This paper presents the capacitive VAr requirements of a three phase pole changing self-excited induction generator and a single phase self-excited induction generator, used as isolated power sources by a constant speed or a variable speed prime mover, to obtain the desired voltage regulation at various values of load and speed. Different performance criteria such as constant terminal voltage or constant air gap flux have been considered. The developed mathematical model using nodal analysis based on graph theory is quite general in nature and can be used for any combination of the unknown variables such as magnetizing reactance (X-M) and frequency (F) or capacitive reactance (X-C) and frequency (F) or capacitive reactance (X-C) and speed (upsilon). The proposed model completely avoids the tedious and erroneous manual work of segregating the real and imaginary components of the complex impedance of the machine for deriving the specific model for each operating modes. Moreover, any element, like the core loss component, can be included or excluded from the model if required. Next, to obtain the capacitive VAr requirements of a three phase pole changing self-excited induction generator and a single phase self-excited induction generator, a fuzzy logic approach is used for the first time to find the unknown variables using the above model. The results are presented in a normalized form so that they are valid for a wide range of machines and would be useful for the design of voltage regulators for such generators. (c) 2006 Elsevier Ltd. All rights reserved.