Variable Speed DrivesIn HVAC a variable speed drive is a speed control system for electric motors. The variable speed drive (VSD) or variable frequency drive (VFD) among other names (adjustable-frequency drives (AFD), AC drives, micro-drives or inverter drives). The application of variable speed drives has brought about a new way of engineering buildings and controls systems HVAC. The flow of water and air can now be regulated precisely using VSD’s and state of the art control systems. By combining VSD’s with state of the art control systems large savings in energy usage can be realized because the exact quantity of flow of water or air can be delivered to satisfy demand. Demand-based control is energy efficient.

Before Variable Speed Drives

In the past when the need to be energy conscious was not so important as it is today HVAC systems for air and water flow usually ran at a constant speed and used mechanical or electrical devices to regulated the flow depending on the demand of the system. Bypasses on piping and duct work, three-way valves, inlet and outlet dampers and vanes, eddy current clutches, variable pitch pulleys, and hydraulic couplings were used to control flow or the speed of the fan or pump. These mechanical/electrical throttling devices were a way of flow control for the flow of air and water and usually added a lot of moving parts to a system that was neither as efficient nor was it as maintenance friendly as modern variable speed drives are today.

Fault Protection with VSD

Many modern VSD’s also add motor protection to the motor eliminating the need for solid state motor starters to monitor motor amps and motor volts. Modern solid state variable speed drives have fault protection such as motor over amperage, motor voltage, and many other variables in an electric motor that a conventional motor starter would not monitor such as back EMF, frequency, phase monitoring, external control faults, and motor winding problems. Always check the specific drive manufacturer’s manual to see what fault protections are available on specific drives.

Energy Savings

Variable speed drives allow fans and pumps to run at lower speeds based on demand. A typical duct system for variable air volume boxes can be controlled on a static pressure set point based on the pressure of a duct pressure transmitter located strategically in the duct work. When the VAV box damper goes to minimum position there is less demand for air flow and the static pressure will rise as the VAV dampers close. The control system responds by sending a signal to the variable drive to slow down. As the drive slows the static pressure in the duct falls back to the set point and the fan motor requires less energy to move the air. Demand is low therefore motor speed is reduced and a savings is realized in energy use.

The same for water flow in a chilled water or a hot water piping system. When the valves to the coils modulate to a minimal position the control system received a higher pressure in from the pressure transmitter in the piping system and responds by sending a signal to the variable speed drive to slow down which slows the pump motor resulting in less energy utilized at the pumps.

VSD Installation Considerations

Some considerations about variable speed drives and their installation and location of installation. Always ensure the drive is installed in a waterproof NEMA rated box. It is not ideal to have piping connections terminate in the top of the box but rather on the side and ideally on the bottom of the box where the drive is located. If it is impractical for the drive to be located in a protective waterproof box make sure the variable speed drive is not installed under piping where it is possible for water damage to the drive. Variable speed drives have solid state electrical components and it is important to keep these components dry.

Additionally, VSD’s can cause problems with harmonics in the electrical systems. Bad harmonics can lead to problems such as conductor overheating, overheating and failure of capacitors, spurious trips of circuit breakers and blown fuses, increased eddy currents in transformers, voltage regulation problems in generators, utility meters reading higher than normal resulting in higher than normal billing, and computers and telephone systems to fail.

To reduce this problem it is important for the design engineer to properly distribute the variable speed drives equally across the power distribution for the building. Harmonics are typically not a problem in the average commercial building unless there are excessive drives using the same bus for power. There are many methods used to reduce harmonics in any facility where there are issues. Consult with a commissioning or electrical engineer for possible solutions if your building experiences these problems.

VSD Conclusion

VSD’s reduce energy consumption in buildings by reducing the amount of energy fans and pumps consume while continuing to maintain comfort. Variable speed drives eliminate the need for bypasses and outdated mechanical or electrical flow control methods that artificially created pressure drops. VSD’s also monitor and respond to faults that can cause problems with motors so they offer motor protection in many cases. These three reasons make VSD’s a necessary component and addition to every building. Variable speed drives have proven they will save money on energy use, maintenance factors, and in motor protection.

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