What is the minimum recommended absorption coefficient value for AC and DC rotating machines?

What is the minimum recommended absorption coefficient value for AC and DC rotating machines? 
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a. IR value decreases some what with an increase I applied voltage. However for machines in good condition substantially the same IR is obtained for any test voltage up to the peak value of the rated operating voltage. 


b. If the IR value decreases significantly with an increase in applied voltage it is an indication of imperfections or fractures of the insulation aggravated by the presence of dirt or moisture or may be due to the effects of dirt or moisture alone, or may result from numerous other phenomena not necessarily associated with any defect or weakness. 


c. IR value for good dry winding continue to increase for hours with constant test voltage continuously applied, however a fairly steady value is usually reached in 10 to 15 minutes. If the winding is wet or dry or dirty the steady value is usually reached in 1 or 2 minutes after the test voltage is applied. 


d. The recommended minimum IR value for AC and DC machines is determined by the following empirical relationship. IR = kV + 1 Where IR = recommended minimum IR in mega ohms at 40°C of the entire machine winding and kV = rated machine voltage in kilo volts. Temperature correction is to be applied, if winding is not at a temperature of 40°C. 


e. IR of the one phase of three phases winding with other two phases earthed, is approximately twice that of the entire winding. Therefore when the three phases are tested separately, the observed insulation resistance of each phase should be divided by two to obtain a value which after correction for temperature, may be compared with the recommended minimum value of IR. 

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