Moving Iron (MI) Instrument

MOVING IRON (MI) INSTRUMENT

It is the commonly used, accurate instrument for both AC and DC measurement. It is divided into two types 1. attraction type and 2. Repulsion type. Moving Iron Instrument consist of a stationary coil which is excited by the current or voltage under measurement. It also consist of a plate or vane of soft iron in case of attraction type in case of repulsion type, two vanes are present.

Vane forms the moving element of the instrument and moves in a magnetic field produced by a stationary coil.

Principle:

Supply is given to the stationary coil it becomes Electromagnet vane moves to increase the flux of the electromagnet because vane tries to occupy a position of minimum reluctance.

Inductance α 1/reluctance

Thus a force is developed for getting high inductance. This force gives the deflecting torque.

Types:

Attraction type MI instrument

The flat stationary coil forms the narrow slot like opening. moving element is a flat disc or plate.

When current flows through the coil, magnetic field is produced around the coil. moving Iron moves from weaker field to stronger field side.

For control torque this instrument uses spring. In panel type, vertical mounted case uses gravity control for T. Damping is provided by air friction damping.

Repulsion Type

Two vanes are present inside the stationary coil, one vane is fixed and another vane is movable.

Both vanes are similarly magnetised when the current flows through the stationary coil, So repulsion occurs between the two vanes.

This type is divided into two

1. Radial vane type and

2. Co-axial vane type.

In radial vane type, vanes are radial strips of Iron.

In co-axial vane type, vanes are sections of co-axial cylinders.

T_c is provided by spring and by gravity control for vertically mounted instrument.

Air friction damping is used.

Eddy current damping is not used because operating magnetic field is very weak.

Torque Equation:-

Torque equation is derived from the energy relations.

Normally current flows through the coil

Now a small increment in current is supplied to the coil, it result in a small increment in deflection.

Mechanical work done = T_d dƟ ………(1)

emf,  e = d/dt (L I)

e = I dL/dt + L dI/dt ………(2)

Electrical energy supplied is given by, multiply I. dt on both sides of equation (2),

eI dt = I² dL/dt. dt + L I dI/dt. dt

eI dt = I² dL + L I dI ……….(3)

Stored energy changes from 1/2 I² L to 1/2 (I + d I)² (L + dL)

Change in stored energy is

1/2 (I² + 2 I dI + d I² (L + dL)

Neglecting second and higher order terms,

IL dI + 1/2 dL ……..(4)

By the principle of conservation of energy,

Electrical energy supplied = increase in stored energy + mechanical work done.

From equation (3), (4) and (1),

I² dL + I L dI = IL dI + 1/2 I² dL + T_d. dƟ

T_d . dƟ = I² dL – 1/2 I² dL

T_d .dƟ = ½ I² dL

T_d = ½ I² dL/dƟ ……….(5)

control torque is provided by spring

T_c = K Ɵ …………(6)

For final steady deflection, T_c = T_d

.. from equation (5) and (6),

KƟ = ½ I² . dL/dƟ

Deflection, Ɵ = ½ I²/K . dL/dƟ

deflection is directly proportional to the square of the rms value of the measuring current. scale is nonlinear.

Torque is unidirectional, so MI instrument is not polarity sensitive.

Error in MI instrument:

(i) Error occurs when used for both AC and DC measurement:

1. Varying flux density causes hysteresis error. it is eliminated by using small Iron pieces.

2. Temperature error, it is eliminated by swamping Resistance.

3. Operating field is weak and it cause stray magnetic field error. itis eliminated by a thin iron shield placed over the working parts.

(ii) Error occurs when used for ac only:

1. Frequency error: it is due to change in reactance of working will, as a result of this error deflection for a given voltage will be low at high frequency than at loco frequency.

This error is minimized by providing frequency compensation.

Frequency compensation is done by connection a capacitor 'C' parallel with the series resistor 'R_s'.

This is possible only upto 125HZ.

2. Eddy carrent error:

Eddy current developed in the moving iron parts causes error.

Extension of meter range:

Comparision between Attraction Type and Repulsion Type MI instrument

Advantage:

1. Used for both dc and AC measurement.

2. Less frictional errors.

3. Low cost compared to other ac measuring instruments.

4. Accuracy

5. Robustness.

Dis-advantage:

1. Non uniform scale

2. It suffers from hysteresis and eddy current error.

3. Calibration is different for AC and DC operation.