INDUCTION
TYPE
SPLIT PHASE TYPE:
PRINCIPLE:
When a disc or drum of a non-magnetic
conducting material is placed in a rotating magnetic field, eddy currents are
induced in disc or drum. The torque produces with the reaction of rotating
magnetic field and eddy current. The relation between torque, eddy current,
rotating flux is given below:
T α φ α i^2
CONSTRUCTION:
This instrument convert single
phase to two phases. This is also called ferraris type instrument
*It
consists of pole which are laminated and placed right angles to each other
opposite pole connected in series the two pairs of poles connected in parallel.
*One
set of coil is connected to inductance and another with a high resistance to
create a phase difference 90. The purpose of both the coils is to measure the
current.
*In
the centre of yoke, the coil is an aluminium drum, inside drum, to strengthen
the magnetic field there is a cylindrical laminated iron core.
WORKING:
When this instrument is fed with supply
due to electromagnets action a rotating magnetic action a rotating magnetic
flux produced. This flux induced eddy current in the disc or drum. The reaction
between flux and eddy current results in production of torque. This torque
deflects the pointer attached to the drum. The controlling torque is provided
by spring action.
SHADED POLE:
PRINCIPLE:
When
a disc or drum of a non-magnetic conducting material is placed in a rotating
magnetic field, eddy currents are induced in disc or drum. The torque produces
with the reaction of rotating magnetic field and eddy current. The relation
between torque, eddy current, rotating flux is given below:
T α φ α i^2
CONSTRUCTION:
This
instrument consists of magnetic core (shaped same as transformer). One end is
placed with band of copper(shaded portion), this makes the two fluxes of shaded
and unshaded portion of differ in phase by 90.
*A metallic disc rotates between pole faces. The
damping is provided by the another magnet. Observe below figure.
WORKING:
When the
instrument is fed with the supply the coil sets up flux, eddy currents are
induced in copper bands. Now there are two different fluxes one at coil and
other at magnetic core. These two fluxes differ by a phase of 90. Flux of eddy
current opposes the flux of magnetic core the remaining working is same as
split phase.
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