Operating Principles of Moving Iron Instruments Ammeters and Voltmeters

Moving Iron instruments

Moving-iron instruments are generally used to measure alternating voltages and currents. In moving-iron instruments the movable system consists of one or more pieces of specially-shaped soft iron, which are so pivoted as to be acted upon by the magnetic field produced by the current in coil.

There are two general types of moving-iron instruments namely:

1.       Repulsion (or double iron) type (figure 1)

2.       Attraction (or single-iron) type (figure 2)

The brief description of different components of a moving-iron instrument is given below:

Moving element: A small piece of soft iron in the form of a vane or rod.

Coil: To produce the magnetic field due to current flowing through it and also to magnetize the iron pieces.

Repulsion type

In repulsion type, a fixed vane or rod is also used and magnetized with the same polarity. Control torque is provided by spring or weight (gravity).

Damping torque is normally pneumatic, the damping device consisting of an air chamber and a moving vane attached to the instrument spindle.

Deflecting torque produces a movement on an aluminum pointer over a graduated scale.

The deflecting torque in any moving-iron instrument is due to forces on a small piece of magnetically ‘soft’ iron that is magnetized by a coil carrying theoperating current. In repulsion type moving–iron instrument consists of two cylindrical soft iron vanes mounted within a fixed current-carrying coil. One iron vane is held fixed to the coil frame and other is free to rotate, carrying with it the pointer shaft. Two irons lie in the magnetic field produced by the coil that consists of only few turns if the instrument is an ammeter or of many turns if the instrument is a voltmeter.

Current in the coil induces both vanes to become magnetized and repulsion between the similarly magnetized vanes produces a proportional rotation. The deflecting torque is proportional to the square of the current in the coil, making the instrument reading is a true

‘RMS’ quantity Rotation is opposed by a hairspring that produces the restoring torque . Only the fixed coil carries load current, and it is constructed so as to withstand high transient current.

Moving iron instruments having scales that are nonlinear and somewhat crowded in the lower range of calibration.

Measurement of Electric Voltage and Current

Moving iron instruments are used as Voltmeter and Ammeter only.

Both can work on AC as well as on DC.

Ammeter

Instrument used to measure current in the circuit.

Always connected in series with the circuit and carries the current to be measured. This current flowing through the coil produces the desired deflecting torque.

It should have low resistance as it is to be connected in series.

Voltmeter

Instrument used to measure voltage between two points in a circuit.

Always connected in parallel.

Current flowing through the operating coil of the meter produces deflecting torque.

It should have high resistance. Thus a high resistance of order of kilo ohms is connected in series with the coil of the instrument.

Ranges of Ammeter and Voltmeter

For a given moving-iron instrument the ampere-turns necessary to produce full-scale deflection are constant.

One can alter the range of ammeters by providing a shunt coil with the moving coil.

Voltmeter range may be altered connecting a resistance in series with the coil. Hence the same coil winding specification may be employed for a number of ranges.

Advantages

1.     The instruments are suitable for use in AC and DC circuits.

2.     The instruments are robust, owing to the simple construction of the moving parts.

3.     The stationary parts of the instruments are also simple.

4.     Instrument is low cost compared to moving coil instrument.

5.     Torque/weight ratio is high, thus less frictional error.

Errors

(i). Error due to variation in temperature.

(ii). Error due to friction is quite small as torque-weight ratio is high in moving coil instruments.

(iii). Stray fields cause relatively low values of magnetizing force produced by the coil. Efficient magnetic screening is essential to reduce this effect.

(iv). Error due to variation of frequency causes change of reactance of the coil and also changes the eddy currents induced in neighbouring metal.

(v). Deflecting torque is not exactly proportional to the square of the current due to non -linear characteristics of iron material.

Attraction type

The basic construction of attraction type moving iron instrument is illustrated bellow A thin disc of soft iron is eccentrically pivoted in front of a coil. This iron tends to move inward that is from weaker magnetic field to stronger magnetic field whencurrent flowing through the coil. In attraction moving instrument gravity control was used previously but now gravity control method is replaced by spring control in relatively modern instrument. By adjusting balance weight null deflection of the pointer is achieved. The required damping force is provided in this instrument by air friction. The figure shows a typical type of damping system provided in the instrument, where damping is achieved by a moving piston in an air syringe.

Theory of Attraction Type Moving Iron Instrument

Suppose when there is no current through the coil, the pointer is at zero, the angle made by the axis of the iron disc with the line perpendicular to the field is φ. Now due current I and corresponding magnetic field strength, the iron piece is deflected to an angle θ. Now component of H in the direction of defected iron disc axis is Hcos{90 - (θ + φ) or Hsin(θ + φ). Now force F acting on the disc inward to the coil is thus proportional to H²sin(θ + φ) hence the force is also proportional to I²sin(θ + φ) for constant permeability. If this force is acting on the disc at a distance l from the pivot, then deflection torque,

Td = Fl cos (θ+Φ)

Thus Td = I² sin (θ+Φ) cos (θ+Φ)

Td = kI² sin 2(θ+Φ)

Where k is constant.

Now, as the instrument is gravity controlled, controlling torque will be

Tc = k’ sin θ

Where k ‘is constant