Contributions to the commutation analysis of uncompensated single phase universal motors

T Technology (General)

Willig, Matthias ; Miller, T.J.E.

University:University of Glasgow

Department:School of Engineering

关键词: Universal Motors, commutation, simulation, carbon brushes, equivalent circuits;   

Others  :  http://theses.gla.ac.uk/4262/1/2013WilligPhD.pdf

来源: University of Glasgow

【 摘 要 】

In this thesis various aspects of the electromagnetic effects influencing the commutation ofuncompensated single phase universal motors are investigated. An introduction to the subjectas well as a review of significant literature on the subject are given. The literature reviewincludes classical text books about commutator motors as well as more recent publicationsabout the mathematical analysis of the commutation of universal motors.Subsequently, the analysis of the most important inductances of the machine is outlined thatcomprises the analytical and numerical calculation as well as the measurement of the machineinductances 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 coilsshorted by one brush, and a strategy to obtain their elements is proposed. This uses a dedicatedfinite difference program to calculate the effective brush resistance between all pairs ofterminals. The treatment of the boundary conditions is critical in this process. The resultingterminal resistances are regarded as combinations of a set of internal resistances and thisnonlinear relationship is inverted to obtain the internal resistors using a modifiedNewton-Raphson method. Results are presented showing the influence of anisotropy andgeometry, and a simplified example of commutation analysis using the model established isgiven.In the next step the arcing phenomenon in analysed mathematically. Equations are given forthe pre-arcing phase, the arcing phase and the post-arc oscillation. Equivalent circuits for thedifferent phases are proposed and the equations are derived strictly from a circuit point ofview. In the analysis a constant arc voltage (confirmed by experimental data and literature onthe subject) and a minimum uncommutated residual current necessary for arc ignition areassumed. 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 performanceaccording 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 speedcharacteristic of a universal motor is shown as well as the torque as a function of the motorcurrent. These measurements were carried out at reduced motor voltage to keep the shaft speedwithin reasonable limits.Furthermore, theoretical and experimental investigations are carried out in order to estimatehow strongly certain rotor coils undergoing rapid current changes affect each other due tomutual coupling and how the mutual coupling changes in the presence of a damping fieldwinding. Several FEA simulations are performed in order to get an insight into the flux patternif rotor coils are acting on each other and the field winding is allowed to impose its dampingeffect on the rotor coils. Simple AC measurements are performed as well as di/dt - tests usinga more complex oscillating circuit for measurements of the change of the di/dt of a rotor coilwith 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 theability of the field winding of an uncompensated universal motor to damp flux variationscaused by the commutation process. The motor is regarded as a harmonic generator with thepower cord and the source impedance acting as a load. Rotational tests are carried out withdifferent loads connected to the field winding and the Fourier spectrums of the field voltageare evaluated.In the final stage a simulation model is described that uses deductions from the previouschapters and that simulates the electromagnetic behaviour of the machine including thecomplex problem of brush commutation. Measured and calculated signals suitable forvalidation of the model were compared in order to evaluate the accuracy of the model withregard to motor performance and commutation analysis.