Tasked with selecting the most reliable motor control solution with the lowest total cost of ownership? You’ll want to remember that it really pays to buy “smart” in this case.
Rising energy prices are motivating industry to explore any number of new methods to reduce operating costs. Energy-efficient motor control solutions are one such method—and a particularly attractive one at that.
Since over 80% of pump and fan applications require control methods to reduce flow to meet demand, these applications can be especially good hunting grounds in the search for 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 (VFDs), 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 optimized efficiency, power factor, isolation transformer cost and losses, regeneration capabilities, synchronous transfer options and specialized intelligent motor control energy-saving features.
Beat peak-demand charges
It’s important to be aware that 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.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.
Consider the following documented real-world successes
Kraftwerke Zervreila, a hydroelectric power generation plant in Switzerland, was causing a 20% under-voltage condition and line flicker on the electrical grid every time it started its 3.5 MW synchronous water pump motors that drew 1,600 Amps 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 200 Amps, greatly reducing its peak energy demand.
The Monroe County Water Authority, in Rochester, NY, invested in a 4160 V, 750 hp Allen-Bradley PowerFlex 7000 medium voltage drive for one of its centrifugal pumps in 2003. In doing so, the Authority achieved annual savings in energy use and peak demand charges of over $23,000. These types of returns are not unusual.
Optimize 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% 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 translate into substantial energy savings.
Energy consumption in centrifugal fan and pump applications follows the affinity laws—meaning that flow is proportional to speed, pressure is proportional to the square of speed and horsepower is proportional to the cube of speed. For example, if an application only needs 80% flow, the fan or pump will typically run at 80% 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 such energy savers.
Take, for example, what happened at the Lewis County General Hospital in Lowville, NY. Management wanted to reduce the amount of energy consumption in the facility’s HVAC system while assuring patients that care and comfort would remain high. Rockwell Automation helped the hospital install a computerbased energy management system to track temperatures and energy use throughout the facility. The system collected data to help assess where the hospital could improve and identified the fans responsible for moving cool air through the HVAC system. Engineers installed Allen-Bradley PowerFlex 400 AC Drives in the system to optimize fan and pump performance throughout the facility. Installing the drives helped the hospital reduce HVAC-related energy costs by 15%.
Energy savings also can be realized by managing input power based on system demand.
Germany’s Vattenfall Europe Mining AG modernized the overburden conveyor systems of its open pit coal mine with 6.6 kV 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 optimized 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 comes from the reduced amount of installed drive power. Before modernization, the conveyor required six fixed-speed controllers at 1.5 MW each, totaling 9 MW to start the motor. The conveyor with a variable speed solution now uses installed power of only three units at 2 MW each, for a total of 6 MW to generate a smooth start.
The power factor 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 .95 power factor or greater throughout a wide operating speed range. An example of the effect of power factor on energy cost compares two 4,000 hp 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 $0.07 per KW hr results in savings of $63,173 annually using the .98 power factor drive system compared to the .95 power factor drive system.
The hidden cost of transformers
Every drive creates harmonic distortion, which creates extra heat in the plant power system and losses to the drive system. Manufacturers can reduce harmonics by using either a phase-shifting and multipulse rectifier transformer or an active front-end rectifier.
Transformers have long contributed to costs of the overall drive system. Some of the negative issues include increasing the size, cost, weight and complexity of the drive system. Transformers produce losses that generate heat and contribute to energy loss. Extra air conditioning is necessary to cool the transformer, which adds to initial capital costs, but also consumes excess power on an ongoing basis.
Engineers can now take advantage of transformerless medium voltage drives. These drives use an active front-end rectifier (AFE) with a line reactor and integral common-mode voltage protection that has a simpler power structure. They help reduce drive system size by 30-50% and lower drive system weight by 50-70%.
Since transformerless medium voltage drives produce fewer losses due to less magnetic components in the line reactor, they also eliminate the need for extra air conditioning.A transformer is about 98.5 – 99% efficient while an AFE line reactor is about 99.5% efficient. This difference of 0.5–1% sounds small, but it can add up to big savings. Engineers can retrofit AFE drives with existing motors, making the drives ideal for process improvement or energy savings projects with existing motors, switches and control rooms, where space is often limited.
Consider the example of a 4,000 hp drive using a 4,000 KVA isolation transformer that resulted in $154,804 in monthly energy costs. After installing a transformerless line reactor drive at the same power rating, energy cost was reduced to only $153,249 per month—an annual savings of $18,660 at an average rate of 7 cents per kW.
Generate your own energy
Another consideration in selecting a drive is regeneration capabilities. Some VFD applications enable users not only to save energy, but to regenerate power, which can be routed back to the system or sold to utilities for additional revenue.
La Union, S.A. sugar mill in Guatemala uses its waste energy to produce power for its factory by burning the sugar cane bagasse in boilers to generate steam. In 2002, La Union expanded its generation capabilities to sell its excess energy to the local utility market.
La Union replaced its steam turbines with more efficient electrical motors and used Allen-Bradley PowerFlex 7000 2300V, 1000 hp, medium voltage variable speed AC drives in the boiler fans and pumps. The new drive and motor set uses 66% less steam to create the equivalent power, and now provides 1,420 kW of electrical power with the same 23,000 lbs. of steam. This brought in additional revenue of $158,480.
Use one drive for multiple motors
Synchronous transfer capability is another way to reduce energy costs. The synchronous bypass method uses only one drive to start and synchronize multiple motors through the process of transferring a load from one source to another by matching the voltage waveform frequency, amplitude and phase relation between the two sources. Using a VFD to start a motor, bring it up to speed and then synchronize it, causes a reduction in full-load current and optimizes the process.
In 2001, Conoco Inc. built a new crude oil pipeline origination/injection station in Montana to pump a wide range of crude oil types at various flow rates, viscosity and density. Operators had five different pumping scenarios to consider. Conoco used two centrifugal pumps at 2,500 hp and 1,500 hp to accommodate the differing flows, and one 2,500 hp Allen- Bradley PowerFlex 7000 VFD with synchronous bypass to control both of the motors.
The economic advantages of the VFD with a synchronous bypass are in both installation and operating costs. A synchronous system for two motors costs 33% less in initial capital outlay compared to multiple drives. It also reduces drive efficiency losses when compared to multiple drive systems.
Extra energy-saving potential
Not all drives have the same capabilities. Intelligent motor control today takes advantage of advanced networking and diagnostic capabilities to better control performance, increase productivity and perform diagnostics, while reducing energy use. Additionally, software features and programmability can further contribute to a drive’s energy savings potential.
Users can now program their VFD to adjust the total acceleration time and current limit and adjust the speed to the load requirement. Current limit on drives is normally set between 105 and 110 percent, whereas using the across-the-line starting method produces current limits of approximately 650 percent. Reducing the inrush current requirements of the plant equals reduced energy use.
Advances in power semiconductor switches like SGCTs (symmetrical gatecommutated thyristors) are designed for high-voltage operation and ensure the lowest switching and conduction losses while maintaining a high switching frequency.
- Power optimization
Power optimizing features optimize the power usage when operating fans and pumps by adjusting the required voltage to the application. This reduces losses for improved motor and drive efficiency.
- Communication software
Software features enable torque limit and integrated architecture through communication connectivity between the drives, starters and soft starters for greater control and optimization.
ROI from energy management
Industry has many energy-saving opportunities. Intelligent motor control solutions, including high-efficiency VFDs, are an important part of an energy efficiency program to optimize equipment and processes and reduce electric energy bills.
Careful evaluation of your facility, your application(s) and the different VFDs available to you are the keys to investing well. Look for drives that use intelligent motor control through advanced technological features, including regeneration, synchronous bypass, transformerless options, software and communications to optimize energy consumption. As the savvy operators of the facilities referenced in this article will attest, making the right decisions can result in significant returns.
The right energy management solutions– like those described here—are investment strategies for long-term reduced operating costs that have typically provided users payback within one to three years.
Richard Piekarz is an electrical engineering technologist and project solutions manager with Rockwell Automation in Cambridge, ON, Canada. A large-horsepower drive specialist with over 20 years experience in the industry, he has written numerous papers on the subject. E-mail:firstname.lastname@example.org Internet: www.rockwellautomation.com