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Variable Frequency Drives

This could be a little on the complicated side to explain in simplified terminology, but if my explanation is not clear, please feel free to call our office for more information or a quotation. Variable Frequency Drives or VFD’s are a motor speed control that will vary the rotational speed of a motor and generate considerable saving in hydro. Your system (if not equipped with VFD) has one speed… full tilt maximum. When a motor starts, the amount of torque (turning force) to start the motor is at its’ maximum and so is the current or amperage of the hydro consumption. Many commercial properties pay for hydro based upon this peak load which could be as much as 4 times the normal running amperage of the motor. Take for example a large commercial property with multiple roof top units that all come out of night set-back at the same time, the peak current draw could cost a fortune payable to the hydro. For this reason alone Energy Management is essential. A VFD eliminates the hydro “spikes” and slowly increases the speed of a motor up to a generally accepted maximum speed of about 80% of the rated speed. This generates a significant savings by itself, but the VFD can do more. If the motor is running at 80% speed there is a 20% reduction in air flow. This also means there is less energy required to heat the air in winter and less to cool the air in summer. It is similar to driving your car in cruise control on a level highway. Further control of the fan speed can be accomplished with a control that measures the air pressure in the system. This method is particularly useful for Make Up Air units in high rise buildings. Typically a make up air unit would run full speed 24 hours a day pumping 100% outside air into the corridors. Imagine the cost to heat -10degree air, in one pass over the burner to 65 or 70 degrees F. in the winter or cool 100 degree air to 75 degrees in the summer. There are formulae for calculating how many BTU’s are required to control the temperatures, but it is unimportant for this understanding at this point. The make-up air unit need not run steady at 100% output if there is reduced demand in the corridors. The “demand” is the amount of air required to maintain a positive air pressure so that odor migration does not occur from the suites. If there is reduced exhaust from the suites from items such as bathroom fans, range hoods and clothes dryers there is less demand for air in the hallways. Systems without pressure control operated VFDs’ to not recognize the changes in demand and operate at full speed, wasting fuel and hydro. There are periods during any 24 hour day when there is less activity in the building at which point a VFD can reduce the fan speed to as little as 30% of peak demand. There is a pay-back to installing a VFD, as little as 2 -3 years and on top of the monthly energy savings there are energy rebates available from Hydro to assist in the installation cost. The cost of operating a make-up unit in a high rise application is part of the common elements and paid out of the condominium fees. Savings as much as 30 to 50% are realized that could generate a reduction in monthly fees. All the work must be inspected and approved by hydro in order to receive the rebate, so you know the work has been installed correctly. The VFD is a control assembly that is mounted in a panel that can be installed in a mechanical room (indoors), outdoors in a weather proof enclosure or in the unit itself if space permits. I have made no mention of price as it would vary greatly depending on the size of the motor(s) being controlled, where the panel is located, wiring involved and a few options that are available. If you have a B.M.S. (building management system) for controlling the equipment in your building a VFD can be easily integrated into your existing system. Each job is different and therefore it would be futile to even provide a price range. Larger horsepower rated motors are more practical to provide with VFDs’, generating a more rapid pay-back, however it is still practical for motors as small as 5 HP. Just remember that the savings are greater than the hydro savings of the motor being controlled. In some cases the motor may have to be replaced if the efficiency and motor windings are unable to accept the VFD. We at Signature Air use Danfoss VFD controls exclusively, as we feel they are the best and have proven their reliability and service when needed to ensure savings for our customers. It costs nothing to get a quotation ( at least with Signature Air Systems ).


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