Wattmeter design
Power in an electric circuit is the product
(multiplication) of voltage and current, so any meter
designed to measure power must account for both of
these variables.
A special meter movement designed especially
for power measurement is called the dynamometer
movement, and is similar to a D'Arsonval or Weston movement
in that a lightweight coil of wire is attached to the
pointer mechanism. However, unlike the D'Arsonval or Weston
movement, another (stationary) coil is used instead of a
permanent magnet to provide the magnetic field for the
moving coil to react against. The moving coil is generally
energized by the voltage in the circuit, while the
stationary coil is generally energized by the current in the
circuit. A dynamometer movement connected in a circuit looks
something like this:
The top (horizontal) coil of wire measures
load current while the bottom (vertical) coil measures load
voltage. Just like the lightweight moving coils of voltmeter
movements, the (moving) voltage coil of a dynamometer is
typically connected in series with a range resistor so that
full load voltage is not applied to it. Likewise, the
(stationary) current coil of a dynamometer may have
precision shunt resistors to divide the load current around
it. With custom-built dynamometer movements, shunt resistors
are less likely to be needed because the stationary coil can
be constructed with as heavy of wire as needed without
impacting meter response, unlike the moving coil which must
be constructed of lightweight wire for minimum inertia.
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