Performance improvement of SVPWM fed sensorless BLDC motor.
The BLDC motor is widely used in applications including appliances, automotive, aerospace, consumer, medical, automated industrial equipment and instrumentation. The BLDC motor is electrically commutated by power switches instead of brushes. Compared with a brushed DC motor or an induction motor, the BLDC motor has many advantages like higher efficiency and reliability, Lower acoustic noise, Smaller and lighter, Greater dynamic response, Better speed versus torque characteristics, higher speed range, longer life. The efficiency and small size of the brushless DC motor is of paramount importance to many industries, from aeronautic and automotive to the medical and military. The desire for alternative fuelled vehicles has breathed new life into the design and availability of electric vehicles and the high-tech motors that power them. It is commonly used in electric vehicles of all types due to its low maintenance, high efficiency and operating speeds, lack of sparking, compact size and quick response. When it comes to the HVAC and refrigeration industries, brushless DC motors have been the trend versus the typically used AC motor. The top cited reason for the switch is the reduction in power required to operate the motor. In addition, to increase overall system efficiency, many fans are now run using a brushless motor. Manufacturers are using BLDC motors for positioning, actuations systems and more. Considered ideal for many manufacturing applications, brushless DC motors are commonly used in industrial automation design. Manufacturing companies and engineers alike cite the motor's power density, good speed-torque characteristics, speed ranges and ease of maintenance as just a few of the benefits. When it comes to motion-control applications, BLDC motor systems are usually preferred, when their benefits outweigh their potentially higher cost. When it comes to the bottom line, they cost less than brush types.
II BldC Motor:
A BLDC motor is a permanents magnet synchronous motor. Position sensors are used to sense the rotor position according to the rotor position inverter control the stator currents thus the speed of motor. The term dc comes in the name of BLDC because its torque speed characteristics are similar to that of dc motors. BLDC requires an electronic commutation circuit instead of mechanical or brushed commutation used in dc motor. A BLDC motor uses a simplified structure with trapezoidal stator windings.There are two types of stator windings: trapezoidal and sinusoidal, which refers to the shape of the back electromotive force (BEMF) signal. The shape of the BEMF is determined by different coil interconnections and the distance of the air gap. In addition to the BEMF, the phase current also follows a trapezoidal and sinusoidal shape. A sinusoidal motor produces smoother electromagnetic torque than a trapezoidal motor, though at a higher cost due to their use of extra copper windings. A rotor consists of a shaft and a hub with permanent magnets arranged to form between two to eight pole pairs that alternate between north and south poles. A BLDC motor accomplishes commutation electronically using rotor position feedback to determine when to switch the current. Feedback usually entails an attached Hall sensor or a rotary encoder but here sensorless control is followed. The stator windings work in conjunction with permanent magnets on the rotor to generate a nearly uniform flux density in the air gap. This permits the stator coils to be driven by a constant DC voltage (hence the name brushless DC), which simply switches from one stator coil to the next to generate an AC voltage waveform with a trapezoidal shape. A single phase electronically commutated BLDC motor is shown in figure1.
[FIGURE 1 OMITTED]
Brushless DC Motor Control:
Brushless DC motors use electric switches to realize current commutation, and thus continuously rotate the motor. These electric switches are usually connected in an H-bridge structure for a single-phase BLDC motor, and a three-phase bridge structure for a three-phase BLDC motor shown in Figure 2.Usually the high-side switches are controlled using pulse-width modulation (PWM), which converts a DC voltage into a modulated voltage, which easily and efficiently limits the start-up current, control speed and torque.
[FIGURE 2 OMITTED]
The BLDC sensorless driver monitors the BEMF signals instead of the position detected by Hall sensors to commutate the signal. The relationship between the sensors' output and the BEMF is shown in Figure3.
BLDC motor drives mostly uses three-phase bridge inverters for supplying power to it. The circuit diagram of a six-step VSI fed to a BLDC motor is shown in Figure4. A controlled voltage is generated by the voltage source inverter with the help of pulse generated from the space vector modulation method. Because of which this is used in variable speed applications. According to the varying pulses the switching time period of switches vary from which a varying voltage is obtained.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Space Vector Pulse Width Modulation:
Pulse Width Modulation is the process of modifying the width of the pulse in pule train in direct proportion to a small control signal. The greater the control voltage the wider the resulting pulse becomes. SVM is a technique used as a direct bridge between vector control (voltage space vector) and PWM. The SVM technique consists of the following steps:
1. Sector identification.
2. Space voltage vector decomposition into directions of sector base vectors Ux, Ux [+ or -] 60.
3. PWM duty cycle calculation.
[FIGURE 5 OMITTED]
Assume [i.sub.sa], [i.sub.sb], and [i.sub.sc] are the instantaneous balanced 3-phase stator currents
[i.sub.sa] + [i.sub.sb] + [i.sub.sc]=0 (1)
Then the stator current space vector can be defined as follows:
[i.sub.s] = k([i.sub.sa] + a [i.sub.sb] + [a.sup.2][i.sub.sc]) (2)
Where a and [a.sup.2] are the spatial operators, a = [e.sup.j2[pi]/3], [a.sup.2]=[e.sup.j4[pi]/3] and Kis the transformation constant and is chosen k=2/3.
Figure6. Shows the stator current space vector projection.
[FIGURE 6 OMITTED]
The space vector defined by Equation 2 can be expressed using the two-axis theory. The real part of the Space vector is equal to the instantaneous value of the direct-axis stator current component, [i.sub.s[alpha]] and whose Imaginary part is equal to the quadrature-axis stator current component, [i.sub.s[beta]]. Thus, the stator current space Vector in the stationary reference frame attached to the stator can be expressed as
[i.sub.s] = [i.sub.s[alpha]] + [ji.sub.s[beta]] (3)
In symmetrical 3-phase machines, the direct and quadrature axis stator currents [i.sub.s[alpha]] and [i.sub.s[beta]] are fictitious Quadrature-phase (2-phase) current components, which are related to the actual 3-phase stator currents.
Vector Control Of Bldc Motor:
High-performance motor control is characterized by smooth rotation over the entire speed range of the motor, full torque control at zero speed, fast accelerations and decelerations. To achieve such control, vector control techniques are used for 3-phase AC motors. The vector control techniques are usually also referred to field-oriented control (FOC). The basic idea of the FOC algorithm is to decompose a stator current into a magnetic field-generating part and a torque-generating part. Both components can be controlled separately after decomposition. The structure of the motor controller is then as simple as that for a separately excited DC motor.
Figure 6shows the basic structure of the vector control algorithm for BLDC Motor. To perform vector control, it is necessary to follow these steps:
1. Measure the motor quantities (phase voltages and currents).
2. Transform them into the 2-phase system ([alpha],[beta]) using Clarke transformation.
3. Calculate the rotor flux space-vector magnitude and position angle.
4. Transform stator currents into the d, q reference frame using Park transformation.
5. The flux ([i.sub.sd]) and stator current torque ([i.sub.sq]) producing components are separately controlled.
6. The output stator voltage space vector is calculated using the decoupling block.
7. The stator voltage space vector is transformed by an inverse Park transformation back from the d,q reference frame into the 2-phase system fixed with the stator.
8. Using space vector modulation, the output 3-phase voltage is generated.
[FIGURE 7 OMITTED]
Stator Voltage & Current Transformation:
Initially the stator current and stator voltages are taken to do the abc to dq transformation. Here three phase to two phase transformation is done.
[V.sub.d] = 2/3 [va cos [theta] + Vb cos ([theta] - 2[pi]/3) + Vc cos ([theta] - 2[pi]/3)] (4)
[V.sub.d] = 2/3 [va sin [theta] + Vb sin ([theta] - 2[pi]/3) + Vc sin ([theta] - 2[pi]/3)] (5)
Vq= [va. sin 6 + Vb sin (e - y) + Vcs in^B - (5)
Since it's a sensorless control only the stator voltage and current values are taken.
Instantaneous Active and Reactive Current Estimation:
This step is done to calculate the reference current with motor terminal parameter values alone.
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)
[V.sub.d]--dc link voltage
[L.sub.d]--Inductance of the motor
Electrical Parameter Estimation:
This estimation is done to calculate the reference speed value from estimated current and speed in above steps.
Speed reference=[I.sub.d] [I.sub.q.sup.*] - [I.sub.d.sup.*][I.sub.q] - ([I.sub.q] - [I.sub.q.sup.*]) (flux/[L.sub.q]) (8)
[omega]e = Kp X Speed reference + [integral] K[iota] x Speed reference where.
[I.sub.d], [I.sub.q.sup.*], [I.sub.q.sup.*], [I.sub.q] are calculated from previous steps
Integration of that speed gives the theta value this speed control gives the torque reference. ([theta] = [integral] [omega]e)
Inverse Park's Transformation:
[V.sub.[alpha]], [V.sub.[beta]] are stationary orthogonal reference frame quantities The transformation from a two axis orthogonal stationary reference frame (fixed frame) to a rotating reference frame is accomplished using inverse Park's transformation. Using the below mentioned formula it is done.
[V.sub.[alpha]] = [V.sub.q]cos[theta] - [V.sub.d]sin[theta] (9)
[V.sub.[beta]] = [V.sub.q]sin[theta] + [V.sub.d]sin[theta] (10)
[FIGURE 8 OMITTED]
The transformation from a two-axis orthogonal stationary reference frame to a three-phase stationary reference frame is accomplished using Inverse Parks transformation as shown in the figure7.
Space Vector Transformation:
A three phase inverter provides a three phase ac supply which could be given to a three phase motor. The switches must be controlled so that at no time are both switches in same leg turned ON or else D supply would be shorted.Each switch have the specified time period of conduction when the sector number is selected using the voltage values obtained from above step
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (11)
Where N= sector number
Only positive values of [V.sub.[alpha]], [V.sub.[beta]] are taken. Each sector has two fixed time periods using that ON and OFF period are calculated.
Overall Block Diagram And Working:
To perform sensorless control the stator voltage and the stator current are the major parameters which are taken to perform control process. The overall block diagram in controlling the motor speed is shown below in figure8
[FIGURE 9 OMITTED]
Taking the stator voltage and current value whole control process is carried out. Initially the stator voltage and current of three phases are taken and abc to dq transformation is done. [V.sub.abc] to [V.sub.dq] and [I.sub.abc] to [I.sub.dq] is calculate with the motor parameters itself the active and reactive current values are estimated. From the calculated and estimated values speed and torque error values are calculated and corresponding control is carried out. The obtained value is transformed from [V.sub.dq] to [V.sub.[alpha][beta]] using inverse clarks transformation Time period and switching angle are calculated with the help of V[alpha][beta], time, and Vdc voltage using SVPWM technique. From which gate pulse generation is done and controlled three phase output voltage of inverter is fed to BLDC motor.
Discussion on Simulation Results:
The overview of simulation diagram and the result obtained are shown below.
[FIGURE 10 OMITTED]
A. Trapezoidal Back Emf Waveform:
[FIGURE 11a OMITTED]
[FIGURE 11b OMITTED]
B. Dynamic Stability Of Bldc Motor:
[FIGURE 12a OMITTED]
[FIGURE 12b OMITTED]
C Three Phase Current Waveform:
[FIGURE 13a OMITTED]
Discussion On Result:
This sensorless techniques aims at reduction of harmonics, torque ripple and obtaining stability as soon as possible. Here 0.1s is the response time within which the stability is attained for any specified speed and torque ripple is greatly reduced in closed loop system whereas in open loop speed varies continuously and torque ripple is greater.
From Current and voltage waveform we can observe that the open loop has more spikes showing much losses in open loop system and switching losses are greater in open loop system.
[FIGURE 13b OMITTED]
D. Torque Curve Of Motor:
[FIGURE 14a OMITTED]
[FIGURE 14b OMITTED]
The performance of the proposed system is executed in MATLAB and simulation results are obtained. It shows that the system stability is attained as soon as possible and torque ripple is greatly reduced. These are the types of applications where a variable speed and keeping the accuracy at a set speed is important. In these applications the load is directly coupled to the motor shaft, and this happens in fans, pumps, air compressors and blowers, which demand low-cost controllers. In these applications the load on the motor varies over a speed range and may demand high-speed control accuracy and good dynamic responses. Home appliances such as washers, dryers and refrigerators are good examples.
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(1) S. Kaliappan and (2) R. Rogini
(1) Assistant professor, Department of Electrical and Electronics Engineering, Kumaraguru College of Technology, Coimbatore-49
(2) PG Scholar, Department of Electrical and Electronics Engineering, Kumaraguru College of Technology, Coimbatore-49
Received 12 January 2016; Accepted 20 March 2016; Available 4 April 2016
Address For Correspondence:
S. Kaliappan, Assistant professor, Department of Electrical and Electronics Engineering, Kumaraguru College of T echnology, Coimbatore-49
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|Title Annotation:||space vector pulse width modulation; brushless DC motor|
|Author:||Kaliappan, S.; Rogini, R.|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||Feb 1, 2016|
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