Moving coil
instruments can be classified into two types:
1. Permanent magnet moving coil
instrument (for DC)
2. Dynamometer type (for both AC and DC)
PERMANENT MAGNET MOVING COIL –
PRINCIPLE OF OPERATION –
The
operation of a permanent magnet moving coil instrument is based upon the
principle that when a current carrying conductor is placed in a magnetic field,
it experiences a force or torque which moves it.
CONSTRUCTION –
The
instrument consists of a permanent magnet and a rectangular coil of many turns
wound on a light aluminium frame inside which is an iron core. A soft iron
cylinder is fixed between the magnetic poles which help to
(i) Make
the field radial.
(ii) Decrease
the reluctance of air path between the poles.
The
rectangular coil on the iron core is attached to a light pointer. The sides of
the coil are free to move in the air gaps between the poles.
CONTROL –
The
controlling torque is provided by two phosphor bronze hair springs. These
springs also lead the current in and out of the coil.
The control
torque can also be provided by using a ribbon suspension and it is considered
to be an advantageous method as it eliminates bearing friction.
DAMPING –
Damping
torque is produced by the movement of the aluminium frame moving in magnetic
field of the magnet.
DEFLECTING TORQUE AND DEFLECTION –
The
deflecting torque produced in the coil is given by
${{T}_{d}}$
= Force * perpendicular distance
$=NBIl\times b$$=NBIA$ N-m.
Where, N =
no. of turns in coil
B = flux density in Wb/m2
I = current in the coil
A = area of the coil
$\Rightarrow
{{T}_{d}}\propto I$
Also, controlling torque ${{T}_{c}}\propto \theta $
(Because such instruments are spring controlled)
As Td
= Tc $\Rightarrow \theta
\propto I$
Therefore,
such instruments have uniform scales.
Damping is
electromagnetic i.e. by eddy currents induced in metal frame over which coil is
wound.
ADVANTAGES –
- They have low power consumption.
- Their scales are uniform.
- They have high torque-to-weight ratio.
- No hysteresis loss.
- Effective and efficient eddy current damping.
- They are not much affected by stray magnetic fields, due to the presence of strong permanent magnets.
DISADVANTAGES –
- Costlier as compared to moving iron instruments.
- Errors occur with the ageing of instrument.
