The stiffening effect of laminated rotor cores on flexible-rotor electrical machines

Almost all large electrical machines have laminated rotors. Any one of these rotors is constructed as a stack of steel laminations pressed onto either a solid shaft or a shaft with arms extending from the shaft outer diameter to the inner diameter of the core. When the margin between running speed and a major critical speed is small, it is critical that the stiffening effect of the stack of laminations on the rotor should be understood. It is obvious that the laminated core will offer some additional stiffening but it will not be as stiff as a solid steel body of the same dimensions. This paper describes how this stiffening effect can be included in a rotordynamic model and how the relevant parameters can be determined from experimental work. One particular outcome is the recognition that the simple rotordynamic models using unbranched shaft elements with fixed section properties are not suitable for representing these rotors. The alternative branched model structure is discussed.