PMMC INSTRUMENT.pptx

Construction, working, pros and cons, application, Advantages of PMMC Instruments, Torque Equation, Controlling torque

1. PMMC INSTRUMENT Dr. Shweta Singh Assistant Professor MUIT, Lucknow

2. Introduction The permanent magnet moving coil instruments are most accurate type for direct current measurements. The action of these instruments is based on the motoring principle. When a current carrying coil is placed in the magnetic field produced by permanent magnet, the coil experiences a force and moves. As the coil is moving and the magnet is permanent, the instrument is called permanent magnet moving coil instrument . This basic principle is called D’Arsonval principle. The amount of force experienced by the coil is proportional to the current passing through the coil.

3. Construction of PMMC Instrument

4. Construction PMMC Instrument consists of the following given component for its proper working. (1.)Permanent Magnet it is a horseshoe permanent magnet that provides magnetic flux. The soft iron pole piece is used to get uniform flux. (2.)Magnetic Coil it is a movable coil made of thin winding wire and has a few numbers of turns placed on the rectangular aluminum frame. this frame along with the coil is attached to the spindle. (3.)Iron Core it is a soft iron core placed in the magnetic field and its function is to minimize the reluctance effect. (4)Pointer and Scale A light pointer is attached to the spindle which moves over the scale and gives the reading. Sometimes a mirror is also provided in the instrument for an accurate reading . (5.)Spring Two Flat spiral spring of phosphor bronze is attached to both sides of the spindle. The function of the spring is to provide controlling torque and supply electric current to the coil. (6.)Jewel it is placed at each side of the spindle which acts as a bearing to minimize the friction effect.

5. Working Principle of  PMMC D’Arsonval Movement Principle : • An action caused by electromagnetic deflection, using a coil of wire and a magnetized field. When current passes through the coil, a needle is deflected. • Whenever electrons flow through a conductor, a magnetic field proportional to the current is created. This effect is useful for measuring current and is employed in many practical meters. • The basic dc meter movement is known as the D’Arsonval meter movement because it was first employed by the French scientist, Jacques Arsene d'Arsonval , in making electrical measurement .

6. Why PMMC instrument cannot be used for AC  These types of instruments are only used for measuring the  DC quantities  as if we apply AC current to these type of instruments the  direction of current will be reversed during negative half cycle and hence the direction of torque will also be reversed  which gives average value of torque zero. The pointer will not deflect due to  high frequency  from its mean position showing zero reading. However it can measure the direct current very accurately.

7. Applications Ammeter : Voltmeter : Galvanometer : Ohm Meter :

8. Advantages of PMMC Instrument The scale of the PMMC instruments is correctly divided. Its range of either voltage or current can be easily extended by using a suitable shunt. Damping force is very effective . The PMMC instruments have high accuracy because of the high torque weight ratio . T here is no hysteresis loss. P ower consumption is low. The PMMC instruments use shelf shielding magnet which is useful for the aerospace applications.

9. D isadvantage of PMMC Instrument The moving coil instrument can only be used on D.C supply as the current reversal produces a torque on the coil. It’s very delicate and sometimes uses an AC circuit with a rectifier. It’s costly as compared to moving coil iron instruments. It may show an error due to the loss of magnetism of the permanent magnet.

10. Thank You