This article presents some defects in the electrical machines and a few methods for appropriate diagnosis. It is part of a broader study that aims to identify electrical or mechanical defects that can occur in electrical machines in operation, a description of their manifestation, prevention and ultimately solutions to remedy them. Intention is that this work becomes a guide to maintenance and repair of the electrical machines that can be used by specialized personnel.It is known that the correct selection of the appropriate machine for each application, the usage of the correct loading and ventilation, the selection of suitable protective elements and the preventive maintenance is the guarantee of extended service life of these machines. It is well known too that a number of concealed defects (both material and regarding the installation) can produce more or less damage, that’s why careful monitoring of electrical machines (especially those of high power) is very important. At least as important are the methods of diagnosis of mechanical or electrical defects, some of which can provide the specialized maintenance personnel with data about the machine itself, before the appearance of defects and without dismantling the machine.The methods used are specific to preventive maintenance such as measurements, verifications, and tests.We conclude that the defects in the electrical machines are multiple, can have internal or external causes, affect nearly all the constructive elements, have specific ways of manifestation, and that their appearance is often preceded by some signs that can warn the operating and maintenance personnel.Compliance with the correct operation scheme of the electrical machines, knowledge of the manifestation of defects, professionalism of the staff, combined with the use of appropriate protective elements may lead to increasing their life span and reduction of the consumption of material and financial resources.