- Electrodynamometer Wattmeter Definition: An electrodynamometer type wattmeter measures electrical power by using the interaction between magnetic fields and electric currents.
- Working Principle: It operates on the principle that a current-carrying conductor in a magnetic field experiences a mechanical force causing deflection.
- Construction: It includes a moving coil connected across the voltage and a fixed coil divided into two parts, connected in series with the load.
- Advantages: This wattmeter can measure both AC and DC quantities, has a uniform scale, and is highly accurate.
- Errors: Common errors include those due to inductance, capacitance, mutual inductance, connections, eddy currents, vibration, temperature, and stray magnetic fields.
An electrodynamometer type wattmeter measures average electrical power through the force between two current-produced magnetic fields. One coil carries a current proportional to load current, and the other carries a current proportional to load voltage. Their field interaction produces torque on the movable conductor. For sinusoidal AC and a nearly resistive voltage-coil circuit, average torque is proportional to VI cosφ, so the pointer indicates real power associated with the magnetic field interaction.
Construction and Working Principle of Electrodynamometer Type Wattmeter
An electrodynamometer wattmeter has fixed current coils, a moving pressure coil, a control system, damping and a pointer mechanism.
The two electrical coil systems are described below.
Moving Coil
The moving coil carries the pointer and is restrained by control springs. A high-value, low-inductance resistor in series limits current and makes the pressure-coil circuit as nearly resistive as practical. The air-cored coil is mounted on a pivoted spindle or suspension. In an electrodynamometer type wattmeter, this pressure coil is connected across the measured voltage, so its current is approximately proportional to voltage and in phase with it.
Fixed Coil
The fixed current coil is commonly divided into two sections connected in series with the load. The two sections provide a working field around the moving coil and space for its movement. They use relatively few turns of conductor sized for the rated load current. These are the current coils of the electrodynamometer type wattmeter. Current rating is specific to the instrument and its range; it must be checked independently of the watt-scale indication, especially at low power factor.
Control System
Two traditional pointer-control methods are:
- Gravity control
- Spring control. Electrodynamometer wattmeters use springs because they provide restoring torque independent of instrument orientation and can also carry current to the moving coil.
Damping System
Air-friction damping is used because eddy current damping would introduce an additional magnetic field and could alter the operating torque.
Scale
The scale is approximately uniform when the rate of change of mutual inductance with deflection is designed to remain nearly constant over the working range.
The following circuit supports the deflecting-torque derivation:
Instantaneous electrodynamic torque is proportional to the product of the two coil currents and the rate of change of mutual inductance with deflection.
Let I1 and I2 be the instantaneous currents in the current coil and pressure coil respectively. The torque is
Here x is the deflection angle.
Let the applied voltage across the pressure-coil circuit be
The high series electrical resistance makes pressure-coil reactance small relative to resistance at the rated frequency. The ideal derivation therefore treats its impedance as the stated electrical resistance, while practical instruments retain a small phase-angle error.
The instantaneous pressure-coil current is I2 = v/Rp, where Rp is the total pressure-circuit resistance.
If load current lags the voltage by φ, the instantaneous current-coil current is
Pressure-coil current is normally small compared with load current, but the selected connection determines whether its power consumption contributes to the reading.
The instantaneous torque is therefore
Average deflecting torque is found by integrating over one period T.
The controlling torque is Tc = Kx, where K is the spring constant and x is the steady deflection.
Advantages of Electrodynamometer Type Wattmeter
The main advantages of electrodynamometer type wattmeter are:
- The scale can be made approximately uniform over its specified working range.
- The air-cored electrodynamic movement can measure both AC and DC power when the instrument is calibrated and connected for the required range.
Errors in Electrodynamometer Type Wattmeter
Following are the errors in the electrodynamometer type wattmeters:
- Pressure-coil inductance shifts pressure-coil current away from the voltage phase, with a proportionally larger power error at low power factor.
- Pressure-coil and series-resistor capacitance changes the circuit impedance and phase angle, especially as frequency rises.
- Unwanted mutual inductance between instrument circuits or nearby conductors can alter torque.
- Connection error includes either pressure-coil power or current-coil loss in the reading, depending on which side of the current coil feeds the pressure circuit.
- Eddy currents in nearby metal create frequency-dependent fields and torque error.
- Mechanical vibration and friction disturb the moving system and pointer reading.
- Temperature changes coil and multiplier resistance, affecting current and calibration.
- Stray magnetic fields affect the weak air-cored operating field unless shielding or an astatic arrangement reduces them.
