The AC withstand voltage test for electric motors is a critical test item for evaluating the insulation strength of core components such as stators and rotors. It is primarily used to detect localized defects in motor insulation that may arise during manufacturing, transportation, storage, or field operation—including moisture ingress, cracks, and insufficient creepage distances in winding insulation. As a destructive test, it applies voltages significantly higher than the motor’s rated operating voltage, requiring strict compliance with national and industry standards throughout the process. Based on GB 50150-2016 “Code for Handover Tests of Electrical Equipment in Electrical Installation Projects” and related preventive testing procedures, this article systematically outlines the key technical points, equipment configuration, and standardized operational procedures for this test.
I. Test Standards and Procedures
The fundamental logic of the AC withstand voltage test is to apply a voltage specified by standard multiples of the motor’s rated voltage to each winding and maintain this voltage for one minute. If no insulation breakdown, surface flashover, or significant change in leakage current occurs during the test, the result is considered合格 (passing). Test voltage requirements vary clearly depending on winding type and motor specifications, necessitating precise adherence to these distinctions, along with fulfillment of prerequisite tests.
(1) Test Voltage Standards for Different Windings
For stator windings with a rated voltage above 1000 V, the AC withstand voltage is calculated using the fixed formula: “twice the rated voltage plus 1 kV.” For example, a motor with a rated voltage of 6 kV would have a handover test voltage of 2×6 + 1 = 13 kV. Small motors with a rated voltage below 1000 V and capacity under 100 kW are not required to undergo mandatory AC withstand voltage testing; completion of insulation resistance measurement and no-load inspection alone meets acceptance criteria.
For wound-rotor motors, the rotor winding test voltage depends on the motor’s direction control capability: for non-reversible motors, the test voltage is 1.5 times the open-circuit voltage of the rotor winding at rest; for reversible motors, the test voltage increases to 3 times the open-circuit voltage at rest.
For synchronous motors, the rotor winding test voltage is set at 7.5 times the rated excitation voltage, with minimum and maximum limits applied—no less than 1200 V and no more than 75% of the manufacturer’s factory test voltage.
(2) Key Pre-test Conditions
The AC withstand voltage test must only be conducted after successful insulation resistance and absorption ratio tests; bypassing these preliminary tests is strictly prohibited. For motors with a rated voltage exceeding 1000 V, if the stator winding insulation resistance falls below 1 MΩ/kV, the withstand voltage test must not proceed. Instead, the winding must first undergo drying treatment or insulation defect repair until insulation parameters meet requirements before testing can begin.
II. AC Withstand Voltage Test Equipment
Equipment selection for the test primarily depends on the motor’s ground capacitance and rated test voltage level. Commonly used on-site equipment includes power frequency withstand voltage testers and series resonant withstand voltage testers, which are suited to different motor testing conditions.
(1) Main Equipment Types and Applicable Scenarios
Power frequency withstand voltage testers consist of two core components: a voltage regulator and a test transformer. By adjusting input voltage, they generate high output voltage. These devices feature simple structure, intuitive operation, and strong stability, making them suitable for low-voltage motors and small-capacity high-voltage motors with low ground capacitance current. However, their limitations become evident when the motor capacity exceeds 1000 kW—the capacitive current between the winding and ground increases significantly, substantially increasing the required capacity of the test transformer. This results in heavier equipment, greater difficulty in supplying power on site, and reduced adaptability.
Series resonant withstand voltage testers are currently the mainstream equipment for on-site testing of high-voltage motors up to 10 kV. Their working principle involves creating a series resonance between an inductor and the ground capacitance of the motor winding. With only a small excitation power, they can generate a standard test voltage far exceeding the input voltage across the test specimen. The device features prominent core advantages, offering overall portability and light weight, along with excellent self-protection performance: when insulation breakdown occurs in the test specimen, the resonant balance state is disrupted, causing the test voltage to drop rapidly and instantly, effectively limiting the damage range caused by insulation failure. Meanwhile, the output voltage waveform exhibits outstanding sinusoidal quality, fully meeting technical requirements for test power waveform as specified by standards.
(2) Core Equipment Parameter Configuration Requirements
When selecting equipment, it is essential to carefully verify two key parameters: output voltage and output capacity. The peak output voltage must cover the highest test voltage requirements of the equipment—for example, a 10 kV motor commissioning test requires 21 kV; thus, the rated output voltage of the supporting equipment should not be less than 30 kV, allowing sufficient safety margin. Output capacity should be calculated based on the motor’s ground capacitance. For on-site testing of 6 kV–10 kV high-voltage motors, configuring a resonance device with 4 kVA–50 kVA capacity can meet the needs of most common operating conditions.
Preferably select test equipment equipped with arc detection functionality, which can accurately capture transient flashover pulses during dielectric withstand tests and effectively identify hidden insulation defects that do not result in complete breakdown, thereby enhancing the comprehensiveness and accuracy of testing.
III. Test Operation Methods and Key Steps
Standardized and standardized test procedures are fundamental to ensuring accurate test results and avoiding safety risks. The complete test process consists of seven critical steps, all of which must be strictly followed throughout execution.
Step 1: Safety Preparation and Electrical Isolation
Disconnect the motor under test from all external electrical circuits, including switchgear, cables, and soft starters, ensuring that each phase winding terminal is exposed, independent, and free from contact. Thoroughly clean dust, oil, and contaminants from the end surfaces of windings and lead wires to prevent surface leakage currents from interfering with test result evaluation. Set up a rigid isolation barrier around the test area, post clear “High Voltage Danger” warning signs, and assign a dedicated supervisor to monitor the entire operation—strictly prohibit unauthorized personnel from entering the test zone.
Step 2: Pre-Test Parameter Verification
Re-measure the motor winding insulation resistance to confirm it meets the minimum requirements specified in regulations, and check the absorption ratio—the ratio of insulation resistance at 60 seconds to that at 15 seconds must not be less than 1.3. If the motor has just been shut down and the winding temperature is elevated, allow the equipment to cool to near ambient temperature before testing, or apply a temperature correction formula to adjust the insulation resistance value. Never conduct tests under high-temperature conditions in violation of safety protocols.
Step 3: Instrument Wiring and Reliable Grounding
Use dedicated high-voltage leads to connect the high-voltage output terminal of the test equipment to the terminals of the phase winding under test. Ensure the leads remain suspended and isolated throughout to avoid corona discharge caused by contact with grounding wires or metal structures. Connect the motor housing, equipment base, and non-test phase windings to a dedicated test grounding network using copper wire with a cross-sectional area of no less than 4 mm², maintaining a grounding resistance below 0.1 Ω. The test equipment’s outer casing must be independently grounded to ensure the overall grounding system is safe and reliable.
Step 4: Pre-Voltage-Raise System Inspection
Confirm that the voltage regulator and frequency source are at zero output initially. After closing the circuit and powering up, observe that the test equipment operates without abnormal noise, and that the voltage and current meter pointers remain stable and unfluctuating. If a series resonant device is used, first perform frequency elevation to precisely locate the system resonance point. Only after the equipment reaches a stable resonant state should the voltage-raising procedure begin.
Step 5: Steady Voltage Rise and Holding Time Measurement
Control the voltage rise rate in stages: prior to reaching 40% of the target test voltage, the rise rate may be freely adjusted; once exceeding 40%, voltage must be increased steadily, with a recommended rate of 3% per second of the target test voltage. Upon reaching the standard-specified voltage level, immediately start timing and maintain the voltage for one minute. During this holding period, operators must maintain a safe distance from the test equipment and continuously monitor its condition through observation windows or video surveillance.
Step 6: Result Evaluation and Controlled Voltage Reduction After the 1-minute holding period, if the equipment shows no smoke, burnt odor, surface flashover, abnormal discharge sounds, and the ammeter pointer does not exhibit periodic large fluctuations, the insulation withstand voltage of that phase winding is considered合格. Then, smoothly reduce the voltage to zero and completely disconnect the test power supply. If insulation breakdown occurs during voltage rising or holding, immediately cut off the power, record the breakdown voltage value and fault conditions, identify and rectify the defects before re-performing the test.
Step Seven: Complete Discharge and Post-Test Re-inspection
Motor windings store a significant amount of charge due to their capacitance to ground; therefore, after testing, a dedicated discharge rod with a current-limiting resistor must be used for thorough discharge. First, discharge through the resistor to buffer the energy release, wait several seconds, then directly ground for discharge—repeat this process at least twice to ensure all residual charge is fully dissipated. After discharging, remeasure the insulation resistance between the tested phase winding and ground, and compare it with pre-test values. Under normal circumstances, there should be no significant drop in insulation resistance; if the value decreases noticeably, it indicates insulation damage during testing, requiring further investigation and analysis of the root cause.
Post time: Jun-02-2026