Page 280 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

When the response settles quickly without any oscillation, the system is said to be critically damped.
The damping force can be produced by i) air friction, ii) fluid friction and iii) eddy currents.
i Air friction damping
The light aluminium piston attached to the moving system of the instrument is arranged to travel with a very small
clearance in a fixed chamber closed at one end. The cross-section of the chamber is either circular or rectangular.
Damping of the system is affected by the compression and suction actions of the piston on the air enclosed in the
chamber. In another method, light aluminium vane is mounted on the spindle of the moving system which moves
in air or in a closed sector-shaped box.

ii Fluid friction daming
Fluid-friction is similar is action to the air-friction. In this type a high viscosity oil is used. Due to greater viscosity of
the oil, the damping is more effective. However, oil damping is not much used because of several disadvantages
such as objectionable creeping of oil, the necessity of using the instrument always in vertical position and its
obvious unsuitability for use in portable instruments.
iii Eddy current damping
Eddy current damping is the most efficient type of damping A thin disc of a conducting but non-magnetic material
like copper or aluminium is mounted on the spindle. The disc is so positioned that its edges, when in rotation, cut
the magnetic flux between the poles of a permanent magnet. Hence eddy currents are produced in the disc which
flow and so produce a damping force in such a direction as to oppose the very cause producing them (Lenz’s
law). Since the cause producing them, is the rotation of the disc, these eddy currents retard the motion of the disc
and moving system as a whole

Wattmeter
Dynamometer type

Dynamo-meter type wattmeter is used to measure power in both AC and DC circuit. Dynamo-meter type
instrument works on the principle of DC motor.ie. whenever a current carrying conductor is kept in a magnetic
field, a force is created and it tends to move the conductor away from the magnetic field.

267

CITS : Power - Electrician & Wireman - Lesson 50-53 CITS : Power - Electrician & Wireman - Lesson 50-53