Implementation of a Single-phase Unipolar Inverter Using DSP TMS320F241

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1 U J.T. 8(4): 995 (pr. 25) Implementation of a Singlephase Unipolar Inerter Using DSP TMS32F24 Narong phiratsakun, Sanjia ao Bhaganagarapu and Kittiphan Techakittiroj Faculty of Engineering, ssumption Uniersity Bangkok, Thailand bstract This paper presents the design and implementation of a singlephase inerter that produces a symmetric ac output oltage of desired magnitude and frequency. diode bridge rectifier is used to rectify the ac line oltage. Unipolar technique is employed to control the output oltage magnitude and frequency. The digital signal processor (DSP) of Texas Instruments TMS32F24 is used for the implementation of the inerter Keywords: Singlephase inerters, digital signal processor (DSP), unipolarswitching scheme, output oltage control of singlephase inerters, Matlab simulation.. Introduction Singlephase inerters are widely used in industrial applications such as induction heating, standby power supplies and uninterruptible supplies. block diagram representation of a singlephase inerter is gien in Fig.. The inerter consists of four switching deices (represented as ideal switches) connected in the form of a bridge. The control scheme is implemented using TMS32F24 DSP controller (Techakittiroj et al. 23) dc GS GS 4 Fig.. Singlephase inerter In the unipolar switching scheme (Ned et al. 995), the output oltage changes between positie and zero, or between zero and negatie oltage leels. To produce a sinusoidal output o = B GS 3 GS 2 B oltage waeform of ariable frequency and amplitude, a sinusoidal reference signal ( ref ) is compared with the triangular waeform ( tri ). The amplitude modulation index (m a ), which controls the rms alue of the output oltage, is defined as m a = ^ ^ ref tri (.) The ref and tri in equation (.) refer to the peak amplitudes of the signals. Leg and B of the fullbridge inerter are controlled separately by comparing tri with ref and tri with ref. The resulting waeforms are used to control the switches as follows: In leg : (.2a) ref > tri : GS on and ref < tri : GS4on and In leg B: (.2b) ref > tri : GS3 on and ref < tri : GS2 on The signals obtained are shown in Figs. 2a and 2b. Note that GS4 and GS2 will be automatically created as the inersion of GS and GS3, respectiely. ^ ^

2 generated from ref and carrier The dead band (T dead ) calculated by phiratsakun (24) and TI (2): T dead = Period x Dead Band Prescaler xpu clock (.4) ount up _PEIOD ount down (a) generated from ref and carrier (b) Fig. 2 signals (a) For leg (b) For leg B The comparison of the reference sinusoidal signal with the triangular waeform is done in the generator of the DSP to generate the control signals for the switching deices along with the inerted signals with the required dead band. 6bit counter register is used to measure the frequency of the triangular wae. centered symmetric signal is used which has maximum count up of 2 6 and count down 2 6. The signals and the control signals generated are gien in Fig. 3. ount up, count down, switching time and dead band are calculated as shown by phiratsakun (24) and TI (2): ount up = ount down = _PEIOD T sw = 2x_PEIODxPU clk (.3) = 2x_PEIODx5 ns a Dead Band b Fig. 3. Symmetric waeform generation 2. ircuit Description The schematic diagram of the inerter circuit implemented is gien in Fig. 2. It has two parts, the control circuit and the power circuit. The shaded part is the control circuit containing the DSP controller TMS32F24 that generates the signals and also proides soft start function. Set point for the modulation index and frequency are set by a computer through serial interface. The low pass filter was designed in such way that the output oltage waeform of the inerter is sinusoidal. To start with, the singlephase inerter with the unipolar switching scheme is simulated using simulink in Matlab and its performance is studied. Later on, the singlephase inerter was implemented using DSP TMS32F24 and its performance was studied. comparison is made of the results obtained through simulation and experimental work under the same operating conditions. 92

3 i NO elay contact Singlephase Diode bridge singlephase inerter L filter 22 5 Hz Supply 45 Softstart ariable ac oltage ariable frequency Modulation set point Unit Interface Sine wae calculation generation Dead time selection Softstart time elay oil: 6 D Frequency Set Point Serial TMS32F24F DSP ontroller Switching Frequency Fig. 2: Oerall schematic diagram of singlephase inerter The tested conditions for the simulation and experimental work are: P rated LN : kw : 22 dc : µf L : 2 mh : µf m a :.8 ref : 5 Hz tri : 7.5 khz T dead : 5.6 µs 3. Simulation Study The schematic for the simulation of unipolar singlephase inerter is gien in Fig. 3. The waeforms of output, filtered output oltage and filtered output current, obtained through simulation are shown in Fig B Diode Bridge.8 Modulation index dc dc_scope Inerter pulses B Fig. 3. MTLB simulation of singlephase inerter _pulse 2 mh L filter ab To Workspace uf il IL To Workspace3 filter lock filter To Workspace out time To Workspace2 output () Filtered output () Output current() Simulation waeforms Time second Fig. 32. waeforms of unipolar inerter (simulation) 4. Experimental The schematic diagrams for the power circuit and control circuit are shown in Figs.4 and 42, respectiely. TMS32F24 DSP controller (Techakittiroj et al. 23) with and output ports is used for the implementation. The four signals hae been fed to the optocoupler (6N37) for the isolation of gate driers. Four discrete MOSFETs (IFP45) are used as switching deices. I2, I gate drier is used to drie the MOSFETs in the upper and lower legs of the inerter. In the power circuit of Fig. 4 the singlephase diode bridge rectifier (66 GW) with a µf D link capacitor ( dc ) is connected to the singlephase ac power supply, 22, 5 Hz to proide 93

4 softstart 22 ohm/ 2 W IFP45 IFP45 LN 22 5 Hz dc dc uf/35 snubber G. uf S S 3 Filter G 3 IFP45 IFP45 G 4 G 2 S 4 S 2 Fig. 4. Power circuit 6N37 Optocouplers 5 5 current. The rms output oltage is measured by rms meter and is found to be 22. Gate drier 5 5 gate pulses 5 5 I2 GS GS4 G G4 DSP ontroller 5 5 G3 5 I/O G2 GS3 5 I2 GS2 elay: OMON G2E oil: 5 D NO elay contact (a) 5 Fig. 42. ontrol circuit a constant dc source to the inerter. The dc must hae high oltage rating (greater than 35 ). snubber capacitor of.µf is connected across the inerter to protect it from high surge oltages. soft start resistor is used to reduce the peak inrush currents, thereby reducing the stress on rectifier diodes and dc. Photographs of the unipolar singlephase bridge inerter constructed in the laboratory are shown in Figs. 43 (a) and (b). Fig. 44 shows the recorded waeforms of unfiltered sinusoidal, filtered output oltage and (b) Fig. 43. Laboratory unipolar inerter board (a) control circuit (b) power circuit 94

5 output () Filtered output () Output current() Waeforms Time second Fig. 44. waeforms of unipolar inerter obtained through experimental work onclusion laboratory model of a unipolar singlephase inerter was successfully implemented using DSP TMS32F24 and tested. The inerter unit consists of four, discrete MOSFETs connected as a bridge and drie with a low cost drier. The experimental result matched with simulation results. lthough any parameters are adjusted for giing fundamental frequency rms output oltage of 22 at 5 Hz. With this unipolar inerter unit, further research on singlephase inerters can be carried out such as soft switching inerters, singlephase UPS and singlephase induction motor dries. eferences phiratsakun, N. 24. Effect of Sampling Time, Switching Frequency and Output Quantization Step on ector ontrolled age Motor, Master Thesis, ssumption Uniersity, Bangkok, Thailand. Ned, M.; Undeland, T.M.; and obbins, W.P Power Electronics onerters, pplications, and Design, 2 nd ed., John Wiley & Sons, New York, NY, US. Techakittiroj, K.; phiratsakun, N., Threeithayanon, W.; and Nyun, S. 23. TMS32F24 DSP Boards for Powerelectronics pplications. U J.T. 6:6873. TI. 2. TMS32F243/F24/242 DSP ontrollers eference Guide, system and Peripherals, Literature Number: SPU276. Texas Instruments, Texas, US. 95