Comparison between analytical and numerical methods of calculatingtooth ripple losses in salient pole synchronous machines

The application of analytical and numerical methods to an electromagnetic problem requiring an accurate representation of saturation is examined. The problem considered is that of tooth-ripple losses in salient-pole synchronous machines for the situation where the pole pitch is much greater than the armature slot pitch so that the applied DC field can be taken as uniform. To calculate these losses, several analytical methods have been developed over a period of many years. Two such methods, one devised by Oberretl (1972) and a modified version of the considerably older one-dimensional approach of Gibbs (1947), are compared with results obtained from the finite-element and finite-difference methods. Using a time-stepping finite-difference calculation, the influence of moving boundaries and the imposed DC field are taken into account for the first time in this tooth-ripple calculation. A saturation factor is defined that allows the designer to calculate the tooth-ripple losses of solid salient-pole synchronous machines for a wide range of machine size taking magnetic saturation into account. To verify the theory, the results are compared with measurements on a small model. These measurements were done using a torque meter placed between the model and a DC drive motor and were cross-checked by the Poynting vector method. Rules and limits are given for the use of the analytical methods     

Temperature rise within the rotor of squirrel cage induction machines with solid iron and laminated rotors during run up and standstill

The electrical and thermal behaviour of induction motors with solid and laminated rotors is compared. In particular, temperature rises within the rotor during standstill and run ups are investigated. The investigation is based on a finite difference time-stepping method, which takes the motion of the rotor into account. The calculation method is verified by comparison with measurements on a 22 kW induction motor. The temperature rise depends upon the position of the rotor bars compared with the stator winding during standstill. Differences up to 50% occur on account of the position. Specialities in the temperature distribution are elaborated for the solid iron rotor and are compared with the results of laminated induction motors.     

Medium frequency shaft voltages in large frequency converter driven electrical machines

The operation of large electrical machines like turbo-generators with ratings of several hundred MVAs by static frequency converters can cause shaft voltages of several hundred volts. The shaft voltages consist of peaks with a time constant of some μs. They occur mainly during the rectifier switching at the grid-linked section of the converter. The voltages are the result of a coupled capacitive and inductive phenomenon. The root cause of these voltages is shown and a machine model of turbo generators for several 10 kHz is developed taking parasitic capacities and eddy currents into account. It is not sufficient to limit the model just to the generator. It is the combination of transformator, cables, frequency converter and generator, which allow for such high voltages. Comparisons with measurements on a 300 MVA generator are made to verify the model. A high level of agreement between measurement and calculation is reached for this recently discovered capacitive inductive shaft voltage phenomenon. Especially the high power class of the equipment involved reveals the underlying physics fairly clearly, but enforces a deep investigation and high predictability of the right counter measures in order to avoid substantial risk exposures.     

Current flow and losses in brushless exciters with polygon-connected windings and dc rectifiers

The current flow within a salient pole brushless exciter with polygon-connected winding is calculated. The winding is connected to a rectifier, which causes transients in the winding during switching. Brushless exciters are over-dimensioned for their rated operation. During ceiling conditions the required voltage may be two times higher and the exciter is strongly saturated. Therefore the normal asynchronous circuit theory is no longer applicable for a detailed study within the exciter itself. A nonlinear finite difference time stepping method with rotating rotor and diode circuits is applied instead. Basic theories for the combination between the circuit theory and field theory are summarized in one mathematical model. Calculation results are compared to measurements on a real power plant exciter for different operation conditions. Rolf Gantenbein has recently passed away.