| Abstract: |
The mam object of this thesis is to demonstrate how energy optimal control can be achieved for doubly fed induction motors, squirrel cage induction motors and synchronous motors with relatively simple control algorithm. The conclusions of the presented work are summarized herein as follows:Optimal performance characteristics of do ubi v fed induction motors An optimal control strategy has been presented based on theoretical and experimental works. The following conclusions are drawn:1- The developed torque of the motor consists of three torque components, two-induction torques and one synchronous torque. The two induction torques depend on the motor parameters, frequencies, the values of stator and rotor voltages while the synchronous torque depends on motor parameters, frequencies, the values of stator and rotor voltages and also depends on torque angle.2- It is found that, by adjusting the primary power factor to unity at all load torques and speeds, the motor develops a torque equals that of a conventional operation, but with lower losses.3- The results present the optimum values of the stator and rotor voltages, which result in a unity power factor operation and thus a higher torque productivity4- The proposed technique of the control does not require any shaftencoder, and therefore control system becomes simple.Optimal Performance Characteristics of Squirrel Cage Induction MotorsIn this thesis three methods of speed control are given. The first method is using a supply with constant V If. The second method is the optimum voltage and frequency control method (OVF method). Themethod is nearly constant over all range of load torque3- The efficiency in OVF A control method is higher than that of OVF one. third method is the optimum voltage, frequency and load angle control method (OVFA method. The following results are concluded:1- Using the constant (VI£) control method, the motor efficiency and torque productivity are small especially at light loads.2- The efficiency in both optimum voltage and frequency (OVF) control method and optimum voltage, frequency and torque angle (OVF A) control especially in heavy loads but the difference in stator CUlTent between them is not high in light loads.4- Using the OVFA control method, the torque per ampere of the motor increases over 200% at light loads than that one of constant (V If).5- The optimum values of the stator voltages and frequency at any load torque and speed in the three method of speed control are givenOptimum Performance Characteristics ofSvnchronous MotorsIn this thesis the efficiency maximization of synchronous motors is achieved by different ways such as:1- Unity power factor operation by controlling the field current with constant supply voltage. In this control method, the motor develops the same torque as the conventional one but with lower losses, which means a higher efficiency motor. This for load torques lower than certain values determined by equation (5.15).2-Unity power factor operation by controlling the supply voltage with constant field current. The losses are not much less than those of the conventional operation because the field losses are constant with load torque change. Further in this technique the required supplyvoltages are higher than the rated value when the field current is fixed at the rated value. Therefore, the field current must be fixed at a value lower than its rated value. This, of course, will reduce the maximum torque.3- Unity power factor operation by controlling both supply voltage and field current. from the analysis of this technique, the following results could be concluded:i) Unity power factor operation at certain load torque can be obtained by different values of supply voltage and field current. However, the optimum values of supply and field current will be those values which develop such torque but with minimum losses.ii) The efficiency of the motor over a wide range of load torque is nearly constant and also higher than conventional one.iii)The optimum values of the stator voltages and field current are also obtained.4- Field orientation control method, in this technique both supply voltage and field current are controlled in such a way that the angleobetween the stator and rotor fluxes is equal to 90 at all loadtorques. The analysis of performance characteristics and maximum torque which can be obtained by this method are presented. The losses and efficiency obtained by this technique are nearly the same compared with that obtained by unity power factor (by controlling supply voltage and field current). However the more efficient results in operation than that of UPF method are obtained.5- Operation at maximum torque point by controlling, both supply voltage and field current. The efficiency of the motor under this type of control is lower than those obtained from using both UPFoperation and field orientation methods. Further, at loads higher than 0.32 P.D, the efficiency using this technique is lower than that in the conventional operation and therefore this technique is undesirable.6- The optimum case. In this technique both the supply voltage and field current are controlled to develop a certain torque but with minimum losses. The results of this case show that, excellent performance characteristics of the synchronous motor have been obtained. The total losses are lower compared with all other techniques. It has been also found that, the performance characteristics of the machine in this method and those using field orientation method are the same at light loads. Further, the optimum values of the stator voltage and field current of this method are also gIven.
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