Logic gates & their Truth Table

     Logic gates are digital circuits that do processing of digital circuits. Gates have one or more inputs but only one output. There are three basic logic gates and they are named as OR gate, AND gate & NOT gate. These three gates can be combined in various ways to perform more complex arithmetic functions based on processed input digital signals.

     There are two types of gates named as sequential gates and combinational gates. One type of logic gates called sequential gates, which have memory function, can process sequence of input digital values and outputs are based on sequences of applied inputs. Example of sequential gates are Flip-flops, Counters & Registers. Another types of logic gates called combinational gates, which do not have memory function, can process only instantaneous digital inputs and outputs are based on inputs applied at the moment of time. Examples of combinational gates are OR gate, AND gate, NOT gate, XOR gate, NAND gate, etc.

     Different types of logic gates which will be discussed here are ;

1. OR gate,
2. AND gate,
3. NOT gate,
4. NOR gate,
5. NAND gate,
6. EXCLUSIVE OR gate, &
7. EXCLUSIVE NOR gate.

OR gate

     This gate can have two or more than two inputs but only one output. OR gate is named so because output signal will be high if any of the input signals are high. Truth table and symbol of OR gate with two inputs are given below ;

OR gate & Its Truth Table

Output of OR gate is "X = A + B".

AND gate

     This gate can have two or more than two inputs but only one output. AND gate shows output only when all the inputs are applied at same time. Truth table and symbol of AND gate with two inputs are given below ;

AND gate & its Truth Table

Output of AND gate is "X = A . B".

NOT gate

     This gate has only one input and one output. NOT gate always gives output opposite to that of input signal i.e., if input signal is 0 then output will be one and vice versa. Truth table and symbol of NOT gate are given below ;

NOT gate & Its Truth Table

Output of NOT gate is "X = Ā".

NOR gate

     This gate can have two or more than two inputs but only one output. NOR gate is a combination of OR gate and NOT gate. NOT gate is connected to the output of OR gate. So the output of NOR gate is always opposite to that of OR gate. Truth table and symbol of NOR gate with two inputs are given below ;

NOR gate & Its Truth Table

NAND gate

      This gate can have two or more than two inputs but only one output. NAND gate is a combination of AND gate and NOT gate. NOT gate is connected to the output of AND gate like NOR gate. So the output of NAND gate is always opposite to that of AND gate. Truth table and symbol of NAND gate with two inputs are given below ;

NAND gate & Its Truth Table

EXCLUSIVE OR (XOR) gate ;

Ex-OR gate & Its Truth Table

EXCLUSIVE NOR gate

Exclusive NOR gate & Its Truth Table

Few other posts seeking your attention are ;

1. Rectifiers Theory @ Industrial Items

2. Hooke's Law @ All Laws of Science & Imporant Formulas

Cloud Computing

Now these days, we get a term to listen very frequently in news papers, TV channels, etc., which is  "Cloud Computing". Now couple of questions arise that what is this Cloud Computing & why everybody is discussing it ?

  

Let’s understand term Cloud Computing

To understand the concept of "Cloud Computing", let us first understand the meaning of cloud in the term "Cloud Computing". Cloud may be referred to as a data center or simply a huge data storage facility, which is well equipped with hardware & software to continuously serve the needs of the users & companies, owned by third party somewhere on the web. Companies store their data on the cloud and their customers or users  can access data stored by the company free of cost or on the basis of pay-per-use and this is called Cloud Computing. The data stored on the cloud can be any application software or information pertaining to some product, company, service, etc. One of the biggest and oldest examples of cloud computing is E-mail provided by Yahoo Mail or Gmail. Google Apps is another example of cloud computing, where users can access various applications free of cost like document viewers, translator, calender, Blogger, Picassa, etc.

          In general, cloud computing is delivering information, application software & other web based services for use by a company to users while hosting them on a cloud rather than investing in their small servers, which in general are expensive, not flexible to accommodate ever increasing future data storage requirements & prone to obsolete as technology is changing every second.

Pay per use – A feature of Cloud Computing

Now, to understand the concept of " pay per use ", the term used above, we can take an example of a small shop keeper, who needs a Tally software or any text editing software for only two or three times in a month not more than half hour or an hour, he would then use, on demand, the required software online simply on his browser using cloud computing and would pay a minimal amount as per usage rather than buying costly software. So one of the important characteristic of cloud computing is on demand usage of data, software, hardware or storage space.

Cloud may be public or private. A public cloud is one which is available for all. Anybody, who wish to use such cloud can pay as per usage to use cloud. Or simply, a cloud owned by a third party is generally a public cloud. Whereas private cloud is available for only one or a limited number of users or customers.

  

Benefits & Limitations of Cloud Computing

So, a question arises here that " why cloud computing ? ". Cloud computing will change future of IT because of its following benefits ;

1.     More Green Technolgoy ; Cloud computing is more green technology because having a number of small capacity servers create more electronic waste and more electricity in running servers & for their air-conditioning is required. So switching to cloud rather than investing in a number of small capacity servers is more environment friendly. Big companies may have large capacity servers by considering their future data storage space requirements but it also produces more electronic waste & consumes more electricity than cloud computing and at the same it is wastage of resources as such large capacity servers may not be used to their full capacity. So, the intent of cloud computing is not just using cloud but utilize the cloud in its full capacity, which specifically makes it more Green Technology.

2.     More Cost Effective ; Investing in costly hardware & software is not required with cloud computing. As I have already discussed above that we can use resources, whether it is hardware or software, on the basis of pay-per-use, so that the capital investment cost is almost zero. In conventional or earlier IT technology, where we maintain servers locally, also require administration staff, which makes it costlier. But in case of switching to cloud owned by third party or service provider, they them-self manage the administration of cloud. Again, if people invest in software, it is required to upgrade very frequently with its latest version. For example, document files created by using MS-Office 2007 is not compatible with its earlier versions. Cloud computing may be the solution of this problem, for example, we may read doc files using google docs free of cost and may prepare reply using existing version of MS-Office with upgrading the same.

3.     Flexibility ; It is generally difficult to handle ever increasing requirements of data storage space, costly hardware & costly software. Cloud computing is flexible enough to accommodate these requirements. As "on demand usage" is the characteristic of cloud computing, we may increase or decrease the usage of resources as per our requirements.

4.   Requirement of High band-width ; Improvements in technology like IT products and especially internet connectivity is the main factor, which is making companies to shift to cloud computing & making it a future technology. Therefore, one important thing to consider is if the internet connectivity is poor or internet connection, which we use for consuming services based on cloud computing, is slow then it can be a worst experience using cloud computing.

Few other posts seeking your attention are ;

BEE Energy Label & its Significance

CURRENT TRANSFORMER, POTENTIAL TRANSFORMER OR CT, PT

Current Transformer

    Current transformers and Potential transformers are widely used in industries to measure alternating currents and voltages, respectively, of high magnitudes and to operate and control protection devices. Current transformers and Potential transformers, like other transformers, have two windings i.e., Primary and Secondary windings. The output of these transformers, i.e., secondary winding remain connected to other instruments like energy meters and other protection devices. Current transformers and Potential transformers reduce high value alternating currents and voltages, respectively, flowing in main transmission line or supply system to very low values in proportions specified. Thus, these transformers provide economic, accurate, easy, simple and safe way of handling supply currents and voltages to get monitored and controlled. The magnitude of outputs of these transformers depends upon the ratio of the transformer. These transformers are also known as Instrument Transformers. Current transformer and potential transformer are also commonly called CT and PT respectively.

CONSTRUCTION OF CURRENT TRANSFORMERS ;

     Current transformers have primary windings, secondary windings and a core of magnetic material like normal transformers. Function of current transformers has to reduce transmission or supply current to low values in specific proportions to be sensed by other display, metering and protective instruments. For this reason, primary windings of current transformers have very few turns and some CTs have only one turn whereas, secondary windings of current transformers have large number of turns. Conductors of primary windings of CTs are of thicker gauges or heavy wires of higher diameters to handle high transmission currents. Primary windings are connected in Series of transmission lines. Some current transformers do not have primary windings instead a transmission line or supply line is passed through a hole of magnetic core wrapped over with conductors of secondary windings. These transformers are available in many shapes and sizes.

CONSTRUCTION OF POTENTIAL TRANSFORMERS ;

        Construction of Potential transformers is similar to Current Transformers, as discussed above, except the number of turns in secondary winding are much less than primary winding just like any step down transformer.

Types of Instrument Transformers ;

                

     Some of the common types of Instrument Transformers are Wire-wound and Ring or Toroidal type transformer. Rectangle CTs & Split-core CTs are also available, so that CTs can be fixed without removing or opening connections. Standard output of Instrument CTs is 1 Amp or 5 Amp and standard output of Instrument PTs are 110 V or 220 V. CTs can be of Oil-immersed as well.

                

      Important characteristics of Instrument transformers, which should be taken into consideration while selecting Instrument transformers are ; i) Input & Output ratio or Transformer Ratio, ii) Standard Load, iii) Accuracy, iv) Rated Voltage, etc.

    Some of the Applications of Instrument transformers are ; i) Metering Inputs, ii) Monitoring Loading and Un-loading of power transformers, heavy motors, etc., iii) Protection against over-loading of various equipment like power transformers, to operate protective relays, circuit breakers and switch gears, iv) Earth fault or leakage protection, etc.

     Current Transformer and Potential Transformer together can be used to measure Power with the help of Watt Meter.

      

SECONDARY OF CURRENT TRANSFORMERS CANNOT BE LEFT OPEN OR KEPT SHORT CIRCUITED ;

     Secondary terminals of Current transformers should not be left open because of high induced e.m.f. in secondary windings. Current transformers work as Step-up transformers which increase input e.m.f. as primary windings have only one or two turns of conductors whereas secondary windings have many hundreds of turns of conductor wrapped around magnetic core thereby magnify e.m.f. or voltage to many times which can be dangerous and source of accident. So secondary terminals of these transformers are kept short circuited when not connected with helping instruments.

     In the end, instrument transformers prove to be very useful in industries in sensing current flow and e.m.f. between two or more terminals of high magnitudes very efficiently, economically, safely and easily. 

     

Few other posts seeking your attention are ;

Coulomb's Law

Engineering Drawing Orthographic Projection

    Engineering drawing is two-dimensional depiction of various phases of actual objects. To make engineering drawings meaningful and executable, different phases of an object are depicted in two-dimension and placed in a particular way using technique of Projection. We need to know following terms before reading about Orthographic Projection ;

What are Projections, Projection Plane, Projection Lines  in Engineering Drawing ?

           

         As discussed above that ED is two-dimensional depiction of various phases of actual object, a phase of object is projected on a plane using imaginary lines and the two-dimensional figure then formed is called Projection or Graphical Projection of object. Imaginary lines, known as Projection Lines, are drawn from various points on contour of the object and are projected to meet at a plane known as Projection Plane.

What is Orthographic Projection in Engineering Drawing ?

        Orthographic Projection, also known as Multi-view Projection, is universally accepted way of depiction of shapes of an object through two-dimensional figures on a plane. Projection Lines are parallel to each other and are orthogonal to plane of projection. Two types of orthographic projections namely, i) First Angle Projection and ii) Third Angle Projection have been explained below ;

First Angle Orthographic Projection ;

      In first angle projection, object is placed in first quadrant so that the Vertical Plane is behind of the object and Horizontal Plane is below the object.

First Angle Projection Method ;

      In first angle projection, projections of an object are drawn in following ways ;

         a)      Top View of object is drawn below Front View.

         b)      Right Side View is drawn towards left side of Front View.

         c)       Bottom View of object is drawn above Front View.  

         d)      Left Side View is drawn towards right side of Front View.

Third Angle Orthographic Projection ;

    In third angle projection, object is placed in third quadrant so that the Vertical Plane is in front of the object and Horizontal Plane is above the object.

Third Angle Projection Method ;

   In third angle projection, projections of an object are drawn, normally, in following ways ;

        a)      Top View of object is drawn above Front View.

        b)      Right Side View is drawn towards right side of Front View.

        c)       Bottom View of object is drawn below Front View.

        d)    Left Side View is drawn towards left side of Front View.

         
     
        
                 Symbols of first angle projection or third angle projection are drawn in right bottom or top corner along-with other descriptions like tolerance, material, name of organization, etc. in engineering drawings, so that engineer before reading drawing will know that the drawing is made using first angle projection or third angle projection.

        Other Posts seeking your attention are ;

        Hooke's Law

        

        Near Field Communication

Industrial Linear Motion Guides

INTRODUCTION TO LINEAR MOTION GUIDES ;

        Linear Motion Guides are those precision machine elements which are designed with friction less, noise less, vibration free and other working characteristics to permit accurate linear relative motion between two machine members. LM Guides are available in two parts, one is LM bearing or carriage and another is LM shaft or rail over which carriage moves. These guides are made in various shapes, sizes and materials depending upon load, length of travel, speed of travel, space availability and working conditions. Linear Motion Guides find applications in almost every industry from General Purpose Machine Tools to Special Purpose Machine, from Medical Equipment to Aerospace and from Transport to Construction. All the LM guides can be classified broadly into three categories depending upon the type of contact in the guide elements during motion i.e., (i) Sliding Contact, (ii) Rolling Contact and (iii) Magnetic field. 

Classification of Linear Motion Guides ;

            Classification based on technology incorporated to achieve motion or the type of contact in LM Guide elements ;

1.  Sliding Contact Linear Motion Guides ;

        In these guides, motion takes place because of sliding action between guide elements i.e., LM block and LM guide rails or rods. These are also known as Plain LM Guides. Sliding Contact LM Guides can be further classified in to two sub-categories, depending upon thickness of lubrication film in between sliding surfaces, as given below ;

a)       Thin Lubrication Film Sliding Contact LM Guides ; A partial contact remains in between guide elements as a thin layer of lubricant sweeps in between them during motion. Low friction coefficient materials are used in construction of this category of Guides. Despite of some advantages like simple in design, low cost, etc., these guides, however, least preferred because of certain disadvantages like  high friction, high heat generation, less efficiency, low life, low load carrying capacity, high maintenance.

b)        Hydrostatic LM Guides ; In these types of bearings a thick layer of pressurized fluid holds the carriage and there is no direct contact between the moving elements. To overcome disadvantages of high friction, high heat, etc. pressurized fluid is supplied in between moving elements forcibly from external source, which makes these guides costly because it requires additional unit to supply fluid, not suitable for applications in clean-rooms as there are always chances of leakage of pressurized fluid, etc.

Examples of Sliding Contact Linear Bearings are i) Spline Shaft & Spline Nut, ii) Slide Rails, iii) Lead Screws, etc.

2.    Rolling Contact Linear Motion Guides ; 

       These LM guides consists of balls or cylindrical rollers or wheels of various shapes or profiles which enable smooth and very low frictional motion at fairly high speeds in between carriages and their guides or rails. Despite of being comparatively costly these LM Guides are most preferred because of certain advantages like trouble free operations even at high speeds, compact design, low maintenance, high load carrying capacity, high efficiency, etc.

Examples of Rolling Contact Linear Motion Guides are ;

i)               LM Guide Blocks and Rails, and ii) Ball Screws, in which hardened spherical steel balls keep on circulating in a continuous end-less grooves of carriages while motion. These spherical balls support the carriage over rail or shaft which is firmly fixed to one of the machine members as well as facilitate smooth movement of loads,

iii)       LM Roller Guide Blocks have cylindrical rollers, which facilitate relative movement in between carriage with greater load and rail, and

iv)         Guide Wheels and Rails, in which wheels with a profile usually Circular groove (semi-circle) or V groove in its outer face along circumference is provided. Also a protrusion of similar profile like Guide Wheel’s groove profile in rail is provided which fits into Wheels and guides them. These are preferred where movements of comparatively greater loads at higher velocities are required.

LM Block

Advantages of Rolling Contact LM Guides ;

a)    High Positional Accuracy,

b)   High Efficiency,

c)    High Speed Motion of loads is possible,

d)   High Loads movement is possible, 

e)   Low Noise,

f)     Less Maintenance,

g)    Fairly good Life.

3.  Magnetic Field Linear Motion Guides ; 

       These guides preferably use electromagnets to provide relative linear movements in between two machine members. Their application is limited to only clean rooms in medical and other such similar clean and tidy places as no contact between parts having relative motion eliminates use of any kind of lubrication. Disadvantages like high cost, space constraints, etc. makes them less popular.

Types of Linear Motion Guides

Different types of LM Guides, which falls in any of the above mentioned categories, available in markets have been explained below ;

1.     Linear Motion Block & Rail ; 

          These blocks are highly precise and compact in design, in which 4 rows of steel balls keep on running in continuous end-less paths. The rolling balls remain in firm touch with 4 corresponding raceways on rail and supports block on rail and enable linear movement along the rail. The re-circulation of balls also permits the motion of load along desired length by introducing additional lengths of rails. These guides come in category of rolling contact LM guides and are available with either spherical hardened steel balls or hardened cylindrical rollers, which also re-circulate or guide wheels to achieve rolling action. These are most commonly used guides because of certain advantages like high accuracy, high efficiency, availability, high speed motion, etc.,

2.     Linear Motion Ball Bearings & Shafts ; 

         These are also a type of LM guides which permits linear relative motion. LM Ball Bearings are made in ring shape which carries continuous end-less raceways along its length to guide hardened spherical steel balls during motion. And as the bearings are in ring shape, they deliver motion through cylindrical shafts.  These bearings are available in various shaft sizes and types like closed-type and open-type, with or without flanges, Pillow blocks, etc. Open-type bearings are made cut open along its length to guide shafts mounting on fulcrums. These guides also come in Rolling Contact LM Guides.

3.    Linear Motion Stroke ; 

         These are also made in ring shape like LM bearings, except it has a drilled ball-cage in which holes are arranged spirally along the surface of cage and so the balls are and steel balls do not re-circulate unlike LM bearings. After inserting balls in holes of cage, holes are mechanically deformed through suitable process usually caulking, which narrows holes and cage retains balls and they do not fall. LM Strokes are also rolling contact type and they are suitable for rotational and reciprocating motions. These are used for small distance and high speed movements. 

4.  Linear Motion Spline Nut & Shaft ; 

         A Spline shaft is a round shaft in which a series of continuous equally spaced grooves are cut along the length of the shaft. Spline shafts, therefore, has teeth along its periphery and gives gear like structure when looking from cross-section area.  Thus, spline nut has similar teeth at its internal surfaces, which fits into corresponding grooves on shaft and hence provide stable sliding motion along shaft length. These are Sliding Contact type LM Guides. These are less efficient, they have high friction, low positioning accuracy. Advantages are ; Low Cost, Good Life, Less Maintenance, etc.

5.   Lead Screw ; 

      These screws normally have square threads instead of V threads as they are more efficient. Lead Screws are used to convert rotational motion of screw into linear motion of Nut. Backlash, low efficiency, high wear are the disadvantages of these screws.

6.   Ball Screw ; 

       These guides like Lead Screws convert rotational motion of screw into linear motion of Nut. Unlike lead screws, these are highly efficient because rolling motion of steel balls in between grooves cut in the nut and screw shaft. Steel balls re-circulate in the groove. Wear rate in these guides are much less.

Other LM Guides available in market are ; Ball Spline, Slide Rail, Guide Wheels and some LM Tables with or without drive.

Applications of Linear Motion Guides ;

Some common applications are ;

LM Block

• Machine Tools like Horizontal Machining Centres (HMC), Vertical Machining Centres (VMC), CNC machine tools, etc.,
• Industrial Robots,
• Medical Equipments,
• Electronic Equipments,
• Special Purpose Machineries.

Selection of Linear Motion Guides ;

 LM Guides are selected on the basis of following points ;

• Load to be moved,
• Moving Speed of Load,
• Space Availability,
• Accuracy,
• Length of Movement,
• Efficiency,
• Working Conditions, &
• Inter-changeability & Easy Availability

A thought from personal experience ; An LM Block was broken during my service to a Diaper Manufacturing Company. The constraint was that the machine manufacturer was Chinese and spare was not available with us or anywhere in Indian market. It was also not possible to get it delivered from China as it would have taken long time. So, I checked model number of LM Block engraved on it and searched it over internet. I found company's catalog and after comparing all the specifications of LM block with Indian manufacturer, I got part number of local brand Block. I ordered block and in that case rail also had to purchase. Just because of that I had put a point of Easy Availability in Points of Selection.

Guides also need to be protected from dust for which manufacturers provide some accessories like bellows to cover rail and open parts of carriage.

Few other posts seeking your attention are ;

1. Hooke's Law

Star Delta Starter Theory

     Electrical motors have been used in industries for quite a long period of time to convert electrical energy into mechanical energy. Three phase induction motors, also called asynchronous motors, are most extensively used motors in industries because of certain advantages like self starting, robust design, simple construction, less maintenance, efficient and comparatively low cost, though there is a problem of peak starting current associated with these motors. Peak starting current can be up to 5 to 7 times of full load current (flc) and sometimes it may become as high as 10 times of flc. However, the problem of peak starting current remains only for few seconds till the motor attains its speed, this problem may become severe especially with the motors above 10 HP. To deal with the problem of peak starting current or inrush current associated with three phase induction motors, many different starters having different mechanism and principle of operation are used. Star-delta starters are used for motors ranges from 5 hp or 3.5 kW. Star delta starters first configure windings of 3 phase motors in star thereby reduce voltage across each winding and then after few seconds these starters configure windings in delta and motors start run at full load voltage without any difficulty.

 Introduction to Star-Delta Concept 

     The magnitude of voltage induced in rotor conductors depends upon the magnetic flux linking with the rotor conductors and the Slip. Slip is the difference between the synchronous speed of the rotating magnetic flux produced from voltage applied to stator windings and the actual rotational speed of the rotor. At the moment of starting, Slip is maximum and decreases as the motor gains speed. Emf, therefore, induced in rotor conductors is high at starting as it is proportional to the Slip. Also magnitude of e.m.f. induced in rotor conductors will become high at the moment of starting, if full rated voltage i.e., line voltage is applied directly to the motor as the strength of the magnetic flux linking with rotor depends upon the voltage applied to the motor.
     Because the impedance of rotor windings being short circuited is very low and voltage induced is very high across windings, the motor current becomes many times of full load current or flc during starting. This high current drawn by the motor may burn motor windings and / or may cause unwanted disturbances in the voltage supply regulation and hence affects other loads adversely connected to the same supply.


   Three phase induction motors are ,therefore, started through appropriate star-delta starters because star-delta starters reduce voltage or start motors at reduced voltage by first configuring windings in star connections to overcome problem of high current surge at starting.

 Working Principle of Star Delta Starter 

     There are three states of Star-Delta starters, a). Star Connected State,  b). Open State, and c). Delta Connected State. During starting time Main and Star Contactors remain closed and complete Circuit. In Star Connected State, voltage applied is reduced to 1/√3 of the Line Voltage across each winding. As and when motor attains good rotational speed, say about 90% of full r.p.m. after few seconds, timer connected in starter disconnects Star Contactor first and then connects Delta Contactor. Between these two, Star connected and Delta connected states, circuit becomes open and motor neither remains in Star nor in Delta State. This is called open transition switching. In Delta connected state voltage applied to windings is equal to Line Voltage.

 Items Required to Make Star Delta Starter 

1. Three Contactors ( One Main Contactor, One Star Contactor and One Delta Contactor ),
2. Over Load Relay ( or OLR )
3. Timer,
4. Fuse Switch Unit ( or FSU ),
5. 2 Pole MCB,
6. Fuse,
7. Start Push Button ( NO )
8. Stop Push Button ( NC )

 Working of Star Delta Starter 

     Referring to the Control Circuit of Star Delta Starter shown here, lets understand working ;

• Switching "Start PB" completes Star circuit by switching ON main contactor "K1" and Star contactor "K2" through Timer "T1, NC". Now motor keeps on running in Star configuration.
• Once circuit is completed and main contactor "K1" is activated, one of its "NO" contact point, connected parallel to "Start PB", becomes close and keeps circuit complete by providing a continuous holding even after Push button is released.
• It should remember that the motor remains in Star state configuration till "Timer T1's NC" remains close. After a time delay is reached, Timer T1's NC becomes open, thereby de-activating Star contactor. Immediately after that Timer T1's NO becomes close, thereby activating Delta contactor "K3" to complete Delta circuit. In the time between switching from star state to delta state, which is normally few mili-seconds, circuit becomes open and motor neither remains in Star nor Delta state. Now motor keeps on running normally at full load voltage. To stop motor, "Stop PB" is pressed and circuit becomes open as all contactors are de-activated.
• Both contactors, star & delta contactors, are also electrically isolated from being accidentally operational by providing one another's NC in series with each other. For example, "K3, NC in series with "K2" contactor and "K2, NC" in series with "K3" contactor. Also Over Load protection is provided through OLR.

 Advantages of Star Delta Starters 

1. Simple in Design, &
2. Comparatively cheaper.

 Dis-Advantages of Star Delta Starters 

1. Open transition switching,
2. Once components are selected and circuit is designed, modifications in design are not possible, &
3. Less Torque at the time of starting than during delta configuration operation.

Few other posts seeking your attention are ;

1.    Near Field Communication technology ( NFC )

               

Industrial Timing Belts

Timing Belts

     Belts are used to transmit power from driver shaft to driven shafts using pulleys mounted on the shafts. Timing belts are essentially used where the timing in motion of one machine member has to be precisely maintained with respect to other machine members. Timing belts have teeth, which fits into corresponding grooves on the timing pulleys and thus avoid any slippage. These belts are more quieter in operation & more cheaper than other such mechanisms & can be used at high speeds without any difficulty. These belts are very efficient as well. 
     Timing belts of various teeth profiles, tooth pitches, pitch lengths, widths, materials & strengths suiting different applications and needs are available in the market. Some manufacturers also provide customized special purpose belts on orders.

Structure of Timing Belts

• Rubber back body protects cords from damage and keeps oil, dirt, grease and other foreign particles away from cords.
• Timing belts carries equally spaced Cords, which are housed between rubber back body and inner profile teeth. These Cords are made of fibre glass and have high tensile strength, which takes load during machine run and protects belt from breaking under load. Polyurethane (PU) belts carries steel wire rope cords, which provides extra heavy duty strength and are used where belt failure rate is high and where the access to change belt needs more time and efforts. 
• There are various types of teeth profiles available suiting different operational needs. Inner teeth are covered with nylon fabric which strengthen teeth against wear and abrasion.

STRUCTURE OF POLYURETHANE TIMING BELT

Nomenclature of Timing Belts

     When replacing belt or placing an order to buy belt from market, a specific naming i.e., nomenclature helps in identifying right size belt. Till late 90s, conventional belts with trapezoidal teeth were in existence and thereafter belts with circular teeth came in market and became popular in industry. For both the belts, different naming scheme is used.

1. Nomenclature of conventional belt with Trapezoidal teeth ;

     There are two series available in Trapezoidal profile teeth belts ;

• Trapezoidal teeth timing belt – Inch Series

         To  define  this belt,  we need belt pitch length, tooth pitch and belt  width.  For  example, if ordering a 15  inches long belt,  whose tooth pitch  is  0.2 inch or  1/5  inch  and width  is  0.75 inch or  3/4 inch  then  nomenclature  will  be  "150 XL 075".  Various  standard tooth  pitches  available are given  in the following table. Tooth pitches can be selected from the following table.

• Trapezoidal teeth timing belt – Metric Series

          To  define  this belt,  we need belt pitch length, tooth pitch and belt  width.  For  example, if ordering a 1500 mm long belt,  whose tooth pitch  is  5 mm and width  is  40 mm then  nomenclature  will  be  "1500 T5 40".  Various  standard tooth  pitches  available are given  in the following table along with their  abbreviations. Tooth pitches can be selected from the following table.

There is slight difference in teeth of belts having pitch start from T and AT like teeth height, etc.

       2.    Nomenclature of timing belt with curvilinear teeth ;

Following types of circular or curvilinear teeth are available ;

• HTD (High Torque Drive) or conventional curvilinear teeth timing belt ;

 

           To define this belt, we need belt pitch length, tooth pitch and belt width. For example, if ordering a 1396 mm long belt, whose tooth pitch is 8 mm and width is 30 mm then nomenclature will be "1396-8M-30".  Belts of various tooth pitches can be selected. Available tooth pitches are 2M, 3M, 5M, 8M, 14M & 20M. 

• STD (Super Torque Drive) or modified curvilinear teeth timing belt ;

           To define this belt, we need belt pitch length, tooth pitch and belt width. For example, if ordering a 896 mm long belt, whose tooth pitch is 8 mm and width is 30 mm then nomenclature will be "896-S8M-30".  Belts of various tooth pitches can be selected. Available tooth pitches are 2M, 3M, 5M, 8M, 14M & 20M.

• Belts with teeth at both sides ;

.         Belts with teeth at both sides are also available. These belts come in use, when drive from motor or gear box has to give to more than one timing pulley rotating in opposite directions. Again available tooth pitches are 2M, 3M, 5M, 8M, 14M & 20M.

        Sometimes when numbers marked on belts are erased due to friction, it becomes difficult to identify belt because it is neither easy nor reliable to measure belt's length as it elongates extensively during time of use and also from the portion it breaks. In such case, we can measure tooth pitch and count the number of teeth. The multiplication of both will give us belt length. Width can be measured easily.

       Open ended belts are also provided by manufacturers which are cut as per size required in zig-zag format called fingers, from both the ends and jointed with the help of punching machine or press machine under application of heat, which is called finger joint. Apart from open ended belts, different manufacturers make special purpose belts as per need and demand of industry. 

Advantages of timing belts ;

1. Timing belts are highly precise & efficient because these belts do not slip like flat belts or V-belts.
2. As there is no metal to metal contact like gears, timing belts are quieter and wear less during operation over the period of time. Also no lubrication is required.
3. Strong, light weight & cheaper.
4. Can be used for high speed applications.
5. Timing Belts can be used for comparatively longer centre distances between shafts than chain drives.

Care for Timing Belts ;

1. In operation (while doing preventive maintenance of the machine, technicians should take care of following points related to timing belts to avoid frequent breaking) ;

• Environment surrounding belts should be clean free from oil, grease, moisture, dirt and other foreign particles.
• Belts wearing from one side that may be because of mis-alignment of pulleys. Pulleys must be re-aligned in such cases.
• When the pulley is too small and the centre distance is also too small then idler must use at the slack side to increase teeth engagement on the smaller pulley and to avoid breaking.
• When belts break too frequently then its tension must be re-adjusted.
• Remove belts and never let belts run idler, if some operation has to stop in machine for a long period of time as it also leads to wear and tear.

     2.  In storage ;

• Store in cleaner place free from oil, grease, moisture, dirt and other foreign particles.
• Do not keep belts in twisted position while storing belts.
• Do not put excessive loads on belts as its teeth may deform, which may affect normal working.

Few other posts seeking your attention ;

>> Coulomb's Law

     More at ; Scientific Laws

>> Shiv Chalisa

     More at ; Bhakti Sangrah