Gain More With VFDs
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Expanding population.
Codes and regulations.
Connectivity and monitoring.
These trends are directly impacting the HVAC/R equipment requirements and market growth. More equipment is needed for new project construction, required retrofits, and building expansions. Just because more or new equipment is required doesn’t always mean that the total cost of ownership has to increase. This is where energy efficiency plays a huge role – and it has never been more important.
Variable frequency drives (VFDs) have been a vehicle for improving a systems energy efficiency for over 40 years. A standard VFD controls a motor based on a linear volts per hertz (V/Hz) relationship. The output voltage waveform of a standard VFD is typically based on pulse-width modulation (PWM). The amplitude, duty cycle, and periodicity of the PWM waveform decide the effective voltage and frequency of the VFD output.
However, this isn’t enough anymore, especially for fan and pump motors in HVAC/R applications or variable torque applications. In these applications, the speed of the motor is reacting to external sources that are a part of the overall system. These external sources could be a temperature sensor in a refrigeration system at a grocery store, a humidity sensor in a data center facility or a pressure sensor monitoring the room pressure in a clean room. A VFD uses the external sources as input data to adjust the output frequency to the motor, in turn, adjusting the speed of the motor. In these applications, there are advantages to varying speed rather than running at 60Hz, full load amps, at all times.
Below are 3 additional considerations to reduce energy costs and become more energy efficient.
Energy optimization software
As mentioned above, a standard VFD modulates the output voltage waveform to adjust the output frequency. With new technologies in current VFDs, more can be done to optimize the energy consumption in variable torque applications than just standard Voltz per Hertz control. Many VFDs have optimization parameters that can be used to tune the input power consumption while maintaining the load’s torque or speed demand. However, some VFDs have built-in algorithms that can conduct the tuning and monitoring automatically. Eaton’s Active Energy Control is an example of this technology.
This algorithm works right out of the box, no tuning or adjusting of parameters is required. The algorithm monitors the output power performance while reducing the input power requirement. To assure the stability of the motor, the algorithm initially sets the drive output voltage at the same level as the voltage based on the linear V/Hz method for the same reference frequency. It then begins to reduce the voltage incrementally to optimize the energy usage. Meanwhile, the algorithm monitors several real-time parameters of the motor to prevent the motor from entering conditions that may lead to instability. When the motor enters the optimal zone of operation, the drive output voltage stays at the same level until there is a change triggered by commands to the drive, such as a change in the reference frequency or a change in some real-time parameters. After the output voltage stabilizes, the drive keeps monitoring the real-time parameters of the motor to prevent instability conditions.
The VFD built-in algorithm can save energy costs up to 10 percent more compared to the optimization parameters as the algorithm is actively monitoring and consistently running the optimization algorithm at all times. This is big savings to realize for systems with multiple loads with the added benefit of no additional adjustments required to maintain the systems performance and reliability.
Electronic bypass control
Many HVAC/R applications are mission critical to a building or facility’s processes. It is suggested to improve the reliability to the system to have a bypass circuit in parallel with the VFD circuit. The bypass circuit provides redundancy to the primary VFD circuit in case of failure or fault conditions. Bypass circuit can include contactors, reduced voltage soft starters or another VFD.
- Most commonly, contactors are used because they are compact in size, reliable and inexpensive. While in bypass mode, the contactors would run the motor at full load amps. No energy savings are seen during contactor bypass mode.
- Often, reduced voltage soft starters are a bypass option for motors that tend to have high inrush currents that can damage mechanical components in the motor or equipment such as water hammer in pumps. The soft starter is able to ramp up the current and eliminate this high inrush current. The soft starter, like the contactor, would run the motor at full load amps once the current ramp process to full load amps is completed.
- Lately, VFDs are becoming a specified bypass solution to provide the highest level of redundancy. The VFD bypass circuit can control the system in the same manner as the primary VFD and continue to communicate to the building management system. By using a VFD bypass circuit, you can resume the energy efficiency and savings similar to the primary VFD.
These three methods of bypass can be controlled two different ways – manually or electronically. Manual bypass control allows a user to switch the VFD into bypass mode or back into VFD mode through a position switch or pushbuttons. This method provides the user a physical control mode, but the control is local to the VFD and manual.
Electronic bypass allows the bypass mode to be controlled by the VFD or an external controller. The controller or VFD can receive signals and electronically put the VFD into bypass mode and close the bypass circuit. Signals could originate from a digital input from a sensor, via communication from the building management controller or from the VFD keypad for local control. This improves the monitoring capability and system processes while reducing the overall cost of the VFD product by removing pilot devices from the front of the enclosure and the labor to assemble.
By using a VFD with built in electronic bypass, you can further reduce the cost of the product since the external controller is not required. You can also add functionality to go into bypass mode in a fault condition providing even more system process protection and increased system reliability.
Utility Rebate Programs
As energy demands rise with increased population, manufacturing and facilities, utilities are offering rebate programs to residential, commercial and industrial customers to promote the use and installation of energy efficient products that can help reduce the energy footprint. These programs include rebates for LED lighting or smart lighting controls, solar PV arrays, and highly efficient HVAC/R equipment and controls – including VFDs.
These programs encourage building owners and facility managers to upgrade equipment to the more energy efficient solution. The initial capital cost of installing the new solutions can be offset by the rebate and the long-term energy savings. VFDs alone can provide a 60 percent energy savings over standard across-the-line starters.
Energy saving calculators can help visualize the long-term cost of a VFD versus a starter. The calculator evaluates the typical fan or pump duty cycle, the initial cost of the VFD, and the motor efficiencies. It conveniently provides a report on the energy saved and reduced carbon footprint. This report includes estimated payback time to better understand the effect of installing a VFD in place of a starter.
By utilizing the utility rebate programs and the energy savings calculator, building owners can see monetized benefits of installing a VFD to control the HVAC/R applications.
For more details on the utility rebate programs in your area, visit www.DSIREUSA.org.
Energy efficiency has become more important to building owners and facility managers as a channel to achieve overall cost reductions. Energy is crucial to most applications, especially mission critical HVAC/R applications. By optimizing the input power required to maintain output performance, utilizing VFD integrated electronic bypass for improved reliability and taking advantage of the rebates available, VFDs installed on variable torque motors are an easy solution to reduce energy cost, increase energy efficiency and improve systems reliability and monitoring.
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- Gain More With VFDs - September 12, 2016
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