Drehgeberlose Identifikation der Rotorlage der elektrisch erregten Synchronmaschine in Mittelspannungsantrieben

For the control of electrically excited synchronous machines, the information of the actual rotor position or the position of a machine flux space phasor is mandatory. Generally, a mechanical angular encoder is installed at the machine shaft which delivers this information, if necessary in combination with an adequate machine model. Thus, all required information is obtained in any arbitrary operating point of the machine. Unfortunately, these sensors are mechanically and electrically sensitive devices. Leading to a degradation of the total reliability of the whole drive system as an error in these sensors normally leads to a failure in the whole drive.
An alternative to the commonly used control approaches with a mechanical sensor (encoder) are the encoderless control schemes. In this case, the required information of the rotor or flux position is determined solely by means of machine models, which only need to be fed by the measured stator and field currents as well as the voltage of the dc-link, which are measured anyways in a classical configuration.
For the encoderless control of electrically excited synchronous machines, several different approaches are necessary for different operating points of the machine. The methods which cover the high-speed operation of the drive can be applied without difficulties to this machine type, but the methods for the low speed range or standstill fail with most machines or cannot be applied in several applications. The first thing, that is discussed in this dissertation are the problems which lead to the failure of the standard methods. Based on this knowledge, two new identification strategies are introduced: one for the initial rotor position and the other for the encoderless control in the low speed and standstill area. Both approaches are based on physical effects which have the potential to work on many different electrically excited synchronous machines, namely the injection of low frequency test signals and the evaluation of the response in the stator or field winding. Both approaches were first simulated, then tested on a low voltage test bench and finally validated on a conventional medium voltage drive.
The approach for the initial rotor position identification leads to an accuracy better than 5° for the identified electrical rotor angle at the measured medium voltage machine. It can be applied at either direct or brushless excited synchronous machines.
The approach for the encoderless control in the low speed and standstill region ensures the permanent and safe operation under load in standstill and low velocities without any drift of the identified angle. However, due to practical constraints this procedure exhibits a moderate accuracy of approximately 30° of the electrical angle on a conventional medium voltage inverter.