Electrical Engineering ⇒ Topic : Electrodynamometer Wattmeter
This type of wattmeter is similar in design and construction to the electrodynamometer type ammeter and voltmeter. The fixed coils are connected in series with the load and carry current in the circuit. These coils are called current coils. The moving coil is connected across the voltage.
It carries current proportional to the voltage. In order to limit the current to a small value, a high non-inductive resistance is connected in series with the moving coil. It is also called pressure coil or voltage coil of the wattmeter.
The fixed coils are divided into two halves of fixed coils and are used as current coils because these coils can be made more massive. These coils can be easily constructed to carry considerable current, as there is no problem of getting the current in or out. The fixed coils are wound with heavy wire.
The moving coil, i.e., pressure coil is mounted on the spindle. It is entirely embraced by the fixed current coils. Spring control is used for the movement. Air friction damping is used for providing damping torque. The moving system carries a light aluminium vane which moves in a sector-shaped box.
The schematic diagram of an electrodynamometer wattmeter is shown in Figure (a).
Figure (a) Electrodynamometer wattmeter
The instantaneous torque of an electrodynamometer instrument is given by
where ip and ic are instantaneous values of current in two coils.
The instantaneous voltage across the pressure coil is given by
where V is the r.m.s. value of voltage.
If the resistance of the pressure coil circuit is very high, the current through the pressure coil circuit is given by
where Rp is the resistance of the pressure coil and /p = V/Rp = r.m.s. value of current in pressure coil.
Let the current through the current coil lags behind the voltage by an angle Φ. The instantaneous value of current through the current coil is given by
Therefore, instantaneous torque is given by,
Therefore, average deflecting torque is given by,
Controlling torque exerted by springs is:
where K is the spring constant and θ is the final deflection.
It is clear from the Eq. (5) that the deflection is directly proportional to the power being measured and the scale is essentially uniform over the range in which dM/dθ is constant.
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