Insulation Resistance Measurement (megger tester)

IR may be measured between a conductor and earth or between conductors. It consists of a permanent magnet rotor, a control coil and a deflecting coil. When permanent magnet rotor is rotate in stator winding, then a magnetic field is develop which is fed to the control coil and deflecting coil through rectifier. 

Control coil and deflecting coil wound in a pivoted core. A pointer is pivoted at the center of rotation of the coils. The two coils are wound at right angles to each other and is connected in such a way that one measures voltage and the other measures current. If the conductor insulation ok, then no change in current & voltage in the control coil and deflecting coil, pivot pointer indicate ~ (actually 1 m ohm). If insulation damage, then change in current & voltage, causes the pivot pointer move to 0. Deflecting circuit resistance coil and control circuit resistance coil equally hold the voltage and current which provide from the permanent magnet rotor.