Electrodynamometer Type Wattmeter, Construction, Working, Errors
Table of Contents
Electrodynamometer
The Electrodynamometer Wattmeter is a device whose operation depends on the interaction between the magnetic fields of moving and stationary coils. It is used to gauge the strength of both AC and DC circuits. The Electrodynamometer Wattmeter operates on a very straightforward and easy principle. Their operation is based on the idea that a magnetic field causes a mechanical force to act on a conductor carrying current. The mounted pointer on the calibrated scale is deflected by this mechanical force.
Construction of Electrodynamometer Wattmeter
These are the key components of the electrodynamometer wattmeter.
Fixed coil –
The load and fixed coil are connected in series. Due to the load current flowing through it, it is regarded as a current coil. The fixed coil is divided into two parts to facilitate construction. And there is a parallel connection between these two components. The uniform electric field created by the fixed coil is necessary for the instruments to function. In order to conserve power, the instruments’ current coil is built to handle current of about 20 amperes.
Moving Coil –
The instruments’ pressure coil is thought to be the moving coil. In parallel with the supply voltage, it is connected. The supply voltage and the current that flows through them are directly inversely related. The moving coil supports the pointer. The spring aids in controlling the movement of the pointer. The coils’ temperature rises as current passes through them. A resistor connected in series with the moving coil is used to control the current flows.
Control –
The instruments’ controlling torque is delivered by the control system. The two different types of control systems are gravity control and spring control. The Electrodynamometer Wattmeter uses a spring control system out of the two. The pointer’s movement is controlled by a spring system.
Damping –
The effect that makes the pointer move less is called damping. The damping torque in this wattmeter is caused by air friction. Because they eliminate the useful magnetic flux, the other types of damping are not utilized in the system.
Scales and pointers –
The instruments’ moving coil moves linearly, so they use a linear scale. The device employs a knife-edge pointer to eliminate parallax errors brought on by mistakes.
Working of Electrodynamometer Wattmeter
There are two different kinds of coils in the Electrodynamometer Wattmeter: fixed and moving coils. The circuit whose power consumption needs to be measured is connected in series with the fixed coil. The moving coil is subject to the supply voltage. And The moving coil is connected in series with the resistor, which regulates the current flowing across it.
The moving coil, which is positioned in between the fixed coils, is where the pointer is fixed. The two magnetic fields are produced by the current and voltage of the fixed and moving coils. The instrument’s pointer is deflected by the interaction of these two magnetic fields. So The power flowing through the pointer directly correlates with how much it deflects.
Electrodynamometer Wattmeter
The figure below depicts the electrodynamometer wattmeter’s circuit diagram.
So The instantaneous torque acts on the pointer of the wattmeter and is given by the equation
Where, ip – pressure coil current
ic– current coil current
dm/dθ – the rate of change of deflection of pointer concerning angle θ
So The voltage across the pressure coil of the circuit is given as
If the pressure coil is only resistive, then the current and voltage are in phase. The equation also provides the current value.
If the current coil lag by a voltage in phase angle Φ, the current through the current coil is given as
ip = √2Isin(ωt-∅)
The pressure coil’s current has a very low value. The total load current is therefore determined by the current that passes through the pressure coil. As a result of the torque acting on the coils,
By integrating the torque from 0 to T limit, the average deflection torque is obtained. The coil’s typical deflection torque is given as
The controlling torque exerted on the spring is given by
Errors in Electrodynamometer
The following are the errors in the Electrodynamometer Wattmeter
- Pressure Coil Inductance – The Electrodynamometer’s pressure coil exhibits some inductance. The pressure coils’ current lags behind the voltage because of inductance. As a result, the wattmeter’s power factor lags and the reading on the meter is high.
- Pressure Coil Capacitance – Along with the inductance, the pressure coil also has capacitances. The instrument’s power factor is raised by this capacitance. Consequently, the reading error is caused.
- Error due to Mutual Inductance Effect – An error is produced by the pressure and current coils’ mutual inductance.
- Eddy Current Error – The coil’s magnetic field is produced by the eddy current that it induces. The coil’s primary current flow is impacted by this field. Therefore, the reading error happens.
- Stray Magnetic Field – The Electrodynamic Wattmeter’s main magnetic field is disturbed by the stray magnetic field. affect their reading as a result.
- Temperature Error – The resistance of the pressure coil will change as a result of the temperature variation. So The temperature change had an impact on the spring’s movement, which produces the controlling torque. As a result, the reading error happens.
Both the electrodynamometer’s calibration for AC and DC measurements are the same.