Repulsion Type Moving Iron Instrument | Advantages| Disadvantages | Application | Scale of MI Instrument

Repulsion Type Moving Iron Instrument:

Fig. 4.7.2 shows the construction of a repulsion type moving iron instrument. The basic principle on which this instrument i works is that if two soft iron pieces, magnetised in the samei direction (north poles of both on one side and south poles on the other side) are placed very near to each other they repel. i The two soft iron strips A and B are shown in Fig. 4.7.2. The strip A is attached to the bobbin over which the coil C is wound. The other strip B is attached to the spindle and therefore it is movable.
When the current to be measured flows through coil C, A and B are magnetised in the same direction and thus repel each other.
Due to this force of repulsion strip B moves away from strip A and therefore the pointer attached to the spindle deflects.

The type of damping is usually the air friction type. It is provided by the piston moving in the air chamber.
The force of repulsion between the two strips depends onitheir relative field strength. As the field strength proportional to the current in the coil, the detlecting torque is proportional to the square of the current.
Due to this, the instrument has a non-uniform scale. In practice, attempt is made to make the scale more uniform by using soft iron pieces of suitable shapes. Similar to the attraction type instrument, this instrument can be used for a.c. as well as d.c. measurement. i

Advantages of moving iron instruments: 

Following are the advantages of these instruments:
Sutable for a.c. as well as d.c. measurements. Simple and robust in construction as the moving system (soft iron piece) is free from current leads. Good accuracy.

Cheaper in cost as compared to permanent magnet moving coil instrument.
A large scale of a length of about 2409. The instrument has high torque to weight ratio.

The instrument can be used for low frequency measurements also. 

Disadvantages : 

The disadvantages are as follows:
Power consumed by the instrument is high as compared to that of the permanent magnet moving coil instrument. The scale is non-uniform.
Due to the presence of iron part in the operating system, error due to the hysteresis effect is introduced. To reduce this effect nickel iron alloys are used.
Similar to other instruments, this instrument also suffers from the errors introduced by friction, temperature variation and lack of balance in the moving system. i
The operating magnetic field being weak, the stray fields affect the working of the instrument. To reduce this effect it
is sometimes necessary to provide a magnetic shield to the instrument.

Applications:

Moving iron instruments are
measurements except for very inexpensive indicators such as i for d.c. charge-discharge indicators in automobiles. They extensively on a.c. where 5 to 10% error is acceptable.

Scale of a MI Instrument:

shows the scale of a MI instrument. Note that the scale is nonlinear.