Variable frequency drives (VFDs) have been widely applied to pumping applications for nearly 30 years to provide variable flow and pressure output to meet changing conditions in commercial buildings. While the affinity laws for centrifugal loads explain the large energy savings potential due to speed reduction, the individual components of this "variable speed system" have not historically been examined or held to any efficiency standard. VFDs are also commonly packaged with options for pump applications like harmonic filters, EMC filters, dV/dt motor filters and contactor bypass, which impact system efficiency. Even VFD parameters adjusted during startup can affect how much power is consumed by both the VFD and the motor. To establish some means of evaluating these systems for efficiency, the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) formed a VFD task force in 2006 to develop a performance standard for VFDs. The result was the publication of AHRI Standard 1210 (IP) and 1211 (SI) Performance Rating of Variable Frequency Drives. The complete standard sets a minimum of three test criteria for VFD manufacturers to publish under the newly published guidelines:
- Drive System Efficiency (percent)
- Motor Insulation Stress, Peak V and Rise Time (µsec)
- Power Line Harmonics, Total Harmonic Current Distortion (THDi) (percent)
Calculating VFD Efficiency
If system efficiency is to be determined for a VFD, one might think it can be as simple as measuring the power into and out of it, then dividing the two measurements to represent it as a percentage value. The AHRI VFD task force quickly realized it was not that simple. Even small changes to the drive setting, such as carrier switching frequency, could have a large impact on VFD efficiency. Likewise, in order to provide a standard method of test, basics like connected motor style, cable characteristics between the drive, and the motor had to be standardized. Electrically, power source impedance, phase imbalance and power factor needed definition in order to establish a standardized system where a user would be able to compare data between compliant drives and be assured that it was collected and published based on the same system.
Figure 1. AHRI standard test setup for drive system efficiency and power line harmonics
(Graphics courtesy of Schneider Electric)
P1 - Closest point less than 1 m to input terminals of the VFD
P2 - Closest point less than 1 m to input terminals of the motor
P3 - Temperature sensor installed on the motor stator winding out of the cooling air circulation path
P4 - Torque/speed sensor between the motor and the load
P5 - Ambient temperature sensor near the cooling air entering the motorDrive Technology
Today's VFDs have three power component sections: an input rectifier, a fixed DC-bus section and a transistorized inverter. Each design can differ slightly in components, as well as how the power devices are controlled—all of which have an impact on their efficiency. In the early 1990s most VFDs were updated to include newer power semiconductors, and insulated-gate bipolar junction transistors (IGBT) were controlled by pulse-width modulation (PWM) algorithms. This change greatly enhanced reliability and reduced the overall size of VFDs, allowing electrical variable speed control to replace mechanical methods of varying flow in pump systems.
Table 1. Typical efficiency values as shown in VFD product literature
Figure 2. Typical VFD circuit diagram
Table 2. Speed/torque test points for drive system efficiency. Please note: Output frequency or other readouts from the VFD shall not be used to determine percent speed. Only those cells that contain CT or VT are test points.VFD Efficiency
While a single value did provide some comparison of VFD efficiency between manufacturers, it did not provide a means of comparison at any point other than full load and speed, where equipment rarely operates in modern variable volume systems. AHRI Standard 1210 and 1211 addresses this issue by requiring test data be published at four speed and torque points representing a variable torque fan or pump motor load, or a constant torque positive displacement pump or compressor loads. Table 2, taken from the AHRI Standard, illustrates data that must be published by manufacturers seeking to be certified.Certified VFDs
With the release of AHRI Standard 1210 and 1211, specifying design engineers, owners and energy managers can compare VFDs on an equal basis. Efficiency has been the driving force behind motor regulations that define minimum levels of acceptable power usage. VFDs now have a similar tool for those trying to maximize system efficiency with detailed operating points that closely define where fans and pumps operate within variable volume systems. As a result, simple, certified test data will be published following third-party verification testing to comply with these new standards.Trusted Certification Program
AHRI Certified products are verified to perform as specified by the manufacturer, and the process of certification holds them accountable for their reported efficiency ratings. This globally recognized, industry-respected approach to product certification carries benefits for consumers and specifying engineers. The performance of equipment in the program is independently measured and verified, allowing for fair product comparisons. Equipment must undergo rigorous testing by independent, third-party laboratories under contract to AHRI. References ANSI/AHRI Standard 1210 (I-P) and ANSI/AHRI Standard 1211 (SI)Reducing Capacitance VFD Input Waveform
An example of how drive technology has been impacted by advanced switching algorithms is seen in drives that reduce DC bus capacitance, allowing a more efficient transfer of power from line to load. When combined with rapid sensing circuits that detect abnormal line condition, a VFD can automatically adjust the motor output waveform by modulating the required amount of voltage and associated energy to the motor, avoiding nuisance trips or loss of speed. Effective conversion of the line power to a custom pulse width modulated waveform without the need for large energy storage inside the drive via capacitors has increased VFD efficiency by up to 5 percent.
