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In 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 control 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 see realization because the exact quantity of flow of water or air can be delivered to satisfy demand.
Demand-based control is energy efficient. VSD’s can be stand-alone or controlled by a building automation system.
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 regulate the flow depending on the demand of the system. Bypasses on piping and ductwork, 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. They 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. 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. Feedback can be monitored through the direct digital control system.
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 is controlled on a static pressure setpoint based on the pressure of a duct pressure transmitter located strategically in the ductwork. When the VAV box damper goes to the minimum position, there is less demand for airflow, 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 setpoint, and the fan motor requires less energy to move the air. Demand is low; therefore, the motor speed is reduced, and savings are realized in energy use.
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 at 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 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 essential 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 excessive drives are 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.
Variable Speed Drives - VSD Conclusion
VSD’s reduce energy consumption in buildings by reducing the number 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 a necessary component and addition to every building. Variable speed drives have proven they will save money on energy use, maintenance factors, and motor protection.
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