Where has all the power gone?

A power analyzer calculates the efficiency of an electrical system like a motor or transformer by measuring both the fundamental and harmonic components of power. At high frequencies and high currents, this becomes challenging: accurate measurement requires high-quality current sensors. These sensors must not only maintain accuracy across the entire frequency range, but their phase shift also needs to be known in order to calculate power correctly. To make matters worse, the phase shift of a current sensor is typically frequency dependent. Unless a sensor has a consistent time delay across all frequencies, accurate power measurements are difficult – regardless of the analyzer used.

Figure 1: Active power and harmonics over the frequency band

What happens if your power analyzer and current sensor combination doesn’t meet high-frequency measurement requirements? When measuring the efficiency of an inverter using SiC semiconductors with a carrier frequency of 50 kHz or higher, your setup might yield efficiency values that exceed reality – sometimes even suggesting efficiencies over 100%. While such an extreme result is an obvious red flag, a more concerning scenario is when the analyzer shows 96% efficiency instead of the true 94% – or 96% instead of the true 98% – due to limitations of the current sensors. This is the real issue: how would you know? The key is to not only rely on the power analyzer's specified accuracies but also consider the current sensors' impact.

As an example, consider the power losses in a motor system. In such systems, harmonics of the main frequency are converted into noise and vibration, which can be identified without direct power measurements. However, high-frequency harmonic power is primarily converted into heat within the motor. Measuring this high-frequency power is challenging due to the motor’s inductive nature: at higher frequencies, the phase angle between voltage and current approaches 90°. Since active AC power is calculated as Voltage × Current × cos(θ), any phase error pushing this phase angle beyond 90° results in a negative measured power loss – appearing as a false gain!

The reason for this inaccurate measurement is an uncompensated phase error of the current sensor. Such phase errors distort the measurement and can lead to incorrect conclusions about a system's energy losses. There is a solution to this: With the right combination of power analyzer and current sensors made for power measurement, phase errors can be corrected ensuring accurate power measurements even for high currents and high frequencies. This lets you know precisely how much power is lost in your system. However, the next crucial question is: where exactly these losses occur – and how to trace them?

Power analyzers offer various features to determine at which frequencies power losses occur. A traditional method is harmonics analysis, which uses the motor’s main frequency as a base. Since motor main frequencies are typically low, harmonics analysis is generally limited to frequencies below 100 kHz. As an example, with a 50 Hz fundamental frequency, the 2000th harmonic corresponds to 100 kHz – which already marks the practical upper limit for harmonic power analysis in most instruments.

For a comprehensive view of power losses across the entire frequency range, an advanced feature like Power Spectrum Analysis (PSA) is required. PSA uses FFT but isn’t restricted to the main motor frequency; instead, it covers the full frequency range of the power analyzer – up to 6 MHz in the case of HIOKI’s PW8001. As a result, PSA opens up new possibilities for identifying losses that may remain undetected with conventional methods.

Figure 2: Visualize power loss over the frequency band with Power Spectrum Analysis (PSA)

To use the PSA (Power Spectrum Analysis) function effectively, two key conditions must be met:

1. Any phase shift introduced by the current sensors must be accurately compensated by the analyzer.
2. The current sensors must provide a flat amplitude response across the entire frequency range.

When both conditions are met, power losses can be accurately identified across all relevant frequencies.

Figure 3: HIOKI power analyzer and current sensors – engineered from a single source

Since HIOKI designs, engineers, and manufactures both the power analyzer and current sensors in-house, these components are optimized for high-frequency power measurements. This high level of accuracy provides confidence in precise determination of power loss and pinpointing where power has gone. This is a critical step toward improving energy efficiency in motors, inverters and other power electronics applications.

Kai Scharrmann, Roy Hali, Ryuji Nishizawa, HIOKI Europe GmbH