportional to speed, pressure is proportional to the square of speed, and horsepower is proportional to the cube of speed. That means if an application only needs 80 percent flow, the fan or pump will typically run at 80 percent of rated speed. But at 80% speed, the application only requires 50% of rated power. In other words, reducing speed by 20% requires only 50% of the power needed at full speed. It’s this cubed relationship between flow and power that makes VFDs energy savers.
Energy savings can also be realised by managing input power based on system demand. Vattenfall Europe Mining AG, in Germany, modernised the overburden conveyor systems of its open pit coal mine with 6.6kV Allen-Bradley PowerFlex 7000 medium voltage VFDs. The drive’s inherent regenerating capability allows fast, coordinated deceleration without the need of braking components and without wasting energy. The optimised conveyor loading (OCL) ensures system efficiency by using a material tracking system across an array of conveyors to continuously adjust speeds so that the conveyor belts are fully and uniformly loaded. A partly loaded conveyor wastes energy and causes unnecessary wear.
Vattenfall’s biggest benefit is the reduced amount of installed drive power. Before modernisation, the conveyor required six fixed-speed controllers at 1.5MW each, totalling 9MW to start the motor. The conveyor with a variable speed solution now uses installed power of only three units at 2MW each, for a total of 6MW to generate a smooth start.
Power factor makes a difference
Power factor and how it affects displacement and harmonic distortion is an important consideration in drive selection. Drives that are near-unity true power factor translate to reduced energy use. Leading drives produce a 0.95 power factor or greater throughout a wide operating speed range. An example of the effect of power factor on energy cost compares two 4,000hp drives, one with a true power factor of .95 and one with a true power factor of .98. The annual operating cost for 8,760 hours of use at
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