A wattmeter is an electrical measuring instrument used to measure electric power in a circuit. It indicates the amount of electrical power consumed or produced by an electrical device. Wattmeters are widely used in electrical laboratories, industries, substations, and power systems for accurate power measurement.
In DC circuits, power can easily be calculated using the formula:
P = VI
However, in AC circuits, power depends on voltage, current, and power factor. Therefore, direct calculation becomes difficult. In such cases, wattmeters are used for accurate power measurement.
Types of Wattmeters
There are mainly two types of wattmeters:
- Dynamometer Wattmeter – Used for both AC and DC power measurement.
- Induction Wattmeter – Used only for AC power measurement.
Dynamometer Wattmeter
A dynamometer wattmeter is the most commonly used wattmeter for measuring both AC and DC power. It works on the dynamometer principle, which states that a mechanical force exists between two current-carrying conductors.
Construction of Dynamometer Wattmeter
The main parts of a dynamometer wattmeter are:
- Fixed Coil (Current Coil) – Connected in series with the load and carries load current.
- Moving Coil (Potential Coil) – Connected across the load through a high resistance and carries current proportional to voltage.
- Control Springs – Provide controlling torque and also carry current to the moving coil.
- Pointer and Scale – Indicate the measured power.
- Air Friction Damping – Used to reduce oscillations.
Working Principle of Dynamometer Wattmeter
When current flows through the fixed and moving coils, magnetic fields are produced. The interaction between these magnetic fields creates a mechanical force that moves the pointer. The pointer comes to rest when the deflecting torque becomes equal to the controlling torque.
Since the direction of current reverses simultaneously in both coils during AC operation, the direction of torque remains unchanged. Therefore, the instrument can measure both AC and DC power.
Deflecting Torque in DC Circuit
For a DC circuit:
Td ∝ I1I2
Since the potential coil current is proportional to voltage:
Td ∝ VI
Thus, the deflecting torque is proportional to load power.
Deflecting Torque in AC Circuit
In an AC circuit:
v = Vm sin θ
i = Im sin (θ − φ)
Average deflecting torque:
Td ∝ VI cos φ
Hence, the wattmeter measures true power in AC circuits.
Advantages of Dynamometer Wattmeter
- Can measure both AC and DC power.
- Uniform scale.
- High accuracy can be achieved.
- Simple operating principle.
Disadvantages of Dynamometer Wattmeter
- Errors occur at low power factor.
- Affected by stray magnetic fields.
- Comparatively costly.
- Weak operating torque.
Errors in Dynamometer Wattmeter
The following errors may occur in dynamometer wattmeters:
- Error due to inductance of potential coil.
- Error due to capacitance in potential circuit.
- Error due to stray magnetic fields.
- Error due to eddy currents.
- Error due to incorrect connection of potential coil.
Induction Wattmeter
An induction wattmeter works on the induction principle and is used only for AC power measurement. Its working principle is similar to induction-type ammeters and voltmeters.
Construction of Induction Wattmeter
The main components are:
- Shunt Magnet – Connected across the supply and carries current proportional to voltage.
- Series Magnet – Connected in series with the load and carries load current.
- Aluminium Disc – Rotates due to induced currents.
- Permanent Magnet – Provides damping torque.
- Shading Rings – Used to adjust phase angle.
- Control Springs – Provide controlling torque
Working Principle of Induction Wattmeter
The shunt magnet produces a magnetic flux proportional to supply voltage, while the series magnet produces flux proportional to load current. These two alternating fluxes induce eddy currents in the aluminium disc. The interaction between the fluxes and eddy currents produces torque, which rotates the disc and moves the pointer.
Advantages of Induction Wattmeter
- Uniform scale.
- Long scale for accurate reading.
- Excellent damping.
- Less effect of stray magnetic fields.
Disadvantages of Induction Wattmeter
- Can measure only AC power.
- Low accuracy compared to dynamometer wattmeter.
- Affected by temperature variations.
- High power consumption.
- Frequency changes affect accuracy.
Applications of Induction Wattmeter
- Used as panel instruments.
- Used in AC power measurement systems.
- Used where frequency variation is small.
Range Extension of Wattmeters
Induction wattmeters can directly handle:
- Current up to 100 A
- Voltage up to 750 V
For higher values:
- Current Transformers (CTs) are used for large currents.
- Potential Transformers (PTs) are used for high voltages.
Difference Between Dynamometer and Induction Wattmeter
| Parameter | Dynamometer Wattmeter | Induction Wattmeter |
|---|---|---|
| Working Principle | Dynamometer principle | Induction principle |
| Supply Type | AC and DC | Only AC |
| Accuracy | High | Moderate |
| Power Consumption | Low | High |
| Effect of Frequency | Less | More |
| Applications | Laboratory measurements | Panel instruments |
Applications of Wattmeter
- Measurement of electrical power in AC and DC circuits.
- Testing electrical appliances.
- Power measurement in laboratories.
- Industrial electrical monitoring systems.
- Power system analysis and testing.
Conclusion
A wattmeter is an essential instrument used for measuring electrical power. Among different types, the dynamometer wattmeter is widely preferred because it can measure both AC and DC power with high accuracy. On the other hand, the induction wattmeter is suitable only for AC circuits and is mainly used as a panel instrument. Understanding the construction, working principle, advantages, disadvantages, and applications of wattmeters is very important for students preparing for competitive exams like SSC JE, RRB JE, GATE, UPPCL JE, and other electrical engineering exams.
FAQs on Wattmeter
What is a wattmeter?
A wattmeter is an electrical instrument used to measure electric power in a circuit.
Which wattmeter is used for both AC and DC?
A dynamometer wattmeter is used for both AC and DC power measurement.
Why is an induction wattmeter used only for AC?
Because its operation depends on electromagnetic induction, which requires alternating magnetic flux.
What is the function of shading rings in an induction wattmeter?
Shading rings adjust the phase angle of the shunt magnet flux to improve accuracy.
What are the main types of wattmeters?
The two main types are dynamometer wattmeter and induction wattmeter.
Which wattmeter has higher accuracy?
Dynamometer wattmeters have higher accuracy compared to induction wattmeters.