Technical tests

1. Purpose of the Resistance Test

The resistance test on stator windings aims to precisely determine the electrical resistance of the conductors that make up the winding itself. This type of measurement is essential for:

• Verifying the integrity of the winding (absence of interruptions or faulty connections);
• Detecting any manufacturing defects, such as cold joints or imperfect welds;
• Identifying phase imbalances that could generate abnormal heating during motor or generator operation;
• Monitoring the condition of the winding over time, useful for maintenance or diagnostics.

1. Principle of Measurement with the Kelvin Method

To obtain an accurate resistance measurement, the Kelvin method (4-wire) is used. Unlike the two-wire measurement, which also includes the resistance of the test leads and contact points, the Kelvin method eliminates these parasitic contributions by measuring only the winding resistance. The system is based on two distinct circuits:

• Current circuit: a pair of wires injects a constant current into the winding under test;
• Measurement circuit (voltage): a second pair of wires, connected directly to the ends of the winding, detects the voltage drop generated by the current.

The resistance R is calculated according to Ohm’s law:

R = V I

where:

• V is the voltage measured across the winding,
• I is the injected current.

1. Advantages of Kelvin Measurement

• High accuracy, even for very low resistance values ​​(typical of copper windings);
• Exclusion of contact and cable resistance, which can be significant compared to the value being measured;
• Repeatability of the measurement, essential for quality control and acceptance testing.

1. Typical Applications

The resistance test is typically performed:

• At the end of the stator production process (end-of-line testing);
• After thermal processing or impregnation to check for any alterations;
• During predictive maintenance of electric motors to compare values ​​over time;
• During fault analysis to identify localized imbalances or short circuits.

1. Conclusions

The Kelvin resistance test is a simple but extremely significant test for evaluating the quality and reliability of an electric winding. When integrated into a testing or maintenance process, it helps reduce the risk of failure and extend the life of electrical machinery.

1. Purpose of the Insulation Test

1. Measurement Method

1. Test Voltages

• In some cases, 250 Vdc can be used for electronic or sensitive circuits.

1. Interpretation of Results

• > 100 MΩ → excellent insulation;
• 10–100 MΩ → acceptable insulation, but should be monitored (depending on motor type and conditions);
• < 10 MΩ → critical or degraded insulation;
• < 1 MΩ → insufficient insulation, possible electrical hazard.

1. Factors Affecting Measurements

• Temperature: insulation deteriorates as temperature increases;
• Humidity: the presence of humidity reduces the values;
• Contamination: dust, oil, salt, or processing residues alter the measurement;
• Presence of partial discharges or microcracks in the insulating materials.

1. Typical Applications

• End-of-production testing of motors and transformers;
• Post-manufacturing or post-impregnation checks;
• Preventive maintenance in industrial plants;
• Pre-commissioning checks of an electrical system;
• Testing after repairs or motor refurbishments.

1. Conclusions

1. Purpose of the Dielectric Strength Test

The dielectric strength test (also known as the “hipot test”) is a fundamental electrical test for verifying the insulation strength of a winding or electric motor when subjected to a voltage higher than the rated voltage.

The purpose is to ensure that there are no discharges or perforations in the insulation under electrical stress conditions. This allows you to:

Detect hidden insulation defects, such as microcracks, impurities, or unwanted conductive paths;

Verify the quality of the insulating materials and the impregnation process;

Ensure the electrical safety of the component before commissioning;

Prevent premature or dangerous failures.

1. Test Method

During the test, a high voltage is applied between the active windings (or between phases) and the ground (or metal frame of the motor). The voltage is applied in alternating current (AC) or direct current (DC), depending on the regulations or internal standards.

The test voltage Vp is calculated based on the nominal voltage Vn, typically according to the formula:

V_p = 1000 + (2 × V_n)

Example: for a 400 V motor, the test voltage will be 1000 + (2 × 400) = 1800 V.

During the test, the leakage current through the insulation is measured. The result is expressed in milliamps (mA).

1. Passing Criteria

The component passes the test if:

No flashovers or perforations occur (no arcing or noise);

The leakage current remains below the maximum specified value;

The insulation level remains stable for the entire duration of the test (typically 1 minute).

1. Usefulness of Up and Down Ramps

The use of gradual up and down voltage ramps is strongly recommended in the following cases:

To avoid sudden thermal or mechanical stress on the insulation;

To reduce the risk of false faults due to rapid triggering;

To obtain a more reliable and stable measurement;

To protect delicate windings or those with unusual geometries.

With ramps, the voltage is gradually increased to the target value, maintained for the test time, and then gradually reduced to zero.

1. AC vs. DC Testing

AC: better simulates real-world operating conditions. It also provides information on the insulation’s ability to withstand alternating stresses.

DC: allows for more stable leakage current measurements and permits the use of more compact instrumentation. However, it does not highlight some defects that are only visible in AC.

1. Typical Applications

End-of-line testing for motors, stators, transformers, and coils;

Verification after repairs or impregnation;

Quality control of production batches;

• Compliance testing with CE, UL, IEC, EN, etc.

1. Conclusions

Dielectric strength testing is a critical test for ensuring the reliability and safety of an insulation system. The application of a controlled high voltage, with leakage current monitoring, allows for the detection of latent defects that cannot be detected with other tests.

The use of ramps improves test safety and insulation protection, making it an essential step in industrial testing processes.

1. What is the Surge Test?

The surge test is a high-voltage impulse test applied to electrical windings (motors, stators, transformers, coils) to verify the dielectric strength between adjacent turns. It is considered the most significant and representative test for evaluating the quality of the enamel insulation of the conductor wire.

Unlike static tests (such as insulation or dielectric strength), the surge test reproduces the actual impulsive stresses that a winding may experience during operation, for example in the presence of switching, overvoltages, or disturbances.

1. How the Surge Test Works

The test involves applying a very rapid voltage impulse to one end of the winding, while the other end is connected to ground. This generates a rapid surge wave that propagates along the wound wire.

In our system, the pulse has an extremely fast front: approximately 30 nanoseconds, creating a strong potential difference between adjacent turns. This electrical stress induces a high electric field between the layers of insulated wire, simulating extreme conditions capable of highlighting even minimal defects in the insulating enamel.

1. What is the purpose of the Surge Test?

The purpose of the surge test is to:

• Verify the seal between adjacent turns, the main weak point in the event of insulation defects;
• Detect microcracks in the wire enamel due to mechanical processing, overheating, or manufacturing defects;
• Identify partial short circuits between turns, which may not occur under normal conditions but can lead to failure during operation;
• Ensure the quality of the winding and impregnation process;
• Make comparisons between symmetrical phases to highlight construction anomalies (comparison of response curves).

1. Test Result

The test result is displayed as an impulse response curve, also called an oscillographic curve. The curves obtained from intact windings show a regular and repeatable trend. Any deviation, damping, or distortion of the curve may indicate:

• Inter-turn discharges;
• Presence of partial shorts;
• Discontinuity in the internal insulation.

In multiphase systems, it is common to compare the responses of the different phases (e.g., U, V, W): evident asymmetries between the curves suggest an anomaly in one of the phases.

1. Advantages of the Surge Test

• Extremely high sensitivity in detecting inter-turn defects;
• Rapid and non-destructive diagnosis (if the test is performed correctly);
• Possibility of automated testing on the production line;
• Predictive testing useful in motor maintenance to identify initial degradation.

1. Conclusions

The Surge Test is a fundamental tool for ensuring the safety and reliability of electrical windings. By simulating real-life overvoltage conditions, it allows for the identification of hidden defects that cannot be detected with other standard tests. The use of high-speed pulses (30 ns in our system) allows for effective and targeted stressing of the insulation, making the test one of the most powerful tools in winding testing and diagnostics.

1. Importance of Rotor Testing

The die-cast squirrel-cage rotor is a critical component in asynchronous motors. The quality of the die-cast aluminum bars and short-circuit rings directly determines the electrical and mechanical performance of the motor.

Defects such as broken bars, porosity, blowholes, or eccentricity can cause:

• Vibrations and abnormal noise;
• Loss of efficiency;
• Non-compliant performance;
• Overheating and breakage over time.

For this reason, it is essential to test each rotor reliably and quickly, either directly in production or during quality control.

1. Testing Method – SM SYSTEM

SM SYSTEM has developed a specific system for the non-destructive testing of rotors, based on a simple yet extremely effective principle.

2.1 Operating Procedure

• The rotor is rotated at approximately 400 RPM.
• A dedicated SM SYSTEM sensor is positioned near the conductor cage.
• The sensor, coupled with a high-resolution sampler, detects the induced signal bar by bar.
• The signal is processed and displayed, allowing for a detailed and rapid analysis of the rotor’s condition.

2.2 What the System Detects

The system allows for the identification of:

• Interrupted or failed fusion bars;
• Blowholes and porosity in the conductors;
• Mechanical eccentricities or non-uniform fusion;
• Even minimal differences in the electromagnetic response between the bars.

1. Analysis Technology

The heart of the system is an advanced software algorithm, which operates in two main modes:

• Basic mode: real-time acquisition and reading of the signal on an oscilloscope, for immediate visual assessment.
• Advanced mode (with SM SYSTEM software): automatic signal processing and digital filtering to identify even very small defects, often invisible to the naked eye.

A “sample stator” can be integrated into the system, simulating real-world operating conditions. This allows for a direct assessment of the rotor’s electromagnetic effectiveness, providing an additional quantitative parameter useful for batch comparisons or for validating the casting process.

1. System Advantages

• Rapid testing: a complete rotor scan takes just a few seconds;
• High reliability, even on defects invisible to the naked eye;
• Simple operation: no need for electrical contacts or complex mechanical connections;
• Bar-by-bar analysis, also useful in the event of partial breakage or localized defects;
• Adaptable to different rotor geometries, even on automated lines.

1. Conclusions

Testing squirrel-cage rotors is a fundamental step in ensuring the quality, reliability, and efficiency of electric motors. The system developed by SM SYSTEM enables the timely identification of structural defects in rotor bars, with precise, non-invasive, and fast measurement, thus helping to reduce non-compliance costs and increase the competitiveness of the finished product.

The SM SYSTEM enables the rapid and precise analysis of squirrel-cage die-cast rotors by reading the induced signals bar by bar. Below are some practical examples of acquired signals, which clearly illustrate the differences between a healthy rotor and one with defects.

ROTORE BUONO

INCLINAZIONE BARRA ERRATA
O LEGA ALLUMINIO POVERA

MAGNETISMO RESIDUO

TRAFILAMENTI TRA LAMIERINI
SALDATURA ALLUMINIO-FERRO

BARRA INTERROTTA

PRESENZA SOFFIATURE
O POROSITÀ

GABBIA ECCENTRICA

Figure: Examples of signals detected on different rotors.

GOOD ROTOR

The signal is regular and symmetrical, with constant amplitudes and a uniform sinusoidal pattern. No anomalies in the bars or rotor geometry.

INCORRECT BAR TILT / POOR ALUMINUM ALLOY

The signal exhibits regular distortion with amplitude variations. This indicates possible geometric (tilted bar) or metallurgical (impure aluminum) problems.

RESIDUAL MAGNETISM

Signal modulation is observed, with irregular and off-center deformations. The rotor exhibits residual magnetism in the laminations, often due to incorrect machining.

LEAKAGE BETWEEN LAMINATIONS / ALUMINUM-IRON WELDING

The signal is inhomogeneous, with sudden drops in amplitude. Possible presence of aluminum infiltration between the laminations or welding spots between different metals, which alter the electromagnetic response.