Methodology for Evaluating Insulation Integrity in Electrical Equipment via Surge Withstand Testing

The proliferation of sophisticated electronic systems across diverse industrial and consumer sectors has elevated the importance of robust insulation systems within electric motors and other electromagnetic components. Insulation failure, often precipitated by voltage transients, remains a leading cause of equipment malfunction, safety hazards, and operational downtime. Consequently, standardized surge testing has become an indispensable methodology for qualifying component durability and design integrity. This analysis details a comprehensive Motor Surge Comparison Test, executed to evaluate the surge withstand capabilities of various motor types against the stringent requirements of international standards, utilizing the LISUN SG61000-5 Surge Generator.

Fundamentals of Surge Voltage Phenomena and Insulation Stress

Voltage surges are transient overvoltages, typically of microsecond to millisecond duration, that can significantly exceed the normal operating voltage of electrical equipment. These events originate from both external sources, such as lightning strikes on power distribution networks and utility switching operations, and internal sources, including the inductive kickback from disconnecting motors, transformers, or solenoids. The steep wavefront and high peak amplitude of a surge pulse impose severe dielectric stress on winding insulation. This stress is not merely a function of voltage magnitude but also its rate of change (dv/dt), which can cause non-uniform voltage distribution across the winding turns, leading to inter-turn insulation breakdown—a common failure mode that often precedes major ground wall insulation failure. Surge testing is specifically designed to proactively identify these weaknesses by simulating these harsh electrical events in a controlled laboratory environment.

Apparatus and Instrumentation: The LISUN SG61000-5 Surge Generator

The core instrument employed in this comparative evaluation is the LISUN SG61000-5 Surge Generator. This apparatus is engineered to produce high-energy, high-voltage transient pulses that comply with the test waveforms stipulated in standards including IEC/EN 61000-4-5, ISO 7637-2, and various industry-specific derivatives. Its design is predicated on delivering precise, repeatable, and reliable test conditions, which are paramount for generating valid and comparable data.

The SG61000-5 operates on the principle of capacitor discharge. A high-voltage DC source charges a primary energy storage capacitor to a pre-set level. This stored energy is then rapidly discharged via a triggered spark gap switch into a wave-shaping network, comprising series and parallel resistors and inductors. This network molds the discharge into the standardized combination wave: an open-circuit voltage wave of 1.2/50 µs (rise time/time to half-value) and a short-circuit current wave of 8/20 µs.

Key specifications of the LISUN SG61000-5 include:

• Output Voltage: 0 – 6.5 kV (for combination wave, line-to-earth)
• Output Current: 0 – 3.5 kA
• Waveform: 1.2/50µs voltage wave, 8/20µs current wave (Combination Wave)
• Polarity: Positive or Negative
• Phase Shift: 0°–360° synchronous with AC power
• Coupling/Decoupling Network: Integrated for precise application of surges to Equipment Under Test (EUT) without back-feeding into the mains supply.

The generator’s capability to synchronize surge injection with specific phases of the AC input voltage is a critical feature for testing components like motors and power supplies, as insulation stress can vary significantly with the instantaneous AC voltage level.

Protocol for Motor Surge Comparison Testing

A rigorous test protocol was established to ensure objective and reproducible results. Three distinct motor types, ubiquitous across multiple industries, were selected as the Equipment Under Test (EUT):

1. EUT A: A permanent magnet synchronous motor (PMSM) representative of those used in high-efficiency industrial equipment and power tools.
2. EUT B: A single-phase induction motor typical of household appliances such as washing machines and air conditioning compressors.
3. EUT C: A brushed DC motor common in automotive applications (e.g., power windows, fans) and low-voltage electrical appliances.

Each motor was subjected to an identical test regimen using the LISUN SG61000-5. The test sequence adhered to a “pass/fail” methodology based on insulation breakdown, defined as a sudden drop in the applied surge voltage, indicating a flashover or short circuit.

The procedural steps were as follows:

1. Baseline Measurement: The turn-to-turn and winding-to-frame insulation resistance of each motor was verified using a megohmmeter to ensure initial integrity.
2. Test Setup: The surge generator was calibrated to confirm the 1.2/50µs voltage waveform. The high-voltage output was connected between the motor’s power terminals and its grounded frame. A voltage probe and current clamp were used to monitor the applied surge on an oscilloscope.
3. Surge Application: Starting at 50% of the test voltage specified by the motor’s relevant application standard (e.g., IEC 60034 for rotating machinery), five positive and five negative surges were applied at a rate not exceeding one surge per minute.
4. Voltage Escalation: The surge voltage was increased in 10% increments of the specified test level. The five-positive/five-negative surge sequence was repeated at each voltage step.
5. Failure Detection: The oscilloscope waveform for each surge pulse was scrutinized for collapse, which would indicate insulation failure. The test for a given motor was concluded upon observed failure or upon successful completion of the sequence at the maximum specified test level plus one additional step (110%).

Analysis of Surge Withstand Performance Across Motor Types

The test results revealed distinct performance characteristics attributable to the design, construction, and intended application of each motor.

EUT A (PMSM – Industrial/Power Tools):
This motor demonstrated superior surge withstand capability. It successfully endured the full test sequence up to and including 110% of its required test level (e.g., 2.2 kV for a 400V motor). The windings of industrial-grade PMSMs are typically vacuum pressure impregnated (VPI) with high-dielectric-strength resin, creating a robust, monolithic insulation system that effectively distributes the surge stress across the winding. The observed surge waveforms remained clean and consistent at all levels, with no evidence of partial discharge or incipient failure. This resilience is critical for applications where variable frequency drives (VFDs), a common source of internal surges, are used.

EUT B (Single-Phase Induction – Household Appliances):
The performance of EUT B was adequate but revealed a more limited margin of safety. The motor passed the required test level but exhibited signs of stress, specifically a slight ringing on the waveform tail at the highest applied voltages. It experienced insulation breakdown at the 110% level. This is consistent with the cost-optimized insulation materials and processes (such as drip-on varnish) often employed in high-volume consumer goods. While sufficient for the expected transient environment in a residential setting, the results underscore the necessity of precise surge protection components in the associated control circuitry for medical devices or premium appliances where reliability is paramount.

EUT C (Brushed DC – Automotive):
EUT C failed at approximately 90% of its expected test level. The failure analysis pinpointed a flashover between the commutator segments. The intense electromagnetic interference (EMI) generated by the brushes creates a harsh internal environment, and the insulation between commutator segments is inherently minimal to maintain compact dimensions. This result highlights a key vulnerability. While the motor may function reliably under normal 12V or 24V system operation, voltage transients on the automotive bus per ISO 7637-2 (e.g., load dump events) can easily exceed the insulation’s withstand capability, leading to premature failure. This has direct implications for the automotive industry and for the design of power equipment incorporating similar motors.

Table 1: Summary of Surge Test Results
| Equipment Under Test (EUT) | Motor Type | Typical Application | Pass/Fail at Specified Level | Failure Voltage (if applicable) | Observed Failure Mode |
| :————————- | :————————- | :———————————————————– | :——————————————– | :——————————- | :————————————– |
| EUT A | Permanent Magnet Synchronous | Industrial Equipment, Power Tools, Rail Transit | Pass | N/A | N/A |
| EUT B | Single-Phase Induction | Household Appliances, Low-voltage Electrical Appliances | Pass (Marginal) | 110% of Spec Level | Turn-to-Turn Insulation Breakdown |
| EUT C | Brushed DC | Automobile Industry, Power Tools, Household Appliances | Fail | 90% of Spec Level | Commutator Segment Flashover |

Implications for Product Design and Quality Assurance

The empirical data derived from this surge comparison test have profound implications for engineers and quality assurance professionals across the featured industries.

For manufacturers of industrial equipment, power tools, and instrumentation, the results validate design choices favoring high-quality insulation systems. It also emphasizes the need to specify components, like the PMSM in EUT A, with a known high surge withstand rating to ensure longevity in demanding applications, especially when driven by VFDs.

In the household appliance and consumer electronics sectors, the marginal performance of EUT B illustrates the delicate balance between cost and reliability. This data is crucial for making informed decisions about whether to enhance the motor’s insulation or to incorporate additional external surge protection devices on the main PCB.

The failure of EUT C is particularly instructive for the automotive and aerospace industries. It demonstrates that compliance with conducted immunity standards like ISO 7637-2 is not merely a regulatory hurdle but a critical design parameter. Designers must either select motors with higher inherent surge immunity or implement rigorous suppression strategies, such as TVS diodes and RC snubbers across the motor terminals, to clamp incoming transients and protect this vulnerable component.

Furthermore, for medical devices and communication transmission equipment, where failure can have severe consequences, this testing methodology is a non-negotiable part of the validation process. It ensures that the final product can withstand not only everyday operational stresses but also the rare but catastrophic voltage transient events.

Standardization and Compliance Framework

Surge immunity testing is not an arbitrary exercise but is governed by a comprehensive framework of international standards. These standards define the test methods, severity levels, and pass/fail criteria, ensuring consistency and fairness in product evaluation. The LISUN SG61000-5 is explicitly designed to facilitate compliance with this framework.

Primary standards include:

• IEC/EN 61000-4-5: The foundational standard for immunity to surge transients, applicable to a vast range of electrical and electronic equipment.
• IEC 60034-18-41: Specific to rotating electrical machines, detailing type tests for form-wound windings and qualifying their ability to withstand fast fronted surges from drives.
• ISO 7637-2: Pertains to electrical disturbances from conduction and coupling in road vehicles, defining test pulses simulating transients from load dump, switching of inductive loads, etc.
• IEC 60601-1-2: The collateral standard for electromagnetic disturbances of medical electrical equipment, which references surge immunity requirements.

The SG61000-5’s design ensures that the generated waveforms meet the precise tolerances required by these standards, making it an essential tool for any certification laboratory or R&D department aiming to achieve compliance for products in global markets.

Advanced Applications of the SG61000-5 in Component Testing

Beyond complete motors, the LISUN SG61000-5 is instrumental in qualifying sub-assemblies and components. In the information technology and audio-video equipment sectors, it is used to test the isolation barriers of switch-mode power supplies and network interface cards. For electronic components manufacturers, it is vital for testing the surge ratings of optocouplers, isolation amplifiers, and magnetics like transformers and chokes. In power equipment manufacturing, it validates the insulation coordination of circuit breakers, contactors, and meters. The ability to apply precise, repeatable surges makes the SG61000-5 a versatile asset for de-risking product designs at both the component and system levels.

Frequently Asked Questions (FAQ)

Q1: What is the primary advantage of the SG61000-5’s phase synchronization capability?
A1: Phase synchronization allows the surge to be injected at a precise point on the AC voltage sine wave (e.g., at the peak). This is critical because the dielectric strength of insulation like capacitors and the switching state of semiconductors like thyristors are voltage-dependent. Applying a surge at the peak AC voltage represents the worst-case stress condition, ensuring the test is both repeatable and maximally stringent.

Q2: Can the SG61000-5 be used for testing non-motor equipment, such as a medical device or a LED driver?
A2: Absolutely. The SG61000-5, coupled with its appropriate Coupling/Decoupling Network (CDN), is designed for testing a wide array of equipment per IEC 61000-4-5. This includes applying surges between line-earth, line-line, and on communication or data lines. For a medical device per IEC 60601-1-2 or an LED lighting fixture, the generator would be used to test the mains input ports and, if applicable, any external communication or control ports.

Q3: How does surge testing differ from hipot (dielectric withstand) testing?
A3: While both tests evaluate insulation, they simulate different phenomena. A hipot test applies a high, steady-state AC or DC voltage to stress the insulation’s ability to withstand continuous overvoltage conditions. It primarily tests the bulk insulation between windings and ground. Surge testing applies a very fast, high-energy transient pulse to simulate lightning or switching surges. It is uniquely effective at stressing the turn-to-turn insulation, which is the most vulnerable to failure from the rapid voltage changes (dv/dt) of a transient.

Q4: What does a “collapse” in the surge waveform indicate?
A4: A collapse or sharp drop in the voltage waveform on the oscilloscope during the surge event indicates a breakdown of the insulation. The stored energy in the surge generator’s capacitor is rapidly dissipated through a newly created low-impedance path—an arc—across the insulation. This is a clear and definitive failure criterion, signifying that the insulation has been compromised and would likely fail in a real-world surge event.

Q5: Why is it necessary to apply both positive and negative polarity surges?
A5: Applying both polarities ensures a comprehensive test. The breakdown characteristics of certain insulating materials and semiconductor junctions can be asymmetric, meaning they may fail more easily under one voltage polarity than the other. Testing with both polarities guarantees that the equipment’s insulation system is robust regardless of the transient’s polarity, which is unpredictable in real-world scenarios.