Reducing Switching Losses in Switched Reluctance Motor (SRM) Starting System

Transcription

1 International Research Journal of Applied and Basic Sciences 2013 Available online at ISSN X / Vol, 4 (7): Science Explorer Publications Reducing Switching Losses in Switched Reluctance Motor (SRM) Starting System NeamatAllah Monsef 1, SeyedZeinolabedinAbbaspour 2, AbdolrezaEsmaeli 3 1. Department of Electrical Engineering, SavehBranch, Islamic Azad University,Saveh,Iran 2. Department of Electrical Engineering, Hormozgan Branch, Islamic Azad University, Hormozgan, Iran 3. Plasma Physics and Nuclear Fusion Research School, Nuclear Science and Technology Research Institute, Tehran, Iran Corresponding Author aesmaeli@aeoi.org.ir ABSTRACT: In this paper, switching losses in Miller convertor of switched reluctance motor (SRM) are studied and a solution is presented for it. Switching losses are reduced with zero voltage or zero current switching. In this paper, switching losses are reduced in the system by presenting a new snubber circuit. After analyzing the desired circuit using software, the desired circuits were simulated. Simulation results confirm switching losses reduction in driving circuit of switched reluctance motor. Keywords: Switched Reluctance Motor, Switching, Switching Losses, Snubber INTRODUCTION Whereas electrical motors are of the most important tools in industry, preparation and presentation of the machine which has lower maintenance cost its highest efficiency and controllability are the goals of many researchers and specialists. In this regard, switched reluctance motor (SRM) have taken special position. This motor has gained high efficiency due to simplicity of different types production, high controllability, high heat tolerance and is high durability in many routine applications, industry and even aviation. For correct rotation and control of these motors, there is need for starter and controller (Sayeed M., 2000). Different starter circuits have been proposed for start of motor each having advantages and disadvantage. These starters are different from different perspectives: such as need for sensors, the number of spare parts, PWM switching controllability and complexity of soft switching control form etc. In general, these starters are classified into two groups of hard switching and soft switching. Different methods are available for control of SRM. In these methods, attempt has been made to use the fewest possible parts for better and more accurate control of these motors. On and off circuits in both motors include two parts(sayeed M., 2000). A. controller switch for connection of source voltage to coils to increase the desired current in them.

2 R V2 V1 L D 1 2 Figure 1.a simple circuit for connection and disconnection in each phase B. Creating a discharge route at time of switch connection so that energy stored in coils of the phase can be used elsewhere. This protects connection and disconnection transistors against strong current shock at time of disconnection. In figure 1, a very simple circuit has been drawn for switched reluctance motor (Murai Y, Cheng J. 1998). In the above figure, general equation which can be written for circuit and coil of stator is as follows: d λ V = Ri + (1) dt Where the voltage applied on any end of coil is V and pressure inside the coil is. If we assume magnetic value of wire as linear and waive resistance value, one can rewrite equation (1) as follows: di dl dθ V = L + i. (2) dt dθ dt Therefore, changes rate of the transformed energy is equal to: dl = + (3) dθ 2 V idt Li. di i. ω. dt The above relation indicates that varying input electrical energy with reluctance motor is spent for increase of magnetic field(li.di) and another part is converted to output mechanical energy. At the end of work of each phase, the stored energy should be retrieved. In this regard, different converters have been proposed for starting switched reluctance motors. Of these converters are diving of two switches in each pole (Sayeed M., 2000), transistor N+1 converter circuit(miller converter ) for N phase motor (Sayeed M., 2000,GairolaPriti S, Paliwal LN.2010), Bifilar converter, C-dump topologies (Sayeed M., 2000),resonance driving circuit ( ). In converter of two switches in each pole, two transistors and two diodes have been used for each phase. Transistors perform switching action so that the upper transistor of each pole is applied for controlling current of that pole and lower transistor prepares current route at proper time depending on position of rotor. Now, in order for energy of coils not to damage transistors at time of switch disconnection, we used diode so that surplus energy in coils is returned by diodes to power supply at proper time. In Bifilar converter, the number of switches and diodes has decreased. It is necessary to note that the primary and secondary converter is winded altogether to provide the maximum coupling on each one of the poles and this type of switching causes not to use space of rotor. In c-dump converters, energy stored in each phase is accumulated in a capacitor and is used on another phase in the next motion of motor. Different topologies of c-dump converter are also mentioned in reference. 1798

3 Basis of resonance converter circuit driving is on C-dump driving. The only difference is replacement of a series of elements with main transistor in C-dump driving to provide soft switching conditions. Energy of capacitor is transferred to DC power supply through resonance circuit. Proposed Method Base circuit of miller converter for three-phase motor is shown in figure 2. This driving is obtained from summarization of two switches driving in each pole (classic) and only one transistor is applied for controlling current which is common among all phases while all of the other transistors receive start command for correct function of each phase. The energy stored in coils is returned only by diode to the main source. Main disadvantage of miller converter is high switching losses like other converters with hard switching. At time of switching, since current of switch becomes zero due to its physical limitations, it cannot become zero instantaneously while voltage on the part increases, its current has not become zero and as a result, product of voltage by current becomes large value. Average power which is lost has direct relationship with the number of switching in time. Therefore, it is necessary to reduce switching losses by adding suitable parts to increase return of driver and reduce damage resulting from high heat of switches. It is possible to omit switching losses completely using zero voltage switching (ZVS) or zero current switching (ZCS)(Murai Y et. al., 1997, Murai Y, Cheng J. 1998, Ching TW et. al., 1998,PanCK.2003, Yu H, Song B, Lai J.1999, Cho IG et. al., 1997). In these methods, product of these two parameters becomes zero by keeping each one of two parameters zero i.e. voltage of two ends of switch or its current at time of switching. Main point is that lateral circuit should start resonance before switching of the main switch. In low working voltage and power, problem of losses is not so serious that one should reduce it. In high voltage and power, resonance and full execution of switching with zero parameters because its result is full deletion of switching losses in figure 3, miller converter with new snubber circuit is shown(panck.2003,yu H, Song B, Lai J.1999).Snubber is placed only on the current control switches which are turned on and off with high frequency and the upper switches control current of phase and have high switching frequency and inductive load, switching losses are considerable in them. To reduce switching losses in these switches, the following topology has been used: In this topology, auxiliaryswitch is turned on shortly before disconnection of the main switch and the main switch remains on for some seconds and this action causes to resonate the resonance elements in a fraction of main period and zero voltage switching is realized. This process is classified into 4 stages which are shown in figure 4. SA DAs D1 D2 D3 + Vd - C DA S1 D1s S2 D2s S3 D3s Figure 2. miller converter 1799

4 Figure 3. General scheme of miller converter with snubber circuit Figure 4. representation of function of snubber circuit in new starter in 4 working states In figure 4a, main switch is being conducted and nominal load current passes trough it and we assume that the system has reached stable state and voltage of two ends of capacitor has reached V d from the previous cycle. In figure 4- b, auxiliary switch S r is being conducted while main switch is also on but it is on the threshold of disconnection. When S r is turned on, resonance circuit including L r and C r is resonated and negative base voltage of capacitor increases to reach voltage (V d ). In figure 4-c, the main switch has been off and because negative base voltage has been higher than power voltage, diode D r1 and diode of transistor body are conducted and resonance conductive current is drawn by conductive load and this diode. By conducting the body diode, negative base voltage of the capacitor remains fixed in V d for a while. In figure 4 d, auxiliary switch is off and load current is closed from the shown route and is charged by this capacitor current for the first stage up to V d. Simulation and results A miller converter along with its control system is shown in figure 8. In a sample switch which has been used, bus voltage is 200 v, switch current is 20 amperes and switching frequency 1800

5 is about 10 khz. to show efficiency of snubber, we assumed the lower switches non-ideal and ascending and descending times of its current are 1 microseconds and 2 microseconds. In figure 6, waveform of voltage and current of two ends of current control switch in miller converter are given. As shown in figure 6, 200-volt voltage is applied to two ends of switch at time of switch connection and in this case, switching action hardly occurs. In figure 7, it is shown that switching losses have considerably decreased. The reason for variability of losses is that the system switching frequency is related to system hysteresis controller. Miller converter circuit with application of snubber along with its control system is shown in figure 8. Waveform of voltage and current of the current control switch at time of snubber circuit connection are shown in figure 9. It is clearly shown that when switch is turned off, voltage will be zero and when it is turned on, voltage will be applied with delay. At time of switch connection, voltage and current will not immediately collide with each other and it should be considered that switches are not also ideal. Here, we assumed that we have delayed disconnection of main switch for 10 microseconds. This action done by the following control system for snubber circuit switch is shown in figure 10. Figure 5. miller converter along with control system Figure 6. waveform of voltage and current of two ends of current control switch 1801

6 Figure 7. diagram of switching losses in miller converter Figure 8. Miller converter circuit with application of snubber along with control system. Figure 9. waveform of voltage and current of the current control switch at time of snubber circuit connection 1802

7 Figure 10. switching control system for model with converter with snubber Function of this circuit is such that when input is passed through edge detector which can be rising or falling and here, we consider falling edge because we want to see when a pulse used for main switch is turned off which can delay off state for 10 microseconds and we turn on snubber switch exactly when the edge appears and after this edge didn t appear and main switch circuit was turned off, this snubber switch remains on for other 10 microseconds and then, it is turned off. Figure 11 shows that voltage of capacitor increases and then the current is passed through the given routes so that its two ends remain zero for acceptable term at time of voltage disconnection and ZVT mode is realized for the main switch. In figure 12, waveform relating to switching losses for miller converter with application of snubber is shown. In this figure, it is clearly shown that its switching losses have been reduced compared with miller converter without snubber. Figure 11. function of ZVT mode for main switch 1803

8 Figure 12. waveform relating to switching losses for miller converter with application of snubber CONCLUSION In this paper, losses in miller converter were studied due to importance of switching losses in high power and voltage in SRM converter. Switching losses are usually reduced using switching structures with zero voltage or current. Here, new snubber was proposed to reduce losses. By doing simulation with MATLAB software, its effect on reduction of losses was observed. REFERENCES Ching TW, Chau KT, Chan CC A Novel Zero-Voltage Soft-Switching Converter for Switched Reluctance Motor Drives, Department of Electrical & Electronic Engineering The University of Hong Kong Pokfulam, Hong Kong, China, Cho IG, Kim WH, Yoo DW, Rim GH Novel Zero Voltage Transition PWM Converter for Switched Reluctance Motor Drives, PESC'97, pp , Gairola Priti S, Paliwal LN A New Powe Converter for SRM Drive IEEE, Murai Y, Cheng J, Yoshida M.1997.New softswitched reluctance motor drive circuit, in Proc. IEEE IAS, vol. 1, pp ,1997. Murai Y, Cheng J "a Simple Soft Switched Switched_ Reluctance Motor Drive" Industrial Electronics Society, IECON `98, pp PanCK.2003.ADSP-basedsoft-switchingconverterfedswitchedreluctancemotordrive, Master Thesis, Department of Electrical Engineering, National Tsing Hua University, ROC, Sayeed M Classification of SRM Converter Topologies for Automotive Application, SAE technical paper series number , March 6-9, Yu H, Song B, Lai J.1999.Design of a Novel ZVT Soft Switching Chopper, IEEE PowerElectronics Specialists Conference 1999, PESC'99, pp