Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Electrical Engineering


A portion of this study analyzes the starting conditions of 5,000 HP motors connected to a 4,160-volt bus. The power to the motors is supplied through a transformer with a nameplate rating of 12/16/20/22.4 WTA, 115-4.16 kV. This study will include from one to five, 5,000 HP motors although the present installation consists only of two motors. The transformer may supply power for five motors running at a steady-state, full-load condition without exceeding the maxi mum power rating. The equivalent impedance of the 115 kV system has been determined from the short-circuit on the 115 kV side of the transformer. The transformer impedance was given as 7.41 per cent on the 12 MVA rating which enabled the impedance value in ohms to be determined. The impedance of the cable has been calculated from information such as the length and number of cables per phase, known for this installation. The method derived by Lathrop et. al. has been followed in the analysis of the cases of the multi-operation of motors in parallel Chapter IV. In this method, special consideration has been given to the changes of voltage and current with speed, during acceleration process. These changes in voltage and in current are mainly due to the changes in motor impedance with motor slip. The calculations of voltages and currents during the acceleration period is followed by the calculation of available acceleration torques and then by calculation of the incremental acceleration time. Finally, total starting time for the motor under acceleration is obtained. The previously mentioned calculations have been done with the help of a digital computer. The program written is simple and easy to apply to a one-motor problem or to a multi-motor problem. The only information required for this application is the motor current and torque curves versus speed, and the pump torque versus speed. Also, the total inertia of motor and pump is required. This program has been used for the analysis of the different cases, detailed in Chapter IV, in order to determine how the sequential starting of the motors affect the motor bus voltage, as well as the available motor torque and consequently the acceleration. Based on the assumption that a 20 percent voltage drop is tolerable on the motor bus at the instant of starting, the study of these cases helps to establish the best scheme possible for sequential starting of these motors. Therefore, when the motor bus voltage is below the 80 per cent requirement for satisfactory operation of the system, shunt capacitors have been considered. These static capacitors will give the kVAR support specially required at the time of starting the motor. After acceleration, the capacitor bank may be disconnected from the bus depending on the number of motors in operation.

Library of Congress Subject Headings

Electric motors, Induction
Electric machines



Number of Pages



South Dakota State University