Permanent magnet moving - coil ammeter

Permanent Magnet Moving Coil (PMMC)

• Permanent Magnet Moving Coil (PMMC) instruments, also known as D'Arsonval meters or galvanometers, are used to measure direct current (DC) quantities, such as voltage and current.

• The instrument which uses the permanent magnet for creating the stationary magnetic field between which the coil moves is known as the permanent magnet moving coil or PMMC instrument.
• It operates on the principle that the torque is exerted on the moving coil placed in the field of the permanent magnet. The PMMC instrument gives accurate result for DC measurement.

Construction of PMMC Instrument:

The main parts of PMMC instruments are:

1.) Moving coil

• The coil is the current-carrying part of the instruments which is freely moved between the stationary field of the permanent magnet.
• The current passes through the coil and deflects it due to which the magnitude of the current or voltage is determined.

2.) Magnet system

• The PMMC instrument uses a permanent magnet for creating stationary magnets.
• The Alnico material is used for creating the permanent magnet because this magnet has a high coercive force.

3.) Controlling system

• In the PMMC instrument, the controlling torque is because of the springs. The springs are made up of phosphorous bronze and placed between the two jewel bearings.
• The current passes through control springs to the moving coil. These coil springs are two in number so if one of them is broken then the current path to the moving coil is disrupted. Hence, the meter would read zero.

4.) Damping system

• The damping torque is used for keeping the movement of the coil at rest.
• This damping torque is induced because of the movement of the aluminum core which is moving between the poles of the permanent magnet.