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JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRICAL ENGINEERING REDUCTION OF SWITCHING LOSSES IN BLDC MOTOR USING SOFT SWITCHING TECHNIQUES 1 MR. D. K. PATEL, 2 MR. J. B. SARVAIYA 1 M.E. 2 Assist. Ivth Sem, Electrical S.S.Engineering College Bhavnagar, , Professor, Electrical Department S.S.Engineering College.Bhavnagar, India, [email protected],[email protected] ABSTRACT : A BLDCM can be described as an inverted brush dc motor with its magnet being the rotor and its stationary windings forming the stator. BLDC Motor is fast gaining popularity because of their superior performance in terms of high efficiency, fast response, light weight, precise and accurate control, maintenance free operation, brushless construction, the operating characteristic of a motor is important for its control, modeling and optimum performance. It is usually supplied by a hard switching PWM inverter, which normally has number of detrimental effect is produce. In order to reduce the effects, many soft switching techniques are designed or develop. The purpose of this paper is to build a modeling and simulation of hard switching and soft switching inverter for estimate or discriminate the switching losses in BLDCM using soft switching techniques, for see reduction in switching losses by using MATLAB SIMULINK. Index Terms :BLDCM (Brushless DC Motor), Hard And Soft Switching, PWM Inverter, MATLAB. I. INTRODUCTION In this paper we propose a simulation model for an entire BLDCM drive and its actual implementation. In this model the trapezoidal back EMF waveforms are modeled as a function of rotor position, so that position can be actively calculated according to the operating speeds. Moreover, the switching function concept is adopted to model the PWM inverter. This in turn results in obtaining the detailed voltage and current waveforms [1]. BDCM has been widely used in industrial applications because of its high power density, As compared to a conventional DC brush motor, BLDC motors are DC brush motors turned inside out, so that the field is on the rotor and the armature is on the stator. In BLDC motor, field excitation is provided by a permanent magnet and commutation is achieved electronically instead of using mechanical commutators and brushes. In BLDC motor, the mechanical ‘rotating switch’ or commutator/brush gear assembly is replaced by an external electronic switch synchronised to the rotor's position.There are two main types of BLDC motors; trapezoidal type and sinusoidal type.In the trapezoidal motor the back-emf induced in the stator windings has a trapezoidal shape and its phases must be supplied with quasi-square currents for ripple-free torque operation. The sinusoidal motor on the other hand has a sinusoidal shaped back-emf and requires sinusoidal phase currents for ripple-free torque operation.The shape of the back-emf is determined by the shape of the rotor magnets and the stator winding distribution. The sinusoidal motor needs a high resolution position sensor because the rotor position must be known at every time instant for optimal operation. It also requires more complex software and hardware. The trapezoidal motor is a more attractive alternative for most applications due to its simplicity, lower price and higher efficiency. [3] Figure 1 Basic block diagram of BLDC II. CONSTRUCTION OF BLDC MOTOR Figure 2 Construction of BLDC Motor ISSN: 0975 – 6736| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 147 JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRICAL ENGINEERING · LA · RA s1 s3 a · Vdc s5 · LB · RB · ia ea · · eb · BLDC works on basic principal of Motor, when current carrying conductor placed in magnetic field it experiences force and its direction is define by Fleming left hand rule and force is define by F=BIL. In BLDC, stator has winding but rotor has no winding so it does not require brushes. Rotor has permanent magnet in cylindrical form which continuously magnetic field generating. BLDC Motor requires controlled alternating cycle voltages which are generated by SPWM Inverter. The main function of inverter is converts fixed direct cycle voltage in to variable alternating cycle voltage. SPWM means sinusoidal pulse width modulation technique is used to operate inverter and reduces harmonic contents. In SPWM, pulse width is changing but amplitude of pulse is not changing. SPWM is one of type of PWM techniques among them SPWM is widely used in industries for controlling purpose. BLDC classified in to two categories, trapezoidal type and sinusoidal type. BLDC is classified by generating back electro motive force (EMF) shape if it is trapezoidal then BLDC is trapezoidal type. The trapezoidal Motor is a more attractive for most applications due to its simplicity, lower price and higher efficiency. Hall sensor uses to sense the position of rotor of BLDC Motor and that signals are given to inverter circuit for controlling purpose. Here we have used back electromagnetic force wave instead of Hall sensors for sensing the position of rotor of BLDC Motor. Back electromagnetic force is depending on radius of rotor, angular velocity of rotor, length of rotor, magnetic flux density of rotor, and no. of turns per phases. III. OPERATION OF BLDC MOTOR The three phase BLDC Motor is operated in a twophase-on fashion, i.e. here third phase is off, so two phases produce the highest torque, which two phases are energized depends on the rotor position. The signal generated from the position sensors in the form of three digit number that change over every 60 degree (electrical deg.) as shown in Fig 3. The figure shows also ideal current and BACK EMF waveforms · b · ib c · ic · LC · RC s4 s2 s6 n ec · · Figure 4 BLDC driver circuit Table 1 Switching sequence Switchin Se Pos. sensor g q. H1 H2 H3 No Interval( s. 0-60 0 1 0 0 deg.) 60-120 1 1 1 0 Switch Phase current 120-180 2 0 1 0 Q3 Q6 Off + - 180-240 3 0 1 1 Q3 Q2 - + off 240-300 4 0 0 1 Q5 Q2 - Off + 300-360 5 1 0 1 Q5 Q4 off - A Closed Q1 Q4 + B C - Off Q1 Q6 + Off - + IV. MATHEMATICAL MODEL OF BLDC MOTOR The 3-phase star connected BLDC Motor can described by following four equation from the Figure 3, ............. (1) .............. (2) .............. (3) .................. (4) Where denote the phase-phase voltages, phase currents and BACK EMF respectively, in three phase a, b and c. = Resistance of winding per phase in ohm = Inductance of winding per phase in m H = Electrical Torque in mNm Load Torque in mNm Figure 3 Ideal back-EMFs, phase currents and position sensor signals Rotor Inertia in g Friction Constant in Nm s (assumed value) ................................................. (5) ISSN: 0975 – 6736| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 148 JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRICAL ENGINEERING ................................................ (6) ............................................. (7) .......... (8) Where are the BACK EMF Constant and the Torque Constant, .................................................... (9) Where Electrical Angle Rotor Angle Gives the Trapezoidal waveform of the BACK EMF. One period of this function can be written as, ... ....... (10) For simplification of implementation in MATLAB, equation 1 - 4 must be written in state space form. Each voltage equation is linear combination of the other two voltage equations only two equations are required. By reducing one equation and eliminating one variable using the current relationship, The voltage equations become, ........... (11) ........... (12) And the complete model is, …….... (13) ........... (14) independent. By moving switching frequency far off the resonant frequency and using a simple buck type voltage converter, we have been able to make a switching converter with very low output impedance. By accepting inherent switching losses, the buck converter’s output voltage is accurately controlled by ‘pulse width modulation ‘(PWM) of the switching transistors. One of the biggest challenge connected to hard switching is certainly connected to electromagnetic noise generated in the switching moment, especially in hard switching circuits the problem is exaggerated by a desire to shorten the switching moment to minimum .The shorter the switching moment, higher the frequency of the noise As the frequency goes up, the more apparent the noise problem and the challenge of controlling the noise increases. Why soft switching inverter is required? · Due to switching loss, · Device under Stress, · EMI Problems, · Effect on Machine insulation, · Machine Bearing Current, · Machine Terminal Overvoltage. This is the main reason why many design engineers choose a ‘soft switching’ design. VI. SOFT SWITCHING In order to minimize the size of necessary reactive power components, we have used relatively high switching frequency: 20 kHz in on module and 35 KHz in another module .By using latest technology within IGBT’s we have been able to reduce the switching loss. Hard switching is opposed to soft switching. When we make soft switching circuits we start out with hard switching circuit and than add circuitry (power components) to make it soft. Softmeans to achieve smooth current /voltage transitions in the switching moment. By ‘hard Switching we simply means that no special circuitry is added to make the circuit soft. In order to get smooth transitions, the fundamental principle for all ‘Soft Switching’ techniques is to switch in a moment at zero voltage and zero current, in the main switching devices. At high switching frequency soft switching techniques (ZVS or ZCS) are used to achieve good efficiency and reduced switching stress. In Zero-Voltage Switching (ZVS), the voltage across device is zero just before turn–on. On the other hand in Zero-Current Switching (ZCS), the current through device is zero just before turn-off. Fig 5 (a) and Fig 5 (b) illustrate the ZVS and ZCS switching trajectory and also shown in Fig 6.Typical switching trajectories of power switches. V. HARD SWITCHING This is the simplest method of switching. It requires less no. of power inductors/capacitors in the circuit. This means we reduce the cost, complexity and power loss of the circuit. The output voltage is load ISSN: 0975 – 6736| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 149 JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRICAL ENGINEERING (a) ZVS trajectory Heat–sinking requirement Hardware count Overall power density EMI problem dv/dt problem Modulation scheme Maturity Cost (b) ZCS trajectory Norm Possibly lower Norm Norm More Possibly higher Severe Severe Versatile Mature Norm Low Low Limited Developing Higher Figure 5 Soft switching trajectory I Safe Operating Area On Hard-switching snubbered Soft-switching Off V Figure 6 Typical switching trajectories of power switches. VII. SOFT COMMUTATION TECHNIQUES As a consequence, switching loses now tend to predominate Conventional PWM power converters when operated in a switched mode operation, the Power Switches have to cut off the load current within the turn-on and turn-off times under the hard switching conditions. Hard switching refers to the stressful switching behavior of the power electronic devices the switching trajectory of hard-switched and soft switched power devices. Dissipative passive snubbers are usually added to the power circuits so that the dv/dt and di/dt of the power devices could be reduced, and the switching loss and stress be diverted to the passive snubber circuits. However, the switching loss is proportional to the switching frequency, thus limiting the maximum switching frequency of the power converters.During the turn-on and turn-off processes, the power device has to withstand high voltage and current simultaneously, resulting in high switching losses and stress. IX. ESTIMATION OF BLDC MOTOR AND INVERTER LOSSES [9] The BLDC motor losses are mainly the winding copper losses and iron losses. Of these, the core or iron loss is the most difficult to compute accurately. The permanent magnet and rotor back iron experience little variation in flux and therefore do not generate significant core loss. On the other hand, the stator teeth and stator back iron experience flux reversals on the order of Bmax (maximum steel flux density) at the fundamental frequency. With the knowledge of Bmax and frequency f, the core loss of the stator can be roughly approximated. · BLDC Motor Losses are below:- VIII. DIFFERENCE Hard Switching Soft Switching A. Copper losses:In a 3-phase BLDC motor, the permanent magnets rotate and the armature remain static..The copper losses are defined as the resistive heating losses that occur in the stator windings. These losses are expressed for a three-phase winding as in (15). Severe Norm Almost zero Possibly Higher ................... (15) B. Stator core losses:There are three components in iron losses which are the eddy-current losses, the hysteresis losses, and the BETWEEN SOFT AND HARD SWITCHING: [1] Table 2 Comparison between hard and soft switching Switching loss Overall efficiency ISSN: 0975 – 6736| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 150 JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRICAL ENGINEERING excess or residual losses. Basically, there are several methods to determine the iron losses. The benefit of having different methods is to compare the results obtained to ensure the accuracy. The iron losses are expressed as in (16). ........ (16) The determination of iron losses requires the knowledge of the flux density variations, the frequency, and the magnetic material characteristics for all the motor magnetic parts. In BLDC motor, iron loss not only appears in the stator but also in the rotor. C. Eddy current losses in the magnetic core:Analytical approach to calculate the eddy current density distribution in permanent magnets is based on known distribution of magnetic flux density in the airgap of BLDC motor. The eddy losses are expressed as in (17). .................. (17) Where, Ke = the eddy-current constant, f = frequency, t = thickness, B = flux density The parameters used can be found from the datasheets provided by the manufacturer of the electric motor. D. Hysteresis losses:The hysteresis is an effect within the ferromagnetic materials. The area between the B-H curve and the B axis represents the work done (per unit volume of material). Theoretically, the total power lost over one complete cycle is proportional to the area within the hysteresis loop. Because this effect is related to an area, hysteresis loss is roughly proportional to the square of the working flux density. In fact, the nonlinearities will, for iron material, reduce this to about B1.6. For ferrite grade 3C8 it is 2.5. The data sheets sometimes have graphs of loss versus flux density on a log scale. These can be used to estimate the index n. The hysteresis losses are expressed as in (18). ..................... (18) Where, Kh = hysteresis constant, f = frequency, B = Flux- density, n = Steinmetz exponent. It is clearly seen that different materials result in different losses as constants used in calculating the eddy current and hysteresis losses are different for different types of material. Therefore the material selection is also important in achieving the lowest iron losses. The iron losses can be calculated as in (19) is the core loss density of the stator material at the flux density, B max and frequency, f . · INVERTER Losses are below:E. Semiconductor Losses:In the IGBT PWM inverter, each switch consists of an IGBT and connected with a feed-back diode in anti-parallel. The losses in an IGBT device are mainly due to conduction, off-state blocking, turn-on switching and turn-off switching. It is assumed that the power switches in the inverter are subject to hard switching and the load is the BLDC motor. F. IGBT off-state blocking losses:The leakage currents passing through an off-state power switch, or diode, whose function is to block the supply voltage from reaching the load, results in offstate power losses and are usually small enough to be neglected G. IGBT on-state conduction losses:When a power switch is turned-on, the voltage drop across the main switching terminals falls to a low (but, finite) on-state value of Vsat : typically 1-3 volts when fully saturated. The conduction losses are computed from the average current, I T avg passing through the on-state switch. The on-state voltage drop, Vsat is across the switch terminal and the losses are as in (20). ........ (20) H. Diode conduction losses:The conduction losses in the feed-back diodes can be estimated with the same functional equation used above to calculate Conduction losses in IGBT power switch. Simply replacing Vsat with diode forward voltage drop VDfd typically 1 V and I D avg substituted for ITavg. The diode current has the same peak value as the IGBT switching current but, flows only during t off . The average diode current, is thus proportional to where is the peak (flat-topped) approximation of the switching current and, D is the converter duty cycle. The diode conduction losses are as below: ..(21) I. IGBT turn-on and turn-off switching losses:The turn-on behavior of the power switch is characterized by the rise time of the current and fall time of the voltage. Similarly the turn-off behavior of the switch is denoted by the rise time of voltage and fall time of the current. The switching losses occur in both the diode and IGBT devices due to the existence of high current and high voltage. The switching losses can be computed as in (22). ......... (19) Where, ρ bi , kg/m3, is the mass density of the back iron, Vst is the stator volume, and Γ (B max, f ), W/kg, ISSN: 0975 – 6736| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 ……...(22) Page 151 JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRICAL ENGINEERING Where, tir = the time for current to rise from zero to peak current, IP at turn-on. tvf = the time for voltage to fall from Vdc to Von at turn- on. tvr = the time for voltage to return to Vdc at turn-off tif = the time for current to fall from Ip to zero at turn-off. X. SIMULATION AND RESULTS: Figure 10 Inverter output with soft switching Figure 7 Simulation diagram for BLDC motor with SPWM inverter Figure 11 BLDC output without soft switching Figure 8 SPWM output Figure 12 BLDC output with soft switching Figure 9 Inverter output without soft switching Figure 13 Three phase Iabc, eabc, Theta without soft switching ISSN: 0975 – 6736| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 152 JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRICAL ENGINEERING XII. REFERENCES Figure 14 Three phase Iabc, eabc, Theta with soft switching Figure 15 Rotor position without soft switching Figure 16 Rotor position with soft switching XI. CONCLUSION Soft switching using IGBT gives low switching losses, higher efficiency, reduce torque ripples and improves speed as compared to hard switching. The waveforms for stator current, rotor speed, torque and voltage for the BLDC motor drive with soft switching are more effective than hard switching. Soft switching converter gives “smoothed waveform with no transient spikes. It reduces switching losses and stress. Soft -switched converter can be operated at the very high frequency. These converters also provide an effective solution to suppress EMI. [1] Comparative Study Of Soft Switching And Hard Switching For Brushless Dc Motor: Neelam Rathi, Aziz Ahmed, Rajiv Kumar: International Journal Of Recent Trends In Electrical & Electronics Engg: 2011 [2] Ieee Transactions On Power Electronics, Vol. 19, No. 2, March 2004.Novel Soft-Switching Inverter For Brushless Dc Motor Variable Speed Drive System.Zhi Yang Pan And Fang Lin Luo, Seniour Member, IEEE. [3] Bond Graph Modeling And Simulation Of Three Phase Pm Bldc Motor By Anand Vaz, S.S.Dhami And Sandesh Trivedi. 14th National Conference On Machines And Mechanisms] (Nacomm09), Nit, Durgapur, India, December 17-18, 2009 [4] P.C.Sen. Principles Of Electric Machine And Power Electronics. John Willy & Sons, 1997. [5] T.J.E. Miller, “Brushless Permanent-Magnet And Reluctance Motor Drives”, Clarendon Press, Oxford, Vol.2, Pp: 192-199, 1989. [6] A Kusko And S.M. Peeran, “Definition Of The Brushless Dc Motor”, Industry Applications Society Annual Meeting, Vol.1, And Pp: 20 – 22, 1988. [7] M.A. Rahman, "Special Section On Permanent Magnet Motor Drives: Guest Editorial," Ieee Trans. On Industrial Electronics, Vol. 43, No. 2, Pp. 245, 1996 [8] Bldc Motor Modelling And Control – A Matlab /Simulink /Implementation, Work By Stefan Baldurson May, 2005. [9] Power Loss Estimation And Simulation Of Bldc Motor Drive System By K. S. Rama Rao And Mayarisma Denci. Books [1st] Jacek F. Gieras, Mitchell Wing: Permanent Magnet Motor Technology Design And Applications, Marcel Decker, Inc, 2002 [2nd] B.K.Bose: Modern Power Electronics And A.C.Drives. ISSN: 0975 – 6736| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 153