The PMMC stands for permanent magnet moving coil and it is also called as D’Arsonval instruments. The PMMC works on the electromagnetic effect. A permanent magnet is used to produce magnetic flux and a coil that carries the current to be measured moves in this field. As the pointer is connected to the coil it gets deflected in proportion with the current.
Working principle :
When a current carrying conductor placed in the magnetic field experiences a force. It is given by the expression is given below,
F=BIL
Where
F=Force in Newton
B= Flux density in Tesla
I= Current in ampere
L= Length of conductor in meter
Construction of PMMC :
Construction of PMMC
The basic construction of PMMC is shown in above fig. a light rectangular coil wound on an aluminum frame is pivoted within the air gaps between the two poles of a permanent magnet and a cylindrical soft iron core. This light rectangular coil carries the current to be measured. Soft iron core provides formation of uniform magnetic field. The aluminum frame supports the coil as well as provides eddy current damping. Two phosphor bronze springs coiled in opposite directions serve as leads for the current in the coil. The springs also provide controlling torque.