Equipment and Test Methods for Partial Discharge Detection of Electric Motors

A complete partial discharge test system for motors typically includes the following core equipment, with specific configurations varying depending on the test object (such as the entire vehicle, stator, or individual coils) and the test standards.

I. Core Components
High Voltage Unit
This includes a non-partial discharge test transformer, a variable frequency power supply, and an impulse voltage generator. It mainly provides a high voltage source for the motor under test, outputting power frequency, high-frequency sine waves, or simulated surge pulse voltages to trigger partial discharges.
Detection and Acquisition Unit
This consists of coupling capacitors, detection impedances, and high-frequency current sensors. It is mainly used to extract the high-frequency pulse signals generated by partial discharges, with the combination of coupling capacitors and detection impedances forming the classic pulse current method configuration.
Measurement and Analysis Host
This includes a partial discharge detector, an industrial computer, and professional software. It can amplify, filter, and analyze the collected signals, display the discharge quantity and discharge patterns, and can also integrate withstand voltage and insulation resistance tests.
Auxiliary and Calibration Equipment
This includes a calibration pulse generator and high-voltage connection devices (such as a high-voltage relay matrix). The calibration pulse generator is used to calibrate the system’s sensitivity to ensure measurement accuracy; the high-voltage connection devices can automatically switch between phases or individual coils for testing.

II. Test Methods
The core of the partial discharge test is to precisely capture and analyze weak discharge signals while applying voltage. The relevant test procedures and key parameters are as follows:
1. Test Items
Partial Discharge Onset Voltage: Gradually increase the voltage until a stable discharge pulse is observed.
Partial Discharge Quenching Voltage: After a discharge occurs, gradually decrease the voltage until the discharge completely stops.
Partial Discharge Quantity Measurement at Specified Voltage: Measure the actual maximum discharge quantity of the test sample at a specified voltage (such as 1.5 times the rated voltage), usually requiring it not to exceed 50 picocoulombs or 100 picocoulombs.
2. Test Circuit Wiring
The standard recommends using the basic circuit of the pulse current method, which is mainly divided into two modes:
Single-phase Testing: Test each phase winding of the motor separately.
Multi-phase Testing: Use a high-voltage switch matrix to automatically switch between three-phase windings for testing, which is more efficient.
3. Specific Operating Steps
Calibration: Inject a known amount of calibration pulses at the test sample end to confirm the overall sensitivity of the test circuit.
Voltage Rise Measurement: Smoothly increase the voltage from a lower level (such as 0.2 times the rated voltage) and monitor the discharge signals in real time.
Recording and Analysis: Record the partial discharge onset voltage, quenching voltage, and apparent discharge quantity at the specified voltage; through the phase-amplitude-frequency relationship diagram, the type of defect can be preliminarily determined, such as internal discharge, surface discharge, or slot discharge.

III. Standard Procedures
The standard system for motor partial discharge tests is highly aligned with international standards, mainly divided into offline testing and online testing. Relevant standards are as follows:
GB/T 20833.1-2021 “Rotating Electrical Machines – Insulation of Windings – Part 1: Off-line Partial Discharge Measurement”: The core standard, which stipulates the general basic norms for off-line partial discharge measurement in a stopped state, covering test circuits, measurement methods, noise suppression, and result evaluation guidelines.
GB/T 20833.2-2025 “Rotating Electrical Machines – Insulation of Windings – Part 2: On-line Partial Discharge Measurement of Stator Windings”: To be implemented in February 2026, it is applicable to the monitoring of partial discharges during motor operation and is used to assess the insulation aging trend.
GB/T 22720.2 (planned revision) “Rotating Electrical Machines – Test for Electrical Insulation Structures (Type II) of Rotating Electrical Machines Supplied by Voltage Source Inverter against Partial Discharge”: This standard is for the insulation structure assessment of variable frequency motors, requiring the insulation system to withstand the repetitive voltage pulses generated by voltage source inverters. GB/T 7354-2018 “Partial Discharge Measurement”: A fundamental and general standard for partial discharge measurement, it stipulates relevant terms, definitions, and basic measurement methods.
For low-voltage and high-frequency motors such as those used in new energy vehicle drive motors, the IEC 60034-18-41 standard (Type I insulation structure) is usually referred to, with a focus on testing the partial discharge inception voltage to ensure that no partial discharge occurs under the voltage stress generated by the inverter. Equipment and Test Methods for Partial Discharge Detection of Electric Motors