Could somebody please tell me what are acceptable megger readings for generator windings. Also what is the standard rule, ohms per volts (somebody told me that it was 100 ohms per volts). e.i. if the generator is a 3 phase 120/208 then the value should be at least 208K ohms. Where can I find this information.
Thanks
IEEE, NEMA, NETA, ANSI, all the same spec. Depends on a few factors but likely your spec is 5M. These are the ANSI minimum acceptable values, and for a generator they need to be corrected to 40 degrees C (This can make a huge difference and is very important). You also need to calulate DAR and PI.
The dielectric absorption ratio or polarization index shall not be less than 1.0. The
recommended minimum insulation resistance (IR 1 min) test results in megohms shall be
corrected to 40? C and read as follows:
1. IR 1 min = kV + 1 for most windings made before 1970, all field windings, and others not
described in 2.2 and 2.3.
(kV is the rated machine terminal-to-terminal voltage in rms kV)
2. IR 1 min = 100 megohms for most dc armature and ac windings built after 1970 (formwound
coils).
3. IR 1 min = 5 megohms for most machines and random-wound stator coils and formwound
coils rated below 1 kV.
NOTE: Dielectric withstand voltage and surge comparison tests shall not be performed
on machines having values lower than those indicated above.
