Successful injection molding process design and simulation

Plastic injection molding is possibly the foremost widely used, yet least thoroughly understood manufacturing technologies in use today. Injection molding of thermoplastics began within the 1930s, long before the scientific study or understanding of the behavior of polymer melting struggling . it had been an empirical, trial by error industry, and therefore the critical element of low-cost, high-value part making—the mold—was designed by engineers who learned more by experience than from textbooks. it had been something of a sorcery .

 

Today, everything is different. the mixture of excellent mathematical models of the rheology of resin melts, a far better understanding of metallurgy and warmth transfer, and therefore the formalization of years of “rules of thumb” has allowed specialist engineers to coach specifically in plastic mold engineering.

 injection moulding machine process

Iterative mold development, however, remains considerably a tooling strategy. The difference today is that this is often done virtually with simulation software. But why? Modelling of fluid flow in closed channels has been finished decades. the rationale is within the resins, says Caitlin Tschappat, Moldflow technical specialist with Autodesk and a specialist polymer engineer.

 

“People don’t realize how complex plastics are and the way they don’t behave like metals. They flow by non-Newtonian principles, with very different flow properties. for instance , they’re highly shear sensitive. It’s complex. many of us check out an easy part and think ‘oh, that’ll be easy to fill.’ that straight forward shape might be one among the toughest parts to manufacture due to many considerations, like warpage,” she explains.

 

At its simplest, injection molding is about orienting cavities in three-dimensional space with a parting line that permits free ejection of the cooled, solid resin parts. Simply determining the situation of the parting line are often nontrivial. Parts with zero or negative draft angles could also be impossible to eject with conventional ejector pins, forcing a designer to use fewer cavities, or non optimal cavity orientation during a mold to urge clean ejection. In some severe cases, there's no thanks to orient the part to facilitate ejection, and core pulls must be used, adding complexity and price .

 injection molding machine working

The complexity doesn’t end there. Co-injection, over molding—especially of TPU’s over commodity thermoplastics—and in-mold decorating all complicate the mold design process.

 

If very high-volume production is important, like within the packaging industry, stack molds may require complex designs with multiple parting lines and a requirement for fast, clean ejection. Symmetry helps, and a 64 or 128-cavity small part mold could also be an easy matter of design recursion. A family mold, however, or parts that need special features like the favored “living hinge,” are often very difficult to style. There also are multiple other issues involving gates, runners and other essential mold features.

 

The advantages of simulation are obvious. Tschappat is an industry veteran with experience within the packaging and automotive industries and has seen this complexity up close.

“Think a few larger part,” she says. “The automotive industry, for instance, has many long, thin parts. For these, you want to consider the ratio with reference to part length to wall thickness, because it would be difficult to fill uniformly without the inclusion of complex runner systems, hot drops and valve gates. On the opposite hand, small parts, like those for medical or electronic applications, can also require an equivalent complexity with reference to the runner systems, as they can also be difficult to fill thanks to small features and limited filling pressures.”

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Modern mold design partially addresses the complexity issue by the utilization of off-the-shelf mold bases, inserts, gates and other standardized components wherever possible. Advanced simulation software like Autodesk Moldflow works with these components to permit accurate approximations of mold performance in gating and cavity filling.

 

Cavity balance is usually a high priority when filling multi-cavity molds, which may sometimes be remedied through the utilization of mold cavity symmetry. These instances also are ready to simplify mold simulations, like how other Finite Element Analyses use symmetry for model simplification.

 

For family molds, or large single cavity molds with complex shapes, simulation makes the difference between a productive and cost-effective tool, and a design that's revised such a lot it “goes through the alphabet.”

 

Before simulation became more widespread, it had been not uncommon to change important mold components on the fly, like quick-fixes to gates for improving mold balance. Unfortunately, these are even as it says—quick-fixes—leading to effects on other aspects like part quality like jetting, knit lines and even dimensional instability. that sort of experimentation may solve a drag , but it frequently requires a complete rethink of the general molding strategy, with new machine parameters which will require many shots to perfect.

 plastic part design for injection molding an introduction pdf

The ability of simulation to attenuate rework not only reduces time spent on the mold, but also the training curve on machine found out with a replacement job. for outlets with a captive press operation, the savings for reduced downtime and improved machine scheduling ability are obvious, except for mold shops there are additional benefits. Rework costs and delivery delays are reduced, customer satisfaction is improved, and therefore the too frequent finger-pointing (who pays for that modification?) are often greatly minimized.

Complexity may be a given with modern injection molding and lots of jobs simply can’t be attempted without advanced simulation. Tschappat describes how she uses Moldflow to deal with higher level problems, saying, “We have many modules within the software counting on what questions you would like answered, whether it's co-injection, two-shot, over-molding or insert molding. for instance , with gas assist, we will help identify where the gas void will settle within the cavity, or with coinjection how two plastics are getting to bond together within the analysis.”

 

Simulation helps address the previously mentioned topic of cavity balance for multi-cavity tools.

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"We see numerous tool cavity layouts when talking with our simulation customers. But the one thing that always seems to throw people off is that if they will escape with molding quality parts from family tools, where each cavity could also be a special part, and technically would wish different processing conditions,” says Tschappat.

 

“Using Moldflow simulation, we model up the mold layout and predict the part quality of the various cavities. Then we experiment with artificially balancing the filling through the utilization of adjusting runner diameters between cavities or changing the runner design, to aim a more uniform filling from cavity to cavity. albeit the filling is balanced, other factors like shear-induced imbalances can occur as a results of the various geometry features. this is often where simulation is basically cool—seeing something that we will not even see with our eyes when actually molding the parts at the press," she adds.

 

The types of runners are another source of complexity. Hot runners are standard for volume production thanks to zero or minimal wasted material, but cold runners leave re-purpose of their runner material toward regrind to feature to material savings through reusing small percentages. But as Tschappat observes, “based on your part, what's the simplest gating scheme? counting on how big your part is, maybe you're employing a fan gate, or if it is a smaller part it'd be alittle pin gate. Then the pressures and therefore the pressure drops are an element throughout the runner system. These are all things that make it complex, yet you've got to weigh them out at the top of the day to work out what's best for your business.”

 

Even a seemingly simple change during a commodity resin can introduce issues. “They teach you in class that if you're getting to be employing a different resin, particularly very dissimilar materials to people who you’re wont to , you ought to build your mold thereto material,” Tschappat says. “We all know that that's not necessarily the practice. you'll start out with a non-filled material then plan to switch to a glass-filled material after manufacturing a couple of shots, for instance . Now you've got to stress about more abrasive decline the tool over time. What does one neutralize those situations? More frequent tool inspections and reworks, maybe even welding up the gate and re-cutting it so it allows for fewer shear of the fibrous material. These kinds of things just take overtime out of the method and slow you down, but it's normal practice.”

 injection molding machine design simulation pdf

The ability to rerun a simulation with a replacement material virtually can flag a molder about potential problems before they translate to expensive tool rework and downtime. Warpage is another common reason for modifications to the tool design. an honest designer who has familiarity with simulation packages like Moldflow can gain insight into root causes of those problems.

 

“Once you actually get comfortable, you'll actually isolate causes of the warpage to raised understand why something is warping,” states Tschappat. “For example, if you're getting tons of warpage thanks to a thicker cross-section because your material's not freezing off and you've got tons of shrinkage therein area, Moldflow allows you to ascertain that then make a design change and rerun an analysis to see how that style change reduces overall warpage. this is often why we are working more and more with part designers. they will use simulation as they design the part to flag these problem areas before escalating to the tool designer, resulting in fewer iterating between the 2 , making them appear as if they're superstars!”

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The cooling issue is critical and is usually harder to perfect than the cavity and runner design. Although 3D printing promises truly conformal cooling, most production molds are cooled by drilled and manifolded channels carrying coolant, usually water or oil, which carries the warmth away by the thermalator.

 

There’s an old rule of thumb that for efficient cooling, set the machine to eject the part at 80 percent of the part’s heat distortion temperature. However, for complex parts, multiple cavities or complexity added by factors like thermoplastic elastomers, coinjection or gas assist, rules of thumb are quickly replaced with empirically derived settings during the mold runoff. Cooling is typically the determining think about overall cycle time, so here time considerably is money.

 

Simulation of cooling channel layout and flow capability are often equally or more important than efficient cavity filling for a high capacity mold, and within the world of injection molding, only a few molds aren't thought of as high capacity, meaning cooling is nearly always a critical factor.

Mold designers are frequently faced with customer requirements which will be difficult or maybe impossible to realize . Most production shops understand their press plate size and tonnage, chiller capacity and target cycle time, but know little about the mold.

Customer expectations can sometimes be unrealistic. “Part designers and mold engineers need to work with each other ,” says Tschappat. “Time is money, and everything must be finished yesterday. It goes through several phases. How are you getting to lay your model out? you've got to think about where you are going to inject resin and locate your gates. then what sort of gate you are going to use. What sort of runner systems? Is it getting to be a hot manifold or are cold runners getting to be sufficient? Or is it a hot to cold runner? what is the ejection unit look like? Where are you able to squeeze cooling in?”

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“Often in my job, you get parts from customers and you ask them, ‘what's your cooling layout look like?’ you'll get a solution thereto question, or they'll not know. Often a designer will just squeeze cooling in there wherever they need extra room left over,” Tschappat adds.

 

 

Even the potential of the machines that run the mold are often assessed critically with mold simulation. Simulation allows a mold designer to run “what if” scenarios which will show a customer where expectations are unrealistic and nudge them to a far better mold design without an argument.

 

Math is definitive. “If you're limited within the types or how large your injection molding machines are, then you're limited to the dimensions of the mold that you simply can put therein unit,” Tschappat says. “And then in fact , what proportion pressure it's getting to fancy fill these parts? is that the machine large enough to not just fill out the cavities, but hold the pressures needed to pack those parts? If it isn't , you are going to possess some problems and you're either getting to need to redesign the tool or reconsider it. Or buy a replacement machine.”

 

This isn’t a theoretical consideration. Tschappat has seen customers who were spared the value of a replacement machine when shown the advantages of a far better optimized mold design.

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Does simulation replace the tooling designer?

 

“No,” declares Tschappat. “I would say there's still an art thereto. When I'm in conversation, I regularly tell people they ought to be using simulation as an additional datum or another datum , and use their experience. This actually ties into that aging workforce, too, as we're beginning to lose that skillset. People are retiring, so how can we build that have base up? Simulation may be a specialized thanks to help. children got to be sponges; if you are a toolmaker or a tool designer, work closely with the older generation to select up a number of their skills.”

 

Simulation software for injection mold design has progressed from “nice to have” to a “must have” for cost effective tooling

“I think it did wonders for the industry because you are able to create an ROI case and see where you'll improve cost savings at the top of the day. Everybody wants a more complex part cheaper, faster, quicker, and by incorporating this type of technology into your workflows and work processes, we will help achieve that goal for our customers.”

how to test transformer using multimeter

 

how to test transformer using multimeter

 Today I will explain, how to test transformer using multimeter. I have one transformer here. That is shown here. It has one primary 230 volt. It has one secondry 180 volt. That has one tapping also. And they are isolated with each other. To check that, We measure 6 types of resistance, in this transformer. First is P,  P means, resistance of primary winding. It will not show short, because wire is very long. 100s of turns wii be there. Some value will come, but it will not be open. Similarlly S1 & S2, S1, we will measure the resistance of this , winding from here to here. That also will not be short, it will show some value. Then S2, we will measure resitance between this & this. Then P & S. In between primary & secondry, we will measure the resistance. It has no connection. It will show open. If there is any failue of insulation, It will not show open. Then, this is core, We measure the resistance primary to core. This is primary winding, this is core. In between these two, we measure resistance. That also will come open. If there is some shorting or insulation breakup Then this will not come open. So we will know, it is damage or not. Similarlly secondry & core. This is secondry. This is core. In between this & this, We will measure the resistance. That again should come open. Now, I have connected my multimeter, across primary. Here. This is showing 65.2 volt, this is not showing zero. This is showing some value. We will write 65.2 volt. Now I have another small transformer. We will check, the primary winding resistance of this also. So I will connect like this, here. Now this is showing open. Open does not means, that this is bad. Open means, I have set value 200 ohms. You see, value of resistance is more than 200 ohm. I will increase the scale. Now showing 919 ohm. So this value much much more, than this. Because this transformer is of small rating. This is 30 VA. This is about 10 VA. 2nd thing, this is very good industrial grade transformer. This is normal commercial transformer. So, after connecting, your multimeter here, We have to change the scale. Just it is showing open, does not mean, it is bad. There is a possibility, That resistance of this winding may be higher. Only thing, it should not be open. Now, I am measureing resistance, across this. This is this terminal. This terminal is this. So resistance across this is, coming 55.8 ohm. I will write 55.8 ohm. Now I will measure the resistance between these two. 160 volt winding. This is 160 volt winding. This is common. So I will change. Now resistance is comming 49.4 ohm. You are seeing. Here resistance is higher 65 ohm. Here resistance is lower. Because turns are more here. The more the turns means, the higher the resistance will be. Lesser the turns, resistance will be less. Across this, turns are very less. They will be for only 20 volt difference. We will measure here also. Let us see how much comes. It is just 7.3 ohms. From here to here, Resistance is very low, only 7 ohm. Because turns are very less. Now, I have connected my multimeter, at this point and this pont, between primary & secondry. This is showing open, open does not mean, That this is OK. Because I have set value 200 ohm. We have to set at high value. Now I have set at 2000K. Still it is showing open, means, In between this & this, It is perfect open. There is no insulation failure. See here. Now I will measure resistance, betwen primary winding & core. Primary is this. Core, this is core. But this core is connected here using a terminal. Industrial transformer will have a connection like this. The core terminal is brought here. And then earth terminal is shown here. So I measure between this ( primary winding, ) and this core point. See, this is showing open. This scale is at higher range. I am checking this primary winding & core. This is showing open means, no insulation failure in primary winding. Now I will check secondry & core. This is core. This point, I will change to secondry. Again showing open. So all 3 are open. This means, there is no insulation failure here or here. Now here, I have shown, between primary & secondry, there will be open. This is called isolating transformer. If it is auto transformer like this, Then primary & secondry are shorted in auto transformer. There is no isolation. Then it will not be open. It will show some value. Now to measure turn ratio, We have to give AC here. With this way, we can not measure. At primary, we will give AC voltage here. This value will depend upon the rating of this transformer. To secodry, we connect a multimeter. And measure the voltage here. And this voltage devided by this voltage, will give this turns, devided by this turns. This is for one tap. For another tap, We have to connect multimeter like this. This voltage, devided by this voltage, will be equal to this turns, devided by this turns. This method, what I have shown, If you check the transformer in the lab or house, that will be not full tests. In production, in companies, They do so many other tests, not only these tests. Today this much only. 

What is zinar diode & working as a voltage regulator

  Welcome. Today I will explain function of zener diode & its applications. This is symbol of zener diode. It has 2 terminals, one is anode, one is cathode. This is special type of diode. In this if we flow the current in forward direction, Forward means anode to cathode, current is flowing like this. Then it behaves like normal diode. But if we apply the voltage in reverse direction, Reverse means, cathod voltage +ve and anode voltage negative, or cathode voltage more than anode voltage. And voltage in reverse direction, becomes equal or greater than break down voltage of zener diode. Then current starts flowing. So in zener diode, current can flow in both the directions. This is characteristic curve of zener diode. This is V. This is I. This operates in 2 ways. This side is +ve biasing. This side is negative biasing. This side it behaves like normal diode. This side it behaves like zener diode. In this side, when voltage exceeds more than 0.7 V, say for silicone diode, current will increase suddenly. But we will discuss here. Zener diode behaviour. In reverse side ,when we apply the voltage, small leakage current flows. When reverse side voltage crosses, break down voltage, this is breakdown voltage, Then current will increase suddenly. And the voltage remains less or more constant. Zener diode has one dynamic resistance. Which is equal to delta V/ delta I. Suppose we take 2 point A & B, This line is not vertical, it has some angle, you see like this. This ∆V is the voltage difference between A & B. And ∆I is the difference of current at point A & B. Ratio ∆V/∆I is called the dynamic resistance. It can be10 ohm, 20 ohm, 100 ohm & so on. Now we come to specifications. This is one example. Say zener diode voltage is 5.6 volt. If you see the data sheet, There will be tolerance say +/- 5%. So 5.6 volt will not be correct value, When you measure, it may be 5.6, + 5% or minus 5% or in between. If you want accurate value, You can take 5 or 10 numbers of zerer diodes, measure the value, and select one which has closure value. Now zener diode has 3 currents. One is test current. One is normal current. 3rd is surge current. Test current means, you will get 5.6 volt at this current. This is the test current. For example if 45mA flows in a zener diode, Voltage measured across this will be 5.6 V. 2nd is 160 mA, Which is continous current it can take. 3rd is surge current. Now you see this current 810 mA, is almost 5 times, compare to 160 normal current. This means for short time, It (zener) can take much more current, say 810mA, this is just example. 810 mA for 10 mSec only, not for longer time. Then power loss. Suppose this zener diode has 5.6 V, Current flowing in this is Iz. Loss will be VzIz. Nor this loss or this current. Current flowing in this, that will decide the loss. I have written 2 figures. 900 mW & 1300 mW. In this case, This zener diode is rated 1300mW at 25 degree. That is this point. This is power derating curve. This is temperature. This is power. Till 25, it is 1300 mW. If you increase the ambient temperature, You have to derate. It can not take 1300 mW. It can take less. For example at 70 degree, It can take only 900 mW. This is just example. So we have to derate. One more thing is there, When they define this power, or this current, They define lead length also. You have to see data sheet carefully. Now we come to applications. This is list of 8 applications. Out of 8, two are shown here. Remaining 6 are in other sheet. Now here I have shown, Zener diode application in waveform clipper. I have connected 2 zener diode in series. But polarity is reverse. In one case cathode is at top. Another case anode is at top. So total voltage will be this voltage 5.6V + this drop 0.7V=6.3V. This is resistor to limit the current. When we give input voltage AC, Output voltage will not exceed more than 6.3V. Output will be red color like this. This drop, this voltage will be 6.3V. Then zener diode application in voltage shift. This is voltage. This is time. Green color is input voltage. This volatge will be always less than this equal to this drop. Suppose at this point, at input voltage is this. So output voltage will be this voltage, minus this zener diode drop. So we get this. Now this is another application of zener diode, in voltage regulation. I have connected one resistor here. This resistor is connected to limit the current in this. And ouput voltage will be equal to zener diode voltage. And current flowing in load will be Vz/RL. But limitation of this circuit is, To much variation in load is not allowed. It has limitation. To improve that this is better way. Here I have connected one transistor. NPN transistor. In zener diode, current flows like this. Now suppose zener diode voltage is 5.6 V, Then voltage at this point at output will be equal to this voltage minus this voltage, Vbe voltage. Suppose Vbe voltage is 0.6 V, Then voltage at this point will be You will get 5 volt here, not 5.6V in this load connected. You can select bigger size of NPN transistor. In this case load variation can be to much. It has lot of range to change the load. When we use this kind of circuit, Then load current flows like this, through transistor. This is load current. And power loss across this transistor, will be load current multiply by drop across transistor Vce. Now this is another application of zener diode, in over voltage protection. This is thyristor. I have connected one zener diode in gate. This resistor is connected to suppress noise. When voltage across this, become equal or more than this zener diode, Zener diode will conduct. And the gate current will flow like this. Because of this gate current, this thyristor will trigger. and this will become short. It will not be fully short because there will be drop of thyristor small drop. Now current flows like this. Now there are 2 ways we can use this. We connect a fuse here. When this becomes short, very high current will flow. And fuse here will blow. 2nd application is, we do not want fuse to blow. We want zero volt itself. Whenever voltage across this, becomes equal or more than this, we want zero volt. In that case fuse is not required. In my one project we wanted like this. We wanted zero volt, whenever this voltage exceeds more than zener diode voltage. Here I am using zener diode, at op amp gain control limiter. I have connected 2 zener diode in series in reverse direction. Assume total voltage is 10V. This is zero, this is zero volt. Normal gain of op amp is R2/R1. If this is not there. When this voltage exceeds more than 10V, Current starts flowing like this. And it will not go more than 10 volt. It provide the dynamic resistance, And limit the voltage at this point. In many control application, This voltage should not go to saturation. If goes to saturation, then response becomes slow. This also protect this IC. As voltage across this will be limited to 10V. Here I have used zener diode as a voltage reference. This is comparator. What happens, When there are so many circuts, Then this supply point will have some spikes. If you connect 2 resistor here, Then spikes will pass over here also. But if we connect a zener diode, Then this point will be stable. So voltage reference will be stable, for the comparator. This resistor I have connected, such that we get the hysteresis. If we do not connect this, This hysteresis function will not be there. This is zener diode application for power supply protection. This is small resistor to limit the current. I have told in the beginning. This zener diode has short time current rating much higher than normal current. That short time rating is used here. Whenever pulse comes here, say spikes comes here. The current flows like this. It can take heavy current. And this voltage will not go more than zener diode voltage. So this R, this C, this zener diode together, provide the power supply protection. But because of this resistance, this voltage will be little lower than this. Or it will have some unregulation. If you can afford that then this circuit is very good for power supply protection. This is meter protection. I have connected one zener diode like this. When voltage across this becomes more than this, then current flows like this. But then full current will flow like this. You have to select proper rating of this zener diode, otherwise it will fail. Or we have to put another fuse or some other thing here, to limit this current. Today we will close here itself.

Why is AC motor used in the train engine

 Why is AC motor used in the train engine

 Why is AC motor used in the train engine? today topic is, why 3 phase AC motors are used, in electric train engine, now days? here, I have shown, technology improvement with time, in electric train engine, Olden days, DC motor were being used, afterwards, around year 2000 in India, 3 phase AC motor along with, GTO based control system came, and after some time, now days, 3 phase AC motor, and IGBT based control system are used, this AC motor, along with IGBT based control, are giving the advantage, olden days, this IGBT based control system, was not there. 1st advantage is, 3 phase AC motor, and IGBT based VFD/VVVF, technology provides, better speed control, and better acceleration control, in the engine, VFD means, variable frequency drive, when this control has, variable voltage also, so we call it, variable voltage variable frequency, now 2nd advantage, 3 phase AC motor, and IGBT based technology, provides unity power factor, and very less harmonics, now 3rd advantage, AC motors used in the train are, squirrel cage induction AC motor, this is easier to maintain, this kind of motor than DC motor, then there will not be, any mechanical contacts like brushes, now 4th advantage, AC motors are lighter than DC motor, for equivalent power, now 5th advantage, more effective regenerative braking, because of more effective braking, less wear and tear, of the braking system will be there, more energy saving will be there, and fast money pay back, payback is due to more saving, in the electricity bill, now I will explain, what is the payback period, pay back period, suppose money invested is Rs. 100, and after investing Rs. 100, we save Rs. 5 per month, in our electricity bill, so, to recover full Rs. 100, it will take 100/5 = 20 months, we will recover full Rs. 100 in 20 months, so this is called the pay back period, 6th advantage is, IGBT based control, with 3 phase AC induction motor, gives better performance, for a given weight and volume, 7th advantage is, AC motor in the train, with IGBT based control provides, better tractive effort, and higher adhesion level, I will try to explain these 2 points now, I have shown a train here, this is engine, this is rail, and these are wheels, when we have a CAR, there will a tyre over the wheel, and road will be rough, so, when we run the motor, or engine of the car, car will start moving, but in the train, there is no tyre over the wheel, so, it is not so easy, that we run the motor, and train start moving, I will explain you now, here, this is rail, this is wheel, gears, and motor, when motor is running, then 1st, power transfers to gears, then wheel, and a tractive force, is generate in the wheel, which try to rotate this wheel, it will not move, because for movement, rail is also required, then between wheel and rail, there will be friction, and there will be weight, like this, this together form adhesion, now this tractive force, and adhesion together, (tractive force is this), and adhesion is, in between wheel and rail, this and this together, make the train to move, here, more the utilization of friction, ( friction is between this and this), train motion will be better, in the case of the AC motor, along with IGBT based control system, utilization of the friction, is much better, here, I have given equations, and definition of adhesion variable, Adhesion variable is the ability, of the engine / locomotive, to convert the available friction, into the usable friction, at the wheel and rail interface, here, you can see 2 trains, running at the same speed, but here, wheel is moving faster than speed, so wheels are slipping here, this slip is due to, tractive effort more than adhesion level, here again, both the trains are moving, at the same speed, but 1 wheel of this train, is moving faster, so this wheel is slipping on the rail, this is due to, tractive effort more than adhesion limit, only for this wheel, in this train, both the trains are moving, with same speed, but one wheel of this train, is not moving properly, speed is less, this is sliding on the rail, this happens, if we apply the brake on the train, this is due to braking effort, is more than adhesion limit, we have to correct, the torque level here, this is tractive effort, and this is speed, this plot is tractive effort vs. speed, and this is adhesion level, this tractive effort, should be as high as possible, but should be less than adhesion limit, other wise wheel will start slipping, so, in the case of AC motor, along with IGBT based control, we can keep tractive effort, at high level, just below the adhesion limit, so utilization of tractive effort, is much more, this is slip control concept, between rail and wheel, this is time, this is torque, train speed is shown using red color, and speed is increasing with time, similarly, wheel wheel speed is shown in blue color, that also in increasing with time, wheel speed and train speed, should be equal, it is shown up to here, at this point, wheel speed, becomes more than train speed, in that case, wheel will start slipping on the rail, this difference is measured, using control system, and value of torque applied is reduced, until slip becomes zero, or speed of wheel, and train becomes equal, at that time, torque value is brought again, to normal condition, this concept of control, works better, with AC motor, and IGBT based control system, 8th advantage is, this one , I explained just now, better tractive effort, and higher adhesion level, by slip and slide control, 9th advantage is, better tractive effort, and higher adhesion level, by different power, to different axle or bogies, I will try to explain this now, during movement of the train, loading or weight distribution, is not uniform on all axles or wheels, so power or torque requirement, is different for different wheel, I have shown in this diagram, that 2 different control. and 2 different motor, are driving wheels, so depending on the requirements, we can adjust, different torque levels, for different wheels, such that wheel will not slip, this problems becomes more, when train is moving on a slope, or train is just starting, now, some big trains, will have more than 1 engine, similarly metro train, will have more than 1 engine, it is distributed in many bogies, suppose this is one engine, and this another engine, in such cases, torque and power requirement, will be different, for this engine, and this engine, then using control, we can set different torque value, here, and here, this means, power will be different, for different bogies or engine, this way, power and tractive force available, can be used more properly, in case of AC motor, along with IGBT based control system, today, we will close now, 

What is VFD & Why VFD is used in the train

  

What is VFD & Why VFD is used in the train.

  Today topic is, What is VFD & Why VFD is used in the train. VFD means varaible frequency drive. In VFD system, speed of motor is controlled by changing frequency. Now here, we have one 3 phase AC motor. This is 3 phase supply. Generally it will be induction motor. Speed of 3 phase AC motor, depends opon the frequency of AC supply. Sp speed of motor can be controlled, by changing the frequency of AC supply. Now synchronous speed of AC motor, is given by formula, Where f is the frequency of the supply. And P is the number of poles. suppose  f is 50 Hz, and P is 2. Then sysnchronous speed will be, Revolution per minute. This is synchronous speed. Actual speed will be little less, About 2 to 5 % less than this. VFD system. Here I have shown the VFD system. This consist of 4 parts. 1st is rectifier. 2nd part is DC link. 3rd part is inverter. 4th part is 3 phase AC motor. Now days, in train engine, we use 3 phase AC motor. Earlier days, DC motor were used. By now days, in latest technology, 3 phase AC motor are used. Input will be 1 phase AC. Rectifier function is, to convert single phase AC in to DC. Rectifier is made using diode also, using IGBT also. Latest technology is IGBT based. Because of IGBT based control, we can get unity power factor here. This is called the DC link. One capacitor is there. Voltage across this capacitor, may be 650 volt DC, may be 750 volt DC or may be 1800 volt. It may be anything depending upon the system. Inverter converts DC in to 3 phase AC. This also made using IGBT nowdays. Now to control the motor speed, frequency of this AC supply need to be controlled. That also is done by this inverter. This inverter does 2 jobs atleast, One is, to make 3 phase AC supply, 2nd function is to control the frequency. But in traction system, there may be so many controls. Simplest one is to control the frquency here, to control the motor speed. This is inverter. It coverts DC into 3 phase AC supply. You may be aware of 3 phase rectifier, which converts 3 phase AC into DC. But inverter is reverse type. This convert DC into AC. There are 6 switches. These are IGBT. They will be becoming ON & OFF in sequence. If this becomes ON, we get +ve voltage here. I have shown here in green color. If this becomes ON, we get negative voltage here. So these switches will be becoming ON & OFF. I have shown only 1, 2, 3, 4, 5, 6 times becoming ON & OFF. Actually these will be becoming ON & OFF many times. When you filter this green color, you get red color, which is AC. Here IGBT is ON for longer duration. So we get more voltage. Here ON period is less. So we get less voltage. So this is AC. I have shown only one phase. There will be 3 phases like this. And this is the time period, for this red color AC. Frequency will be equal to 1/T. If you want motor speed more, so we want frequency more, So we have to reduce T. So this will have to be reduced. So this inverter does 2 functions mainly. One is to generate 3 phase AC, And 2nd is to control the frequency. VFD controls. The types of controls for VFD, can be basically devided in 2 parts. One is scalar control or V/f control. 2nd is vector control. In scalar control, we keep V/f constant. V/f decides the flux. Flux we try to keep constant for better performance. Suppose we want motor speed double. We will increase the frequency to double. Then voltage also, we will make double. Suppose we want motor speed half. Then f will become half. Then V also will become half. But still speed will become half. This scalar type VFD control, are low performance type. They are simple control. And price also is low. In market, generally for small size motor, 3 phase motor, we get VFD control, those are scalar type. Because price is low. 2nd is vector control. These types are used in train engine nowdays. They have high performance. They have very omplex control. Price is also very high. And good steady & transient response. When motor speed is changing. We call it transient condition. When motor speed is steady, We call steady response. In both conditions, this vector control system will give good respose. Now benefits of VFD. First is, performance is very high. Then energy saving. VFD system consumes less electricity. In case of train & railways, electricity expenditure are very high. They want to reduce it. To reduce the electricity expenditure, They use vector type VFD. Then regenerative breaking is possible here. What is regenerative breaking ? When train is moving, It stores lot of energy because of very high mass of the train. So when we apply the break, that whole energy or part of the energy, is converted back to main power. That is called the regeneration or regenerative breaking. Means, train energy is converted, into electrical energy back to AC system. Then very good transient response. Transient means, just now I told, When speed is changing. When motor is starting, train is about to start from station, That will be transient response. So VFD control provides very good transient response. So we should feel less jerk. Very good steady respose. Steady means, constant speed. When train is running at constant speed, Then also respose of the system is very good. That, when we use vector type control. That is very complex system. High power factor. If we use IGBT based recrifier in VFD system, Then unity power factor, or about unity power factor can be achieved. Soft starting, when we start the train, that time we should start slowly, not with jerk. That is the meaning of soft starting. This is required in train, when train starts. It should not move suddenly. It should move slowly. Then torque control. We will not go in detail about torque. But as an example, when we start any vehicle, say train. Then lot of torque is required just to start. That is why, we use 1st gear during staring of the car. Because 1st gear gives more torque. So torque control is possible in VFD. That to, in vector type VFD system One more thing, I will tell about transient. That input supply will be changing. When input supply changes, that is also called transient condition. During that time, VFD system has very good transient response. Today we will close now.