The most complex and crucial step in the production of high-voltage motors is the manufacturing of the motor coils. Turn-to-turn and withstand voltage tests are essential during coil manufacturing. Some motor manufacturers use sampling inspection methods for coil testing, while many others use individual coil inspection to ensure the electrical performance of the high-voltage motor meets requirements.
For motors with rated voltages of 3000V-15000V, the impulse voltage withstand level is calculated according to the standard formula, and the resulting value is rounded up. It is divided into two cases: specified lightning impulse voltage withstand and specified steep wavefront impulse voltage withstand. For motors with rated voltage levels (3 , 3.15 , 3.3, 4, 6, ..., 13.8 , 15), the standard provides specific values and formulas for the specified impulse voltage withstand level and the power frequency voltage withstand (RMS value), divided into three cases: specified lightning impulse voltage withstand, specified steep wavefront impulse voltage withstand, and power frequency voltage withstand.
Standard withstand voltage for lightning impulses
Lightning impulse voltage is tested by artificially simulating the waveform and peak value of lightning current to verify the ability of electrical equipment insulation to withstand lightning impulse voltage. Based on actual measurements of lightning discharges, the lightning waveform is considered a unipolar biexponential curve with a wavefront of a few microseconds and a tail of tens of microseconds. Most lightning strikes are negative polarity. National standards worldwide define the standard lightning impulse wave as follows: apparent wavefront time T1 = 1.2 μs ( also known as wavefront time); apparent half-wave peak time T2 = 50 μs (also known as wave tail time). The permissible deviations between the voltage peak value and waveform generated by the actual test device and the standard waveform are: peak value, ±3%; wavefront time, ±30%; half-wave peak time, ±20%. The standard lightning waveform is usually expressed as 1.2 /50 μs.
The standard specifies that when withstands lightning impulse voltage, the applied impulse voltage shall be calculated according to formula (1) and rounded to the nearest thousand.
UPL=4UN+5…………(1)
In formula (1):
UPL-------Applied impulse voltage, kV;
UN-------The rated voltage of the motor to which the tested coil belongs, in kV.
Steep wavefront impulse voltage
The level is determined according to the apparent wavefront of 0.2 μs . When the voltage is specified to withstand steep wavefront impulse voltage, the applied impulse voltage is calculated and rounded according to formula (2).
UPD=0 . 65×（4UN+5）=0 . 65UPL…………(2)
In formula (2):
UPD—Applied impulse voltage, kV;
UN—Rated voltage of the motor to which the tested coil belongs, in kV.
Power frequency withstand voltage test
When the power frequency withstand voltage is specified, the applied impulse voltage is calculated according to formula (3) and rounded up. This method is the most commonly used test method by motor manufacturers.
US = 2UN + 1…………(3)
In formula (3)
Us—Applied impulse voltage, kV;
UN—Rated voltage of the motor to which the tested coil belongs, in kV.
Methods for withstand voltage tests between coil turns and main insulation
●Inter-turn test
A surge voltage was applied five times between the two leads of the coil using a turn-to-turn meter. The peak value of the applied surge voltage was obtained by equation (2).
●Main insulation withstand voltage test
The impact resistance level of the main insulation is tested using power frequency voltage or impulse voltage. During the test, the coil under test is embedded in a slot or wrapped with a grounded conductive strip around the straight portion of the coil. For ease of operation, many motor manufacturers use the steel ball method, in which the straight edge of the coil is embedded in a steel ball with a diameter of no more than 1 mm.
(1) Power frequency voltage test: Apply a power frequency voltage of (2UN+1)kV between the coil lead and ground for 1 minute. Then increase the voltage at a rate of 1kV/s to reach 2(2UN+1)kV. Immediately reduce the voltage to zero at a rate of at least 1kV/s. No breakdown occurs during the test to meet the requirements.
(2) Impulse voltage test: An impulse voltage is applied between the coil lead and ground. The wavefront time of the impulse voltage is 1.2 μs, and the number of impulses is 5. The peak value of the test voltage is calculated using equation (1).
