Permanent magnet moving coil( P.M.M.C.)Instrument
These instrument work on principle of D.C motor . According to this law when current carrying conductor lies in magnetic field then a force work current carrying conductor.
This force is directly proportional to conductor current
F,, is directly proportional to ,,I
where,, F, is force
,I, is current of conductor
2. Construction of permanent magnet moving coil
1. Permanent magnet
It is made in U shape which made of alnico or alcomax material. it provide uniform magnetic field .
2. Soft iron core
It uses between permanent magnet and it provide support to coil . it is also provide low reluctance path for magnetic field.
3. Pole shoe or Pole pieces
It only use for provide constant magnetic field to coil.
4. Magnetic coil
5. Control spring
Its work is dove-lope controlling torque in instrument. it connect with spindle.
Air friction damping used in permanent magnet moving coil
It is made from steel Rode. which connect various equipment like as coil ,control spring , damper etc. it connect between two ball bearing .
8. Pointer and Scale
It is made from Aluminum and its weight is low . In this type instrument scale being uniform.
3. Working principle of permanent magnet moving coil
When we provide supply of instrument then current will flow in coil . which cause a magnetic field dove lope in coil and deflection torque work on coil . which deflect of moving system . By use control spring and damper generate controlling torque and damping torque when controlling torque is equal to deflecting torque then original reading show on scale.
4.Exasperation for various torque
1. Defection torque is directly proportional to coil current
5. Use of permanent magnet moving coil
1. Use as an ammeter
This type instrument used as Ammeter by use resistance. A resistance connect in parallel with PMMC instrument. which called shunt resistance.
2. Use as a Voltmeter
It is used as voltmeter by use of resistance . A resistance connect in series. which called series resistance.