What is PMMC Instrument?

What is PMMC Instrument?

There are different types of electrical machines which we use in our daily life. Whenever these machines are not working properly then we have to check what is the problem for this purpose sometimes or normally we need to measure some parameters so for this we need some instruments to measure them so then PMMC ( permanent magnet moving coil ) is one of those instruments.

• PMMC  stands for "permanent magnet moving coil".
• It is a simple and frequently used instrument on ships with sophisticated names.
• Used where exact measurement is required also as an aid while maintaining electrical equipment.
• also known as D'alvanometer. because it is a kind of galvanometer that works on the principle of D'Arsonval
• These instruments use permanent magnets to create a stationary magnetic field in the coils.
• Then it is used with the moving coil that is connected to the electric source for generating deflection torque according to the Fleming left-hand rule.

Working of PMMC Instrument

• Working Principle of PMMC Instrument is simple whenever there is a flow of current in the moving coil inside the stationary magnetic field by permanent magnets there is a force there is a deflection torque generated because we know a magnetic field exerts a force on current-carrying wire inside the magnetic field according to the fleming's left-hand rule. thus there is a deflection torque is produced and we providing a damping force by the spring for maintaining the pointer at equilibrium on desired reading.

Construction of PMMC Instrument

The important parts of PMMC are as follows;

Moving Coil

•  It is an essential component of the PMMC instrument. This designing of the coil can be done by wounding copper coils on a rectangular block among the magnetic poles.
•  The rectangular block is made up of Aluminium and can be called Alumnim former rotted into the jeweled bearing. 
•  So it permits the coil to turn freely.
•  Once the current is supplied throughout these coils then deflection takes place within the stationary magnetic field and according to the deflection, we measure the voltage or current magnitude. 

Note: The aluminum is a non - metallic former, used to measure the current, and metallic former including high electromagnetic damping is used to calculate the voltage.

Magnet System

•  It includes two high-intensity magnets otherwise a 'U' Shaped magnet-based design.
•  The designing of these magnets can be done with Alnico and Alcomax for higher superior field intensity and coercive force.
• In several designs, an extra soft iron cylinder can be arranged among the magnetic poles to create the field identical; while decreasing air reluctance for increasing the strength of the field.

Control

• The deflection of the pointer is controlled by the control springs which provide equal opposite torque to balance the pointer in equilibrium position.
• Springs are fabricated by phosphorous bronze. These springs are arranged among the two jewel bearings.
• The spring provides the lane to the lead current to supply in and out of the moving coil. 
• The torque can be controlled mainly due to the delay of the ribbon.

Damping Torque

• generated by using aluminum cor's movement within the magnetic field.
• Because of the movement of coil within the magnetic field eddy currents can be generated within the aluminum former. this generates damping force otherwise to torque to resist the motion of the coil.
• So the deflection of the pointer reduces gradually and the pointer lasts at a permanent position which helps to take the right measurements.

Pointer and Scale

•  In this instrument, the connection of the pointer is done throughout the moving coil.
• So the movement of the coil results in the movement of the pointer and hence which gives the reading on the scale.
• the pointer is made up of a light material so it can easily move with the moving coil.
• Sometimes there is a chance of parallax error which is decreased by properly arranging the pointer on the blade.

Sources of Error in PMMC

• temperature effects
• getting old of the instrument cause error to the main part of the instrument like a magnet, moving coil, and spring.
• errors can be reduced by connecting swamping resistance in series using moving coil. here swamping resistance is nothing but a resistance with less temperature coefficient which reduces temperature effects on the instrument.

Torque Equation

The equation involved in the PMMC instruments is the torque equation.

                                              Td = NBLdl

where,

'N' is the no. of turns in the coil

'B' is the density of flux within the air gap

'L' and 'd' are vertical as well as horizontal lengths of the surface

'I' is the flow of current in the coil

                                       G = NBLd

The restoring torque can be provided to the moving coil is done by the spring which we say,

                                        Tc = Kθ (‘K’ is the spring constant)

The Final deflection cab be done through the equation Tc = Td

Substitute the values of Tc and Td in the above equation, then we can get

                                        Kθ = NBLdl

we know  G = NBLd

Kθ = Gl

θ= Gl/K

I = (K/G) θ

So we conclude that the deflection torque is directly proportional to the flow of current in the coil in PMMC