The effect of power factor on the stability of the doubly fed machine.
thesisposted on 19.11.2015, 08:57 by I. A. Alston
The doubly fed machine is similar to the slip ring induction motor except that both the stator and the rotor are simultaneously supplied with polyphase a.c. The machine develops a synchronous torque at a certain speed (called the 'speed of synchronous operation'), which is proportional to the sum or the difference of the frequencies of the two supplies (depending on the relative phase sequence). When operating synchronously, the machine exhibits several attributes, the most important of which is its ability to act as a variable speed drive (speed variation is caused by variation of one of the supply frequencies) wiith a speed-independent-of-load characteristic. It is, however, unstable at certain speeds. This work uses methods similar to that of Prescott and Raju (Proc. I.E.E. 1958) to analyse the doubly fed machine. An equation is produced relating the speed of synchronous operation of the machine to its supply frequencies and the machine is assumed to operate at (or about) this speed. Equations are also produced, using the method of superimposition, relating the various currents, emf's and torques in the machine to the machine parameters and to the supply voltages and frequencies. This is done in such a way that studies of the dynamic behaviour of the machine could be made. Workers in the past have either defined the ratio of the rotor voltage to the stator voltage in an arbitrary manner or just tried several values. In seeking an equation to define this ratio is it sensible to stipulate the condition that the minimum current (for that load-speed combination) should be drawn from the supplies. This stipulation resolves into two criteria: one of unity power factor in the stator supply and one of unity power factor in the rotor supply, which can be expressed mathematically as two equations. Only one of these criteria may be used at any one time (the choice of which one to use depends on practial rather than theoretical considerations). The machine is now completely specified. To verify the validity of the various expressions, a series of experiments and computer simulations were executed. (The latter being based on the expressions with the parameters of the test machine, used in the former, as data.) The results of these experiments and computer simulations did not only show good agreement between practice and theory but also showed that several advantages ensued from the use of a unity power factor criterion. These advantages are: an increase in the stable speed range of the machine, an increase in efficiency, a possible increase in the usable torque range (at any particular speed), a reduction in the currents drawn from the supplies. These advantages are suggested by qualitative arguments, which use the similarities between the doubly fed machine and its, more well known, special cases. The analysis mentioned above was required to quantify them. The performance of the doubly fed machine under conditions of unity power factor (both stator and rotor power factor conditions), and the variation OF the rotor voltage (for a fixed stator voltage) to maintain these conditions, form the latter part of the work. Comparisons are also made between unity power factor conditions and non unity power factor conditions. The final part of the project was the building and testing of a simple feedback device designed to automatically adjust the rotor voltage to maintain unity stator power factor for all speeds and torques. Many practical difficulties were overcome by using this device including those associated with the starting and synchronising of the machine.