Some important details that are often overlooked in the motor withstand voltage test
The terminology of the withstand voltage test is the dielectric strength test (hereinafter referred to as the withstand voltage test), which is an important inspection item for motor products. For most motor manufacturers, the project will be inspected in the electrical semi-finished products and inspection tests. However, due to the inconsistency of the state of the tested object, the voltage peaks applied by the two links are different.
The withstand voltage test has two kinds of AC and DC voltages, so it is divided into two types: AC voltage resistance test and DC voltage resistance test. The two cannot replace each other. For small and medium-sized motors, unless otherwise specified, it can be understood that only the AC voltage resistance test is required. Due to space limitations, this article only briefly describes the AC voltage resistance test.
First test on new windings
The test equipment adopts a special withstand voltage tester. The choice of the instrument should be selected according to the specifications of the measured motor. The test method, peak test and related precautions include:
● The high voltage output terminal of the step-up transformer is connected to the tested winding, and the low voltage terminal is grounded. The grounding wire of the test site must be inspected regularly to prevent accidents caused by poor grounding.
● The motor casing (or core) to be tested and the windings without high voltage should be grounded reliably.
●The test pressurization time is divided into 1min and 1s; for the finished motor, the pressure test is carried out according to the method of 1min, and the voltage value is specified according to GB755.
● For motor stator and wound motor rotor semi-finished products, it should be increased compared with the whole machine product, which is specified in the technical conditions of the product.
● In order to prevent the test winding from storing charge discharge and injuring the test personnel, the test should be completed, and the test winding should be discharged to the ground before the wiring can be removed. This is especially necessary for a larger capacity motor.
Finished motor withstand voltage test pressure peak
This regulation does not include DC motors, synchronous motors, asynchronous motors that are not permanently short-circuited (or synchronous induction motor rotors), excitation motors, and a combination of motors and devices.
● For motors with rated power not exceeding 1 kW and rated voltage not exceeding 100 V, the test pressure is as shown in equation (1).
U test = 2UN + 500.....................(1)
In formula (1):
U test - motor withstand voltage test voltage peak, unit V;
UN—The rated voltage of the motor in V.
● Rotating electric machine with rated power less than 10000kW, the test pressure value is according to formula (2), but the minimum can't be lower than 1500V; the code is the same as formula (1).
U test = 2UN+1000.....................(2)
● Rotating electric machine with rated power not less than 10000kW and rated voltage not exceeding 24kV, the test pressure value is according to formula (3), where the code is the same as formula (1).
U test = 2UN + 1000.....................(3)
● For rotating electrical machines with rated power not less than 10000kW and rated voltage greater than 24kV, the test voltage value shall be as specified in the agreement.
Provisions for voltage application under different test methods
● In the 1min method test, the applied voltage should never start more than half of the test voltage, and then gradually increase to the full value evenly or 5% of the full value of each step. The time taken for this process should be no less than 10s. The power is turned off after the pressurization reaches 1 min and then gradually reduces the voltage to half of the test voltage. This requirement is implemented in a voltage tester and the motor manufacturer does not need to make settings.
● The method of 1s is limited to mass-produced motors with rated power ≤200kW and rated voltage UN≤lkV; when tested by this method, the test voltage is higher than 20% of the value specified by the lmin method, that is, 1.2% of the test value of the 1min test method. Times.
Repeated withstand voltage test and repair motor withstand voltage requirements
The withstand voltage test has different degrees of damage to the motor winding due to the accumulation effect, and generally no repeated test is performed. When testing must be carried out, it should be reduced to 80% of the initial test voltage. In order to eliminate the moisture and other factors of the motor, the normal state of the motor should be ensured by the drying and insulation resistance detection means before the withstand voltage.
For repairing the motor, if it is a new winding, the voltage can be applied according to the new winding test. If the winding is partially repaired, the applied voltage is performed at 75% of the new winding.
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