Wound field synchronous motors are used in high capacity applications especially in power plants due to the fact that they have a controllable power factor and a higher full efficiency than the induction motors of corresponding rating. The speed control of synchronous motor drives mainly involves the control of frequency. This frequency control may be implemented in (a) True synchronous mode where frequency is an independent variable or (b) self-control mode where frequency is adjusted in accordance with the rotor speed. This thesis investigates the application of self-control to one of the ID (Induced Draft) fan drive of about 1.4 MW capacity installed at NTPC Unchahar thermal power station in U.P. Two different control techniques have been employed to this wound filed synchronous motor drive operating in self-control mode namely: (i) Direct Torque Control (DTC) using a Voltage Source Inverter (VSI) (ii) Load Commutated Inverter (LCI) fed motor operating in current-fed configuration.

DTC of a synchronous motor involves direct control of the flux linkages and electromagnetic torque by application of either current or voltage optimum switching vectors. Here, a VSI has been employed to achieve DTC. To start with, an equation-based model of wound field synchronous motor has been developed. This model is validated initially by starting this motor using damper windings from a normal 3-phase supply. Subsequently, this model is used to develop a Direct Torque Controlled VSI fed wound field synchronous motor drive. The dynamic response of the DTC drive has been analyzed under different transient operating conditions such as starting, load torque disturbance and reference speed change. The results obtained from the simulation of this model have been presented which show the feasibility and excellent performance of this proposed scheme. This portion of the work was completed in Part I of the project work.

Modeling and analysis of an LCI fed wound field synchronous motor drive has been taken up in Part II of this project. Many of the NTPC plants use LCI fed synchronous motor for the FD and ID fans and in this context, the analysis of this drive is very relevant. Starting and low speed operation of a LCI fed drive has been very interesting and challenging to simulate. Results obtained from simulation of starting, load and speed disturbance have also been presented here. The results obtained in Part I and Part II of the project gives a clear comparison of the two schemes employed for the control of a wound filed synchronous motor drive.