The fixed coil in a dynamometer wattmeter is the

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Understanding the Dynamometer Wattmeter Coils

A dynamometer wattmeter is a versatile instrument used for measuring power in both AC and DC circuits. It operates based on the interaction between magnetic fields produced by two sets of coils: a fixed set and a moving set.

Construction of a Dynamometer Wattmeter

The basic construction involves:

Fixed Coils: These coils are designed to carry the main circuit current or a proportional part of it. They are wound with thick wire and are fixed in position.
Moving Coil: This coil is wound with fine wire and is connected across the voltage (or pressure) of the circuit. It is mounted on a spindle and is free to move within the magnetic field produced by the fixed coils.

The deflection of the moving coil is proportional to the product of the currents flowing through the fixed and moving coils, which ultimately relates to the power being measured.

Identifying the Fixed Coil in a Dynamometer Wattmeter

In a dynamometer wattmeter, the fixed coils serve the purpose of the current coil. They are connected in series with the load in the circuit where power is being measured. The total current flowing through the circuit passes through these fixed coils (or a specific fraction if shunts are used), creating a magnetic field proportional to the circuit current.

Conversely, the moving coil acts as the pressure or voltage coil. It is connected in parallel across the supply voltage. A series resistor is usually connected with the moving coil to limit the current through it and make its impedance highly resistive.

The interaction between the magnetic field of the fixed current coils (proportional to current I) and the magnetic field of the moving pressure coil (proportional to voltage V) produces a torque on the moving coil. This torque is proportional to VIcosφ (in AC circuits), which is the active power.

Summary of Dynamometer Wattmeter Coils

Here is a breakdown of the two main coils:

Coil Type	Position	Function	Connection	Carries Current Proportional To
Fixed Coils	Stationary	Current Coil	In Series with Load	Circuit Current (I)
Moving Coil	Movable	Pressure/Voltage Coil	In Parallel with Load	Circuit Voltage (V)

Based on the construction and function, the fixed coil in a dynamometer wattmeter is identified as the current coil.

Revision Table: Dynamometer Wattmeter Essentials

Component	Role	Characteristics
Fixed Coils	Current element	Heavy wire, series connection, generates field ∝ Current
Moving Coil	Voltage element	Fine wire, parallel connection, generates field ∝ Voltage
Control Torque	Springs	Opposes deflecting torque, provides restoring force
Damping Torque	Air Damping	Prevents oscillations, brings pointer quickly to rest

Additional Information on Wattmeter Usage

Dynamometer wattmeters are known for their accuracy on both AC and DC circuits and are often used as calibration standards. However, they have certain limitations:

They consume significant power due to the current and pressure coils.
They can be affected by frequency variations in AC circuits.
Their scale is not uniformly linear at lower deflections due to the torque being proportional to the product of currents.
Air damping is typically used, which is less effective than eddy current damping but suitable because the coils are not mounted on a metallic former.

Understanding the role of each coil, especially the fixed current coil and the moving pressure coil, is fundamental to comprehending how a dynamometer wattmeter functions to measure electrical power.