A Review: Speed Control of Brushless DC Motor

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A Review: Speed Control of Brushless DC Motor Anurag Dwivedi and A. N. Tiwari ABSTRACT: The dc motor is deficient in many aspects that require attention to be focused. This is turn give rise to some other alternative type of the dc motor design, which negotiates the problems like lack of periodic maintenance , mechanical wear outs, acoustic noise, sparking and brush effect, thus result in less defect in dc motor. Various emphatic studies has been made on synchronous dc motor with brushless commutators. Now day’s researchers are confined towards the development of brushless dc motor as it is proving superior to the conventional dc motor type.

KEY WORDS: BLDC motor, PWM, SenselessControl, full speed range, PM- Permanent Magnet, PI- Proportional Integrated controller, VR- Variable Reluctance.

INTRODUCTION The permanent magnet brushless motor can be classified upon to the back-EMF waveform, where it can be operated in either brushless AC (BLAC) or brushless DC (BLDC) mode.

Usually the BLAC motor has a sinusoidal back-EMF waveform and BLDC motor has a trapezoidal back-EMF. In modern electrical machines industry productions the brushless direct current (BLDC) motors are rapidly gaining popularity. BLDC motors are used in industries such as Appliances, HVAC industry, medical, electric traction, road vehicles, military equipment, hard disk drive etc. DC motors has its long history. It has been used in the industries for many years now. They provide simple means and precise way of control. In addition, they have high efficiency and have a high starting torque versus falling speed characteristics which helps high starting torque and help to prevent sudden load rise. But with such characteristics, The dc motor have some deficiencies that needed to be attended to which gave rise to design of some other alternative types of dc motors for example, the lack of periodic maintenance mechanical wear outs, acoustic noise , sparking, brushes effects are some of the problems that were needed to overcome the defects in dc motors.

Research Scholar, E-mail: anurag.srmcem007@gmail.com Associate Professor, E- mail:amardee@rediffmail.com Department of Electrical Engineering, MMM Engineering College, Gorakhpur-273010

As a result emphatic studies have been made on synchronous dc motors with brushless commutators. So, current researches have been tailored towards developing brushless direct current motors, which are fast becoming alternatives to the conventional dc motor types. Types of speed control of BLDC motor drive using Sensors, Sensor less Control Method, Digital Signal Controller, Digital Signal Processors (DSPs), Adaptive Neuro-Fuzzy Controller, Fuzzy Logic PI Controller etc. BLDC motors, also called Permanent Magnet DC, Synchronous motors, are one of the motor type that have more rapidly gained popularity, mainly because of their better characteristics an performance . The control of BLDC motors can be done in sensor or sensor less mode, but to Low-cost Hall-effect sensors are used. Accelerometers have been extensively applied to measure motor position and speed. Reduce overall cost of actuating devices, sensor less control techniques are normally used. As a result, many researchers have been reported for sensor less drives that can control position, speed, and/or torque without shaft-mounted position sensors. Conventional sensor less control methods can be classified into four categories. First, the open phase current sensing method is a technique for detecting the conducting interval of freewheeling diodes connected in anti parallel with power transistors. Secondly, the method detecting the third harmonic of back-EMF is the technique to remove all the fundamental and other poly-phase components through a simple summation of three phase voltages. Thirdly, the back-EMF integrating method is a technique applying the principle that integration is constant from Zero Crossing Point (ZCP) to 30°.In a Digital signal Controller to provide further cost effectiveness and ease of design a high performance 16-bit digital signal controller (DSC) is used.

Sensor less control using the back EMF zero crossing technique is utilized, eliminating the need for hall sensors thus further reducing cost and increasing reliability. The speed is

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IJBSTR REVIEW PAPER VOL 1 [ISSUE 6] JUNE 2013 varied using the pulse width modulation (PWM) technique. In Digital Signal Processor, the speed control of brushless dc motor drive employing PWM technique using TMS320F240 digital signal processor. BLDC is widely used because of its high mechanical power density, simplicity and cost effectiveness. The complete controller for BLDC is developed using TMS 320F240 digital signal processor, which has the special features for digital motor control. New adaptive Neuro-Fuzzy Controller (NFC) is used speed control of Brushless DC (BLDC) motor drives. it requires less calculation as compared with the conventional fuzzy and/or neural networks, used for electrical drive applications. The proposed controller is used for speed and/or torque control of a BLDC motor drive. In order to demonstrate the NFC ability to follow the reference speed and to reject undesired disturbances, PI controller for speed control of BLDC motor. The controller uses three fuzzy logic controllers and three PI controllers. The output of the PI controllers is summed and is given as the input to the current controller. The current controller uses P controller. The fuzzy logic control is learned continuously and gradually becomes the main effective control. In this paper controlling of speed in BLDC motor using controllers are presented.

PRINCIPLES OF THE BLDC MOTOR Mathematical Model of BLDC Motors Modeling of a BLDC motor can be developed in the similar manner as a three-phase synchronous machine. Since its rotor is mounted with a permanent magnet, some dynamic characteristics are different. Flux linkage from the rotor is dependent upon the magnet. Therefore, saturation of magnetic flux linkage is typical for this kind of motors. As any typical three-phase motors, one structure of the BLDC motor is fed by a three-phase voltage source as shown in Fig. 1. The source is not necessary to be sinusoidal. Square wave or other wave shape can be applied as long as the peak voltage is not exceeded the maximum voltage limit of the motor. Similarly, the model of the armature winding for the BLDC motor is expressed as follows. đ?‘‘đ?‘–đ?‘Ž đ?‘Łđ?‘Ž = đ?‘…đ?‘–đ?‘Ž + đ??ż + đ?‘’đ?‘Ž đ?‘‘đ?‘Ą đ?‘‘đ?‘–đ?‘? + đ?‘’đ?‘? đ?‘‘đ?‘Ą đ?‘‘đ?‘–đ?‘? đ?‘Łđ?‘? = đ?‘…đ?‘–đ?‘? + đ??ż + đ?‘’đ?‘? đ?‘‘đ?‘Ą Or in the compact matrix form as follows. đ?‘Łđ?‘? = đ?‘…đ?‘–đ?‘? + đ??ż

đ?‘Łđ?‘Ž đ?‘… + đ?‘?đ??ż đ?‘Łđ?‘? = 0 đ?‘Łđ?‘?

0

0 đ?‘… + đ?‘?đ??ż 0

đ?‘’đ?‘Ž 0 đ?‘–đ?‘Ž đ?‘–đ?‘? + đ?‘’đ?‘?

0 đ?‘’đ?‘? đ?‘… + đ?‘?đ??ż đ?‘–đ?‘?

Where La = Lb = La =L=Ls –M La is the armature self-inductance M is the mutual inductance Armature resistance R a = R b = R c =R đ?‘Łđ?‘Ž ,đ?‘Łđ?‘? , đ?‘Łđ?‘? terminal phase voltage [V] đ?‘–đ?‘? , đ?‘–đ?‘? , đ?‘–đ?‘? , : motor input current [A] đ?‘’đ?‘Ž , đ?‘’đ?‘? , đ?‘’đ?‘? : motor back emf [V] p in the matrix represents

đ?‘‘ đ?‘‘đ?‘Ą

Figure 1- BLDC motor schematics Figure.1 shows the overall system conďŹ gurations of the threephase BLDC motor drive. The three phase inverter topology is a six-switch voltage-source conďŹ gurations with constant dclink voltage (Vdc), which is identical with the induction motor drives and the permanent magnet ac motor drives. The analysis is based on the following assumption for simplIďŹ cation.

METHODS FOR CONTROLLING SPEED OF BRUSHLESS DC MOTOR Different methods for controlling the speed of brushless DC motor are: i) Sensors based control method ii) Sensor less control method iii) Digital control method iv) Genetic based proportional control.

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IJBSTR REVIEW PAPER VOL 1 [ISSUE 6] JUNE 2013 A. Sensors based control method for brushless DC motor:

Figure 3: Hall Sensor Control of BLDC motor

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 producing a measurable voltage between the two sides of the conductor.

To rotate the BLDC motor the stator windings should be energized in a sequence. It is important to know the rotor position in order to understand which winding will be energized following the energizing sequence. Rotor position is sensed using Hall-effect sensors embedded into the stator The connecting principle between the brushless motor and this sensor is reminiscent of the miniaturized magnetic angular encoder based on 3-D Hall sensors. A permanent magnet is fixed at the end of a rotary shaft and the magnetic sensor is placed below, and the magnet creates a magnetic field parallel to the sensor surface. This surface corresponds to the sensitive directions of the magnetic sensor. Three-phase brushless motors need three signals with a phase shift of 120° for control, so a closed-loop regulation may be used to improve the motor performance.

Problem related to speed control of brushless D.C. motor drive using sensors.

1. Low-cost Hall-effect sensors are usually used.

2. Electromagnetic variable reluctance (VR) sensors 3. Accelerometers have been extensively applied to measure motor position and speed.

B. Sensorless control of BLDC motor: Brushless dc (BLDC) motors, with their trapezoidal electromotive force (EMF) profile, requires six discrete rotors to position information for the inverter operation. These are typically generated by Halleffect 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. so sensor less 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

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2. The quasi square wave armatures current are mainly characterized through their maximum amplitude value, which directly controls the machine torque.

3. The inverter performance is very much reliable because there are natural dead times for each transistor.

D. Genetic based proportional control method: To regulate the motor speed at the desired level, a speed regulation scheme is proposed. Although disturbances can be caused by several events, e.g. supply change, sudden load change, etc, in this paper the speed regulation under mechanical load disturbances is only our particular study. To regulate the speed level, the proportional control scheme is employed and can be summarized as shown in the block

Figure 4 Sensor less Driven system.

C. Digital control of BLDC motor drive: The drive consists of speed controller, reference current generator, PWM current controller, position sensor, the motor and IGBT. The speed of the motor is compared with its reference value and the speed error is processed in proportional – integral (PI) speed controller. The output of this controller is considered as the reference torque. The BLDC motor is characterized by a two phase ON operation to control the inverter. In this control scheme, torque production follows the principle that current should flow in only two of the three phases at a time and that there should be no torque production in the region of Back EMF zero crossings. The following figure describes the electrical waveforms in the BLDC motor in the two phases ON operation. Commutation provides the creation of a rotation field. As explained, it is necessary to keep the angle between stator and rotor flux close to 90° for a BLDC motor to operate properly. Six-step control creates a total of six possible stator flux vectors. The stator flux vector must be changed at a certain rotor position. Hall sensors usually sense the rotor position. The Hall sensors generate three signals that also comprise six states. Indeed the drive characteristic and control methods are very similar for the trapezoidal and sinusoidal machines and in both cases the motor must be energized with controlled currents that are synchronized with rotor position. In this paper presented a simple and efficient modulation control system, which allows having good current waveform. To fulfil these objectives, a BDCM is used because of following advantages: 1 .The position sensor system for the shaft needs only to deliver six digital signals for commanding the transistor of the inverter.

Figure 5 Closed-loop speed regulation scheme.

To demonstrate the proposed speed regulation scheme, the proportional control (P-controller) is introduced. As obviously found in other controller design problems, the proportional gain (k) is the key to accomplish this regulation. From Fig. 4the Proportional controller output is assigned by (19). To find an appropriate value of k, some efficient search algorithm such as genetic algorithms, is employed. ∆ωc = k p ∆ω Where ∆ωc is proportional controller output kp is gain of proportional controller ∆ω is the speed error There exist many different approaches to tune controller parameters. The GAs is well-known [6] there exist a hundred of works employing the GAs technique to design the controller in various forms. The GAs is a stochastic search technique that leads a set of population in solution space evolved using the principles of genetic evolution and natural selection, called genetic operators e.g. crossover, mutation, etc. The GAs is selected to build up an algorithm to tune kp parameters. The procedure to perform the proposed parameter tuning is described as follows. First, time-domain results of the motor speed obtained by simulating the BLDC motor system in MATLAB™ [9] are collected.

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IJBSTR REVIEW PAPER VOL 1 [ISSUE 6] JUNE 2013 Second, the Genetic Algorithms (GADS TOOLBOX in MATLAB™) [9]is employed to generate a set of initial random parameters. With the searching process, the parameters are adjusted to give response best fitting close to the desired response in the abc reference signals. To perform the searching properly, its objective function is the key. In this paper, the objective function is defined by using the power loss function.

Figure 6 Hall-Effect Sensors CONCLUSIONS This paper presents a review based on application on different control strategies using different controllers for the speed control of BLDC motors. Here the first controller is superior to the second one in the sense the reduced cost and easily realized for industrial application for simple algorithm. In a digital control system specialised hardware peripherals provide efficient motor control with limited support circuitry, reducing the cost and complexity of the motor control hardware. In genetic base control the load torque disturbance applied to the BLDC motor operation, the rotor speed can be regulated to operate within ±5% speed error band.

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