How to measure VFD efficiency
Energy efficiency is high on the agenda. VFDs are enablers on higher efficiency in drive systems, especially in flow control. However, how to deal with VFD efficiency? In particular, how to measure VFD efficiency and verify the figures provided by the manufacturer?
There is a motivation to reduce the environmental impact and obviously also the cost advantage. VFDs are key to reduce the power consumption of industrial motors.
First, we shall stress again that the energy savings come from the flow control principles that the VFD enables. The VFD losses are secondary. Yet, considering the entire life cycle the VFD efficiency also has certain role in the LCC calculation.
VFD efficiency measurement
There is no direct way to measure efficiency of VFD or other drive component. In fact, the setup is based on power measurement and the efficiency is calculated afterwards (strictly speaking we shall not talk about efficiency measurement but rather efficiency calculation or efficiency determination). It means that the resulting VFD efficiency is substantially impacted by the accuracy of power measurement.
Remark
According to IEC standards the efficiency of the semiconductor converter is defined as a ratio of the output active power to the input active power of the converter.
We would like to stress that the definition is based on active power and not on apparent power as sometimes incorrectly assumed.
Unsuitable measurement setup
As reported in [1], many people have the measurement of input and output power in mind when thinking of how to determine the VFD efficiency. They mostly forget to take the measurement tolerances into account. Unfortunately, those tolerances have dramatic impact on the accuracy of the efficiency determination. Even if the measurement error is small, it may have devastating impact on the calculated VFD efficiency. Remember that VFD efficiency is expected to be roughly in the range of 97% to 99%. That means only 1% to 3% of VFD losses. Having 0.5% error on each the power measurement cannot be neglected.
Recommended measurement method
Luckily, there are better methods to determine the VFD efficiency than by calculation from input and output power. One method, commonly used in VFD test campaigns, is the segregated loss method. Simply stated, measuring the input (or output) power and losses leads to much more accurate results of VFD efficiency than basing the efficiency on input and output power measurement.
1. Input/output power measurement (not recommended)
Accuracy of measuring instruments
This method measures the input and output power of the VFD. For example, a power analyzer may be used. In any case, the power analyzer or other device needs current and voltage measurement as well as the power factor. These measurements inherently include certain measurement error. For example, a quality current transducer (CT) is of accuracy class 0.2. That means that the measurement error is 0.2%. That is quite precise measurement. Nonetheless, the voltage measurement includes certain error as well. And finally, the determination of the power factor, i.e. the angle between voltage and current is also not error-free. Combining all these factors we get an overall accuracy of the power measurement. Combined error of 0.5% can be considered as a good result. Yet, as we will see, such error is enough to compromise the calculated efficiency value significantly.
Multiple systems to measure
Moreover, there may be few additional challenges. Many VFDs have a multi-pulse diode rectifier on the grid side. Measuring the input power would require measuring each 3-phase system individually ad summing them up. For example, a 24-pulse DFE would require 4 sets of measurement instruments. Another challenge is the quality of the waveform. On the primary side of the transformer the waveform is close to a sine wave thanks to the multi-winding design and phase shifting. However, on the secondary side the waveforms are far less good. It is essentially a 6-pulse diode rectifier with its characterstic spectrum rich in harmonics. An alternative is a measurement on the transformer primary side. By doing that you only need to measure one 3-phase system and avoid the issue with large harmonic distortion. However, such setup would include the transformer efficiency and losses that may be fairly difficult to separate.
Non-sinusoidal inverter voltage waveform
Measuring the output power may be even more difficult. The inverter output waveform can contain fair portion of harmonics. Moreover, the voltage waveform is some kind of stairwave for most voltage source inverters. To measure the active power one needs to precisely determine the fundamental waveform (without harmonics as they don’t contribute to active power) and the power factor. Reaching overall 0.5% accuracy is usually not realistic.
2. Measurement using segregated loss method (recommended)
As we have just described, the method using input and output power measurement is highly inaccurate. So it does not surprise us that a better alternative is suggested. The IEC standards related to converters refer to a segregated loss method.
Before going into details we just have a short take away: VFD efficiency determined by measuring VFD losses will always be much more precise than a method using input and output power. This is a fact. Even if the loss measurement has pretty large error, e.g. 10%, it still provides better results.
The segregated loss measurement method is especially useful for liquid cooled drives. Those VFDs evacuate 90% to 97% of their total heat losses through the liquid cooling circuit. By measuring the flow rate and inlet/outlet temperatures the losses can be easily determined.
Description of segregated loss method can be found e.g. in IEC 61800-4.
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Comparison of measurement methods
We will now illustrate the accuracy of determination VFD efficiency by comparing both mentioned methods on a specific example.
Let’s take a medium voltage VFD with 5’000 kW rated output power. The task is to determine the VFD efficiency at the rated operation point.
Input/output power method
Segregated loss method
Comparing both above tables we can clearly notify the major difference. We measure the same VFD with 5’000 kW output power and 70 kW losses.
With the input/output power measurement, a measurement error of just 0.5% is enough to spread the real efficiency of 98.62% to a range of 97.64% to 99.61% (min and max value or worst case and best case). That is a huge variation or huge uncertainty, respectively. With a measurement error of 1.0% we can even obtain VFD efficiency exceeding 100%.
In contrast, the segregated loss method is far less sensitive to measurement accuracy. With considerable measurement error of 10% on the losses the VFD efficiency only varies in the range of 98.47% to 98.76%.
Real life experience
I have participated in test campaigns of medium and large MV drive systems. There I have witnessed myself calcuated VFD efficiency exceeding 100%. And that happened in a respected third party power laboratory. It was not a garage style experiment but something done in a professional, well-equipped test field. The VFDs were of a good quality. However, they were no perpetuum mobiles. The result was clearly influenced by inaccuracy of the power measurement. In fact, the otuput power for all specified operation points was higher than the input power.
In those tests our goal was mainly to verify the temperature rise of the drive components. For that purpose the test setup was OK. We have also measured the losses of the VFD. Using that approach the VFD efficiency was obviously less than 100%.
What do the standards say?
IEC 60146-1-1 gives some hints how to measure VFD efficiency, such as:
“The power efficiency may be correctly obtained either by measurement of a.c. power and
d.c. power or by calculation or measurement of internal losses.”
This first statement is very general. It basically says that the converter (VFD) efficiency can be determined based on power measurement, by calculation or by loss measurement. Of course, a calculation is an alternative to a measurement. However, to make the calculation trustworthy is shall be backed by measurements.
The same standard also clearly defines what losses shall be included when determining the converter efficiency and what not (see chapters 6.2.2.2 and 6.2.2.3 of IEC 60146-1-1).
Power loss determination for assemblies and equipment is classified as type test.
According to IEC 61800-4:
“Typically, efficiency is established by means of calculations or measurements or combinations of both.”
With regard to loss measurement the IEC 60146-1-1 suggests that the tolerance on the measured losses shall be less than 0.002⋅P (< 0.2% of output power). Such criteria is normally not difficult to fulfill. For example, having VFD efficiency 98.5% means that the total losses acount for 1.5% of the output power. Having 10% measurement tolerance means 0.15% with respect to the output power. That is less than 0.2% as recommended by the standard.
IEC 60146-1-1 also requires that all the apparatus included in the determination of efficiency shall be stated.
IEC 61800-4 clearly states when comparing a) the determination of losses by direct measurement of input and output power and b) the direct measurement of losses:
“By directly measuring the losses, method b) gives more accurate results, and is preferable in
the case of systems characterized by high efficiency values.”
Summary
Accuracy of the measurement of VFD efficiency heavily depends on the measurement method and its inherent tolerances. As shown in this post, method based on direct measurement of input and output power results in large uncertainty and in extreme case may lead to determined efficiency exceeding 100% (best case) or being much lower than expected (worst case). Reatively small measurement tolerance, such as 0.5%, can dramatically undermine the accuracy and credibility of the whole measurement campaign. The higher is the real VFD efficiency the more sensitive is this method with respect to measurement tolerances.
In contrast, efficiency measurement based on segregated loss measurement is much more accurate. Even if we allow relatively high tolerance on the loss measurement, for example 10% relative tolerance (error) on the losses, the method still delivers superior results when comparing it with input/ouput power measurement. Especially for liquid cooled drives, this method is easy to apply.
Conclusion
Determination of VFD efficiency requires a suitable setup to be selected. Attention shall be paid to measurement error and its impact on the results. Efficiency determined based on input and output power measurement is prone to be inaccurate and to deliver erroneous results. With state of the art instrumentation the error is simply too large.
Measuring the VFD losses is much better way and this method is recommended by standards. For liquid cooled VFDs the losses into the cooling medium can be determined with a reasonable accuracy.
References
[1] VFD efficiency over 100% – A discovery of perpetuum mobile? https://mb-drive-services.com/vfd-efficiency-over-100-a-discovery-of-perpetuum-mobile/
[2] How to calculate energy and cost savings, https://mb-drive-services.com/how-to-calc-energy-and-cost-savings/
Standards
IEC 60146-1-1
IEC 60146-2
IEC 61800-4
