The technical term for withstand voltage testing is dielectric strength testing (hereinafter referred to as withstand voltage testing). It is an important inspection item for motor products. Most motor manufacturers will conduct this inspection in two stages: electrical semi-finished products and inspection and testing. However, due to the inconsistency of the state of the tested object, the peak voltage applied in the two stages will be different.
Because withstand voltage tests can be performed using either AC or DC voltage, they are further divided into AC voltage withstand tests and DC voltage withstand tests, and these two tests are not interchangeable. For small and medium-sized motors, unless otherwise specified, it can be understood that only AC voltage withstand tests are required. Due to space limitations, this article will only briefly describe AC voltage withstand tests.
First test of the new winding
The testing equipment uses a dedicated withstand voltage tester. The instrument should be selected according to the specifications of the motor being tested. The test method, peak value, and related precautions include:
● The high-voltage output terminal of the step-up transformer is connected to the winding under test, and the low-voltage terminal is grounded. The grounding wire at the test site must be inspected regularly to prevent accidents caused by poor grounding.
●The casing (or core) of the motor being tested, as well as the windings without high voltage, should be reliably grounded.
● The test pressure time is divided into two types: 1 minute and 1 second. For finished motors, the withstand voltage test is performed using the 1 minute method, and the voltage value is in accordance with the specific value specified in GB755.
●The number of semi-finished products, such as motor stators and wound-rotor motors, should be increased compared to the number of finished products, as specified in the product's technical specifications.
● To prevent the stored charge in the test winding from discharging and injuring the test personnel, the test winding must be discharged to ground before disconnecting the wiring after the test is completed. This is especially necessary for motors with larger capacity.
Peak voltage applied during the withstand voltage test of the finished motor
The regulations do not include DC motors, synchronous motors, asynchronous motors (or synchronous induction motor rotors) with non-permanent short circuits, excitation motors, and complete sets of motors and devices.
● For motors with a rated power not exceeding 1kW and a rated voltage not exceeding 100V, the test voltage value shall be in accordance with formula (1).
U<sub>test</sub> = 2UN + 500…………………（1）
In formula (1):
U-test—Peak applied voltage during motor withstand voltage test, in V;
UN—Rated voltage of the motor, in volts (V).
● For rotating motors with a rated power of less than 10000kW, the test voltage value shall be in accordance with formula (2), but the minimum shall not be lower than 1500V; the code in the formula is the same as that in formula (1).
U<sub>test</sub> = 2UN + 1000…………………（2）
● For rotating motors with a rated power of not less than 10000kW and a rated voltage of not more than 24kV, the test voltage value shall be in accordance with formula (3), where the code is the same as in formula (1).
U<sub>test</sub> = 2UN + 1000…………………（3）
● For rotating motors with a rated power of not less than 10000kW and a rated voltage greater than 24kV, the test voltage value shall be as specified in the agreement.
Specifications for applied voltage under different test methods
● During the 1-minute method test, the voltage should begin at no more than half of the test voltage, and then gradually increase to the full value evenly or in steps of no more than 5% of the full value. This process should take at least 10 seconds. After 1 minute of voltage application, the voltage should be gradually reduced to half of the test voltage before the power is turned off. This requirement is implemented in the voltage tester, and the motor manufacturer does not need to make any further settings.
●The 1s method is only applicable to mass-produced motors with a rated power ≤200kW and a rated voltage UN≤1kV. When using this method, the test voltage should be 20% higher than the value specified in the 1min method, that is, 1.2 times the pressure value applied in the 1min test method.
Repeated withstand voltage test and withstand voltage requirements for repairing motors
The withstand voltage test can cause varying degrees of damage to the motor windings due to cumulative effects, and is generally not repeated. If the test is necessary, it should be performed at 80% of the initial test voltage. To eliminate factors such as moisture in the motor, drying and insulation resistance testing should be performed to ensure the motor is in normal condition before the withstand voltage test.
For repairing a motor, if it is a brand new winding, the voltage can be applied as if it were a new winding. If it is a partial repair of the winding, the voltage should be 75% of that of a new winding.
