![]() For example, it can measure the altitude of an airplane, spacecraft, etc., from the earth or sea surface. It is used to measure the altitude or distance of an object above a fixed level such as the earth' surface, water surface, etc. Thus the torque on the moving coil will not change rather it will be unidirectional.It is also known as altitude meter. This means that as the direction of magnetic field changes, the direction of current in moving coil also changes. Therefore, the direction of current in the moving coil is same as that in the fixed coil. If you carefully observe the figure above, you will notice that Fixed Coil as well as Moving coil is in series. Actually this method of reversing the magnetic flux is used in Electrodynamic or Electrodynamometer type instrument. Now suppose, if we were able to reverse the direction of magnetic flux each time the direction of AC quantity changes through the moving coil then we can have a unidirectional torque. Due to the inertia of instrument, the needle cannot follow the rapid change in direction of torque at power frequency and therefore merely vibrates around zero position. But there is something called inertia, you better know. Thus ideally the needle of PMMC instrument shall swing back and forth around zero position. Suppose the torque on moving coil for positive half cycle of AC quantity is clockwise then it will change to anticlock wise direction in the negative half cycle. ![]() ![]() ![]() Because of this change of magnetic flux, the torque on moving coil will also change in each half cycle of AC quantity to be measured. Since the direction of current is changing in each half cycle, there will be a corresponding change in the direction of magnetic flux. Working Principle of Electrodynamic Type Instrumentįor understanding the working principle, first ask yourself what will happen if we connect a PMMC instrument for the measurement of AC quantity? Obviously, your answer will be there will be no deflection in the needle of PMMC instrument. To provide magnetic shielding, normally electrodynamic type instrument is enclosed in a high permeability alloy. Therefore it becomes very important to protect the instrument from the effect of external magnetic field. The magnetic filed in electrodynamic type instrument is very week of the order of 0.005 to 0.006 Wb/m 2. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |