the determination of the temperature index of enamelled and tape wrapped winding wires
IEC 60172-2020 pdf download.Test procedure for the determination of the temperature index of enamelled and tape wrapped winding wires
Méthode d’essai pour a determination de l’indice de temperature des fils de bobinage émaillés et enveloppés de ruban.
Thermal endurance values obtained from test specimens subjected to an average of less than eight or more than twenty cycles at the exposed temperature may not be reliable and should not be used to predict the temperature rating of the enamelled wire. Therefore, a shorter or longer cycle time than those given in Table 3 may be chosen for certain exposure temperatures, to ensure that the average number of cycles to failure falls within this range.
After the specimens have been subjected to a particular cycle, the time may be appropriately increased or decreased to control the number of cycles required to reach the time to failure.
Test specimens shall be exposed to a minimum of three and preferably four exposure temperatures. The lowest temperature, recommended at 20 °C above the desired thermal class, shall be one which results in a time frame to failure of more than 5 000 h. The highest exposure temperature shall have a value of at least 100 h to be considered a valid data point. Exposure temperatures should not be more than 20 DC apart. The accuracy of the temperature index predicted from the results will improve as the exposure temperature approaches the temperature to which the insulation is exposed in service.
7 Test voltage and Its application
The voltage to be applied shall be an AC voltage and shall have a nominal frequency of 50 Hz or 60 Hz of an approximately sine-wave form, the peak factor being within the limits of ± 5 % (1,34 to 1,48). The test transformer shall have a rated power of at least 500 VA and shall provide a current of essentially undistorted waveform under test conditions.
To detect failure, the overcurrent indication device shall operate when a current of 5 mA or more flows through the high-voltage circuit. The test voltage source shall have a capacity to supply the detection current (5 mA or more) with a maximum voltage drop of 10 %.
The test specimens are removed from the ovens and cooled to room temperature. Each specimen shall be subjected to a proof voltage according to the average thickness of the enamel as specified in Table 3 for specimens according to 5.1.1 and in Table 4 for specimens according to 5.1.2. In the case of self-bonding wires, the self-bonding layer is included in the increase in diameter due to the Insulation,
8.1 Specimen failure time
The failure time of an individual specimen at one exposure temperature is determined by calculating the mid-point between the total hours of exposure temperature at which the specimen failed the proof voltage and the total hours of exposure of the previous cycles. This assumes that the specimen would probably have failed the proof voltage at some point In the middle of the last temperature exposure cycle. Thus, the specimen failure time is the sum of the total hours at the time to failure, minus half the hours of the last exposure cycle.
8.2 Time to failure
The time to failure of a set of specimens at one exposure temperature shall be calculated by using either the median value or the logarithmic mean value. For many materials, the median value is statistically valid. In most cases, use of the median will significantly reduce testing time, since the test ceases once the median value has been obtained.
When using the median va’ue. the time to failure is calculated as follows:
Where there are a total number of,, specimens in a set of specimens, the time to failure of the set equals:
a) the specimen failure time of specimen number (n + 1)/2: if n is odd:
b) the mean value of the specimen failure times of specimen number n/2 and (n + 2)/2; if n is even.
For instance, if,, is 12, the time to failure of the set would be the mean value of the specimen failure times of the sixth and the seventh specimen. For convenience, it is suggested that when the median value is used for ca’culating the time to failure of the set, the total number of specimens of a set be odd, thus simplifying calculation.
When using the logarithmic mean value, the time to failure is calculated by dividing the sum of the logarithms of the specimen failure times of the set by the total number n of specimens in the set. The antilogarithm of this mean value is the time to failure of the set.
8.3 LinearIty of data
To avoid misleading extrapolations (see 8.4), the correlation coefficient shall be calculated as shown in Annex B, to provide a measure of linearity.
If the correlation coefficient r is equal to or greater than 0,95. the data are said to be linear and the data points will be reasonably close to a straight line. In the event that the correlation coefficient is less than 0,95, the data are said to be non-linear and an additional test shall be performed at a temperature below the lowest previous temperature.IEC 60172-2020 pdf download.