# Moving Iron Instruments – Attraction and Repulsion Type

- Link: https://studyelectrical.com/2019/11/moving-iron-instruments-attraction-and-repulsion-type.html
- Published: 2019-11-24T06:40:39+00:00
- Author: Tesla

The most common ammeters and voltmeters for laboratory or switch-board use at power
frequencies are the **moving iron instruments**.

These instruments can be constructed to measure current and voltage to the accuracy
needed in most engineering works and still be cheap as compared with any other type
of AC instrument of the same accuracy and ruggedness.

![Moving Iron Ammeter](https://studyelectrical.com/wp-content/uploads/2019/11/Moving-
Iron-Ammeter.jpg)

Moving Iron Ammeter

| **Basic Range** | 10 mA – 100 A | 
| **Usage** | DC and AC moving iron ammeters and voltmeters |

There are two basic **types of Moving Iron Instruments** (MI instruments)

 1. **Attraction-type moving-iron instruments**
 2. **Repulsion-type moving-iron instruments**

## Working of Moving Iron Instruments

A plate or vane of soft iron or of high permeability steel forms the moving element
of the system. This iron vane is so situated that it can move in a magnetic field
produced by a stationary coil.

The coil is excited by the current or voltage under measurement.

When the coil is excited, it becomes an electromagnet and the iron vane moves in
such a way so as to increase the flux of the electromagnet. This is because the 
vane tries to occupy a position of minimum reluctance.

Thus the force (or torque) produced is always in such a direction as to increase
the inductance of the coil (As inductance increases as reluctance decreases).

![Moving Iron Meter](https://studyelectrical.com/wp-content/uploads/2019/11/Moving-
Iron-Meter.jpg)

Moving Iron Meter

The current to be measured, in general, is passed through a coil of wire in the 
moving iron instruments. In the case of voltage measurement, the current which is
proportional to the voltage is measured.

The number of turns of the coil depends upon the current to be passed through it.
For the operation of the instrument, a certain number of ampere-turns is required.

These ampere-turns can be produced by the product of a few turns and large current
or reverse.

## Attraction Type Moving Iron Instruments

The attraction type of moving iron instrument depends on the attraction of an iron
vane into a coil carrying the current to be measured.

The figure shows an attraction-type moving iron (MI) instrument.

![Moving iron instruments](https://studyelectrical.com/wp-content/uploads/2019/11/
Moving-iron-instruments.png)

An attraction type moving iron instrument

A **soft iron vane IV** is attached to the moving system. When the current to be
measured is passed through the **coil C**, a magnetic field is produced. This field
attracts the eccentrically mounted vane on the spindle towards it. The spindle is
supported at the two ends on a pair of** jewel bearings**. Thus, the **pointer PR**,
which is attached to the **spindle S** of the moving system is deflected. The pointer
moves over a **calibrated scale**.

![Internal parts of moving iron instrument](https://studyelectrical.com/wp-content/
uploads/2019/11/Internal-parts-of-moving-iron-instrument.jpg)

Internal parts of moving iron instrument

The control torque is provided by two **hairsprings S1 and S2** in the same way 
as for a [PMMC instrument](https://studyelectrical.com/2019/11/permanent-magnet-moving-coil-pmmc-instrument.html),
but in such instruments, springs are not used to carry any current.

**Gravity control** can also be used for vertically mounted panel type MI meters.
The damping torque is provided by the movement of a **thin vane V** in a closed **
sector-shaped box B**, or simply by a vane attached to the moving system.

**Eddy current damping cannot be used** in MI instruments owing to the fact that
any permanent magnet that will be required to produce Eddy current damping can distort
the otherwise weak operating magnetic field produced by the coil.

If the current in the fixed coil is reversed, the field produced by it also reverses.
So the polarity induced on the vane reverses. Thus whatever be the direction of 
the current in the coil the vane is always be magnetized in such a way that it is
attracted to the coil. **Hence such an instrument can be used for both direct currents
as well as alternating current.**

![Moving Iron Instruments - Sectional View](https://studyelectrical.com/wp-content/
uploads/2019/11/Moving-Iron-Instruments-Sectional-View.png)

Moving Iron Instruments – Sectional View

A sectional view of the actual instrument is shown in the figure. When the current
to be measured is passed through the coil or solenoid, a magnetic field is produced,
which attracts the eccentrically mounted disc inwards, thereby deflecting the pointer,
which moves over a calibrated scale.

## Repulsion-type Moving-Iron Instruments

In the repulsion type, there are two vanes inside the coil. One is fixed and the
other is movable. These are similarly magnetized when the current flows through 
the coil and there is a force of repulsion between the two vanes resulting in the
movement of the moving vane.

Two different designs for moving iron instruments commonly used are as follows:

 1. Radial Vane Type
 2. Co-axial Vane Type

### **Radial Vane Type**

In this type, the vanes are radial strips of iron. The strips are placed within 
the coil as shown in the figure.

![Radial vane repulsion type moving iron instrument](https://studyelectrical.com/
wp-content/uploads/2019/11/Radial-vane-repulsion-type-moving-iron-instrument.png)

Radial vane repulsion type moving iron instrument

The fixed vane is attached to the coil and the movable one to the spindle of the
instrument. The instrument pointer is attached to the moving vane spindle.

![Radial Vane instrument](https://studyelectrical.com/wp-content/uploads/2019/11/
Radial-Vane-instrument.png)

As current flows through the coil, the generated magnetic field induces identical
polarities on both the fixed and moving vane. Thus, even when the current through
the coil is alternating (for AC measurement), there is always a repulsion force 
acting between the like poles of fixed and moving vane. Hence deflection of the 
pointer is always in the same direction irrespective of the polarity of the current
in the coil.

The amount of deflection depends on the repulsion force between the vanes which 
in turn depends on the amount of current passing through the coil. The scale can
thus be calibrated to read the current or voltage directly.

### **Co-axial Vane Type**

In these types of instruments, the fixed and moving vanes are sections of coaxial
cylinders as shown in the figure.

![Coaxial vane repulsion type moving iron instrument](https://studyelectrical.com/
wp-content/uploads/2019/11/Coaxial-vane-repulsion-type-moving-iron-instrument.png)

Coaxial vane repulsion type moving iron instrument

Current in the coil magnetizes both the vanes with similar polarity. Thus the movable
vane rotates along the spindle axis due to this repulsive force. Coaxial vane type
instruments are moderately sensitive as compared to radial vane type instruments
that are more sensitive.

![Concentric vane type instrument](https://studyelectrical.com/wp-content/uploads/
2019/11/Concentric-vane-type.png)

Concentric vane type instrument

Moving iron instruments have their deflection is proportional to the square of the
current flowing through the coil. These instruments are thus said to follow a square-
law response and have non-uniform scale marking.

Deflection being proportional to the square of the current, whatever be the polarity
of the current in the coil, deflection of a moving iron instrument is in the same
direction. Hence, moving iron instruments can be used for both DC and AC measurements.

The video below shows the components inside a repulsion type moving iron ammeter.

## Torque Equation of Moving-Iron Instruments

To deduce the expression for the torque of a moving iron instrument, energy relation
can be considered for a small increment in current supplied to the instrument. This
results in a small deflection dθ and some mechanical work will be done. Let T_(d)
be the deflecting torque.

_∴ Mechanical work done = Torque × Angular displacement_

_= **T_(d ). dθ**_

Due to the change in inductance, there will be a change in the energy stored in 
the magnetic field.

Let I be the initial current, L be the instrument inductance and θ is the deflection.
If the current increases by dl then it causes the change in deflection dθ and the
inductance by dL.

In order to involve the increment dI in the current, the applied voltage must be
increased by:

_e = dϕ/dt = d/dt (LI) = I (dL/dt) + L (dI/dt)_

**The electrical energy supplied is _eldt = I^(2)dL + ILdl_**

Substitute the value of _edt_ from above equation

The current is changes from _I to (I + dI)_, and the inductor _L to (L + dL)._

Therefore the stored energy changes from _½I^(2)L_ to ½_(I + dI)_^(2)_(L + dL)_

Hence the change in energy storage = ½_(I + dI)_^(2)_(L + dL) – ½I^(2)L _

As dI and dL are very small, neglecting the second and higher order terms in small
quantities, this becomes

** _IL.__dL + ½I^(2)dL_**

From the principle of conservation of energy,

_Electrical energy supplied = Increase in stored energy + Mechanical work done._

_I^(2)dL + ILdl = IL.dL + ½ I^(2)dL + T_(d ). dθ_

_∴ T_(d ). dθ = ½I^(2)dL_

**Deflecting torque, _T_(d  )_= _½ I^(2)(dL/dθ)_**

where _T_(d)_ is in newton-meter, _I_ is in ampere, _L_ is in henry and _θ_ is in
radians.

The moving system is provided with control springs and in turn, the deflecting torque
_T_(d)_ is balanced by the controlling torque

_T_(c) = k θ_

where _k_ is the control spring constant (N-m/rad) and _θ_ is the deflection in 
radians.

At the final steady position, _T_(c)_ = _T_(d)_

_k θ = ½ I^(2 )(dL/dθ)_

**_∴ Deflection, θ = ½ (I^(2)/k)( dL/dθ)_**

Hence, the deflection is proportional to the square of the RMS value of the operating
current. The deflection torque is, therefore, unidirectional whatever may be the
polarity of the current.

### Advantages of Moving Iron Instruments

The important advantages of moving iron instruments are

 1. Robust construction and relatively cheap
 2. Suitable for measuring both dc and ac
 3. Can withstand overload momentarily

### Disadvantages of Moving Iron Instruments

The following are the disadvantages of moving iron instruments

 1. As the deflection is proportional to I^(2), hence the **scale of the instrument
    is not uniform**. It is cramped in the lower end and expanded in the upper portion.
 2. It is affected by stray magnetic fields.
 3. There is a hysteresis error in the instrument. The hysteresis error may be minimized
    by using the vanes of nickel-iron alloy. Read [Errors in Moving Iron Instruments](https://studyelectrical.com/2022/09/errors-in-moving-iron-instruments.html)
 4. When used for measuring ac the reading may be affected by the variation of frequency
    due to the change in reactance of the coil, which has some inductance. With the
    increase in frequency, iron loses and coil impedance increases.
 5. Since a large amount of power is consumed to supply I^(2)R loss in the coil and
    magnetic losses in the vanes, it is not a very sensitive instrument.

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## Comments

### By: Beula Crigger

- Link: https://studyelectrical.com/2019/11/moving-iron-instruments-attraction-and-repulsion-type.html#comment-4372
- Published: 2020-05-02T11:50:09+00:00

Nice article. I didn’t get such a complete explanation about moving iron instruments
anywhere in internet. You explained everything in this article. Thank you