Some Special Operation Modes of Synchronous Machine Systems at Constant Speed and Variable Speed

Abstract

As is stated in Chapter 2, by use of d, q, 0 axes can get the differential equations with constant coefficients for symmetrical faults of synchronous machines at synchronous speed, but can’t for unsymmetrical faults or variable speed. Therefore, in this chapter it is necessary to introduce other reference axes systems first, such as α, β, 0 axes; 1, 2, 0 axes; d _c,q _c,0 axes; F, B, 0 axes; F _c,B _c,0 axes. Then use α, β 0 axes to analyse two phases sudden short circuit of synchronous machines, inclusive of stator and rotor loop parameters and currents, overvoltage, pulsating and average torques. Furthermore, use 1, 2, 0 axes to study asynchronous operation of synchronous machines to get corresponding equivalent circuits, currents, torques and negative sequence reactance that depends upon the operating modes. In a similar way, analyse asynchronously starting of synchronous motors to get starting torque and pull-in one and suggest the optimal time for applying a dc voltage to excitation winding. Analysis of combination system for synchronous machine and capacitances is also an interesting topic, in which by use of Herwitz’s, Routh’s and Mihainov’s criterions determine the salient-pole synchronous self-excitation region, asynchronous and repulsion synchronous self-excitation regions, explain the mechanism to cause those self-excitation regions, and illustrate why the sub-synchronous resonance occurs. Cycloconverters can change the source frequency to lower frequency that is under 1/3 –1/2 of source frequency, and cycloconverter-fed synchronous motor systems with field-oriented control are widely used to drive slow speed and large capacity production machines because of their some advantages over the ac-dc-ac frequency converter. Therefore, their mathematic patterns and performance analysis belong to another important topic. Taking a steel-mill driven by a 4 000kW-cycloconverter-fed synchronous motor as example, analyse the steady-state and transient performance of the cycloconverter-fed synchronous motor system, simulative and experimental results being in accord with each other and thus giving a suggestion that is how to dispose damping bars. Some special power supply systems, such as those used on ships, tractions, and aircraft, are often required to provide high quality ac and dc electricity simultaneously, so analysis of synchronous generators with ac and dc stator connections is also distinct. The ac-dc generator has five suits of independent star-connected 3-phase windings in its stator, one is called ac winding to supply ac power and the others belong to dc windings, which are connected to four 3-phase bridge rectifiers respectively, and then the dc output sides of the four 3-phase bridge rectifiers are in parallel. By use of multi-loop model analyse the ac-dc generator to compared with experimental data, both results being in accord with each other.