To study the construction and working principle of PMMC and moving iron instrument

Construction and working principle of PMMC and moving iron instrument

Aim

To study the construction and working principle of PMMC and moving iron instrumen.

Apparatus

Meter Demonstration ckt, Connecting Wire, D.C Power supply

Theory

PMMC

PMMC

A Permanent Magnet Moving Coil (PMMC) meter - also known as a D'Arsonval meter or galvanometer is an instrument that allows you to measure the current through a coil by observing the coil's angular deflection in a uniform magnetic field.

Construction

A PMMC Equipment consists of two main parts; moving coil and a permanent magnet along with other parts. These parts are explained below:

1. Magnet System

The instrument consists of two high intensity, high coercive force magnets or a big U‒shape magnet based on design. These magnets are made up of Alcomax and Alnico for higher coercive force and better field intensity.

In many designs, an additional soft iron cylinder is placed in between the magnetic poles to make the field uniform; while reducing air reluctance to increasing field strength.

2. Moving Coil

It is one of the main components of permanent magnet moving coil equipment; and is made up of copper coils wounded to a rectangular block in between the magnetic poles. Made up of Aluminium; the rectangular block is called Aluminium former pivoted to the jewelled bearing. It is what allows the coil to rotate freely.

Non‒metallic former like that of aluminium is used for current measurement; while metallic former with high electromagnetic damping is used to measure voltage.

3. Control

Two spring made of phosphorus bronze acts as a control system for the permanent magnet moving coil. These springs are mounted on the jewel bearing of PMMC; providing the essential controlling torque. The controlling torque produced is mainly due to ribbon suspension. They oppose the force of deflection; so the electromagnetic force (of Moving Coil) came in equilibrium with the spring tension.

This helps in keeping the pointer at a fixed position after an initial deflection. These control springs also serve the purpose of providing lead current paths in and out of the system.

4. Damping System

Damping torque is produced in the PMMC equipment by the movement of aluminium core in the magnetic field. It keeps the pointer at rest after the initial deflection. This helps in proper measurement without fluctuations.

Due to the movement of the coil in the magnetic field; eddy current is produced in the aluminium former. this produces the damping force / Torque which opposes the further motion of the coil. Slowly the pointer deflection reduces and finally at a fixed position.

5. Scale and Pointer

The pointer connected to the moving coil moves moving coil moves over a marked scale. The pointer moves along with the coil deflection to show readings marked on the scale. A pointer is a simple construction with light weight design and twisted section to reduce parallax error.

A Parallax error can be further reduced by proper alignment of pointer blades to the initial scale.

Working Principle of a PMMC Instrument

When a current caring conductor is placed in a magnetic field; it experiences a force perpendicular to the field and the current (Fleming Left Hand Rule). This force tends to move the conductor. According to Fleming left‒hand rule; if your left‒hand thumb, fore finger, and middle finger are at 90 degrees to each other. Then the magnetic field would be along with the fore finger, current across the middle while the force along with the thumb.

When current flows in the coil on the aluminium former; a magnetic field is produced in the coil in proportion to the current flow. This electromagnetic force along with a static magnetic field from the permanent magnet produces the deflection force in the coil. The spring then produces the controlling force to oppose further deflection; thus helps in balancing the pointer.

Then damping force is produced in the system by the movement of aluminium core in the magnetic field. It keeps the pointer fixed to a position after it reaches equilibrium with the controlling and deflection torque; providing better precision in measurement.

Torque Equation

As we know that torque is defined as:

Torque = force * perpendicular distance

In case of an electromagnetic circuit, force is given by NBIL

where,

N = No. of turns in coil;

B = Flux density;

L = length of the coil;

I = current flowing across the coil

Therefore now torque becomes T = NBIL * D

оr

T = NBI * A

or

T=G*I

A ‒ Area

G= NBA (constant)

As we know that deflecting torque T_d = controlling torque Tc

and

Tc=K*θ

GI= Kθ

This equation shows that the deflection of a PMMC instrument is directly proportional to the electric current flowing across the coil.

Error in PMMC Equipment

1. Error due to magnetism

Permanent magnet loses their magnetism with time; this is called magnet aging. With plenty of heat and vibration on the ship (especially Engine Room); There is a reduction of magnetism due to accelerated aging. This decrease in magnetic strength reduce the coil deflection affecting the readings.

2. Error due to Temperature Difference

Moving Coil of PMMC instrument is made up of copper wires; the temperature coefficients of copper wire is known to be 0.004 per degree Celsius. So with increase in temperature, there will be a high increase in its resistance altering the actual reading.

3. Error due to Spring

Aging leads to weakening of spring tension; this results in decreased deflection of the moving coil. This error is opposite to that of the error due to magnetic aging and sometimes cancel each other to reduce much difference in the final readings.

Advantages of PMMC Equipment

1. High weight to torque ratio.

2. It has pointer deflection proportional to the current; which makes the scale more uniform over an arc of 270 degrees.

3. It consumes much less power than other alternatives.

4. No hysteresis loss.

5. Unaffected by a stray magnetic field; perfect equipment for on‒board applications.

6. All‒purpose equipment; can be used as an ammeter, voltmeter, and galvanometer.

Disadvantages of PMMC Equipment

1. It only works for Direct current (D.C).

2. It's costly than its other alternatives.

3. It can show false reading due to the above stated reasons (cause of errors in permanent magnet moving coil instrument).

Moving Iron:

Moving Iron:

The instrument in which the moving iron is used for measuring the flow of current or voltage is known as the moving iron instrument.

It works on the principle that the iron place near the magnet attracts towards it. The force of attraction depends on the strength of the magnet field. The magnetic field induces by the electromagnet whose strength depends on the magnitude of the current passes through it.

Construction and Working

In Moving Iron Instruments, a plate or van of soft iron or of high permeability steel forms the moving element of the system. The iron van is so situated that it can move in the magnetic field produced by a stationary coil.

The stationary coil is excited by the current or voltage under measurement. When the coil is excited, it becomes an electromagnet and the iron van moves in direction of offering low reluctance path. Thus the force of attraction is always produced in a direction to increase the inductance of coil. Mind that as the van follows the low reluctance path, the net flux in air gap will increase which means increased flux linkage of coil and hence inductance of coil will increase. It shall also be noticed that, the inductance of coil is variable and depends on the position of iron van.

Classification

There are two types of moving iron instrument, Attraction and Repulsion type.

1. Attraction Type

The instrument in which the iron plate attracts from the weaker field towards the stronger field such type of instrument is known as the attraction type instrument.

Construction of Attraction Type Instrument

The stationary coil of the attraction type instrument is flat and has a narrow opening. The moving element is the flat disc of the iron core. The current flow through the stationary coil produced the magnetic field which attracts the iron coil. The iron vane deflects from the low magnetic field to the high magnetic field, and the strength of the deflection is directly proportional to the magnitude of the current flow through it. In short, we can say that the iron coil attracts towards in.

The attraction type instruments use spring, which provided the controlling torque. The deflection of the coil is reduced by the aluminium piston which is attached to the moving coil.

2. Repulsion Type Instruments

The repulsion type instrument has two vanes or iron plates. One is fixed, and the other one is movable. The vanes become magnetised when the current passes through the stationary coil and the force of repulsion occur between them. Because of a repulsive force, the moving coil starts moving away from the fixed vane.

The spring provides the controlling torque. The air friction induces the damping torque, which opposes the movement of the coil. The repulsion type instrument is a non‒polarized instrument, i.e., free from the direction of current passes through it. Thus, it is used for both AC and DC.