Monitor and Measure the Subharmonics Generated by a Phase Inverter

  The phase inverter is one of the more mysterious components in an amplifier, as it works to split an AC signal from a preamp into two equal strength current signals with opposite phases. Fender amps featuring blackface or silverface finishes were well known for using 12AX7 tubes in their phase inverters to mitigate distortion in this stage. 1. Subharmonic Current Measurement An AC-DC converter produces both characteristic and non-characteristic harmonics in its output, known as subharmonics, typically of frequencies lower than its fundamental frequency. Imperfect converter operation causes these subharmonics, which can significantly decrease IT performance. Idealistically, subharmonic current generated by converters should be additive. Unfortunately, due to various factors - IT quality, power loss in transformer core, magnetic flux ripple in rotor core etc - this cannot be guaranteed. To reduce subharmonic current, an optimal PI controller must be used to manage the phase inverter's input current. This involves not only limiting inductor current at its output, but also changing its phase relation to that of an underlying power amplifier's switching pattern. As illustrated in FIG. 11, solid signals (ias, ibs and ics) correspond to three phase currents while the dotted signals represent subharmonic d-q currents (ids -- sub s* and iqs -- sub s*). Each of the filtered signals are combined into the composite inverter output inverter current command before further modifications (task 618) are made using stationary frame fundamental voltage commands and subharmonic adjustment voltage commands. With an RF-coupled PI controller featuring on-chip phase detector and tunable delay cells, the dp/dq control process achieved 40dB SFDR for both fundamental and subharmonic components, providing stable peak current mode control across a broad range of operating conditions and helping the phase inverter deliver superior slope compensation performance. The original differential PM signal passes through tunable delay cells and the phase detector before it is converted to a rail-to-rail CMOS-type signal by way of conversion. From there, multiple-phase PM signals PS1 to PSq are distributed to multiple local phase generators 821 to 82p with individual control settings; these generate multiple phase signals consisting of original PM signals delayed with regard to original timing as well as fundamental/subharmonic adjustment voltage commands dependent upon respective control settings. 2. Subharmonic Frequency Measurement Switching subharmonics generated in parallel systems of high power low switch frequency auxiliary inverters have the ability to negatively influence the uniformity of fundamental wave power distribution, leading to equipment damage or accidents. Therefore, it is imperative to suppress their generation in these parallel systems, using an effective control method which effectively inhibits this generation, reduces circulating current between inverters in parallel connection and increases reliability of these inverter parallel systems in parallel connection. Conventional methods of suppressing switch subharmonics generation include increasing inductance value, decreasing switching frequency, and employing pulse width modulation technology. However, these techniques have limitations, including not being suitable for higher power inverters and necessitating large changes to inductor size. An increase in inductance increases switching losses for an auxiliary inverter and adds weight to the circuit, further compounding issues of reduced efficiency and weight. A new method is presented here for preventing switch subharmonics in parallel systems by taking advantage of voltage transformer (VT) characteristics to prevent their formation. At present, a control process 600 receives multiphase current signals (task 602) from an inverter and transforms these into stationary frame currents (idsss and iqss s), which contain both fundamental motor frequency component as well as subharmonic oscillation components. Filtered stationary frame currents extracted subharmonic oscillation components which then are fed into stationary frame current regulation (task 616). The d-axis subharmonic current error (ids - iq) is determined by subtracting ids s - iqs s from input voltage of gain element 310 and summarizing at summation element 306. From here, an output is fed directly into slope compensation (task 614) of each inverter in order to prevent any tendency towards subharmonic oscillation within one switching cycle. 3. Subharmonic Power Measurement Many power electronic devices produce both characteristic and non-characteristic harmonics due to improper converter operation. Non-characteristic harmonics, commonly referred to as subharmonics, typically consist of lower frequencies than that of the converter's fundamental frequency and can create DC/DC stability issues in power supplies; for instance, buck regulators with continuous inductor current and duty cycles above 50% can produce instability which in turn produces subharmonic oscillations that may be difficult to suppress. Subharmonics in electric motors can contribute to low frequency torque components that cause shuddering on speed changes, and introduce power losses which reduce overall efficiency. To mitigate such negative side effects, it is vital to measure both interharmonic and subharmonic spectral content to detect and identify them. This measurement method utilizes a spectrum analyzer to detect the frequency and shape/magnitude characteristics of an audio signal, along with any related shape or magnitude attributes. With this information in hand, a system can then identify each component's power as proportional to their frequency - using this knowledge, it can then identify their amplitude by subtracting it from total power of signal thereby providing its phase. As a rule, higher harmonic amplitudes generally carry more energy. Subharmonics however may produce much smaller amplitudes yet still cause significant disruptions in power systems. Therefore, it's vitally important that power measurements for these harmonics be conducted regularly in order to make sure that they do not interfere with other signals. In order to alleviate this issue, phase inverters can employ a control architecture which filters stationary frame current commands to remove low frequency subharmonics. An adaptive filter may be employed on input currents to abstract subharmonic current waveforms without impacting other control signals. One way of accomplishing this goal is through the implementation of a polar phase-interleaving architecture. This method balances out the phases of digital input signal paths to neutralise any subharmonics and reduce the matching network requirement. FIG. 1 shows one such phase-interleaved architecture implementation. Here, the differential PM signal 80 first passes through a current mode logic (CML) buffer before being converted into a rail-to-rail CMOS-type PM signal. This PM signal is then distributed to multiple phase generators 821 to 82p with different control settings, each independently creating multiple phase signals PS1 to PSq which combine original PM signal delay time with subharmonic components of fractions of carrier frequency. 4. Subharmonic Waveform Measurement Many power electronics devices produce unwanted subharmonics which lead to distortions in current waveform and voltage waveform, degrading overall power quality while heating up components which shortens their lifespans. Therefore, it is vital for accurate and efficient power systems that these subharmonics be measured regularly - this requires knowing the phase and frequency of currents within a system; which can be accomplished using harmonics software specifically developed for power harmonics measurements. However, these software tools fail to accurately measure subharmonic oscillation due to inaccuracy in data sampling and may fail to account for control dead time and voltage drop - making it hard to accurately ascertain voltage, current and phase in systems. Therefore, in this paper the steady-behavior of a differential boost DC-AC inverter used for producing sinewave AC voltage is examined using discrete time models with Floquet theory with quasi-static approximations as an approach; unwanted subharmonic oscillation in parameter space are identified along with its boundaries in parameter space. Step one of identifying subharmonic oscillation is determining the relationship between inverter's switching frequency and fundamental frequency, and phase-frequency relation of subharmonic oscillation. To do this, perform marker peak to reference level calibration on UUT to produce graph of multiphase inverter currents that contains sinusoidal characteristic of fundamental frequency along with low-frequency oscillation component that corresponds with undesirable subharmonics. After simulating an inverter and evaluating its control architecture and parameters, its fundamental and subharmonic adjustment voltage commands are combined into modified stationary frame voltage commands for feeding into a moving average filter and producing subharmonic current components in stationary frames; this filtering process is determined by window size. Results of the analysis demonstrate that a moving average filter can accurately estimate stationary frame fundamental and subharmonic current components, providing effective estimations that can be used to control an inverter. Furthermore, different pulse width modulation strategies were evaluated and their effects are also compared.