Contributions to the commutation analysis of uncompensated single phase universal motors

Willig, Matthias (2013) Contributions to the commutation analysis of uncompensated single phase universal motors. PhD thesis, University of Glasgow.

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Abstract

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 air
bearings 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 are
carried 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.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Universal Motors, commutation, simulation, carbon brushes, equivalent circuits
Subjects: T Technology > T Technology (General)
Colleges/Schools: College of Science and Engineering > School of Engineering
Supervisor's Name: Miller, Prof. T.J.E. and Cossar, Mr. Calum
Date of Award: 2013
Depositing User: Matthias Willig
Unique ID: glathesis:2013-4262
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 29 May 2013 08:00
Last Modified: 30 May 2013 10:46
URI: http://theses.gla.ac.uk/id/eprint/4262

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