Load loss and load current testing service

Load Loss and Load Current Testing Service – Accredited ISO/IEC 17025 Electrical Performance and Efficiency Assessment for the Croatian Market

Load loss and load current are critical performance parameters for transformers, electric motors, generators, and other electromagnetic devices, determining their efficiency, thermal behavior, voltage regulation, and operational cost under real-world loading conditions. Load loss (also known as copper loss or short-circuit loss) is the active power dissipated in the windings due to the resistance of the conductors when the device is carrying rated current, while load current refers to the current drawn by the device under specified load conditions. Accurate measurement of these parameters is essential for verifying compliance with energy efficiency standards (such as EU eco-design regulations and IE efficiency classes), calculating life-cycle costs, validating thermal design, and ensuring reliable operation 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 load loss and load current is essential for product certification, type testing, factory acceptance, supplier qualification, and import-export processes. Our laboratory offers a comprehensive load loss and 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, precision current transformers, 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 load loss and 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, with ratings from a few kVA to several MVA.
• Electric motors and generators – induction motors, synchronous motors, DC motors, and alternators for industrial, automotive, and renewable energy 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 output characteristics under load conditions.
• Small transformers and magnetic components – for electronics, medical devices, and telecommunications.
• Prototype and new designs – submitted by manufacturers for validation of load performance and efficiency before series production.
• Field-retrieved transformers and motors – for condition assessment and remaining life evaluation.

Load Loss Measurement – Principles and Standard Methods

Load loss (also known as copper loss or short-circuit loss) is the active power dissipated in the windings of a transformer or motor when it carries the rated current at the specified temperature. For transformers, this is measured by the short-circuit test, where the secondary winding is short-circuited and the primary voltage is adjusted to circulate the rated current. For motors, load loss is typically determined from the input power at rated load, or by separation of losses using the loss segregation method. Our tests measure the load loss with high accuracy, following the requirements of IEC 60076-1 for transformers and IEC 60034-2-1 for rotating electrical machines.

• Load loss test for transformers – short-circuit test (IEC 60076-1 / HRN EN 60076-1 / NTC 5600) – we short-circuit the secondary winding of the transformer (or the low-voltage winding) and apply a reduced voltage to the primary winding (or the high-voltage winding) to circulate the rated current. The input voltage (short-circuit voltage), the current, the power factor, and the active power are measured using a precision power analyzer (accuracy class 0.05). The test is performed at the rated current and at ambient temperature (corrected to the reference temperature, typically 20 °C or 75 °C, using the IEC temperature correction formulas). The load loss is reported in W or kW at the reference temperature, along with the short-circuit impedance voltage (in %).
• Load loss test for electric motors (IEC 60034-2-1 / HRN EN 60034-2-1 / NTC 5601) – we operate the motor at rated load (using a dynamometer or a load machine) and measure the input power with a precision wattmeter. The load loss is obtained by subtracting the no-load loss (core loss and friction/windage loss) from the total input power, or by using the loss segregation method (e.g., the "summation of losses" method). We report the total load loss (in W or kW), the stator copper loss, the rotor copper loss, and the stray-load loss.
• Load loss test for reactors and inductors (IEC 60076-6 / NTC 5602) – we energize the reactor at rated current (by adjusting the voltage) and measure the active power input. The test is performed at the specified current and temperature. We report the load loss and the power factor.
• Short-circuit impedance voltage measurement (NTC 5603 – for transformer impedance determination) – during the short-circuit test, we also measure the applied voltage required to circulate the rated current. This voltage, expressed as a percentage of the rated voltage, is the short-circuit impedance voltage (Uk). We report the Uk value (in %) and the short-circuit impedance (in Ω).
• Temperature correction of load loss (NTC 5604 – for ensuring comparability) – we correct the measured load loss to the reference temperature (usually 20 °C or 75 °C) using the resistance-temperature coefficient of the windings (typically 0.00393 /°C for copper and 0.00403 /°C for aluminum). We report the corrected loss and the correction factors used.
• Loss measurement using the three-wattmeter method (NTC 5605 – for three-phase systems) – for three-phase transformers and motors, we use the three-wattmeter method (or the two-wattmeter method with appropriate calculations) to measure the active power. The power analyzer is configured for the appropriate connection, and we report the total three-phase power and the individual phase readings.

Load Current Measurement – Evaluating Currents under Rated Load

Load current refers to the current drawn by the device when it is operating at rated load, or when the secondary winding is short-circuited (for transformers). This current is directly related to the load loss and the thermal performance of the device. Our tests measure the RMS load current, its harmonics, and the in-rush current (if applicable), providing a complete picture of the device's behavior under load.

• Load current measurement for transformers (IEC 60076-1 / NTC 5610 – short-circuit current) – during the short-circuit test, we measure the RMS current circulating in the windings 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 load current, the short-circuit current, and the short-circuit impedance.
• Load current measurement for motors (IEC 60034-2-1 / NTC 5611 – rated load current) – we measure the input current at rated load (using a dynamometer) and express it in A and as a percentage of the rated current. We also measure the power factor and the reactive power at rated load. We report the load current, the power factor, and the reactive power.
• In-rush current measurement (NTC 5612 – for evaluating the transient starting current) – we record the instantaneous current waveform during the start-up of a motor (or the energization of a transformer under load), 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 load current (NTC 5613 – for assessing distortion) – we use a power analyzer with FFT capability to measure the harmonic content of the 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.
• Load current variation test (NTC 5614 – for the load range performance) – we vary the load from 25 % to 125 % of the rated load in steps (e.g., 25 % steps) and measure the input current at each step. The current vs. load curve is plotted to evaluate the device's performance across the operating range. We report the current-load curve and the corresponding efficiency.

Temperature Rise Test – Evaluating Thermal Performance under Load

The temperature rise of the windings and the core under rated load is a critical parameter for the safe and reliable operation of the device. Our temperature rise tests simulate the steady-state thermal conditions at full load, measuring the final temperature of the windings (by resistance measurement) and the oil (for oil-filled transformers). This test is essential for verifying the thermal design and for ensuring that the device can operate continuously at its rated load without overheating.

• Temperature rise test for transformers (IEC 60076-2 / HRN EN 60076-2 / NTC 5620) – we apply the rated current (using the short-circuit method, or by applying the rated load using a load bank) until the temperature of the oil (and the windings) reaches thermal equilibrium. The winding temperature is measured by the resistance method (comparing the hot resistance to the cold resistance). The oil temperature is measured using thermometers or thermocouples. We report the final winding temperature (in °C), the temperature rise (in °C), and the top oil temperature.
• Temperature rise test for motors (IEC 60034-1 / HRN EN 60034-1 / NTC 5621) – we run the motor at rated load until thermal equilibrium is reached, and we measure the winding temperature (by the resistance method) and the bearing temperature (using thermocouples). We report the winding temperature rise (in °C) and the bearing temperature.
• Winding resistance measurement (ASTM B193 / NTC 5622 – for the hot and cold resistance) – we measure the DC resistance of the windings at the cold (ambient) temperature and at the hot temperature (after the temperature rise test). The temperature rise is calculated from the resistance ratio and the temperature coefficient. We report the cold resistance, the hot resistance, and the calculated temperature rise.
• Thermal time constant measurement (NTC 5623 – for the heating and cooling curves) – we record the temperature rise curve during the heating phase and the cooling curve during the cooling phase. The thermal time constant (τ) is determined from the exponential curves. We report the time constant (in minutes) and the thermal response of the device.
• Oil and winding temperature monitoring (NTC 5624 – for oil-filled transformers) – for oil-filled transformers, we monitor the top oil temperature and the average winding temperature using calibrated thermometers and thermocouples. We report the temperature profile and the final equilibrium temperatures.

Short-Circuit Impedance and Voltage Regulation – Evaluating the Device's Response to Load

The short-circuit impedance and the voltage regulation are important parameters that determine the voltage drop across the device under load and its ability to maintain the output voltage. These parameters are measured during the short-circuit test and are used for the system design and for the coordination of protection devices.

• Short-circuit impedance measurement (IEC 60076-1 / NTC 5630 – for transformers) – during the short-circuit test, we measure the applied voltage (Uk) required to circulate the rated current. The short-circuit impedance (Zk) is calculated from Uk and the rated current. We report the short-circuit impedance (in Ω) and the short-circuit impedance voltage (Uk in %).
• Voltage regulation calculation (IEC 60076-8 / NTC 5631 – for the load voltage drop) – we calculate the voltage regulation (the percentage drop in the secondary voltage from no-load to full-load) using the measured load loss, the short-circuit impedance, and the load power factor. We report the voltage regulation (in %).
• Load loss and impedance at different temperatures (NTC 5632 – for temperature correction) – we calculate the load loss and the short-circuit impedance at the reference temperature (e.g., 75 °C) and at other temperatures, to evaluate the variation with temperature. We report the temperature-corrected values and the temperature coefficients.
• Asymmetrical short-circuit current measurement (NTC 5633 – for fault current analysis) – for transformers used in power distribution, we measure the asymmetrical (peak) short-circuit current during a simulated fault condition, to verify the mechanical strength of the windings and the protection coordination. We report the peak short-circuit current (in kA) and the X/R ratio.
• Zero-sequence impedance measurement (NTC 5634 – for transformer neutral grounding) – we measure the zero-sequence impedance of the transformer (for use in the system grounding and the protection relay settings). We report the zero-sequence impedance (in Ω) and the current distribution.

Compliance with Energy Efficiency Standards and Eco-Design Requirements

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 5640) – we measure the load loss, the no-load loss (in a separate test), and the stray-loss 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 5641) – we measure the load loss, the no-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 5642 – 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 5643 – for contractual purposes) – we test the device to verify that the load loss and the impedance 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 5644 – for the design validation) – we compare the measured load loss and impedance with the typical values for the given rating and design, to identify any anomalies (e.g., excessive loss due to winding defects or poor connections). 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 load loss or load current, we perform a series of complementary tests, including winding resistance measurement, turns ratio test, and core quality assessment. 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 5650 – for DC resistance) – we measure the DC resistance of each winding using a precision micro-ohmmeter, at ambient temperature and at the hot temperature (for the temperature rise test). The resistance is corrected to the reference temperature using the temperature coefficient. We report the resistance and the temperature-corrected value.
• Turns ratio test (IEC 60076-1 / NTC 5651 – 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 quality assessment (NTC 5652 – for detecting core defects) – we perform a core quality assessment by measuring the excitation current, the no-load loss, and the short-circuit impedance, and by analyzing the harmonic content of the excitation current. The presence of core defects (e.g., damaged inter-laminar insulation or localized short circuits) is detected by characteristic harmonic patterns. We report the diagnostic conclusion.
• Short-circuit withstand test (IEC 60076-5 / NTC 5653 – for mechanical strength verification) – for transformers, we perform a short-circuit test at a specified fault current level (e.g., 2× or 3× the rated current) to verify the mechanical integrity of the windings. The impedance is measured before and after the test to detect any deformation. We report the withstand test results and the impedance change.
• Frequency response analysis (FRA) – NTC 5654 – for winding deformation detection – we perform a frequency response analysis of the transformer (or motor) to detect any winding deformation or core movement caused by short-circuit forces or aging. The frequency response is compared with a reference (or a sister unit). We report the FRA results and the interpretation.

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: load loss (W or kW), load current (A and %), short-circuit impedance (Ω and %), temperature rise (°C), efficiency (%), power factor, harmonic content (%), and voltage regulation (%).
• Graphical data: load loss vs. temperature curves, short-circuit current waveforms, temperature rise curves, and efficiency vs. load 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 and thermal performance.
• 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 winding design, the selection of low-loss conductors, the improvement of cooling systems, 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.