title: Contributions to the commutation analysis of uncompensated single phase universal motors
Creators: name: Willig, Matthias
Publisher: name: University of Glasgow
Year: 2013
Subjects: subject: Universal Motors, commutation, simulation, carbon brushes, equivalent circuits.

description: In this thesis various aspects of the electromagnetic effects influencing the commutation of uncompensated single phase universal motors are investigated. An introduction to the subject as well as a review of significant literature on the subject are given. The literature review includes classical text books about commutator motors as well as more recent publications about the mathematical analysis of the commutation of universal motors. Subsequently, the analysis of the most important inductances of the machine is outlined that comprises the analytical and numerical calculation as well as the measurement of the machine inductances using different measurement techniques. Moreover, a brush model for commutation analysis of small commutator motors is presented. Equivalent circuits of the brush are established for the cases of one coil shorted and two coils shorted by one brush, and a strategy to obtain their elements is proposed. This uses a dedicated finite difference program to calculate the effective brush resistance between all pairs of terminals. The treatment of the boundary conditions is critical in this process. The resulting terminal resistances are regarded as combinations of a set of internal resistances and this nonlinear relationship is inverted to obtain the internal resistors using a modified Newton-Raphson method. Results are presented showing the influence of anisotropy and geometry, and a simplified example of commutation analysis using the model established is given. In the next step the arcing phenomenon in analysed mathematically. Equations are given for the pre-arcing phase, the arcing phase and the post-arc oscillation. Equivalent circuits for the different phases are proposed and the equations are derived strictly from a circuit point of view. In the analysis a constant arc voltage (confirmed by experimental data and literature on the subject) and a minimum uncommutated residual current necessary for arc ignition are assumed. Those quantities are adopted from reviewed literature and used in the calculations. The design of a motor test bench is described that allows to measure the motor performance according to the principle of the reaction dynamometer. The load machine is mounted on airbearings to minimize possible torque errors in the measurements. A measured torque speed characteristic of a universal motor is shown as well as the torque as a function of the motor current. These measurements were carried out at reduced motor voltage to keep the shaft speed within reasonable limits. Furthermore, theoretical and experimental investigations are carried out in order to estimate how strongly certain rotor coils undergoing rapid current changes affect each other due to mutual coupling and how the mutual coupling changes in the presence of a damping field winding. Several FEA simulations are performed in order to get an insight into the flux pattern if rotor coils are acting on each other and the field winding is allowed to impose its damping effect on the rotor coils. Simple AC measurements are performed as well as di/dt - tests using a more complex oscillating circuit for measurements of the change of the di/dt of a rotor coil with and without the presence of an active field winding. Additionally, investigations arecarried out in order to analyse the influence of power cord and source impedances on the ability of the field winding of an uncompensated universal motor to damp flux variations caused by the commutation process. The motor is regarded as a harmonic generator with the power cord and the source impedance acting as a load. Rotational tests are carried out with different loads connected to the field winding and the Fourier spectrums of the field voltage are evaluated. In the final stage a simulation model is described that uses deductions from the previous chapters and that simulates the electromagnetic behaviour of the machine including the complex problem of brush commutation. Measured and calculated signals suitable for validation of the model were compared in order to evaluate the accuracy of the model with regard to motor performance and commutation analysis.