TEST & MEASURMENT
Measuring efficiency in modern motor drives, when every fraction of a per cent matters
As motor-drive efficiency moves beyond 98 per cent, identifying the next improvement becomes increasingly dependent on the quality of the measurement behind it.
E
lectric motors account for a substantial share of industrial electricity consumption, making improvements in motor and drive efficiency increasingly important for equipment manufacturers and end users alike. Over recent decades, tighter efficiency requirements and advances in motor technology have driven the transition from fixed-speed systems towards variable frequency drives and higher-efficiency motor topologies.
For manufacturers such as Invertek Drives, this progress creates a new engineering challenge. Its Optidrive variable frequency drives are used in applications ranging from pumps and fans to HVAC systems, elevators, cranes and conveyors, controlling motor loads from 370 W to 250 kW, with higher-power products planned. But as efficiency increases, improving it further becomes progressively harder.
When measurement uncertainty becomes part of the design challenge
When engineers are moving from 90 per cent to 95 per cent efficiency, a small amount of measurement uncertainty may have relatively little influence on the conclusion. When a drive already operates above 98 per cent, however, the engineering team may be trying to identify improvements of only a fraction of a percentage point.
At that point, the uncertainty of the measurement itself must be considerably smaller than the improvement being investigated. Otherwise, engineers cannot confidently determine whether a change in efficiency comes from the new design or from the measurement chain. This is precisely the challenge faced by Invertek’s R&D team when evaluating successive generations of its Optidrive products. Accurate power measurement therefore becomes part of the development process rather than simply a final verification step.
Measuring the complete energy conversion path
To calculate drive efficiency, Invertek measures electrical power entering the drive and the power delivered at its output. But
provide important information about the behaviour of the drive and its interaction with the wider power system.
Invertek therefore uses harmonic analysis alongside efficiency measurements to understand both power integrity and power conversion performance. For transient investigation, its engineers also use a Yokogawa DL950 Isolated Oscilloscope to examine inverter output waveforms and capture fast electrical events.
The broader lesson is that modern motor- drive development increasingly depends on combining several measurement perspectives: precision power, mechanical performance, harmonics and transient behaviour.
From precision measurement to scalable test systems
understanding the complete system requires more than two electrical measurements. Its test configuration also includes motor speed and torque, allowing engineers to evaluate drive, motor and overall system efficiency. Invertek uses Yokogawa’s WT5000 Precision Power Analyzer for this work, with a measurement configuration covering two drive inputs, two outputs and mechanical measurements at the motor shaft. This simultaneous approach matters because efficiency is a ratio. If input, output and mechanical quantities are measured under different operating conditions, even small variations in load can influence the result.
Efficiency is not the only measurement that matters
Motor drives also must operate reliably in real electrical environments. Distortion and harmonics at the input and output can
32 OCTOBER 2026 | ELECTRONICS FOR ENGINEERS
As motor and inverter development evolves, another requirement is becoming increasingly important: integrating these measurements efficiently into automated test environments. This is one of the areas Yokogawa is addressing with its new WT1500 Precision Power Analyzer. Rather than replacing the ultra-high-precision role of instruments such as the WT5000, the WT1500 extends Yokogawa’s power-measurement portfolio towards compact, scalable and automated test systems.
The WT1500 supports up to four power and four motor measurement channels in a 2U platform and can work with both encoder and resolver signals. Its wide-bandwidth input option reaches 2 MHz, while multi-unit synchronization enables larger systems to scale beyond a single instrument. For development teams, the choice of instrument ultimately depends on the measurement objective. Some applications demand the highest possible precision to distinguish extremely small efficiency improvements; others place greater emphasis on repeatable automated testing, channel density and integration with a wider test system.
What does not change is the principle demonstrated by Invertek: when efficiency gains become smaller, confidence in the measurement becomes more important.
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