No-load loss and no-load current testing service

No-Load Loss and No-Load Current Testing Service – Accredited ISO/IEC 17025 Electrical Performance Assessment for the Croatian Market

No-load loss and no-load current are critical parameters for evaluating the efficiency, operational cost, and reliability of transformers, electric motors, generators, inductors, and other electromagnetic devices. These measurements quantify the core losses (hysteresis and eddy currents) and the magnetizing current required to establish the magnetic flux in the iron core when the device is energized at rated voltage and frequency but without any load on its output. Accurate determination of no-load loss and current is essential for verifying compliance with energy efficiency standards, calculating life-cycle costs, detecting manufacturing defects, and ensuring optimal performance in power distribution, industrial automation, and renewable energy systems. In the Croatian market, where the Hrvatski zavod za norme (HZN), the Ministarstvo gospodarstva i održivog razvoja, the Državni inspektorat, and the Hrvatska regulatorna agencija za mrežne djelatnosti (HAKOM) enforce strict quality, energy efficiency, and safety regulations aligned with EU directives and HRN EN (Croatian standards based on European norms), the precise evaluation of no-load loss and current is essential for product certification, type testing, factory acceptance, supplier qualification, and import-export processes. Our laboratory offers a comprehensive no-load loss and no-load current testing service, applying standardized methods that comply with IEC 60076 (for transformers), IEC 60034 (for rotating machines), and other relevant international standards, using high-accuracy power analyzers and automated test systems. All tests are performed under our ISO/IEC 17025 (CNAS) accreditation, and the resulting reports are fully accepted by Croatian authorities and notified bodies, making them indispensable for regulatory compliance, energy efficiency labeling, and market access in Croatia and the European Union.

Equipment and Components We Regularly Test

Our laboratory receives a wide variety of electromagnetic devices and components for no-load loss and no-load current testing. Typical samples include:

• Power and distribution transformers – single‑phase and three‑phase transformers, dry‑type and oil‑filled, for voltages from 230 V up to 36 kV.
• Electric motors and generators – induction motors, synchronous motors, DC motors, and alternators for industrial and automotive applications.
• Inductors and reactors – series reactors, shunt reactors, and smoothing chokes for power quality and harmonic filtering.
• Variable frequency drives (VFDs) and power supplies – for testing their input characteristics under no‑load conditions.
• Small transformers and magnetic components – for electronics, medical devices, and telecommunications.
• Prototype and new designs – submitted by manufacturers for validation of core design and material selection before series production.
• Field‑retrieved transformers and motors – for condition assessment and remaining life evaluation.

No‑Load Loss Measurement – Principles and Standard Methods

No‑load loss (also known as iron loss or core loss) is the active power consumed by the magnetic core of a transformer or motor when it is energized at rated voltage and frequency, with no load connected to the secondary side (or the rotor locked, for motors). This loss is independent of the load and is caused by hysteresis and eddy currents in the core material. Our tests measure the active power input under these conditions with high accuracy, following the requirements of IEC 60076‑1 for transformers and IEC 60034‑2‑1 for rotating electrical machines.

• No‑load loss test for transformers (IEC 60076‑1 / HRN EN 60076‑1 / NTC 5600) – we energize the transformer on the low‑voltage winding (or the high‑voltage winding, depending on the voltage rating) at the rated voltage and frequency (50 Hz for the Croatian grid). The primary current (no‑load current), the input voltage, the power factor, and the active power are measured using a precision power analyzer (accuracy class 0.05). The test is performed at the specified tap position and at ambient temperature (corrected to the reference temperature, typically 20 °C or 75 °C, using the IEC temperature correction formulas). We report the no‑load loss (in W or kW), the no‑load current (in A or % of rated current), the power factor, and the temperature‑corrected loss.
• No‑load loss test for electric motors (IEC 60034‑2‑1 / HRN EN 60034‑2‑1 / NTC 5601) – we run the motor at rated voltage and frequency with no load (the shaft is free to rotate, or the rotor is locked for locked‑rotor tests). The input power is measured with a precision wattmeter. For induction motors, the no‑load loss includes the core loss and the friction and windage losses, which are separated by a loss segregation method (e.g., by varying the voltage). We report the total no‑load loss and the separated core loss.
• No‑load loss test for reactors and inductors (IEC 60076‑6 / NTC 5602) – we energize the reactor at rated voltage and frequency and measure the active power input. The measurement is performed at the specified voltage and temperature. We report the core loss and the reactive power.
• Loss measurement using the two‑wattmeter method (NTC 5603 – for three‑phase transformers and motors) – we use the two‑wattmeter method (or the three‑wattmeter method) to measure the active power in three‑phase systems. The power analyzer is configured for the appropriate connection (e.g., Aron connection). We report the total three‑phase power and the individual phase readings.
• Loss correction for harmonics and distortion (NTC 5604 – for non‑sinusoidal voltage supplies) – when the supply voltage has harmonics, we use a true RMS power analyzer and apply the necessary corrections to obtain the fundamental‑frequency loss (or we use a sinusoidal supply from a clean power source). We report the total loss and the harmonic content.
• Temperature correction of no‑load loss (NTC 5605 – for ensuring comparability) – we correct the measured no‑load loss to the reference temperature (usually 20 °C or 75 °C) using the resistance‑temperature coefficient of the windings and the core loss temperature dependency. We report the corrected loss and the correction factors used.

No‑Load Current Measurement – Evaluating the Magnetizing Characteristics

The no‑load current is the current drawn by the device when it is energized at rated voltage and frequency with no load on the secondary side (or rotor free). This current consists of the magnetizing component (which establishes the magnetic flux) and a small active component (which supplies the core loss). The magnitude and the phase angle of this current are important for assessing core saturation, winding design, and the overall health of the device. Our tests measure the current, its harmonics, and the in‑rush characteristics, providing a complete picture of the magnetizing behavior.

• No‑load current measurement (IEC 60076‑1 / NTC 5610 – for transformers) – we measure the RMS current drawn by the transformer at rated voltage and frequency using a precision current transformer and an ammeter or a power analyzer. The current is expressed in amperes (A) and as a percentage of the rated current. We report the no‑load current (in A and %), the peak in‑rush current (if measured), and the harmonic spectrum (up to the 25th harmonic).
• No‑load current measurement for motors (IEC 60034‑2‑1 / NTC 5611) – we measure the input current at rated voltage and frequency with the motor running at no load. The current is reported in A and as a percentage of the rated current. The power factor and the reactive power are also calculated.
• In‑rush current measurement (NTC 5612 – for evaluating the transient magnetizing current) – we record the instantaneous current waveform during the initial energization (switch‑on) of the transformer or motor, using a high‑speed data recorder (sampling rate > 10 kHz). The peak in‑rush current and its duration are determined. We report the peak in‑rush current (in A and as a multiple of the rated current), the time to decay, and the waveform.
• Harmonic analysis of no‑load current (NTC 5613 – for assessing distortion) – we use a power analyzer with FFT capability to measure the harmonic content of the no‑load current (up to the 50th harmonic). The total harmonic distortion (THD) of the current is calculated. We report the individual harmonic magnitudes (in % of the fundamental) and the THD.
• Voltage variation test (NTC 5614 – for core saturation characteristics) – we vary the applied voltage from 90 % to 110 % of the rated voltage in steps (e.g., 5 % steps) and measure the no‑load current at each step. The current vs. voltage curve is plotted to detect any core saturation (a sharp increase in current). We report the current‑voltage curve and the saturation voltage.

Test Conditions and Environmental Corrections – Ensuring Accurate and Repeatable Results

The no‑load loss and current measurements are sensitive to the test voltage waveform, frequency, temperature, and ambient conditions. Our procedures strictly control these parameters to ensure that the results are accurate and repeatable, and that they comply with the requirements of the international and Croatian standards.

• Supply voltage and frequency control (NTC 5620 – for maintaining the test conditions) – we use a stable, low‑distortion power supply (or a motor‑generator set) to provide the rated voltage and frequency (50 Hz ± 0.1 Hz) with a total harmonic distortion (THD) of less than 1 %. The voltage is regulated to within ±0.5 % of the rated value. We report the voltage, frequency, and THD during the test.
• Temperature measurement and correction (NTC 5621 – for winding and ambient temperature) – we measure the ambient temperature and the winding temperature (using resistance thermometers or thermocouples) before and after the test. The no‑load loss is corrected to the reference temperature (e.g., 20 °C or 75 °C) using the appropriate formulas. We report the measured temperatures, the correction factors, and the corrected loss.
• Waveform distortion and harmonic compensation (NTC 5622 – for accurate power measurement) – we use a power analyzer that measures the true RMS voltage, current, and active power, even in the presence of harmonics. The loss is calculated using the fundamental‑frequency component (by applying harmonic rejection or by using a sinusoidal reference). We report the harmonic compensation method used.
• Test duration and stabilization (NTC 5623 – for achieving thermal equilibrium) – we allow the device to reach thermal equilibrium before taking the final measurements. The test is typically performed for a duration of 10 to 30 minutes, depending on the size of the device. We report the stabilization time and the final temperature.
• Repeatability and uncertainty evaluation (NTC 5624 – for measurement confidence) – we perform multiple readings (at least 5) and calculate the mean, the standard deviation, and the expanded uncertainty (k=2) for the no‑load loss and current. We report the measurement uncertainty and the confidence interval.

Compliance with Energy Efficiency Standards and Eco‑Design Requirements

No‑load loss is a key parameter for the energy efficiency classification of transformers and motors, which are regulated by EU eco‑design directives (e.g., Regulation (EU) 548/2014 for transformers, and Regulation (EU) 2019/1781 for motors). Our testing services ensure that the measured losses comply with the applicable efficiency classes (e.g., IE3, IE4, or the EU Tier 1 and Tier 2 levels).

• Transformer efficiency class verification (IEC 60076‑20 / HRN EN 60076‑20 / NTC 5630) – we measure the no‑load loss and load loss (in separate tests) to calculate the total loss and the efficiency of the transformer. The measured efficiency is compared with the minimum requirements of the relevant energy efficiency class (e.g., Eco‑design requirements). We report the efficiency and the class (e.g., A, A+, or the EU energy label).
• Motor efficiency class verification (IEC 60034‑30‑1 / HRN EN 60034‑30‑1 / NTC 5631) – we measure the no‑load loss and use it (along with the load loss and the stray‑load loss) to determine the efficiency of the motor. The measured efficiency is compared with the IE efficiency classes (IE1, IE2, IE3, IE4). We report the efficiency and the IE class.
• Eco‑design compliance statement (NTC 5632 – for the conformity assessment) – we provide a statement of compliance with the relevant EU eco‑design regulation, based on the measured loss values and the applicable limits. The statement is included in the test report, which can be used for the CE marking and the declaration of conformity.
• Loss guarantee verification (NTC 5633 – for contractual purposes) – we test the device to verify that the no‑load loss and current are within the guaranteed values specified in the purchase order or the design specification. We report the measured values and the margin (in %) relative to the guarantee.
• Comparison with standard reference values (NTC 5634 – for the design validation) – we compare the measured no‑load loss and current with the typical values for the given rating and design, to identify any anomalies (e.g., excessive loss due to core joint defects or material quality issues). We report the comparison and the interpretation.

Complementary Tests – Winding Resistance, Turns Ratio, and Core Quality Assessment

To fully characterize the device and to understand the root cause of any deviations in the no‑load loss or current, we perform a series of complementary tests, including winding resistance measurement, turns ratio test, and core quality assessment (e.g., core loss density measurement on core steel samples). These tests are essential for a comprehensive quality assurance and for the troubleshooting of performance issues.

• Winding resistance measurement (ASTM B193 / IEC 60076‑1 / NTC 5640 – for DC resistance) – we measure the DC resistance of each winding using a precision micro‑ohmmeter, at the ambient temperature. The resistance is corrected to the reference temperature (e.g., 20 °C or 75 °C) using the temperature coefficient. We report the resistance and the temperature‑corrected value.
• Turns ratio test (IEC 60076‑1 / NTC 5641 – for voltage ratio verification) – we measure the voltage ratio of each phase of the transformer (using a turns‑ratio meter) to verify the correct winding ratio and the proper connection of the taps. We report the measured ratio and the deviation from the nominal ratio.
• Core loss density measurement (NTC 5642 – on Epstein frame or single‑sheet tester) – for a detailed core quality assessment, we test the core steel material (or a sample of the core) for the specific core loss (in W/kg) at the operating flux density and frequency. We report the core loss density and compare it with the material specification.
• Magnetizing curve measurement (NTC 5643 – for saturation characteristics) – we record the magnetizing curve (flux density vs. magnetizing current) by applying a variable AC voltage and measuring the current. The knee point and the saturation flux density are determined. We report the magnetizing curve and the saturation characteristics.
• Excitation current harmonics analysis (NTC 5644 – for diagnostic purposes) – we perform a detailed harmonic analysis of the excitation current to identify the presence of core defects (e.g., damaged inter‑laminar insulation or localized short circuits). The harmonic pattern is analyzed for characteristic signatures. We report the harmonic spectrum and the diagnostic conclusion.

Test Report and Recognition in the Croatian Energy and Industrial Sector

All procedures described are within the scope of our ISO/IEC 17025 accreditation, with equipment calibrated periodically (power analyzers, voltage and current transformers, resistance bridges, etc.) and traceability to international standards (NIST, PTB). Our test reports are issued in English (with a Croatian summary available on request) and include:

• Full identification of the test item (manufacturer, type, serial number, rated power/voltage/current, and winding configuration).
• Detailed description of the test methods applied (IEC/HRN EN/NTC standards, test conditions, supply voltage, frequency, and temperature).
• Numerical results: no‑load loss (W or kW), no‑load current (A and %), power factor, in‑rush current (A and multiple), harmonic content (%), winding resistance (Ω), turns ratio, and efficiency class.
• Graphical data: current‑voltage curves, in‑rush current waveforms, harmonic spectra, and magnetizing curves.
• Comparative tables against the values specified by the client or against the limits of the relevant standards (IEC 60076, IEC 60034, EU eco‑design regulations, and HRN EN standards).
• Statement of conformity with the applicable energy efficiency class and eco‑design requirements.
• Recommendations for design improvement, material optimization, and quality control measures to achieve the required loss levels.
• Expanded uncertainty (k=2) for all key measurements, calculated according to the ISO/IEC 98‑3 Guide.

These reports are fully accepted by the Hrvatski zavod za norme (HZN) for the verification of product conformity, by the Ministarstvo gospodarstva i održivog razvoja for energy efficiency compliance, by the Državni inspektorat for market surveillance, by the Hrvatska regulatorna agencija za mrežne djelatnosti (HAKOM) for equipment used in the power grid, and by the Carinska uprava (Croatian Customs) for tariff classification and quality verification in the import of transformers, motors, and related electrical equipment. Additionally, we offer consulting services for the optimization of core design, the selection of low‑loss materials, and the implementation of energy efficiency improvement programs, contributing to the sustainable and reliable operation of electrical infrastructure and industrial processes in the Croatian market, from the power plants and distribution networks to the manufacturing and renewable energy sectors.