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.