INTERNATIONAL JOURNAL OF PURE AND APPLIED RESEARCH IN ENGINEERING AND TECHNOLOGY A PATH FOR HORIZING YOUR INNOVATIVE WORK SENSORLESS BLDC MOTOR CONTROL IN MATLAB SIMULINK ANKITA A KANEKAR, V. K. JOSEPH Electronics and Telecommunication Engg. Goa College of Engineering Farmagudi, Goa. Accepted Date: 27/02/2013 Publish Date: 01/04/2013 Keywords Brushless DC Motor, Back EMF ZCD, Sensorlesscontrol, Trapezoidal emf. Corresponding Author Ms. Ankita A Kanekar Abstract Brushless DC Motor drives have made a successful entrance into various sectors of industry such as aerospace, automotive and home appliances due to its simple structure. The accurate knowledge of the rotor position is required for good performance of brushless DC motors the need for the rotor angle information in BLDC has been satisfied by use of some form of rotor position sensor. The position sensor used in BLDC drives have the disadvantages of additional cost, electrical connections, mechanical alignment problems, and disadvantage of being inherent source of unreliability. These bottlenecks results in several sensor less technique in recent years. A proposed sensor less scheme is used to overcome the disadvantages of sensored scheme. The rotor position detection can be estimated even at standstill and running conditions. The method which is proposed in this project is Back EMF ZCD with low cost, effective and simpler controller. In this paper we present a mathematical model of BLDC motor and show the values of various technical parameters using MATLAB/SIMULINK. In this paper the simulation is carried out for 120 degree mode of operation. The test results show the performance of BLDCM which are highly acceptable. Finally a PID controller is applied for closed loop speed control under various loading conditions.
INTRODUCTION During the last two decades, a lot of research on sensor less control technique for BLDC motor has been conducted. Using of Permanent Magnet in electrical machines have so many benefits and advantages then electromagnetic excitation machines these are zero excitation losses result in high efficiency, simple construction, low cost less maintenance and high torque or high output power per unit volume. Due to high power to weight ratio, high torque, good dynamic control for variable speed applications, absence of brushes and commutator make Brushless dc (BLDC) motor, bestchoice for high performance applications. Due to the absence of brushes and commutator there is no problem of mechanical wear of the moving parts. As well, better heat dissipation property and ability to operate at high speeds make them superior to the conventional dc machine. However, the BLDC motor constitutes a more difficult problem than its brushed counterpart in terms of modelling and control system design due to its multiinput nature and coupled nonlinear dynamics. Due to the simplicity in their control, Permanent-magnet brushless dc motors are more accepted used in highperformance applications. In many of these applications, the production of ripple-free torque is of primary concern. Therefore, if the waveforms of the phase back EMF and phase current are perfectly matched, torque ripple is minimized. In this paper finally closed loop speed control is done by using PID controller under various loading conditions. PRINCIPLE OF OPERATION In conventional BLDC motor during bipolar operation, at any time across DC bus, two phases come in series. Only half of the DC bus voltage is applied to each phase, resulting in addition of torque constant on both phases there by achieving high starting torque. But speed will be limited. To get higher speed, full DC bus voltage is to be applied to each phase. This can be achieved in unipolar operation, where each phase conducts only in one direction which in turn reduces the starting torque. order to get high torque, motor should operate in bipolar mode and to get high speed motor should operate in unipolar mode. Shifting of modes between unipolar and bipolar
operation is achieved based on speed requirement. In bipolar operation first 3 legs are active and the 4th leg is inactive. By switching on Q1 and Q4, phase A conducts in positive direction and phase B conducts in negative direction. By switching off Q4 and switching on Q6, a free-wheeling path is established through phase B, diode D3, switch Q1 and Phase A as shown in Figure-1. By switching off Q1 and switching on Q3 and Q6, the free-wheeling energy in positive conducting phase A flows through resistor Rs, D2, phase A, phase C, and Q6, as shown in Figure2. SENSORLESS CONTROL OF BLDC MOTOR Brushless dc (BLDC) motors, with their trapezoidal electromotive force (EMF) profile, requires six discrete rotor position information for the inverter operation. These are typically generated by Hall- effect switch sensors placed within the motor. However, it is a well-known fact that these sensors have a number of drawbacks. They increase the cost of the motor and need special mechanical arrangements to be mounted. Further, Hall sensors are temperature sensitive, and hence limit the operation of the motor. They could reduce system reliability because of the extra components and wiring. sosensorless method is the reliable method used in harsh environments.there are three independent methods for determining the Hall configuration.the selection of which method to use will depend on the
information provided. 1. Hall Based Commutation Sequence Provided. 2. Back EMF Waveforms. 1: Hall Based Commutation Sequence Provided Either method conveys adequate information about driving the motor phases based on Hall Effect sensor states. The relationship between the Hall Effect sensors themselves is always consistent. In other words the Hall Effect sensor sequence seen in can be found in all motors with 120- degree Hall Effect sensors when the motor rotates. However, the direction of rotation, CW or CCW, necessary to produce this relationship can vary across different motors. Very often the binary state of the three Hall Effect sensors will be combined to create a 3-bit binary word. The mapping between the Hall states and the three-bit word. Below the binary word representation in is tables that represent the states of the MOSFETs of the half-bridges. A+ = Phase A high side MOSFET closed A- = Phase A low side MOSFET closed B- = Phase B high side MOSFET closed C- = Phase B low side MOSFET closed C+ = Phase C high side MOSFET closed D- = Phase C low side MOSFET closed If the state of a MOSFET for a particular Hall state is not defined then it is assumed to be 0pen. For example during Hall state 1-0-1, MOSFETs A-, B+, C+ and C- are all open. Below the table of MOSFET states in is a diagram of the relative voltages through each motor phase based on the Hall states (and subsequent MOSFET states). For instance in Hall state 1-0-1, the path of the current begins at the voltage source, flows through the high side MOSFET of phase A, through motor winding A, through motor winding B, through the low side MOSFET of phase B, and finally to the ground plane. 2 :Back EMF When a BLDC motor rotates, each winding generates a voltage known as back electromotive Force or back EMF, which opposes the main voltage supplied to the windings according to Lenz s Law. The commutation time is determined by the rotor position. Since the shape of back EMF indicates the rotor position, it is possible to determine the commutation timing if the back EMF is known. The phase current is in
phase with the phase back EMF. If the zero crossing of the phase back EMF can be measured, we will know when to commutate the current. As mentioned before, at one time instant, since only two phases are conducting current, the third winding is open. This opens a window to detect the back EMF in the floating winding. The proposed back emf detection method describes that, instead of detecting the zero-crossing point (ZCP) of the nonexcited motor back electromagnetic force (EMF)or the average motor terminal to neutral voltage, the true zero-crossing points of back EMF are extracted directly from the supply with simple RC circuits and comparators. In contrast to conventional methods, the neutral voltage is not needed and the diode freewheeling currents in the non-conducted phase are eliminated completely; therefore, the commutation signals are more accurate and insensitive to the common-mode noise. As a result, the proposed method makes it possible to achieve good motor performance over a wide speed range and to simplify the starting procedure. A power supply is given to the inverter. The three phase output of the inverter is applied to the motors stator windings. From the supply, voltage divider is connected, with the RC low pass filter and a zero crossing detector circuit to produce the back EMF for three phases. Low pass filter is a filter that passes low frequency signals but attenuates higher frequency signals. The actual amount of attenuation for each frequency varies from filter to filter. The Back EMF signals are send to the zero crossing detector the positional pulse are produced. A voltage divider (also known as a potential divider) is a simple linear circuit that produces an output voltage (Out) that is a fraction of its input voltage (Vin). SIMULATION RESULTS Here simulation is carried out for four cases. A. Case-1: BLDC with open loop control When the motor is in standstill conditions the back EMF will be zero, so rotor Position cannot be determined by sensorless method. So initially the motor is started by applying external gate pulses to the Mosfets in inverter circuit in proper commutation sequence so that motor rotates with
unidirectional torque. The speed of a BLDC motor is proportional to the voltage applied to the motor. When using digital control, a pulse-width modulated (PWM) signal is used to generate an average voltage. Figure5:open loop speed and torque at 25hz frequency B. Case-2: BLDC with closed loop PID control on no Figure3 : open loop control for loaded condition Load Sensorless Method Figure4 : Table of conclusion Figure6 : circuit for close loop control under no load
Figure7 Output waveforms of the speed of the motor. Figure 10.Output waveforms of the currents. C. Case-3: BLDC with closed loop PID control for Increasing load Here reference speed is taken as 1500 rpm Figure-8.Back EMF of the BLDC motor. the motor reaches the reference speed very quickly with PID control. Here load torque is increasing from 0 to 5 N-m at time t = 0.15 sec. Figure-9 Output waveform of the torque of the motor.
Figure-11 Output waveforms of the speed of the motor Figure-14 Output waveforms of the currents. Figure-12.Back EMF of the BLDC motor. D. Case-4: BLDC with closed loop PID control for decreasing load Figure-13 Output waveform of the torque of the motor. Figure-15Output waveforms of the speed of themotor
the suitability of the proposed method. Simulation of the proposed method is done by using MATLAB/SIMULINK. The performance evaluation results show that this modeling is very useful in studying the high performance drive before taking up the dedicated controller design concept for evaluation of dynamic performance of the Figure-16 Output waveform of the torque of themotor. motor. Simulation results are shown for various loading conditions. REFERENCES 1. Anand Sathyan, Nikola Milivojevic, Young-Joo Lee, Mahesh Krishnamurthy, Ali Emadi,"An FPGA-Based Novel Digital PWM Control Scheme for BLDC Motor Drives",IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,VOL.56, NO. 8, AUGUST 2009. Figure-17 Output waveform of the torque of 2. Cheng-Tsung Lin, Chung-Wen Hung, and the motor. Chih-Wen Liu, Senior CONCLUSIONS Member,IEEE."Position Sensorless Control for Four Switch Three-Phase Brushless DC A simple technique to detect back EMF Motor Drives",IEEE TRANSACTIONS ON using RC filter and comparator is defined. POWER ELECTRONICS, VOL. 23, NO. 1, This method provides an amplified version JANUARY 2008. of the back EMF. Detection of Rotor Position is determined using Back EMF ZCD. 3. Meenal K,K Sobana,A Vanisri.Dr Simulation results are shown which validate Devarajan,"FPGA BASED INDIRECT
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