Variable frequency drives accomplish part load control by varying electric motor speed, significantly reducing energy waste.
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Many electric motor-driven devices operate at full speed even when the loads they are serving are less than their capacity. To match the output of the device to the load, some sort of part load control is in use for the majority of their life. Examples include pumps, fans, conveyors, injection molding machines, air compressors and chillers.
Many part load control strategies waste energy. The most efficient method of part load control, resulting in minimal wasted energy, is the variable frequency drive (VFD).
VFDs accomplish part load control by varying electric motor speed. Energy savings of 50 percent or more over other part load control strategies are common.
At the heart of an electric motor are the stator and the rotor. The stator and rotor contain pole pairs wound with copper wire. When a current is applied, a magnetic field is generated and the north/south field rotates through the stationary stator as the rotor spins to catch up to the rotating field. The spinning of the rotor provides the torque necessary to drive a load.
An electric motor turns at a given speed depending on the number of poles in the motor and the frequency of the alternating current applied. Motor speed can be changed by changing the alternating current frequency.
Nearly all variable frequency drives manufactured today are referred to as pulse width modulation drives. These drives contain electronic circuitry that converts the 60 Hertz line power to direct current, then pulses the output voltage for varying lengths of time to mimic an alternating current at the frequency desired.
The majority of variable frequency drive applications are for centrifugal pumps and fans. The savings potential for these devices is the largest since the theoretical input power varies with the cube of fan/pump speed and volume.
For example, a fan operating at half speed will require only about 13 percent of full speed power. Losses in the variable frequency drive will reduce savings somewhat, but the savings are still very impressive.
Air and water flow control is accomplished by either of several methods, including recirculating a portion of the flow, throttling, variable inlet vanes, and variable frequency drives.
Recirculating part of the flow results in the fan or pump operating at full volume all the time. Only a portion of the flow is used for the system or process and the rest is recirculated back to the inlet of the fan or pump. This is the least efficient means of controlling flow.
Throttling essentially chokes the outlet of the pump or fan to decrease flow much like holding your thumb over the end of a garden hose. The pressure increases and the flow decreases. This results in some energy savings over a constant volume recirculating system but is still wasteful.
Variable inlet vanes apply only to fans and compressors, not pumps. Inlet vanes control flow by pre-spinning the air entering a fan wheel or compressor impeller in the direction of rotation, which effectively varies its capacity.
Variable frequency drives can also be applied to what are called constant torque loads. Unlike the fan and pump power which varies with the cube of speed, constant torque applications vary power in direct proportion to speed. This results in lower savings for a given reduction in speed but there are still significant savings available in some applications.
Examples of constant torque systems include conveyors and hydraulically driven injection molding machines. Positive displacement pumps and compressors are also constant torque machines. Variable frequency drives provide the most efficient part load control of rotary screw air compressors, and are gaining market share in the industry.
Variable frequency drives can be installed with manual or automatic bypasses. In the early days of the variable frequency drives, bypasses were more common since variable frequency drives were not as reliable as they are at the present time. The bypasses were installed in the event of a drive failure to ensure the system or process would remain on line.
Bypasses are still available but not always installed. The criticality of the application must be considered in each case to determine whether the added cost and security of a bypass is warranted.
Harmonic filtering may be necessary in some applications. Variable frequency drives can produce harmonics that can make their way back to the rest of the building and interfere with sensitive electronic equipment and machines. Line reactors can be used on smaller drives of 20 hp and less to dampen and mitigate harmonics.
For larger applications, an isolation transformer may be warranted. The isolation transformer can be installed either on the drive or on the piece of equipment to be protected from the harmonics.
Because the equipment being protected may be much lower in power, it may be most economical to isolate the piece of equipment from the building supply.
Due to the decline in costs over the past five to ten years, and the potential for significant energy savings, variable frequency drives can be cost effective on a very large range of applications.
Variable air volume systems should always have a variable frequency drive installed to control volume. A variable frequency drive serving a variable air volume system operating for a typical 3,000 hours per year will pay for itself in two to five years for a return on investment of roughly 20-50 percent. Variable frequency drives on larger motors will offer a higher return on investment.
For “custom” applications associated with process fans or pumps, almost any scenario where the flow is being reduced to 90 percent or less of full volume for 4,000 hours per year or more is a good candidate for a variable frequency drive. Variable frequency drives installed to reduce flow significantly can easily pay for themselves in under a year for a return on investment of over 100 percent.
Circulating pumps for hot water heating systems are a good application for variable frequency drives, provided the hot water temperature is not reset according to building heating loads or outdoor air temperature.
Hot water temperature reset is an automatic lowering of water temperature when the heating loads on the building decrease. When this happens more flow is required to meet the heating demands of the zones in the building. This keeps the required flow higher, which reduces variable frequency drive savings.
It should be noted though that disabling hot water temperature reset is not advised to achieve pumping savings. Hot water reset results in lower heating costs and allows the temperature controls to work considerably better which provides a more comfortable building.
Chilled water circulating pumps provide good opportunity for savings for variable frequency drives. Although the cooling season in the Midwest is fairly short, in most cases the pumps run continuously, including during light cooling loads outside normal business hours.
When retrofitting an existing system, there is often an added cost for achieving savings for a chilled water pump application. Typical chilled water systems have three-way control valves at the cooling coils which maintains a constant flow through the pump and chiller.
To achieve variable flow savings, the valves will need to be converted to two-way valves so that reduced pumping volume can result during periods of low cooling loads.
Additionally, a primary/secondary chilled water system may have to be established to maintain the required minimum flow through the chiller. Check with the chiller manufacturer.
Almost without exception, variable frequency drives should be installed on circulating loop pumps for geothermal heat pump systems. The long hours of annual operation, particularly at low heating and cooling loads, provide lots of savings potential even for small pumps less than five horsepower.
Learn more about geothermal heat pumps
Retrofitting a variable speed drive on a hydraulic plastics injection-molding machine requires a controls package that is customized for the application.
Savings are typically in the 33-50 percent range, which over the typical long operating hours of injection molders can quickly generate savings for a high rate of return.
Cooling towers can be a good application for variable speed drives. The savings for cooling towers are generated by operating the fan(s) at lower speeds for longer periods of time as opposed to cycling the fans on and off at full speed. This reduces the energy consumption and in some periods may reduce billed demand.
Some chilled water applications that use a cooling tower may have condenser water temperature reset where the condenser water temperature is lowered during periods of low wet bulb temperature (or dewpoint). This saves chiller energy and although the tower fan will have to run faster to achieve lower condenser water temperature, the chiller savings more than offset the extra tower fan energy.
Variable frequency drives offer a number of solutions that may save little energy. In some cases a variable frequency drive is used to provide a soft start for an electric motor to help prevent equipment damage.
Although this will limit a momentary inrush current associated with bringing a motor to full speed almost instantly there is negligible energy savings and demand savings unless the motor continues to operate at lower speed during typical operation.
Variable frequency drives are making their way into systems and equipment for energy savings and as a means of part load control. In the future, expect more centrifugal chillers to be installed with variable frequency drives as opposed to inlet vane control, as well as an increased use of variable frequency drives for screw air compressors.
As prices continue to fall, variable frequency drives will be used more frequently for soft start capabilities and even for balancing fluid systems as opposed to using balancing (throttling) valves.
The convenience of adding a drive to precisely and easily control equipment speed rather than complex, expensive, and maintenance intensive mechanical drives, or using guesswork to control processes, will continue to increase demand for this technology.
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