Calum Cossar

Multiple sets of solutions for harmonic elimination PWM bipolar waveforms: analysis and experimental verification

Multiple sets of solutions for the selective harmonic elimination pulse-width modulation method for inverter control exist. These sets present an independent solution to the same problem but further investigation reveals that certain sets may offer an improved overall harmonic performance. In this paper, a minimization method is discussed as a way to obtain these multiple sets of switching angles. A simple distortion harmonic factor that takes into account the first two most significant harmonics present in the generated waveform is considered in order to evaluate the performance of each set. The bipolar waveform is thoroughly analyzed and two cases are considered; single-phase patterns which eliminate all odd harmonics and three-phase counterparts which eliminate only the nontriplen odd harmonics from the line-to-neutral pattern but such harmonics are naturally eliminated from the line-to-line waveform. Experimental results support the theoretical considerations reported in the paper.

Design of a synchronous reluctance motor drive

A segmental-rotor synchronous reluctance motor is used in a variable-speed drive with current-regulated PWM control. The low-speed torque capability is compared with those of an induction motor, a switched reluctance motor, and a brushless DC permanent magnet (PM) motor of identical size and copper weight. The results suggest that many of the desirable properties of the switched reluctance motor can be realized with the synchronous reluctance motor, but using standard AC motor and control components. The torque capability is lower, but so is the noise level.<<ETX>>

On Attaining the Multiple Solutions of Selective Harmonic Elimination PWM Three-Level Waveforms Through Function Minimization

Selective harmonic elimination pulsewidth modulation techniques are some of the control methods used in voltage/current source converters. However, challenges such as the task of finding all the multiple sets of solutions of the switching angles for a given problem may be difficult to deal with. In this paper, a direct minimization of the nonlinear transcendental trigonometric Fourier functions in combination with a random search is discussed. The unipolar (three-level) waveform is used to illustrate the proposed method confirming its ability to find multiple sets of solutions, including a case where 51 angles are sought for single- and three-phase applications. A simple harmonic distortion factor is studied for each set of solutions to assess their performance against the noneliminated harmonics. The results presented both at theoretical and experimental level are in close agreement and confirm the robustness of the proposed approach.

Improved digital current control methods in switched reluctance motor drives

This paper proposes a method to avoid current feedback filters in fast digital-based current loops in switched reluctance drives. Symmetrical pulsewidth modulation (PWM) and synchronized sampling of the phase current allow a noise-free current sampling with no antialiasing filter. This paper also proposes more efficient methods to chop the two transistors in the asymmetric inverter used with switched reluctance drives. A fast field-programmable gate array (FPGA)-based test system is used for validation of the new methods. Test results show a significant improvement in dynamic and steady-state current loop control compared with traditional methods. The new chopping method is found to reduce the switching losses and increase the drive efficiency.

Line-start permanent-magnet motor single-phase steady-state performance analysis

This paper describes an efficient calculating procedure for the steady-state operation of a single-phase line-start capacitor-run permanent-magnet motor. This class of motor is beginning to be applied in hermetic refrigerator compressors as a high-efficiency alternative to either a plain induction motor or a full inverter-fed drive. The calculation relies on a combination of reference-frame transformations including symmetrical components to cope with imbalance, and dq axes to cope with saliency. Computed results are compared with test data. The agreement is generally good, especially in describing the general properties of the motor. However, it is shown that certain important effects are beyond the limit of simple circuit analysis and require a more complex numerical analysis method.

A General Model for Estimating the Laminated Steel Losses Under PWM Voltage Supply

The new model is based on a modified Steinmetz equation and employs a hysteresis-loss multiplicative coefficient and a combined coefficient for eddy-current and excess losses, both coefficients being variable with induction and frequency. The material model coefficients are first identified through multifrequency tests with sine-wave excitation. The iron-loss increase due to pulsewidth-modulation supply is estimated using global waveform parameters of the nonsinusoidal voltage. The study includes three different grades of non-grain-oriented electric steel. The data cover a wide range of fundamental frequency from 10 to 600 Hz and induction from 0.05 to 2 T. The errors of the computational model are small at relatively low fundamental frequency and increase thereafter. The main advantages of the model are its simplicity of use and minimal data requirements.

End Space Heat Transfer Coefficient Determination for Different Induction Motor Enclosure Types

In this paper, the determination of the end space induction motor heat transfer coefficients is presented, and the methodologies used are examined closely. Two ldquoad hocrdquo prototypes have been built and a test bench completed. This paper reports the setup of the test procedures and results obtained in detail. As the end windings are the hottest points of the motor, particular care has been devoted to the determination of the heat transfer coefficient concerning the end-winding structure. The results obtained are of fundamental importance for the determination of the thermal resistances between end windings and end caps. These can then be used in thermal networks usually adopted in thermal model analysis.

A General Model of the Laminated Steel Losses in Electric Motors with PWM Voltage Supply

A procedure is described for identifying a general mathematical model of core losses in ferromagnetic steel when the voltage supply is non-sinusoidal, i.e. PWM inverter-fed type. This model has a hysteresis loss multiplicative coefficient variable with frequency and induction and a combined coefficient for eddy-current and excess losses that is also variable with frequency and induction. The effect of the PWM supply voltage over the core losses is modeled using factors that depend on the average rectified and rms voltage values. Validation was performed on a number of different samples of non-grain oriented fully and semi-processed steel alloys.

A Vibration-Based Condition Monitoring System for Switched Reluctance Machine Rotor Eccentricity Detection

We propose a condition monitoring strategy for the detection of rotor eccentricity in switched reluctance machines. It uses vibration measurements and harmonic analysis to generate a Fourier series for the vibrations. Assessment of the change in key vibration components allows development of an eccentricity index that can give an indication of increasing eccentricity. The method discriminates between static and dynamic eccentricity. We have tested the method in simulation, using finite-element analysis, and verified it experimentally. Although mechanical resonance is an issue, a method of measurement over a number of speeds illustrates that it can be overcome.

Fault-tolerant operation of single-phase SR generators

This paper studies fault-tolerant operation of multipole single-phase switched reluctance generators (SRGs), in particular, an 8/8-pole switched reluctance machine. The multipole single-phase SRG system is advantageous for reduced cost and higher efficiency compared to polyphase equivalents. However, using the classical phase-leg topology, a phase fault may prevent generating operation completely, since redundancy in the number of phases does not exist like polyphase systems. A new power converter topology which connects two coil banks in parallel is proposed for higher fault tolerance with minimum additional cost. Faulty coils can be disconnected with the proposed converter and the SRG can continue generating operation after coil faults with reduced output power. Output power per coil current under faults is studied. Open- and short-circuit coils are studied through linear analysis, finite-element analysis and static torque measurement. Generated currents under faults with the proposed converter are measured. The capability of the system to disconnect faulty coils dynamically is also shown.