 So, far we have discussed about you know basically using vibration monitoring as a tool for finding out the health condition of machines and then we saw how wear debris analysis or oil contaminants analysis could be used to pinpoint the fault in machines. Now, in this class and so on we will be discussing about the other techniques of condition monitoring which constitute towards the 10 percent of the monitoring methods throughout the industry in the world and especially motor current signature analysis has come up as a very emerging technology in condition based maintenance and we at IIT Kharagpur have contributed a lot to it through our research over the last decade and in fact, in this class and the subsequent class I will be also presenting some results from our research which we at IIT Kharagpur. And before I spend or discuss on motor current signature analysis, I would like to in this class talk about how do you find out the defects in electrical motors. As you know 90 percent of the cases in systems in machines the driven driving unit is an electrical motor. So, fault in an electrical motor is of paramount importance. So, it has to be detected beforehand so that we do not have an unnecessary down time along with motors I will be discussing about transformers as well. So, well if I have to classify the electrical machines there could be many different types of electrical machines by machines I essentially mean motor or a generator particularly if you go to the steam power plants or the hydro power plants you will see large generators. And typical capacity large type of generators would be around 660 megawatts, hydro generators would be about 313 megawatt and so on and of course, if you go to the plants large rolling wheels etcetera we will be having large induction motor or synchronous motor of the around of 10 megawatts in power. And then we have the AC commutator motor around 900 kilowatt and invariably in many plants and steel plants, paper mills, cement plants you will see these kind of motors are used and this is the single most prime mover in their plant. For example, just to begin with if I give you an example of a cement plant essentially we have a motor through a coupling it drives a gear box and then you know actually to this gear there is actually a large kiln I am drawing it in series, but actually it is matched with the gear box. This is the rotary kiln which rotates approximately about 2 to 3 rpm, but it could be carrying loads of you know several tons maybe 10 to 20 tons of material. The idea as you know in a cement plant all the mix is added there is a feeding device here and then it is turned with a hard grinding media and then you will have the powder cement coming out. But our focus is we know how to find out the faults in cracks in the kiln which is a common type of failure in the kilns failures in the gear boxes or the couplings are known to us through vibration monitoring we can do that, but my focus here is on what happens if this motor is at fault how do you find out fault condition in the motor. Because if the motor shuts down your plant is shut is not it. So, let us see and this is the case of a cement plant same is true in rolling mills, paper mills, conveyor drives, the powering unit are these motors. So, motors are usually an AC commutator motor typically 900 kilowatt in power of course, there are larger methods we could be using and they are almost designed to run at constant speed and this gear box because you know from a rpm of 1440 rpm you are finally, rotating the kiln at 2 to 3 rpm. So, you can imagine the amount of speed reduction which is being brought out what about the gear box. So, this is a heavy gear box multistage gear box and so on. Now realizing the importance of fault detection in motors we will see what are the possible different types of faults which can occur in an electrical motor. Of course, before that I is again we have the DC machines, DC machines particularly in your railway locomotives, the traction motors, generators and when you go to the naval frigates the gas turbines are used to power the propulsion unit also this gas turbines are also used to drive a generator which is used to power the electrical supplies the ship even in fact, there are some ships wherein we have a gas turbine driving a generator this generator will drive again a motor which is going to drive a propeller shaft. All electrical drive systems are there of course, you know there are advantages and disadvantages because once you have this electrical drive systems in a ship they say that you know the load at one end is pretty strong or pretty heavy load. So, there can be a situation of stability and instability in a ship but anyway. So, what are the major components of an electrical machine by in this machine here I am usually referring to a motor I am not talking about an electrical transformer right now we will discuss about the transformer later in this class. So, essentially an electrical machine has a stator which actually gives this magnetic field to give this magnetic field we need to have the copper conductors of the windings of course, they have to be insulated and then of course, we have the rotors along with its conductors and rotor body is actually a long shaft on which these conductors are placed. Rotors has to be supported on the bearings and this bearings are actually put in the enclosure and then the motor is sealed of course, it to give a supply to the rotor we have either the brushes, the prints are commuted. So, failure in any of these will eventually lead to the failure of the motor even a failure of a motor bearing is also very very important to consider. With this let me just give you a pictorial view of a motor we just opened up in the lab for the so, that you all are able to see this is how an electrical motor looks in the inside this is the frame of the motor this is the stator and once you give supply electrical supply to this there is a magnetic field in this. Because of this magnetic field there will be flux induced in the rotor, rotor is nothing but again series of conductors put together this is each one is a conductor and these are very soft material because they need to have good magnetic properties and this is the shaft rotor shaft which actually sticks out of the motor and you can see one bearing at this and there is another bearing at another end and this is how it is placed. So, and this rotor here this diameter here is about 6 inches, but let me tell you there could be motors this could be about 6 meters. Now, you could you could move a truck inside it I am not kidding, but motors can be that big, but also motors can be very small that depends on the power and so on. Obviously, big motors require lot of other functions in the sense because of the eddy current losses there will be lot of heat generation. So, you would have seen a motor when it runs it becomes hot and if the temperature if the heat is not dissipated the temperature is going to rise once the temperature rises it may have many effects it may heat up the coils it may degrade the insulation it may damage the bearings because the lubrication in the bearings will get baked ok once this lubrication gets baked they will be hard particles and if you know hard particles they are going to scrape the inner surface and the bearings will become rough. So, one leads to another when you have fault developing in an electrical motor. So, you you will see that in a large electrical motors they constantly monitor the temperature of the drive end and on the non drive end bearing because you would have seen at the end of the motors particularly there are so this is the rotor here actually at the end there is a this is a ventilation blower. So, basically it is it is going to suck in air and then pass through this and then here the motor air is going to come out. So, forced cooling is done to cool the coils by coils and in the rotor coils and the stator coils cool the bearings. There are class of motors which specify the because there are motors which run continuously around the clock 24 by 7 ok continuously in a plant of only they take breaks only for maintenance otherwise they run continuously there are motors which run for short spans. So, you can imagine if this heat is not dissipated I mean this temperature is going to build up typically the bearings are maintained to be in the safe side they should be around 75 degree Celsius ok. And in fact, in some special motors there are alarms which lets you know that the if the bearing temperature is increasing either the motor shuts down or we have to have extra cooling unit to cool the motors. So, this could be a serious problem because this because once the motor heats up there may be unequal differential expansion one conductor may for some reason expand then the other and this differential expansion will lead to misalignment will lead to having no concentric concentricity between the rotor and the stator one will deform more than the other. Say for example, this is a view because of thermal expansion or some other reason there is a change in the air gap because essentially this is the stator this is the rotor this is your air gap. If your air gap changes the magnetic flux will change and once this magnetic flux changes the torque which is responsible for rotating the rotor is going to change. So, many things would happen because excuse me because of this ok. So, for some reason I have a different air gap flux is going to change torque is going to change if torque changes I will be having a pulsating pulsating speed I mean this was suppose to have an uniform speed by uniform speed like this, but it may so happen that suddenly there may be a this spacing is going to change. So, this may become like this and again pull up. So, speed change will occur torque changes means your forcing function is no longer steady forcing function is changing the dynamics is going to change and over the period if this goes unnoticed this is going to be a fatigue load on the machine. So, you see just one component or aspect it is if it is left unchanged or unnoticed it is going to have a domino's effect and that is the why we have failures in machines. Anytime you see a defect initiating you take remedial measures to remove it mitigate it otherwise if it goes unchanged it is going to affect the downstream operations and that is true for anything. As I was telling you this is a motor this is an electrical motor about the same as a 900 kilowatt of capacity and you see this unit here this is nothing but the blowers where in the air comes in to cool such a big motor and this is driving a gear box actually and if you see this is a gear coupling. In a gear coupling as I was telling you earlier the coupling allowances for the thermal expansion as the motor is fixed and in the gear is fixed there are bearings on which these rotors because of thermal expansion can slide and other if they do not slide if they are fixed they are going to bend and the bend again you know the same problem their gaps are going to change then we will have pulsating forces. So, this is why in large machines we have gear coupling. Gear coupling basically there are two gears one over the other and they will just slide a little sliding is there. So, that takes care of the thermal expansion and this usually again this is used to drive a conveyor system and you can see the dimensions as a technician standing here and you can see how big this motor is compared to the person here. This is another view of this motor which is driving a multi stage gear box. In fact, you can see three shafts here 1, 2, 3 and you can see the direction it is a 90 degree arrangement here it is a bevel gear arrangement power is coming this direction and then power is going out in this direction and you will see these machines are maintained the alignment has to be perfect. Otherwise, if there is an misalignment again the rotor at one end of the motor is going to have an excessive force and the bearing at one end may get damaged and this is the view of this motor another motor which is having a massive reduction gear box and this is actually that rotary kiln of that cement plant I was talking about and you can see the dimension compared to this person and this motor rotates at 1440 rpm but this kiln is only rotating at 2 to 3 rpm. So, there is a heavy reduction gear box as well here. So, critical to a plant is this gear these motors if the motors die your plant does not operate. So, you can imagine your ships will not get unloaded your cement will not be produced steel mill will not work. So, you can have serious effect to everybody I mean can imagine life without electrical motors nowadays it is just not possible. I mean even in small on watches to you know even rocket engines we have to rely on electrical motors and that is a very very important I would say mechanical component electrical component any component you call it is a very very important engineering component which we have to know and which we have to care for. So, with this kind of a background about electrical motors as you know now this rotors can have faults staters can have faults and the bearings of the motor obviously they can have faults and you know how to find out the fault in bearings. So, what we did in this laboratory we have an motor which is driving mechanical unit this is the rotor which is supported on two bearings and we are now concerned with this motor. So, in the we have motors with seeded defects by seeded defects I mean there could be faulty rotors typically faulty rotors means you know you have seen those rotor conductors like this slots. So, rotor bar broken is a common mechanical fault what happen if the rotor bar one of the rotor bar is broken the current which is supposed to flow through this conductor is not going, but to balance the load the current in the other conductors are going to increase momentarily. So, this will give rise to an unbalance torque and because of this torque you will see there will be modulations. Modulations will occur in the current waveform and you know from your study of signal processing modulations in the time domain will give side bands in the current spectrum. So, once we see side bands around the supply frequency usually typically supply frequency is F e by supply frequency I mean supply frequency to the electrical motor and typically in our country it is 50 hertz, but now with this many of the motors they use nowadays variable frequency drives for what is known as V F D's. So, that motor can be run at any speed by the way for supply frequency in our country is 50 hertz, but in say in USA it is 60 hertz and though we find out the fault we can still do our good old vibration monitoring on the motors I mean nobody has asked us not to do. We can do vibration monitoring put an accelerometer on the motor I can put accelerometer here to do vibration monitoring and here what will happen you will see that sometimes I will see peaks at supply frequency and also at twice supply you will see the reason why we are getting this twice supply frequency electron in this class we can do vibration monitoring, but in this class since you already know what vibration monitoring is and all you do is put an vibration sensor close to the bearings of the motor record the time history and then do an F F D, but we can also very easily monitor through what is known as the current monitoring. In this experiment here to measure the current which we are using actually an hall effect sensor here if you can see a sensor is like a clamp on a conductor basically something like this is like a spring loaded thing and then all you have to do is the conductor actually this is this is this is spring loaded the conductor will actually come here and then you will get a voltage. So, if I am measuring a current I I will get a voltage here because of the hall effect I will get a voltage which is proportional to the current and with time I can be measuring the current I can be measured usually hall effect sensors have a sensitivity of may be 10 millivolt per ampere etcetera. The reason we are using hall effect sensors is because they have high frequency range all the way from you know may be 0 or DC to about 500 hertz I can measure with such hall effect sensors. Traditionally we measure current by ammeters, but ammeters are very very poor frequency response the only measure around the 50 hertz supply frequency and all we get is an RMS indicator which only gives us the RMS value of the current. And, but the hall effect sensor I get the time history I can record the time measure and record time history of the current. Once I get the time history of this current the entire waveform is available with me. So, with a signal analysis system I can do the current spectrum analysis and then try to know whether side bands are there whether side bands are not there and so on. So, this becomes a very very convenient way to measure the condition of the electrical motor rather than doing a vibration mounting. If I do a vibration mounting I am not sure that sometimes I will get side bands around supply frequency that may not happen. I may see only the peaks at supply frequencies and twice supply frequency, but that is why the measuring and monitoring current is become a very very powerful and convenient technique of monitoring the motor faults. Imagine you are sitting somewhere in the control room where in an electrical motor is may be miles away. All you could do is just lay a current conductor from your motor. So, here you need not go to the motor. I will tell you why and how. Suppose we have a motor and you are sitting somewhere in a control room. Obviously, to drive the motor it requires a supply and the supply cable is may be going from here. I have just done one conductor. So, very easily you can measure at this end measure the current waveform. So, now you can imagine the power of motor current measurement of motor current. Remotely motor is elsewhere motor is underground, motor is over the ground, motor is miles away from you, but as soon as we have access to the supply lead going to the motor you just measure the current waveform. You will record the current waveform and then do your signal processing to find out the condition of the motor and this is why motor current signature analysis has become very very powerful and it is we are not even going close to the motor. If I was to measure the vibration I have to go at least close to the motor to mount an accelerometer and that is not required right now if I am if I am going through this is it not. So, in in this simulator which we have used in the laboratory of course, I have an hall effect sensor and you can see this hall effect sensor is connected to a signal analyzer where we are analyzing the current which has been measured by this and you can see the time history of the current and the spectrum. Now, I was just mentioning that what is the source of this twice line frequency in electrical motors. You will see if you multiply this two waveforms power is nothing, but current times the emf if I multiply this the e i cosine 2 pi f t and cosine 2 pi f t minus phase difference occurs phase between the current and the emf. I will get this expression one is the cosine phi plus cosine 2 omega where omega is 2 pi f. So, one produces the steady torque which is responsible for the motor and there is also an unsteady torque at twice supply line twice the supply line frequency that is why when you do the vibration measurements or the current measurements you may sometimes learn up with strong 2 times the supply frequency peaks which comes up. Many times people ask me this questions why am I in f f t hertz motor why am I getting an 100 hertz peak and this is the reason this is the mathematical reason behind it. There is nothing, but multiplication of two trigonometric functions which gives a 2 omega term. Now, let me go back to how this current analysis can be done to find out the rotor faults perhaps this equation is not coming clear I will just write to write this down here. This is means if there is a broken rotor bar this means the frequency in the current spectra is given by this where this is the k is an integer s is the slip p is the pole pair in the motor. So, in any AC motor because slip will occur because there is a difference between the synchronous speed and the rotor speed pole pairs are there and this is all functions of the supply frequency I will get the frequency of the rotor bar given by this expression and they will actually come up as side bands in the current spectrum and this can be calculated. Now, if a rotor bar why does it break what are the possible reasons? One is thermal stress due to overload in hot spots there will be a linear expansion and then there will be hot spots etcetera. Magnetic stresses created due to electromagnetic forces on balanced magnetic pull I know I have just told you because if the air gap is not maintained there will be pulsating magnetic field during the manufacturing process all of the bars we are not having the same stress relieving. So, some could be having residual stress because I am rotating there is a centrifugal forces and thus the stresses there could be moisture abrasion by chemicals contamination that could lead to the wear and tear of the rotor bar. There are so many loose components in the rotor bar they are actually fixed in a slot few of them could be lose and they could be rattling and then they will if it dig. So, a good motor takes care of these issues are not there in a good motor if you go to the market for the same power same performance you will get motors with varying amount of price that is one of the reasons the quality because somebody would have taken care of that I have good uniform winding uniform air gap good bearings things are not lose there are enough provision for cooling etcetera. So, this is why you know motors these are the possible reasons why this rotor bars can fail and if this rotor bar few rotor bars fail if one goes unnoticed fine if ten goes unnoticed you will see a lot of them creating such an unbalanced torque that it is going to have effect on the operation of the plant. I have seen many a cases even the supply frequency to the electrical motor creates lot of problem in the plant I will tell you how and why. Say for example, if the plant I am supposed to get 50 hertz supply frequency, but because of our power grid supply I am getting sometimes 49 hertz sometimes 47 hertz and so on and sometimes 49.5 etcetera. So, there is a constant fluctuation of the frequency. So, the motor speed is going to fluctuate fluctuate. So, in effect if motor is driving a mechanical unit it is also going to fluctuate. So, this is going to give rise to fluctuating torque on the. So, there will be lot of torsional vibrations which will lead to fatigue failure. I have seen many a places where gear boxes rotate a torsion bar. Torsion bars are actually long rigid bars with good amount of rigidity. They have sheared at the 45 degree axis because the torsional shear stress was maximum and then it failed because of what is the cause it was the fluctuating speed of the motor. So, there are of course, in modern machines there are many safeguards to prevent that the system itself will trip you have seen how the grid failure had a down effect one by one the grids fail. The grids fail the power if you do not take up the load speed frequencies again going to increase. So, there is always in the power generation place they have to always maintain balance the load and the production rate. So, they have to monitor the 50 hertz if you have ever been to power plant you will see they constantly monitor the supply frequency or the power generation frequency a 50 hertz. If I put in more steam and there has to be a governors to limit it that it will run at 50 hertz. Otherwise if I do not govern it speed will increase power supply frequency is going to increase. We all take it for granted, but if you have if you know of a generator generator is supposed to run at the same constant speed. If I give in more fuel or more steam whatever to a steam generator it is going to increase you would have notice that in an ice engines if I give in the fuel flow is more and throttle if you press the throttle more your car moves faster because engine is moving faster. But imagine if your generators was going to move faster your supply frequency going to change. So, these are things which we cannot take it for granted. I have been to many plants where they say the machine has failed because of the bearing failed or the gearbox failed. But we have found out the cause was actually the supply frequency which was been changing and the supply power power power generating units are actually at fault. So, good power supply to a plant is very very important. So, in this example here in the laboratory what we did is we ran this motor through a V F D drive and not at the 50 hertz power supply, but at 47 hertz in a variable frequency drive with 0.67 percent slip. And if you see a normal motor there are hardly any side bands these are the side bands which are showing up at 47 hertz. And once we have three broken bars you can see these are the values of the key these side bands will increase and you can see it in decibels that the peaks are going to increase which are the teletyl signs that the rotor bar has failed. There could be cases where the motors could have an static eccentricity this is the rotor this is the stator this has been exaggerated here and they have an eccentricity A. What are the possible reasons behind this static eccentricity? One is the stator core is not a perfect circle, but oval out of tolerances at the end frames that house the bearings. So, bearings themselves are not aligned incorrect position of the rotor or the stator at the commissioning stage many times the rotors are so heavy that they have to be put right at the site they have to be installed and there could be an incorrect installation one foot is at an higher ground than the other and there is an angular deviation. So, all these lead to the static eccentricity between the rotor and the stator and because of this eccentricity there will be an air gap variation and again we will see frequencies coming up in this current spectra because of static eccentricity. Similarly, when it is rotating I will have a dynamic air gap eccentricity the eccentricity is moving with time you can see how this thing is moving this could be because the shaft is bent soft is bent so it is wobbling. So, one is the bent rotor soft other is the bearing where there could be misalignment there could be mechanical resonance at critical speed though a good motor designer would have ensured that the operating speed is not at its critical speed or away much away from the critical speed. Once we have this kind of defects in the electrical motors they all reflect in the current spectra. So, looking at the frequencies in the current spectra we can find out the faults and there are equations also here how the supply frequency because of the rotor eccentricity again this is perhaps not visible I will write it here many of these are this is the rotors and this is an integer these are all constants depending on the motor this is a typical value plus minus and d for eccentricity for dynamic eccentricity this is about 0.25. So, these are again some integers. So, researchers have found this formula by which I will see the dominant frequency current spectrum due to eccentricity I can see this in the spectrum. I will give you another and this could be because of static eccentricity because of dynamic eccentricity. Another fault which happens in motor is this bearing faults bearing faults also reflect in the motor current see the will be nothing, but the supply frequency plus minus some number and these are nothing, but the bearing frequencies you know if the bearing has an inner rest defect outer rest defect spin ball spin etcetera these equations can come these frequencies can be found out here and these are integers. So, in the supply frequency you will see side bands about the of the bearing frequencies and this is will be the and this is the supply frequency. So, in effect if you do faults in electrical motors can be easily detected by current signature analysis all you have to do is you have to look out for this frequency of the rotor bars frequency because of eccentricity frequency because of the bearings and so on and we are just in the three equations in the previous slides how they can be estimated. So, that in the current signature analysis they will be actually modulated. So, we have to do an envelope analysis which we have discussed earlier or which is what is known as the T modulation T modulation of the current to find out these frequencies and with load with defect severity this the side bands is are going to increase in their magnitude. Another defects which occurs in motors nowadays that we are using V F D drives in electrical motors because of this V F D drives you know V F D drives there are lot of high frequency high frequencies in the ripples I have this motor has a bearing this is the this is the rotor here I am just doing one side what happens there will be a current flowing in this conductor and which gives a voltage as a loop and particularly at the bearing what happens and these are very high frequencies and they could be having voltages about you know typically about 40 to 70 volts 40 to 70 volts and they are going in this loop here what happens at the bearings if you will see they will there will be a dielectric film which is actually an insulator. So, each time the voltage increases there is actually a spark across this outer rays which is attached to one and the inner rays which is attached to the shaft and actually the shaft shifts here the bearing shaft shifts here. So, what happens whenever this bearing is sitting there will be kind of sparking occurring between the bearing and the shaft this spark actually is responsible for eroding the shaft. So, bearing was sitting here you can see this goes undetected particularly the variable frequency drives because of this high frequency ripples they will be sparking on the surface the shafts become worn out like this and sparks shafts become weak. So, how do you take care of it is you have to produce an alternate path to this current by grounding this here by grounding the shaft. So, by sorry by shaft is here you have to ground the shaft shaft with the grounding process. So, that the current flows through that route and there is no discharge taking place. Now it is of course you know people are using VFT drives and they are coming up with situations where the bearings are getting having faults pitting marks or shafts are failing. And this we did in our lab is you know basically we meant measured just the voltage on the shaft here of course we are measuring it and then we recorded it voltage across motor shaft and ground it is a normal case, but when you have the VFT drives at different frequencies we have seen how that these frequencies are there and then they discharge and they actually damage. These peaky frequencies these are as high as 40 to 70 volts and they damage the motor shafts. So, motor current or motor can be faults can be detected through these techniques which we discussed mostly through signature analysis though you can also do vibration monitoring of electrical motors. And then how do you monitor the condition of transformers basically you would have seen in a transformer essentially is nothing but core of coil which is there and this coil is actually kept in a tank full of oil. And this oil is to be maintained for a particular height and because of again the eddy current losses etcetera this current degenerates heat has to be dissipated and that is why these are like fins you know you will have seen in many transformers this there are tubings etcetera which carry nothing but oil and this is a tank wherein oil is there. And you would have heard of scenarios where there is a power outage because of a transformer burst. Why did that burst happen because the transformers was dry the maintenance crew has not ensured that the level of the oil which has to should be there was not there and that is the reason why transformers burst. So, in our laboratory here what we are trying is we are trying to measure the vibration of a transformer of course, there is no secondary discharge here we have just done the primary excitation and this is a very very high 100 mega watt transformer which we are trying to analyze. You can we are measuring the noise here and here through because obviously you cannot go near the transformer we used a laser beam to measure the vibrations and here the sound intensity probe. But usually the condition monitoring of transformers is done by monitoring the condition of the oil you know we discussed about oil analysis. So, by monitoring the condition of the oil whether it has its retained physical properties or chemical properties we can know periodically by taking samples of the transformer oil whether it has got contaminated with moisture whether it is time to change its change the oil etcetera is how these transformers are taken care of. Again for in electrical switch gears when there is contact disc and non-contact lot of heat generations and we will discuss in the class on thermography how through thermal analysis or temperature monitoring we can also find out faults in electrical systems. Thank you.