Moving Iron [MI] Instruments

MOVING IRON [MI] INSTRUMENTS

Types of MI Instruments

All moving iron instruments may be classified, under the following two major groups.

They are:

1. Attraction type

2. Repulsion type

1. Attraction Type

Construction

A moving iron, wended on a spindle, is situated inside a coil of wire. The moving iron is CAM shaped and consists of two or three thin discs of soft iron. It is eccentrically which pivoted on the spindle. The spindle carries a pointer which moves over a graduated scale. The spindle is pivotated to jewelled bearings. The construction detail is shown in fig (6.8).

Working

The coil of wire be connected in series with the circuit if the instrument is an ammeter or in parallel, if it is a voltmeter. If an ammeter, the coil will carry either the load current or a definite fraction of it. If a voltmeter, the coil will carry a current proportional to the voltage across the circuit. In both cases a magnetic field will be set up inside the coil. The soft iron piece is, therefore, attracted and so it tends to move from the weaker magnetic field outise the coil, to the stronger field inside it. Since the moving is eccentrically pivoted the spindle is turned and so the pointer deflects.

Controlling and Damping

Spring control is almost universally used, but gravity control can be used in panel type instruments which are vertically mounted. A hair spring, made of phosphor bronze, is attached to the spindle so that when the spindle turns, the spring test coiled and exerts a controlling torque. Damping is by air friction. A piston, attached to the spindle, moves inside an air chamber and gives the necessary damping.

2. Repulsion Type

In repulsion type of moving iron instruments, there are two vanes inside the coil, one fixed and the other moveable. These are similarly magnetised when current flows through the coil. So there is a force of repulsion between the two vanes resulting in the movement of the moving vane.

There are two different types in repulsion type instruments. They are,

(i) Radial vane type

(ii) Co‒axial vane type

(i) Radial vane type instrument

The Fig. 6.9 shows the radial vane repulsion type instrument. Out of the other moving iron mechanisms, this is the most sensitive and has most linear scale.

Construction

There will be two vanes which are the fixed and movable vanes. The fixed vane will be attached to the coil. Whereas movable vane is attached to the spindle and suspended in the induction field of the coil. The needle of the instrument is attached to this vane.

Working

Eventhough the current through the coil is alternating, there is always repulsion between the like poles of the fixed and the movable vane. Hence the deflection of the pointer is always in the same direction. The deflection is effectively proportional to the actual current and hence the scale is calibrated directly to read amperes or volts. The calibration is accurate only for the frequency for which it is designed because the impedance is different for different frequencies.

(ii) Coaxial vane type instrument

Construction

In this form of repulsion iron instruments, fixed and moving vanes are used, which are sections of co‒axial cylinders. This design is referred to as the concentric‒vane or coaxial‒vane form of instrument. The arrangement is shown in Fig. 6.10. The stationary and moving vanes have shapes similar to those shown in the development of the cylinders. The fixed vane made tongue shaped to obtain more or less uniform scale.

Working

The current in the coil sets up a magnetic field inside the coil, where the fixed vane and moving vane are situated. Since both the irons are influenced by the same magnetic field, they are similarly magnetized. Hence the moving vane is repelled from the fixed vane, so the spindle turns and the pointer deflects.

(iii) Torque Equation of moving Iron Instruments

Let,

I= initial current in the coil.

ϕ= angle of deflection

dϕ = change in deflection

dl = Increase in current in the coil

dL = change in inductance

The change in voltage due to change in current,

e = d/dt (LI)

= I dL/dt + L dI/dt                   ………..(6.1)

Electrical energy supplied in the small interval of time dt = eI dt    ………..(6.2)

Substituting eqn (1) in (2) we get

= I² dL + IL dt

Initial energy stored = 1/2 LI².               ………..(6.4)

Energy stored after the current is measured

 dI = ½ (I + dI)² * (L+ dL)              ………..(6.5)

= 1⁄2 (I² + 2IdI + dI²) (L + dL)

Change in stored energy = ½(I²+2IdI+dI²)(L+dL) ‒ ½I²L.

= ½(I²L+2ILdI+LdI²) + ½(I²dL +2IdIdL+dI²dL) ‒ ½I²L.

= ILdI +  ½Ldl² + ½I²dL + IdIdL + ½dI²dL

Neglecting second and higher order terms, since they are comparatively very small, then the above equation becomes.

= ILdI + ½I²dL                  …………(6.6)

Mechanical work done = T_d dθ                   …………(6.7)

From the principle of conservation of energy,

Electrical energy supplied = Increase in stored energy + Mechanical work done

From eqn. (3), (6) and (7) we get.

We know, T_c = Sθ

At equilibrium position,

T_c = T_d

K_Sθ = 1/2 . I² . dL/dθ

K_Sθ = 1/2 . I²/K_S . dL/dθ

Since Td ∝ I² it can be used for measurement of both DC and AC,

(iv) Reason for using a MI Instrument on both AC and DC

The deflection ϕ, in moving iron instruments is proportional to the square of the operating current. From this, it is evident that the deflection is always positive. That is, the iron vanes are so magnetised that there is always a force of attraction in case of attraction type and repulsion in case of repulsion type independent of the direction of current. The moving iron instruments are unpolarized instruments i.e., instruments which are independent of the direction in which current passes through them. And also MI instruments read the RMS value of the current or voltage. In DC the RMS value is equal to average value. Therefore these instruments can used on both AC and DC.

Operation principles of MI Instruments

In this type of instrument, the coil is fixed through which current passes. The moving iron is a flat disc, which is mounted between the fixed coils. When the current passes through the coil, the moving iron is moved either by force of attraction or repulsion.

Errors in MI Instruments

There are two types of errors which occurs in moving iron instruments. They are,

1. Errors which occurs with both AC and DC.

2. Errors which occurs with AC only.

1. Errors with both AC and DC

i) Hystersis

Moving iron instruments tends to read higher for descending values of current (voltage) than for ascending values. This is due to the hysteresis effect of the ion parts. That is the value of flux density is different for the same value of current when ascending and descending.

This error can be minimised by making the parts small so that they demagnitize themselves quickly.

ii) Temperature Error

The effect of temperature changes on MI instrument arises, chiefly from the temperature co-efficient of spring. The error may be 0.02 percent per °C. For voltmeter both the temperature co-efficient of spring and temperature co‒efficient of voltmeter may balance each other.

iii) Strong magnetic fields

The errors due to stray magnetic fields may be appreciable as the operating magnetic field is weak and hence can be easily distorted. Such errors depend upon the direction of the stray magnetic field relating to the field of instrument. These errors can be minimised by using an iron case or a thin iron shield over the working parts.

2. Errors with AC only

(i) Frequency errors

Changes in frequency cause errors due to change of resistance of the working coil and also due to changes of magnitude of eddy currents set up in the metal parts of instruments.

(ii) Error due to reactance

The change of reactance of instrument coil is important in case of voltmeters where an additional resistance R_S is used in series with the instrument coil. This error may be compensated by connecting a suitable capacitor across the series resistance.

(iii) Error due to eddy current

Eddy current errors are caused due to mutual inductance between the working coil and the metal parts. At low frequencies the eddy current error increases with square of the frequency and at high frequencies the error is practically constant. For these reasons MI instruments are unsuitable for frequencies above 100 Hz.

Advantages and Limitations of MI Instruments

Advantages

1. Universal use

These instruments can be used for both AC and DC.

2. Less Friction Errors

Errors due to friction are quite small as torque/weight ratio is quite high in these instruments.

3. Cheapness

The fact that a single type of moving element could cover the entire range is one reason, that M.I instruments can be built at less cost than other types.

4. Robustness

These instruments are robust owing to simple construction and also that there are no current carrying moving parts.

5. Accuracy

These instruments are capable of giving an accuracy within the limits of precision and industrial grades.

6. Scale

Moving iron instruments are now available with 240° circular scales. The increased scales length being a certain advantage.

Limitations

1. The scale of M.I instrument is not uniform and is cramped at the lower end and therefore accurate readings are not possible at this end.

2. Errors: These instruments are subjected to serious errors due to hysteresis, frequency changes and stray magnetic fields.

Comparison between PMMC and MI Instruments

PMMC Instruments

1. Moving system carries the operating current.

2. Permanent magnet is used to produce the magnetic field required.

3. Spring control is used.

4. Eddy current damping.

5. Deflecting Torque is proportional to the operating current, T_d ∝ I.

6. Uniform scale.

7. Polarised

8. Read the average value.

9. Magnetic shielding is not required.

10. No hysteresis error.

11. Costlier than M.I but cheaper than Dynamometer type.

MI Instruments

1. Moving system does not carry the operating current.

2. Electro‒magnetic field is used.

3. Gravity control is used.

4. Air friction damping.

5. T_d is proportional to the square of the operating current, T_d ∝ I².

6. Non-uniform scale.

7. Unpolarised

8. Reads the rms value.

9. It has to be shielded.

10. Hysterisis error will occur in AC measurement.

11. Cheapest

Extension of Instrument Range

1. Extension of current range for coil ammeters

An ammeter shunt is merely a low resistance that is placed in parallel with the coil circuit of the instrument in order to measure fairly large current. The greater part of current in main circuit is diverted through the shunt. The circuit diagram for a shunt and milli‒ampere meter for measuring large currents is shown in Fig. 6.11.

The main circuit current I will split into two parts. I_m is the low current flowing through the ammeter and I_sh is the heavy current is flowing through the shunt.

Construction of Shunt

Shunt for lower current range can be housed inside the instrument case but above 200 amps the shunts are provided outside the meter.

The shunts used in dc instruments. The temperature co‒efficient of resistance must be very low and as far as possible equal to that of the instrument coil so that multiplying power of the shunt shall be independent of temperature. The resistance of the shunt should not vary with the time of usage. The thermo electric emf of the shunt should be very small and it must be capable of carrying required current without appreciable heating.

The shunt material is usually manganic in order that the resistance changes due to change in temperature may be small as possible, and also low thermo‒electric emf with copper.

Multi‒range Ammeters

The current range of DC ammeter, may be further extended by a number of shunts selected by a range switch for the measurement of different current of full scale deflection in the meter. Such meter is called a multi range ammeter.

The Fig. 6.12 shows the circuit diagram of multirange ammeter. The circuit has for shunts R_sh1, R_sh2, R_sh3 and R_sh4 are connected in parallel with the meter to give 4 different current ranges I₁, I₂, I₃ and I₄. Sometimes when larger current are measured, the connections are brought out to terminal post which is marked for different current ranges 10, 25, 50 amps etc. These ranges are used first from highest current range to the lowest range to avoid the damage of the meter when unknown value of the current is measured.

2. Extension of voltage range for moving coil voltmeters

A millivoltmeter may be converted into voltmeter to the ranges of 100 V, 150 V, 250 V, 500 volts etc., by connecting a series resistance in it is shown in fig (6.13). This series resistance and the coil is connected in series and put across the circuit whose voltage is to be measured. The multiplier limits the current through the meter so that it does not exceed the value for full scale deflection and prevents the damage of coil.

Construction of Multipliers

The essential requirements of multiplier are

1. their resistance should not change with time.

2. they should be non‒inductive type when it is used in ac supply.

3. Low thermo‒electric emf with copper. The material used for multipliers are mangenin etc.,

The common method of winding non‒inductive resistance is the bifilar winding. The bifilar winding is made by taking wire in double wound. For single range voltmeter, the series resistance is bifilar winding.

For multi‒range voltmeter a high resistance of single layer winding is wound on a thin mica or other insulating card. Multipliers case for voltages upto 500 volts for higher voltage the multiplier may be mounted separately outside the case to avoid excessive heating inside the case.

Multi‒range voltmeter

The multirange voltmeter is shown in fig (6.14) in which the connections are made at the junctions of resistances R₁, R₂, R₃ and R₄ in series to obtain the voltage ranges V₁, V₂, V₃ and V₄. These connection are brought out to terminal post on the instrument and the instrument is connected to the proper desired level range. The series resistance for the voltage ranges V₁, V₂, V₃ and V₄ can be calculated.