Analysis and Design of a Novel Three-Level LLCC Inverter ...

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supply, a novel scheme was developed by combining a singlephase three-level PWM inverter and a LLCC filter shown in Fig. 1. It is designed in a way to reduce the total harmonic distortion (THD) of the motor voltage, and to compensate locally for the reactive power of the motor, since the motor is supplied via a long cable due to its displacement from the power supply. metallic blocks connected with eight piezo-ceramic multilayer stacks with alternating polarization. The structure is excited by the piezo stacks at its mechanical eigenfrequency which is designed for 35 kHz (called tangential mode) so that each metallic block would oscillate in the plane of the ring. The structure is able to oscillate also orthogonally to the surface of the stator rings and rotor disk (normal mode), Tangential direction CsT L sT L mT C mT R pT C pT L pT u CpT R mT Inverter filter motor Tangential piezoelement Tangential Mode Normal direction Normal piezoelement Rotor disk Metallic block L sN L mN C mN CsN Normal Mode R pN C pN L pN u CpN R mN Inverter filter motor By combining the three-level PWM inverter and aforementioned resonant filter, the driving voltage of the multi-mass piezoelectric motor can even be varied in a suitable frequency range for characterizing the built actuators; though the output filter shows an optimized filter performance at minimized volume and weight, compared to the classical resonant inverters. Voltage and current cascade controls were designed to act as inner control loops of the whole piezoelectric brake actuator control system. A hysteresis current controller [13] [14] is employed in the inner control loop to improve the dynamic behaviour. Summarily, the proposed single-phase three-level LLCC inverter is studied in this contribution, and the employed cascaded digital voltage and hysteresis current controller are discussed in detail. Simulations as well as experimental results are presented. Fig. 2 Operating Principle of a Multi-Masse Ultrasonic Motor excited by normal piezo stacks at the same frequency as the tangential mode, but with appropriate phase shift. Thus, elliptical movements of the metallic blocks will result that generate thrust by temporary clamping of the disks [1] [5]. The operating peak voltage of the fundamental for both modes is 270V, and the maximum power consumption is 1.5kW for tangential mode and 60W for normal mode. II. PRINCIPLE OF MOTOR OPERATION AND DRIVING SCHEME The multi-mass ultrasonic motor (MM-USM) consists of two pairs of stator rings and two rotor discs connected to the shaft. One rotor disc is sandwiched between a pair of stator rings, as shown in Fig. 2. Each stator ring houses eight Fig. 3 Frequency response u Cp /u filter
Fig. 4 Cascaded voltage and hysteresis current control scheme III. THREE-LEVEL INVERTER WITH LLCC-FILTER The topology shown in Fig. 1 consists of a hybrid threelevel PWM inverter followed by a LLCC-type filter with an isolating transformer and the respective capacitances and inductances of the wiring cables being integrated as part of the filter. The left leg of the PWM inverter is composed of four MOSFETs (S1–S4) used to generate high frequency PWM output voltage. S5 and S6, forming the right leg of the PWM inverter, are employed to alternate the polarity of the exciting waveform at the fundamental frequency. The frequency response of LLCC-filter is shown in Fig. 3. By its inspection the advantages of LLCC-filters, such as robustness to parameter variation e.g. of the piezo capacitance and simple controllability compared with LC-filters become obvious. The second resonant frequency of the filter can be set at a higher frequency compared with the inverter with resonant modulation, since the important 3rd and 5th harmonics are largely reduced by the PWM strategy so there is no need to suppress these frequencies by the filter. As a result, smaller and lighter filter components can be used, and the dynamic response is improved. Moreover, the bode diagrams of Fig. 3 show that the operating frequency of designed three-level PWM controlled LLCC-type filter can be varied within a range of 20-60kHz without any need to adopt the filter components, compared to narrow band 30-40kHz with the design of the resonant controlled inverter. This advantageous property can be utilized, if an actuator is to be characterized or driven versus a large bandwidth. IV. CONTROL SCHEME The power supply control loop acts as inner control loop of the whole piezoelectric brake actuator control system. Therefore the task of the inner loop is to control the voltage amplitude and operating frequency of the tangential and normal modes of the motor along with the phase angle between these two modes. A cascade voltage and current control scheme was designed to satisfy the brake system requirements and provide the flexibility for commissioning. As shown in Fig. 4, the reference variables are voltage u Cp * (u Cp,s *, u Cp,c *), frequency f p * and phase angle Dj, the feedback signals are current i filter voltage u Cp and voltage u Pi of the piezoelectric element. By means of a demodulation, the feedback signals are decomposed into sine and cosine components, yielding the Fourier coefficients at fundamental frequency. In this way, the requirements of signal processing are largely reduced, because the amplitudes of these signals are only slowly varying compared with the fundamental oscillation. Fig. 5 Idealized switching network of three-level inverter
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Analysis and Design of a Novel Three-Level LLCC Inverter ...

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