CHAPTER 2 BRUSHLESS DC MOTOR

Transcription

1 25 CHAPTER 2 BRUSHLESS DC MOTOR 2.1 INTRODUCTION A motion system based on the DC motor provides a good, simple and efficient solution to satisfy the requirements of a variable speed drive. Although dc motors possess good control characteristics and ruggedness, their performance and applications in wider areas is inhibited due to sparking and commutation problems. Induction motor do not possess the above mentioned problems, they have their own limitations such as low power factor and nonlinear speed torque characteristics (Ramu Krishnan 2009). With the advancement of technology and development of modern control techniques, the Permanent Magnet Brushless DC (PMBLDC) motor is able to overcome the limitations mentioned above and satisfy the requirements of a variable speed drive. The permanent magnet machines have the feature of high torque to size ratio. They possess very good dynamic characteristics due to low inertia in the permanent magnet rotor. Permanent magnet machines can be classified into dc commutator motor, Permanent Magnet Synchronous Motor (PMSM) and Permanent Magnet Brushless DC (PMBLDC) motor. The permanent magnet dc commutator motor is similar in construction to the conventional dc motor except that the field winding is replaced by permanent magnets. PMBLDC motors are generated by virtually inverting the stator and rotor of PM DC motors. The DC term does not refer to a DC motor. These motors are actually fed by rectangular AC waveform.

2 26 The PMSM and PMBLDC motors have similar construction with poly-phase stator windings and permanent magnet rotors, the difference being the method of control and the distribution of windings. The PMSM motor has sinusoidally distributed stator windings and the controller tracks sinusoidal reference current. The PMBLDC motor is fed with rectangular voltages and the windings are distributed so as to produce trapezoidal back emf (Kenjo & Nagamori 1985). The advantages of using brushless DC motor are as follows, High Speed Operation - BLDC motors can operate at speed above 10,000 rpm under loaded and unloaded conditions Responsiveness and Quick Acceleration - Inner rotor BLDC motors have low rotor inertia, allowing them to accelerate, decelerate, and reverse direction quickly High Reliability - BLDC motors do not have brushes, have life expectancies over 10,000 hours High Power Density - A good weight/size to power ratio 2.2 COMPONENTS OF BLDC MOTOR Figure 2.1 shows the structure of BLDC motor that are the ideal choice for applications that require high reliability, high efficiency and high power to volume ratio (Chang-liang Xia 2012). Generally, BLDC motors are considered to be high performance motors that are capable of providing large amounts of torque over a vast speed range. Figure 2.2(a) and Figure 2.2(b) show the cross sectional view of DC and BLDC motors which implies that the derivative of the most commonly used DC motor, the brushless DC motor share the same torque and speed performance curve characteristics.

3 27 Figure 2.1 Structure of Brushless DC Motor Figure 2.2 Cross Sectional View of Motors The coils are attached to the stator and the commutation is controlled by electronics. Commutation times are provided either by position sensors or by coils Back Electromotive Force (emf) measurements. Brushless DC motors usually consist of three main parts: a Stator, a Rotor and Hall Sensors.

4 Stator Similar to an Induction motor, the BLDC motor stator is made up of laminated steel stacked up to carry the windings as shown in Figure 2.3. Windings in a stator can be arranged in two patterns, i.e. a star pattern (Y) or delta pattern ( ). The major difference between the two patterns is that the Y pattern gives high torque at low speed and the pattern gives low torque at low speed. This is because in the delta configuration, half of the voltage is applied across the winding that is not driven, thus increasing losses and in turn, efficiency and torque. Figure 2.3 Stator in a BLDC Motor Cross sectional views of slotted and slotless BLDC Motors are shown in Figure 2.4(a) and Figure 2.4(b). An advantage of the brushless configuration in which the rotor is inside the stator is that more crosssectional area is available for the power or armature winding. At the same time the conduction of heat through the frame is improved. A slotless core has lower inductance, thus it can run at very high speed. Because of the absence of teeth in the lamination stack, requirements for the cogging torque also go down, thus making them an ideal fit for low speed too (when permanent magnets on rotor and tooth on the stator align with each other then, because of the interaction between the two, an undesirable cogging torque develops and causes ripples in speed).

5 29 Figure 2.4 Slotted and Slotless Motor The main disadvantage of a slotless core is higher cost because it requires more winding to compensate for the larger air gap. The magnetization of the permanent magnets and their displacement on the rotor is chosen so that shape of the back emf (the voltage induced into the stator winding due to rotor movement) is trapezoidal. This allows the DC voltage of a rectangular shape, to create a rotational field with low torque ripples. The motor can have more than one pole-pair per phase. Proper selection of the laminated steel and windings for the construction of stator are crucial to motor performance. An improper selection may lead to multiple problems during production, resulting in market delays and increased design costs Rotor Depending upon the application requirements, the number of poles in the rotor may vary. Figure 2.5 (a) and Figure 2.5 (b) show the 4 and 8 pole of the permanent magnet rotor respectively. Increasing the number of poles give better torque but the cost has to be reduced with the maximum possible speed (Jang & Lee 2005). Another rotor parameter that makes an impact on

6 30 the maximum torque is the material used for the construction of permanent magnet, higher the flux density of the material and higher the torque. (a) Four Pole (b) Eight Pole Figure 2.5 Permanent Magnet Rotor The rotor in a BLDC motor consists of an even number of permanent magnets. The number of magnetic poles in the rotor also affects the step size and torque ripple of the motor. More poles give smaller steps and less torque ripple. Any of these PMBLM rotor configurations can be selected on the basis of application and power rating. The flux density of the rotor is high due to the construction of permanent magnet, hence there are no losses in rotor because of no winding present in core. Figure 2.6 Rotor in a BLDC Motor

7 31 The permanent magnets go from 1 to 5 pairs of poles. The rotor can vary from two to eight pole pairs with alternate North (N) and South (S) poles. Based on the required magnetic field density in the rotor, the proper magnetic material is chosen to make the rotor. Ferrite magnets are traditionally used to make permanent magnets. As the technology advances, rare earth alloy magnets are gaining popularity. The ferrite magnets are less expensive but they have the disadvantage of low flux density for a given volume. In contrast, the alloy material has high magnetic density per volume and enables the rotor to compress further for the same torque. The rotor of brushless DC motor with one and two pair of poles are represented in Figure 2.6(a) and Figure 2.6(b). Also, these alloy magnets improve the size-to-weight ratio and give higher torque for the same size motor using ferrite magnets Hall Sensors These kinds of devices are based on Hall-effect theory, which states that if an electric current carrying conductor is kept in a magnetic field, the magnetic field exerts a transverse force on the moving charge carriers that tends to push them to one side of the conductor. A build-up of charge at the sides of the conductors will balance this magnetic influence thus producing a measurable voltage between the two sides of the conductor. The presence of this measurable transverse voltage is called the Hall-effect because it was discovered by Edwin Hall in For the estimation of the rotor position, the motor is equipped with three hall sensors. These hall sensors are placed every 120, with these sensors, 6 different commutations are possible. Phase commutation depends on hall sensor values. Power supply to the coils changes when hall sensor values change. With right synchronized commutations, the torque remains nearly constant and high.

8 32 Figure 2.7 Hall Sensor Phase Commutation of BLDC Motor Figure 2.7 shows the phase commutation of BLDC motor depending on hall sensor. It is possible to determine when to commutate the motor drive voltages by sensing the back emf voltage on an undriven motor terminal during one of the drive phases. The obvious cost advantage of sensorless control is the elimination of the Hall position sensors. However the usage of BLDC motor with sensor is applicable for some applications Phase Commutation To simplify the explanation of how to operate a three phase BLDC motor, a typical BLDC motor with only three coils is considered. As previously shown, phases commutation depends on the hall sensor values. When motor coils are correctly supplied, a magnetic field is created and the rotor moves. The most elementary commutation driving method used for BLDC motors is an ON-OFF scheme, a coil is either conducting or not conducting. Only two windings are supplied at the same time and the third

9 33 winding is floating (Jan & Kim 2006). Connecting the coils to the power and neutral bus induces the current flow. This is referred as trapezoidal commutation or block commutation. Figure 2.8 Three Phase Bridge Inverter Figure 2.8 shows the three phase bridges of inverter to run the BLDC Motor. To command brushless DC motors, a three phase bridges is used. For motors with multiple poles the electrical rotation does not correspond to a mechanical rotation. A four pole BLDC motor uses four electrical rotation cycles to have one mechanical rotation. The back emf of the BLDC Motor can drive the inverter by detecting the zero crossing point of the back emf, then commutate the inverter power switching devices. The two power switching device turn ON at any instant for 60 degree and the commutation occurs by next pair conducted for the continuous operation of Motor.

10 34 Table 2.1 Hall Sensor Truth Table Hall Sensors Values Phase Switches 101 U-V Q1;Q4 001 U-W Q1;Q6 011 V-W Q3;Q6 010 V-U Q3;Q2 110 V-W Q5;Q2 100 W-V Q5;Q4 The strength of the magnetic field determines the force and speed of the motor. By varying the current flow through the coils, the speed and torque of the motor can be adjusted. The most common way to control the current flow is to control the average current flow through the coils. PWM is used to adjust the average voltage and thereby the average current, inducing the speed. Table 2.1 shows the operation sequence of a BLDC motor with Hall Sensors. The proposed scheme utilizes the back emf difference between two phases for BLDC sensorless drive instead of using the phase back emf. Figure 2.9 shows the equivalent circuit of a Y connection BLDC motor and the inverter topology. Figure 2.9 Circuit Diagrams of BLDC Motor with Inverter

11 35 Figure 2.10 Phase Back EMF of BLDC Motor The zero crossing points of the back emf in each phase may be an attractive feature used for sensing, because these points are independent of speed and occur at rotor positions where the phase winding is not excited. However, these points do not correspond to the commutation instants. Therefore, the signals must be phase shifted by 90 electrical degree before they can be used for commutation. The detection of the third harmonic component in back emf, direct current control algorithm and phase locked loops have been proposed to overcome the phase-shifting problem. Figure 2.10 shows the phase back emf of BLDC motor. The commutation sequence with back emf difference estimation method is that positive sign indicates the current entering into the stator winding and the negative sign indicates the current leaving from the stator winding. At any instant two stator windings are energized and one winding will be in floating.

12 DYNAMIC MODEL OF BLDC MOTOR The derivation of this model is based on the assumption that the induced currents in the rotor due to stator harmonic fields are neglected and the iron and stray losses are also neglected (Krishnan 2009). Damper windings are not usually a part of PMBLDCM where damping is provided by the inverter control. The motor is considered to have three phases even though the derivation process is valid for any number of phases shown in Figure Equations (2.1), (2.2) & (2.3) implies the voltage equation of the stator windings. dia Van Raia La ea (2.1) dt dib Vbn Ri b b Lb eb (2.2) dt dic Vcn Ri c c Lc ec (2.3) dt Figure 2.11 Dynamic Model of BLDC Motor

13 37 where, V an, V bn and V cn : phase voltage in volts i a, i b and i c : phase current in amps e a, e b and e c : phase voltage back-emf in volts R a, R b and R c : phase resistance in ohms L a, L b and L c : phase inductance in henry Equation 2.4 is the mechanical equation that relates the machine's angular velocity to the developed electromagnetic torque, load torque, and motor parameters. d Tem BJm TL (2.4) dt T em ki (2.5) t a e a k (2.6) e where, T em : developed electromagnetic torque in Nm : rotor angular velocity in rad/sec B : viscous friction constant in N-m/rad/sec J m : rotor moment of inertia in Kg-m 2 T L : load torque in Nm k e : back emf constant

14 38 Equation (2.7) The voltage equation can be written in Laplace domain as shown in V () s R I () s L si () s E () s an a a a a a V () s I ()[ s R sl ] E () s an a a a a (2.7) The Laplace transform of Equation (2.6) is E () s k () s a e (2.8) Equation (2.9) The Equation (2.8) is substituted in Equation (2.7), which gives V () s I ()[ s R sl ] k () s an a a a e (2.9) From Equation (2.9), phase current can be written as I V () s k () s R sl an e a () s a a (2.10) The electromagnetic torque in the Laplace domain are T () s B() s J s() s T () s em m L (2.11) T () s k I () s em t a (2.12) Using Equation (2.11), the angular velocity of motor is T () s T () s B sj (s) em L m (2.13)

15 39 torque equation as, Using Equation (2.10) and (2.12), it is possible to express the V () s k () s T s k R sl an e em() t a a (2.14) From the above equations it is possible to derive the transfer function kt () s JmLa Van() s 2 JmRa BL a BRa ktk e s s JmLa JmLa (2.15) Equation (2.15) gives the transfer function of BLDC motor, from that desired performance of the system can be easily achieved. 2.4 TORQUE - SPEED CHARACTERISTICS There are two torque parameters used to define a BLDC motor, peak torque and rated torque. During continuous operations, the motor can be loaded up to rated torque. This requirement comes for brief period, especially when the motor starts from stand still and during acceleration. During this period, extra torque is required to overcome the inertia of load and the rotor itself. The motor can deliver a higher torque up to maximum peak torque, as long as it follows the speed torque curve. Figure 2.12 shows the torquespeed characteristics of a BLDC motor. As the speed increases to a maximum value of torque of the motor, continuous torque zone is maintained up to the rated speed after exceeding the rated speed the torque of the motor decreases. The stall torque represents the point on the graph at which the torque is maximum, but the shaft is not rotating. The no load speed, ω n, is the

16 40 maximum output speed of the motor (when no torque is applied to the output shaft). If the phase resistance is small, as it should be in an efficient design, then the characteristic is similar to that of a shunt DC motor. Figure 2.12 Torque vs Speed Characteristics of BLDC Motor The speed is essentially controlled by the voltage, and may be varied by varying the supply voltage. The motor then draws just enough current to drive the torque at this speed. As the load torque is increased, the speed drops, and the drop is directly proportional to the phase resistance and the torque. The voltage is usually controlled by chopping or PWM. This gives rise to a family of torque/speed characteristics in the boundaries of continuous and intermittent operation. The continuous limit is usually determined by heat transfer and temperature rise. The intermittent limit may be determined by the maximum ratings of semiconductor devices in the controller, or by temperature rise. In practice the torque/speed characteristic deviates from the ideal form because of the effects of inductance and other parasitic influences. The linear model of a DC motor torque/speed curve is a very good approximation.

17 BENEFITS OF BRUSHLESS TECHNOLOGY Broad operating range: Eliminating the brushes is a definite plus: It not only extend the motor's service life and reduces maintenance, but also eliminates the speed restrictions inherent to "brushed" DC motors. BLDC motors can attain speeds of more than 60,000 rpm. More importantly, the power circuit components that are required to convert from alternating to direct current provide the basis for variablespeed drive, making BLDC motors well-suited for applications that require speed control over a wide operating range. Higher efficiency: Using permanent magnets in the rotor helps to keep the rotor small and inertias low. Without current flow (and the associated losses) in the rotor, the motor generates less heat. Whatever heat produced dissipates more efficiently from the brushless motor's wound stator to the outer metallic housing through the "brushed" motor's shaft or rotor-stator air gap. Flexible design: The DC power supply permits a motor design with any number of phases in the stator. Although three-phase configurations are most common, two and four phased configurations also are used, energization of coils are flexible. As an example, two windings can be energized with the third off at any instant in a three phase BLDC configuration. Energizing the coils in pairs simplifies the control design, which lowers first cost, and provides motor torque about 10 percent more than energizing the windings sinusoidally.

18 42 Table 2.2 and Table 2.3 show the comparison of BLDC Motor with Brushed DC Motor and BLDC Motor with Induction Motor. The necessity of the comparison will extract the performance of the PMBLDC Motor. Table 2.2 Comparison of BLDC Motor with Brushed DC Motor Feature BLDC Motor Brushed DC Motor Commutation Maintenance Electronic commutation based on Hall position sensors Less required due to the absence of brushes Life Longer Shorter Brushed commutation Periodic maintenance is required Moderately flat At higher Speed/Torque Flat Enables operation at all the speed, brush friction Characteristics speed with rated load increases, thus reducing useful torque Efficiency High Moderate Output Power/ Frame Size Rotor Inertia Speed Range Electric Noise Generation Cost of Building High Reduced size due to superior thermal characteristics. Because BLDC has the windings on the stator, which is connected to the case, the heat dissipation is better Low, because it has permanent magnets on the rotor. This improves the dynamic response Higher No mechanical limitation imposed by brushes/commutator Low Higher Since it has permanent magnets, building costs are higher Moderate/Low The heat produced by the armature is dissipated in the air gap, thus increasing the temperature in the air gap and limiting specs on the output power/frame size Higher rotor inertia which limits the dynamic characteristics Lower Mechanical limitations by the brushes Arcs in the brushes will generate noise causing EMI Low Control Complex and expensive Simple and inexpensive

19 43 The comparison of the proposed method with Induction motor shows the advantage of the proposed model with the conventional field and the Table 2.3 which represents the need of the BLDC motor replacement. Table 2.3 Comparison of BLDC Motor with Induction Motor Features BLDC Motors Induction Motors Speed/Torque Flat Enables operation at all Characteristics speeds with rated load Nonlinear Lower torque at lower speed Output Power/ Frame Size Rotor Inertia Starting Current High Since it has permanent magnets on the rotor, smaller size can be achieved for a given output power Low Better dynamic characteristics Rated No special starter circuit required Moderate Since both stator and rotor have windings, the output power to size is lower than BLDC High Poor dynamic characteristics Approximately up to seven times of rated Starter circuit rating should be carefully selected Control Requirements A controller is always required to No controller is required keep the motor running. The for fixed speed; a controller same controller can be used for is required only if variable variable speed control speed is desired Slip No slip is experienced between stator and rotor frequencies The rotor runs at a lower frequency than stator 2.6 TYPICAL BLDC MOTOR APPLICATIONS BLDC motors find applications in every segment of the market. Such as, appliances, industrial control, automation, aviation and so on

20 44 (Padmaraja Yedamale 2003). One can categorize the BLDC motor control into three major types such as Constant loads Varying loads Positioning applications Applications with Constant Loads These are the types of applications where variable speed is more important than keeping the accuracy of the speed at a set speed. In these types of applications, the load is directly coupled to the motor shaft. For example, fans, pumps and blowers come under these types of applications. These applications demand low-cost controllers, mostly Operating in open-loop Applications with Varying Loads These are the types of applications where the load on the motor varies over a speed range. These applications may demand high-speed control accuracy and good dynamic responses. In home appliances, washers, dryers and compressors are good examples. In automotive, fuel pump control, electronic steering control, engine control and electric vehicle control are good examples of these. In aerospace, there are number of applications, like centrifuges, pumps, robotic arm controls, gyroscope controls and so on. These applications may use speed feedback devices and may run in semi-closed loop or in total closed loop. These applications use advanced control algorithms, thus complicating the controller. Also, this increases the price of the complete system.

21 Positioning Applications Most of the industrial and automation types of application come under this category. The applications in this category have some kind of power transmission, which was mechanical gears or timer belts, or a simple belt driven system. In these applications, the dynamic response of speed and torque are important. Also, these applications may have frequent reversal of rotation direction. A typical cycle will have an accelerating phase, a constant speed phase and a deceleration and positioning phase. The load on the motor may vary during all of these phases, causing the controller to be complex. These systems mostly operated in closed loop. There was three control loops functioning simultaneously: Torque Control Loop, Speed Control Loop and Position Control Loop. Optical encoder or synchronous resolver are used for measuring the actual speed of the motor. In some cases, the same sensors are used to get relative position information. Otherwise, separate position sensors may be used to get absolute positions. 2.7 SUMMARY The necessity of the BLDCM in application is based on the efficiency, reliability requirements for variable speed drives. Comparing to conventional dc motor, the BLDC Motor is most efficient and less maintenance due to the elimination of commutator and brushes. To detect the rotor position, it is essential to provide three Hall sensors which makes complexity. The BLDCM play a vital role in many applications due to high torque to weight ratio and it has linear torque speed characteristics. Finally, a dynamic model is performed to validate the desired performance of the BLDCM system.