Advantages of PMMC

What is PMMC?

A Permanent Magnetic Moving Coil (PMMC) meter – also known as a Galvanometer or D’Arsonval meter is an instrument that measures the current through the coil by the coil’s angular deflection....

Working Principle of PMMC

A PMMC type instruments works on the principle which is the current carrying conductor is placed in a magnetic field, it is acted upon by a force which tends to move it to one side and out of the field. It works on the electromagnetic effect ,as the coil carries the current and magnet is used to produce the flux ,a pointer is also connected to coil which gets deflected according to proportion of the current in that coil . The force in the field generates the deflection force, a damping force generates which maintains the pointer stable and thus equilibrium attained by controlling and deflection torque in order to provide the accuracy in the environment....

Construction of PMMC

The Permanent Magnet (M) and a Rectangular Coil (C) which consists an insulated copper wire wound on the light aluminium frame which is fitted with a polished steel pivot resting in jewel brings. The Magnet is made of Alnico and has soft-iron pole pieces (PP) which are bored out cylindrically. The Rectangular coil (C) is free to move in air gaps between the soft iron pole pieces and a soft iron cylinder (A) supported by a brass plate. Construction of PMMC...

Function of Central Core

To intensifying the magnetic field by reducing the length of the air gap between the soft iron pole pieces and a soft iron cylinder across which the Magnetic Flux has to pass. To give a radial Magnetic Flux of uniform density, there by enabling the scale to be uniformly divided. The Movement of the coil is controlled by two Phosphor Brome hair springs (BB), which additionally serve the purpose of leading the current in and out of the coil. The two springs which are spiraled in opposite direction for neutralizing the effect of the change in temperature....

Types of Errors in PMMC

In PMMC, error can be occurred that affects the accuracy of the measurement. The most common error occurs in moving coil instruments are due to the following types of errors....

Deflecting Torque

When the current is passed through the coil, the forces are set up on its both sides which produce deflection torque. If the current measured in ampere (I) the current passing through the coil....

Controlling Torque

Controlling torque of the spring (Tc) ∝ Angular deflection θ Tc ∝ θ Tc = C × θ...

Applications of PMMC Instruments

The PMMC system can be converted into an instrument to measure the A.C as well as D.C quantities like current, voltage and resistance....

Temperature Compensation

Temperature may be compensated by the use of series and shunt resistors. The Swamping resistor is made up of manganin and combined with copper in the ratio of 20:1 to 30:1....

Characteristics of PMMC

Full scale deflection current (IM) : It is the current needed to deflect the pointer all the way to right to the last mark on the calibrated scale. The maximum value of the measured quantity for which a measuring instrument is calibrated. Internal resistor (RM) : It is D.C Ohmic resistance of the wire of the moving coil, RM ranges from 1-2 ohm for 30mA movement to 2 KΩ for 50k.Internal resistor refers to the opposition to the flow of current offered by the cells and batteries themselves resulting in the generation of heat. Sensitivity (S) : The sensitivity of PMMC instrument instrument is 10kΩ/v per volt. This instrument is used in rectifier type voltmeter with half wave rectification the sensitivity will became kΩ/v....

Advantages of PMMC

No hysteresis loss High torque weight ratio Range can be extended with shunts or multipliers. Low power consumption The scale of this instrument is divided equally . High accuracy ....

Disadvantages of PMMC

PMMC instrument is costlier. This instrument cannot be used for A.C. measurements. Errors due to ageing of control springs and permanent magnets, friction and temperature....

Conclusion

In this article, we studied about PMMC instrument and its working principle. The PMMC instrument is a valuable and versatile device used for measuring direct current (DC) in various applications. The PMMC instrument can be used to measure various electrical parameter like current, resistance, voltage and etcetera by adding additional relevant component to the circuit. The measurements through the PMMC can be read by using the deflection angle of pointer along a scale....

FAQs on PMMC

Why PMMC does not use for A.C instruments ?...