megger.pptx

megger

1. MEGGER

2. MEGGER The term megger or megohmmeter refers to an instrument for measuring high voltage electrical insulation. “Megger” is the brand name of the first portable insulation measuring instrument. Also, insulation resistance tester known as megger is small in size, portable and popular Practically, it is used in operation of continuity, short circuit and open circuit tests of electrical installation.

3. A Megger has a crank shaft with clutch mechanism. We can vary the speed of cranking with a help of clutch. This arrangement is located between magnets. The whole setup is called DC Generator. On the left side of the DC generator, there is a scale showing the resistance values from 0 to infinity. The circuit consists of two coils Coil-A and Coil-B. Both the coils are connected to the DC generator

4. Megger operates on the principle that current flowing through a conductor placed in a magnetic field experiences a torque. Mathematically, the force is proportional to the  current  and the  magnetic flux. Where,  Vector Force  = force and direction of the current and magnetic field. Case (I):  If the resistance has a higher value then the pointer on the device coil will move towards infinity. Case(II):  If the resistance value is low then the pointer will indicate zero resistance.

5. WORKING When the current flows from the generator, through the pressure coil, the coil tends to set itself at right angles to the field of the permanent magnet. When the test terminals are open, corresponding to infinite resistance, no current flows through deflection coil. The supply to the meter is usually given by a hand driven permanent magnet dc generator.

6. Thus the pressure coil governs the motion of the moving element making it move to its extreme anticlockwise position. The pointer comes to rest at the infinity end of the scale. When the test terminals are short circuited (zero resistance), the current from the generator flowing through the current coil is large enough to produce sufficient torque to overcome the counter-clockwise torque of the pressure coil. Due to this, pointer moves over a scale showing zero resistance.

7. When the high resistance to be tested is connected between terminals T1 and T2 the opposing torques of the coils balance each other so that pointer attains a stationary position at some intermediate point on scale. The scale is calibrated in mega ohms .

8. Advantages of Hand Operated Megger Still keeps important in such high-tech world as it’s an oldest method for IR value determination. No external source required to operate. Cheaper available in market. Disadvantages of Hand Operated Megger At least 2 person required to operate i.e. one for rotation of crank other to connect  megger  with electrical system to be tested. Accuracy is not up to the level as it’s varies with rotation of crank. Require very high care & safety during use of the same.

9. LCR meter LCR meters  can be understood as a multimeter, this is because it can measure  resistance, inductance, capacitance  as per the requirement.  The significant component of LCR meter is the  Wheatstone bridge  and  RC ratio arm circuits.   The component whose value is to be measured is connected in one of the arms of the bridge. There are different provisions for the different type of measurements.

10. If the value of resistance is to be measured, then Wheatstone bridge comes into picture while the value of inductance and capacitance can be measured by comparing it with standard capacitor present in RC ratio arm circuit.

11. Working of LCR Meter The bridge is adjusted in null position in order to balance it completely. Besides, the sensitivity of the meter should also be adjusted along with balancing of the bridge. The output from the bridge is fed to  emitter follower circuit. The emitter follower is  widely used as a buffer amplifier to reduce the loading on the previous stage and provide a lower impedance output for any following circuits .  

12. The output from emitter follower circuit is given as an input to  detector amplifier. The significance of detector amplifier can be understood by the fact that if the measuring signal is low in magnitude, it will not be able to move the indicator of  PMMC  meter. Thus, in order to achieve the sustainable indication, we need to have a high magnitude measuring signal. The  rectifier  is used in the circuit to convert the AC signal into DC signal . When the bridge is provided with AC excitation then at the output end of the bridge the AC signal needs transformation into DC signal.