Dynamometer Type Wattmeter
This is also similar in design and principle to the
dynamometer type ammeter and voltmeter.
Pic 1
Construction: In the dynamometer type instrument is used as a wattmeter,
the fixed coil, which is divided into two equal portions, the fixed coils are
connected in series with the load and carry the circuit (or load) current,
therefore it is also called as current coil. The moving coils is connected
across the load and carries the current proportional to the voltage across the
circuit. Therefore, it is known as pressure coil or potential coil.
A high non-inductive resistance ‘R’ is connected in series
with the moving coil to limit current through it.
The magnetic field of the
fixed and moving coils react on one another causing the moving coil to turn
about its axis. The movement of the coil is controlled by hair springs also
lead the current into and out of the moving element. Damping torque is provided
by the air friction.
Working: When the instrument is connected in the circuit as shown in the below
fig. the main circuit current I1 passes through the current coil and
the current, I2 is proportional to the voltage to be measured passes
through the pressure coil. Due to the current in the two coils. Mechanical
force exists between them. The result is that the moving coil moves causing
that pointer to deflect on a graduated scale. The pointer move till deflecting
torque is equal to controlling torque.
The reversal of current through the fixed and moving coils
reverses the field due to both the coil hence, the deflecting torque remains
unchanged. Therefore these instruments can be used on A.C power measurement.s
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