Permanent magnet moving coil instruments - Engineering notes

Permanent magnet moving coil instruments:

These instruments are employed either as ammeters or voltmeters and can be used for d.c work only.

Principle of  Permanent magnet moving coil instruments:

This type of instrument is based on the principle that when a current carrying conductor is placed in a magnetic field, mechanical force acts on the conductor. The coil placed in magnetic field and carrying operating current is attached to the moving system. With the movement of the coil the pointer moves over the scale.

Construction of Permanent magnet moving coil instruments:

It consists of a powerful permanent magnet with soft iron pieces and light rectangular coil of many turns of fine wire wound on aluminium former inside which is an iron core as shown in fig. The purpose of coil is to make the field uniform. The coil is mounted on the spindle and acts as the moving element. The current is led into and out of the coil by means of the to control hair springs, one above and the other below the coil. The springs also provides the controlling torque. Eddy current damping is provided by aluminium former.

Permanent magnet moving coil instrument

Working of Permanent magnet moving coil instruments:

When the instrument is connected in the circuit, operating current flows through the coil. This current coil is placed in the magnetic field produced by the permanent magnet and therefore, mechanical force acts on the coil. As the coil attached to the moving system, the pointer moves over the scale.

It maybe noted that if current direction is reversed, the torque will also be reversed since the direction of the field of permanent magnet is same. Hence, the pointer will move in the opposite direction, i.e it will go on the wrong side of zero. In other words, these instruments work only when current in the circuit is passed in a definite direction i.e. for d.c circuit only.

It is worthwhile to mention here that such instruments are called permanent magnet moving coil instruments because a coil moves in the field of a permanent magnet.

Deflecting torque of Permanent magnet moving coil instruments:

When current is passed through the coil, a deflection torque is produced due to the reaction between permanent magnetic field and the magnetic field of the coil as shown in the fig.

Let B = flux density in the air gap between the magnetic poles and iron core.

l = active length of the coil side in meters.

N = number of turns of coil.

If a current of i amperes flows in the coil in the direction shown, then force F acting on each coil side is given by

F = BI / N newtons

Td = Force * perpendicular distance

= F * 2r newton - meters

where r is the distance of coil side from the axis of rotation in meters

Td = NBIl * 2r newton - meters

If A ( = l * 2r) is the surface area of the coil, then

Td = NBIA newton - meters

Deflection torque = Ampere turns on coil (NI) * Area of coil (A) * flux density (B)

Td  proportional  I

Since the control by springs, therefore controlling torque is proportional to the angle of deflection i.e

Tc  proportional  I

The pointer will come to rest at a position when,

Td = Tc

Deflection  proportional  I

Thus, the deflection is directly proportional to the operating current. Therefore, such instruments have uniform scale.

Advantages of Permanent magnet moving coil instruments:

1. uniform scale.

2. Very effective  eddy current damping

3. Power consumption is low.

4. No hysteresis loss

5. As working field is very strong, therefore, such instruments are not affected stray fields.

6. Such instruments require small operating current.

7. Very accurate and reliable.

Disadvantages of Permanent magnet moving coil instruments:

1. Such instruments cannot be used for a.c measurements.

2. Costlier as compared to moving iron instruments.

3. Some errors are caused due to the ageing of control springs and the permanent magnet.