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ELECTRIC POWER UNIVERSITY FACULTY OF CONTROL & AUTOMATION ASSIGNMENT ON SPECIALIZED ENGLISH Topic: Making A Study About DC Motor Instructor: Ths Mai Hoang Cong Minh Group: Student’s name: Nguyen Hoang Anh - Code: 18810430122 Nguyen Le Truong An - Code: 18810430184 Le Tuan Anh - Code: 18810430164 Dang Van Dat - Code: 18810430179 Luong Quoc Dai - Code: 18810430183 Class: D13TDH&DKTBCN2 HaNoi,6/2021 Table of Contents Introduction Chapter 1: General introduction of DC motor 1, What is a DC Motor? 2, Classification of dc motors 2.1, Separately Excited DC Motor 2.2, Series excited DC motor 2.3 Shunt excited DC Motors 2.4 Compound excited DC motor 3, The advantage of a DC motor 4, Disadvantages of DC motor Chapter 2: Structure and working principle of DC motor 1, Structure of a DC motor .9 1.1, Stator 1.2, Rotor 11 2, Working Principle of a DC Motor 14 2.1, Starting of DC Motors .17 2.2, Necessity of Starter for a DC Motor 18 2.3, Starting Methods of DC Motor 19 Chapter : Applications of DC Motors .22 3.1, Applications of DC motor .22 3.2, Applications of Series Excited DC motor .23 3.3, Applications of Shunt Excited DC motor 24 3.4, Applications of Compound Excited DC motor 24 3.5, Applications of Separately Excited DC motor 25 Chapter : Conclusion 26 References 27 Table of Figures Fig 1.1: Separately Excited DC Motor Fig 1.2: Series Excited DC Motor .4 Fig 1.3: Shunt Excited DC Motor .5 Fig 1.4: Compound Excited DC Motor .6 Y Fig 2.1: Structure of DC Motor Fig 2.2: Yoke 11 Fig 2.3: Field Winding 11 Fig 2.4: Armature Winding .12 Fig 2.5: Armature core, Brush and Commutator .13 Fig 2.6: Commutator 14 Fig 2.7: Working principle of DC Motor 16 Fig 2.8: Position of main field Fm and rotor field Fr 17 Fig 2.9: Motor Action .17 Fig 2.10: Three-point shunt motor starter 20 Fig 2.11: Four-Point DC motor starter 21 Introduction Nowaday, the DC motor still occupies an important position in the automatic control system of electric drive, it is widely used in the system that requires high accuracy, wide adjustment area and complex regulation complex Along with the progress of human civilization, we can witness the rapid development of both the scale and the level of modern production In that development we can also clearly recognize and asserts that electricity and power-consuming machines play an indispensable role It is always one step ahead as a premise but also as a key to determining the success of an industrial production system There is not a country, a production that does not use electricity and electric machines Today, our country's economy is on a strong development trend towards industrialization - modernization And in modern production, DC machine is still considered an important machine It can be used as an electric motor, generator or in other working conditions Among them, electric motor is increasingly widely used in industries from small to large DC motor has very good speed control effect, so the machine is widely used in industries with high requirements for speed regulation such as steel rolling, mining, transportation Therefore, learning and applying DC motors is an extremely necessary job for each of us engineers in particular and the automation industry in general Chapter 1: General introduction of DC motor 1, What is a DC Motor? A DC motor is an electric motor that runs on direct current (DC) electricity A direct current (DC) motor is a type of electric machine that converts electrical energy into mechanical energy DC motors take electrical power through direct current, and convert this energy into mechanical rotation DC motors use magnetic fields that occur from the electrical currents generated, which powers the movement of a rotor fixed within the output shaft The output torque and speed depends upon both the electrical input and the design of the motor 2, Classification of dc motors Classification of DC motors When considering DC motors as well as DC generators, they are classified according to the way the motor is excited Accordingly, we have commonly used DC motors: +) Separately Excited DC Motor: Armature and excitation are supplied from two separate sources +) Shunt Excited DC motor: The field winding is connected in parallel with the armature +) Series Excited DC motor: The excitation winding is connected in series with the armature +) Compound Excited DC motor: Consists of excitation windings, one in parallel with the armature and one in series with the armature 2.1, Separately Excited DC Motor Fig 1.1: Separately Excited DC Motor In separately excited DC motor, separate supply Provided for excitation of both field coil and armature coil The field coil is energized from a separate DC voltage source and the armature coil is also energized from another source Armature voltage source may be variable but, independent constant DC voltage is used for energizing the field coil So, those coils are electrically isolated from each other, and this connection is the specialty of this type of DC motor Both in shunt wound dc motor and separately excited dc motor field is supplied from constant voltage so that the field current is constant Therefore, these two motors have similar speed -armature current and torque – armature current characteristics In this type of motor flux is assumed to be constant Speed of this type of dc shunt motor is controlled by the following methods: - Field control methods - Field rheostat control - Armature control methods Separately excited dc motors have industrial applications They are often used as actuators This type of motors is used in trains and for automatic traction purposes 2.2, Series excited DC motor Fig 1.2: Series Excited DC Motor Series DC motors are a group of self-excited DC motors in which the field coil is connected in series to the armature winding and thus a higher current pass through it A series DC motor mission is to turn the electrical energy into mechanical one based on electromagnetic law In this process, the cooperation between the magnetic field present around a current-carrying conductor and an outside field results in a rotational motion on an output shaft A series DC motor converts electric energy to mechanic energy based on the electromagnetic principle In this type of DC motor, a power supply terminal sits at one end of the armature and field coils By applying the voltage, power starts in these terminals and passes through armature and field windings As conductors in this winding are huge, they have minimal resistance As a result, the motor gets a huge supply of power from the terminals By the flow of this large current in the armature and field coils, a strong magnetic field is produced that creates a massive torque in the shafts This strong torque, in turn, spins the armature and produces the target mechanic energy The series DC motors are used where high starting torque is required and variations in speed are possible For example – the series motors are used in the traction system, cranes, air compressors, Vaccum Cleaner, Sewing machine, etc 2.3 Shunt excited DC Motors Fig 1.3: Shunt Excited DC Motor Shunt DC motors are a type of direct connection motors in which the shunt field winding is only shunt to the armature winding and not the series field winding Shunt DC motors are also known as compound wound DC motors Shunt DC motors provide excellent speed regulation, as the shunt field can be excited separately from the armature windings This feature of shunt direct connection motors provides simplified reversing controls Because of their self-regulating speed capabilities, shunt DC motors are ideal for applications where precise speed control is required Keep in mind, however, that they cannot produce high starting torque, so the load at startup must be small Applications that meet these criteria and are suitable for shunt DC motors include machines tool such as lathes and grinders, and industrial equipment such as fans and compressors 2.4 Compound excited DC motor Fig 1.4: Compound Excited DC Motor Compound DC motors or compound wound motors have both series and shunt field windings These types of motors offer good starting torque, but they may have control problems in variable speed drive applications These motors can be connected to two arrangements: cumulatively and differentially The cumulative type connects the series field to aid the shunt field, providing higher starting torque but less speed regulation The differential type has good speed regulation and usually operates at a constant speed Because of the capabilities of the compound motors to provide high starting torque and also speed regulation and control it has many applications in different areas Below we listed some of the applications of a compound DC motor: - Compound motors due to their ability to perform better on heavy load changes are used in elevators - Due to their high starting torque and better speed control for pressure variations, they are used in shears and punches - This kind of motors because of the high starting torque and heavy-duty load is used in steel rolling mills - Again due to the capacity of driving heavy loads, they are used in the printing press and cutting machines - They are also used in stamping presses to provide high starting torque - Their good speed control and high starting torque make them a great choice to be used in mixers The small DC machines whose ratings are in fractional kilowatt are mainly used as control device such in techno generators for speed sensing and in servo motors for positioning and tracking 3, The advantage of a DC motor The advantage of a DC motor is that it can be used as an electric motor or generator in different working conditions But the biggest advantage of DC motors is speed regulation and overload capacity If the asynchronous motor itself cannot meet the requirements, or if it is, the cost of associated converters (such as frequency converters ) is very expensive, then the DC motor does not Wide and precise adjustments can be made, but the power circuit structure, the control circuit is simpler, and the quality is high Different sizes of DC motor parts will create different DC motors, suitable for different needs As mentioned before, small ones can be used in toys, tools, and home appliances and larger ones are used in the elevator and hoists and propulsion of electric vehicles Although AC motors decreased the selling of DC motors on account of simple generation and transmission with fewer losses to long distances, needing less maintenance and can be operated in explosive atmospheres, DCs still are utilized in where ACs can’t fulfill the needs DCs have their own unique features and importance in industries that make up for lots of other advantages that AC motors have over them DC motors are suitable for low-speed torque, or when having an adjustable speed and constant is necessary In other words, with DC motors the speed can be controlled over a wide range That means they offer a wide range of speed control both below and above the rated speed This feature of DC motors can be got in shunt types By armature controlling and field controlling, you can enjoy this unique advantage of DC motors over AC motors Moreover, DCs have a very high and strong starting torque compared to normal operating torque Therefore, DCs are used in electric trains and cranes having overwhelming burdens in the beginning conditions In addition to the above-mentioned advantages, DC motors have smaller converters and drives as well as higher motor power density Not to mention that they have full torque at zero speed! Being around on the market for more than 140 years, DC motors are often more affordable than AC motors and have a simpler and more efficient design Plus, their maintenance is easy and takes little to no time If you redesign your current installation to use an AC motor, it will cost way more than just simply replacing the DC motor inside the installation So, you not only repair your system by installing a new unit inside but save lots of money Needless to said that such small replacement saves time uniformly over the entire periphery of the core The slot openings a shut with fibrous wedges to prevent the conductor from plying out due to the high centrifugal force produced during the rotation of the armature, in presence of supply current and field Used for magnetic conduction, usually 0.5mm thick electrical engineering steel sheets coated with thin insulation on both sides and then pressed tightly to reduce losses caused by eddy currents On the steel foil, there is a groove shape stamping so that after pressing it, the wire is put in In the medium and higher engines, people also stamp the ventilation holes so that when pressed into the iron core, it is possible to create axial ventilation holes Fig 2.5: Armature core, Brush and Commutator 1.2.3, Brushes of DC Motor The brushed DC motor generates torque directly from DC power supplied to the motor by using internal commutation, stationary permanent magnets, and rotating electrical magnets The brushes of DC motor are made with carbon or graphite structures, making sliding contact over the rotating commutator The brushes are used to relay the current from external circuit to the rotating commutator form where it flows into the armature winding So, the commutator and brush unit of the DC motor is concerned with transmitting the power from the static electrical circuit to the mechanically rotating region or the rotor 1.2.4, Commutator of DC Motor The commutator of DC motor is a cylindrical structure made up of copper segments stacked together, but insulated from each other by mica Its main function as far as the DC motor is concerned is to commute or relay the supply current from the mains to the armature winding housed over a rotating structure through the brushes of DC motor 13 The commutator is a split ring made up of Copper segments, Commutator is another DC motor part The operating system of a DCs is based on the interaction of the two magnetic fields of rotating armature and a fixed stator As the north pole of the armature is attracted to the south pole of the stator and south pole of armature is attracted to the north pole of the stator, a force is produced on the armature which makes it to turn the process in which the field in the armature windings is switched to produce constant torque in one direction is called Commutation the commutator is a device connected to the armature enabling this switching of current Different segments of its cylindrical structure are insulated from each other by Mica The commutator is designed to commute the supply current to the armature winding from the mains The commutator passes through the brushes of the DC motor The basic purpose of commutation is to certify that the torque acting on the armature is always in the same direction Naturally, the generated voltage in the armature is alternating, the commutator converts it to the direct current To control the direction the electromagnetic fields are pointing to, the commutator turns the coils on and off On one side of the coil, the electricity should always flow away, and on the other side, electricity should always flow towards This ensures that the torque is always produced in the same direction Fig 2.6: Commutator 1.2.5, Shaft Shaft: The part on which the armature iron core, commutator, propeller, ball bearing The machine shaft is usually made of good carbon steel 2, Working Principle of a DC Motor The operation of a DC motor is based on the principle that when a current carrying conductor is placed in a magnetic field, a mechanical force is experienced by it The 14 direction of this force is determined by Fleming’s Left-Hand Rule and its magnitude is given by the relation: F = BIL Newton  Fleming’s left-hand rule: Fig.: Fleming’s left hand rule Fleming’s left-hand rule is use to determine direction of force acting on the armature conductor of DC motor Statement: “If you keep your left hand in a way that the thumb, first finger and middle finger are at right angle to each other” as shown in fig: “first finger represent direction of magnetic field, middle finger represent direction of the current in conductor, thumb will represent direction of force act on the conductor” For simplicity, consider only one coil of the armature placed in the magnetic field produced by a bipolar machine [see Fig 2.7a] When DC supply is connected to the coil, current flows through it which sets up its own field as shown in Fig 2.7b By the interaction of the two fields (i.e., field produced by the main poles and the coil), a resultant field is set up as shown in Fig 2.7c The tendency of this is to come to its original position i.e., in straight line due to which force is exerted on the two coil sides and torque develops which rotates the coil 15 a, Main field b, Field due to current carrying coil c, Resultant field Fig 2.7: Working principle of DC Motor Alternately, it can be said that the main poles produce a field Fm Its direction is marked in Fig 2.8 When current is supplied to the coil (armature conductors), it produces its own field marked as Fr This field tries to come in line with the main field and an electromagnetic torque develops in clockwise direction as marked in Fig 2.8 In actual machine, a large number of conductors are placed on the armature All the conductors, placed under the influence of one pole (say, North pole) carry the current in one direction (outward) Whereas, the other conductors placed under the influence of other pole i.e., south pole, carry the current in opposite direction as shown in Fig 2.9 A resultant rotor field is produced Its direction is marked by the arrowhead Fr This rotor field Fr tries to come in line with the main field Fm and torque (Te) develops Thus, rotor rotates 16 Fig 2.8: Position of main field Fm and rotor field Fr Fig 2.9: Motor Action It can be seen that to obtain a continuous torque, the direction of flow of current in each conductor or coil side must be reversed when it passes through the magnetic neutral axis (MNA) This is achieved with the help of a commutator Function of a Commutator The function of a commutator in DC motors is to reverse the direction of flow of current in each armature conductor when it passes through the M.N.A to obtain continuous torque 2.1, Starting of DC Motors To start a DC motor, when it is switched–ON to the supply with full rated voltage, it draws heavy current during starting period (more than its rated value) This excessive current overheats the armature winding and may even damage the winding insulation Therefore, during starting period a resistance called starter in connected in series with the armature circuit to limit the starting current 17 2.2, Necessity of Starter for a DC Motor The starting of DC motor is somewhat different from the starting of all other types of electrical motors This difference is credited to the fact that a DC motor unlike other types of motor has a very high starting current that has the potential of damaging the internal circuit of the DC motor if not restricted to some limited value This limitation to the starting current of DC motor is brought about by means of the starter Thus the distinguishing fact about the starting methods of DC motor is that it is facilitated by means of a starter Or rather a device containing a variable resistance connected in series to the armature winding so as to limit the starting current of DC motor to a desired optimum value taking into consideration the safety aspect of the motor E b  V  I aR a or I aRa  V  E b Where, E is the supply voltage, Ia is the armature current, Ra is the armature resistance The armature current is given by the relation Ia  V  Eb Ra When the motor is at rest, the induced emf Eb in the armature is zero (Eb v N) Consequently, if full voltage is applied across the motor terminals, the armature will Ia  V Ra draw heavy current ( ) because armature resistance is relatively small This heavy starting current has the following effects: (i) It will blow out the fuses and prior to that it may damage the insulation of armature winding due to excessive heating effect if starting period is more (ii) (ii) Excessive voltage drop will occur in the lines to which the motor is connected Thus, the operation of the appliances connected to the same line may be impaired and in some cases they may refuse to work To avoid this heavy current at start, a variable resistance is connected in series with the armature, called a starting resistance or starter, and thus the armature current is limited to safe value is reduced (I a  (I a  V ) Ra  R V  Eb ) Ra  R Once the motor picks up speed, emf is built up and current After that the starting resistance is gradually reduced 18 Ultimately, whole of the resistance is taken out of circuit when the motor attains normal speed Another important feature of a starter is that it contains protective devices such as overload protection coil (or relay) which provides necessary protection to the motor against over loading and no-volt release coil 2.3, Starting Methods of DC Motor To avoid the above dangers while starting a DC motor, it is necessary to limit the starting current So, a DC motor is started by using a starter There are various types of dc motor starters, such as 3-point starter, 4-point starter, no-load release coil starter, thyristor controller starter etc 2.3.1, Three-Point Shunt motor Starter The schematic connection diagram of a shunt motor starter is shown in Fig 2.10 It consists of starting resistance R divided into several sections The tapping points of starting resistance are connected to number of studs The last stud of the starting resistance is connected to terminal A to which one terminal of the armature is connected The + ve supply line is connected to the line terminal L through main switch From line terminal, supply is connected to the starting lever SL through over load release coil OLRC A spring S is placed over the lever to bring it to the off position, when supply goes off A soft iron piece SI is attached with the starting lever which is pulled by the no volt release coil under normal running condition The far end of the brass strip BS is connected to the terminal Z through a no volt release coil NVRC One end of the shunt field winding in connected to Z terminal of the starter An iron piece is lifted by OLRC under abnormal condition to short circuit the no-volt release coil The negative supply line is connected directly to the other ends of shunt field winding and armature of the DC shunt motor 19 Fig 2.10: Three-point shunt motor starter Operation First of all the main switch is closed with starting lever resting in off position The handle is then turned clockwise to the first stud and brass strip As soon as it comes in contact with first stud, whole of the starting resistance R is inserted in series with the armature and the field winding is directly connected across the supply through brass strip As the handle is turned further the starting resistance is cut out of the armature circuit in steps and finally entire starting resistance is cut out of armature circuit 2.3.2, Four-point Starter For speed control of DC shunt or compound motors, a rheostat (variable resistor Rh) is connected in series with the field winding, as shown in Fig 2.11 In this case, if a three-point starter is used and the value of Rh is so adjusted that the current flowing through the shunt field winding is very small It may be seen that the same current flows through the no-volt release coil, then the magnetic strength of the coil may be insufficient to hold the plunger at its ON position This is an undesirable feature of a three-point starter This feature makes a three-point starter unsuitable for such applications Accordingly, a four-point starter is designed, as shown in Fig 2.11, in which the current flowing through the no-volt release coil is made independent of the 20 shunt field circuit Figure 2.11a shows a four-point starter used with a shunt wound motor whereas Fig 2.11b shows a starter used with a compound wound machine a, Four-Point Shunt motor starter b, Four-Point Shunt motor starter Fig 2.11: Four-Point DC motor starter Operation The working of a four-point starter is similar to a three-point starter with slight changes.In this case, when the plunger touches the first stud, the line current is divided into the following three parts: (i) First part passes through starting resistance and armature (as well as in (ii) series field for compound motors) Second part passes through the field winding (and speed control resistance if (iii) applied) and The third part passes through no-volt release coil and protective resistance connected in series with the coil 21 Chapter : Applications of DC Motors In the previous chapter, we went to learn the structure and working principle of DC motor In this section, we will introduce the applications of DC motors commonly used today 3.1, Applications of DC motor Generally, because of some specific advantages of each type of DC motors, there are various uses of them At home, small ones are used in tools, toys and many household appliances Some other applications of DCs include conveyors and turntables and in industries, huge DCs usages consist of braking and reversing applications We tried to bring some specific examples as the DCs applications: 3.1.1, Pumps Hydraulic pumps as an essential industrial tool are used in almost all industries such as construction, mining, manufacturing and steel DC motors because of their variable speed control and also excellent starting torque are used to empower these kinds of pumps Most of the time lower-cost brushless DCs are used in pumps which make it far easier to maintain on such a large industrial scope 3.1.2, Toys Due to the fact that small DC motors are easy to use and considerably rugged, they are the best choice of manufacturer and hobbyists for children’s toys such as remotecontrol cars and trains Toys that require different range of speed and types of movements need a motor with a wide variety of voltages Manufacturers find all of these specifications in DCs 3.1.3, Electric Cars Another application for DCs is in electric cars DC motors because of their energy efficiency and durability are one of most favorite options for electric vehicles Moreover, many hobby use DCs because of their great and higher starting torque particularly series-wound motors, and their variable speeds with voltage input 3.1.4, Robots For many hobbyist and engineering robots are any electromechanical devices designed to one or more specific tasks To activate things like tracks, arm or cameras, DC motors are one of the most achievable and reasonable choices with lower 22 costs The specifics like high torque and durability and also efficiency make DCs ideally suitable for robotics 3.1.5, Hydraulic pump: As an essential industrial tool, used in almost every industry including; construction, mining, steel manufacturing… DC motors are used to power these pumps because of their easy variable speed control and excellent response to travel 3.1.6, Pumping system, water pump The rotating shaft of the rotor is attached to the impeller to absorb water to pump the whole system used for daily life, agricultural irrigation system High pressure pump systems are often installed with air vents to increase pressure and are used in industrial pumping and fire protection systems Pumps are the leading known application of electric motors 3.1.7, Ventilation system, fan, exhaust fan In buildings, factories, schools, hospitals, all must have ventilation systems to provide air for work and living Similar to a pump, the motor draws air from the outside in, after passing through the air filter, the air is brought to all parts of the building Then, there is an electric motor placed to make the exhaust fan suck the air out The air convection makes the workplace and living space airy Both cool, have enough air to use, and prevent moisture and disease, bacteria In addition, today people also apply in parts of computers, electric vehicles, freight vehicles in industrial parks and thousands of applications of electric motors in life and production In the context that the future will reduce fossil fuel consumption, electric motors are not new but a future trend As per the characteristics of DC motors, different types of DC motors are applied for different jobs as explained below: 3.2, Applications of Series Excited DC motor DC series motor is suitable for both high and low power drives, for fixed and variable speed electric drives Because of its high starting torque, this motor uses Series motors: The characteristics of a series motor reveal that it is variable speed motor i.e., the speed is low at higher torques and vice-versa Moreover, at light loads or at no-load, the motor attains dangerously high speed It is, therefore, employed: (i) Where high torque is required at the time of starting to accelerate heavy loads quickly 23 (ii) Where the load is subjected to heavy fluctuations and speed is required to be adjusted automatically As such the series motors are most suitable for electric traction, cranes, elevators, vacuum cleaners, hair driers, sewing machines, fans and air compressors, etc Note: The series motors are always directly coupled with loads or coupled through gears Belt loads are never applied to series motor, because the belt may slip over the pulley or it may break Then the motor will operate at light loads or at no-load and will attain dangerously high speed which may damage the motor 3.3, Applications of Shunt Excited DC motor Shunt motors: From the characteristics of a shunt motor we have seen that it is almost constant speed motor It is, therefore, used: (i) Where the speed between no-load to full load has to be maintained almost constant (ii) Where it is required to drive the load at various speeds (various speeds are obtained by speed control methods) and any one of the speed is required to be maintained almost constant for a relatively long period As such the shunt motors are most suitable for industrial drives such as lathes, drills, grinders, shapers, spinning and weaving machines, line shafts in the group drive, etc 3.4, Applications of Compound Excited DC motor The cumulative compound motor has high starting torque characteristic Also, it has good speed regulation at high speed so it uses in Presses, Electric shovels, Conveyors, Elevators, Compressors, Rolling mills, Heavy planners Compound motors: The important characteristic of this motor is that the speed falls appreciably on heavy loads as in a series motor, but at light loads, the maximum speed is limited to safe value It is, therefore, used; (i) Where high torque is required at the time of starting and where the load may be thrown off suddenly (ii) Where the load is subjected to heavy fluctuations As such the cumulative compound, motors are best suited for punching and shearing machines, rolling mills, lifts and mine – hoists, Heavy Planers, etc 24 3.5, Applications of Separately Excited DC motor Separately excited motors: Very accurate speeds can be obtained by these motors Moreover, these motors are best suited where speed variation is required from very low value to high value These motors are used in steel rolling mills, paper mills, diesel – electric propulsion of ships, etc 25 Chapter : Conclusion Learning and researching about DC motors is a necessary job It helps us better understand their structure, principles as well as their wide application Because of different types of DC motors, there are many applications for this type of DC motor in all areas of life: in televisions, industrial machines, in FM radio, DC drives, copiers, especially in the transportation industry transportation, and devices that need to control the speed of continuous rotation over a large range……All around us, small DC motors are used in tools, toys and various household appliances In industry, the applications of DC motors include conveyors and turntables, etc., the use of large capacity DC motors in applications such as braking and reversing In general, DC motors have achieved proud results in the process of formation and development Although each type has its advantages and disadvantages, in general, DC motors have many different uses and are an integral part of all areas of our lives today 26 References 1, Electrical Machines Book 2, Wikipedia.com 3, electrical4u.com 4, circuitglobe.com 5, electricaleasy.com 27