A wattmeter is an instrument used to measure the power consumed in an electrical circuit. It consists of two coils: the current coil (CC) and the potential coil (PC). For accurate measurement of power, the potential coil is made highly resistive and the current coil is made highly inductive. But why is this done? Let’s understand in detail.
Construction of a Wattmeter
A wattmeter has two main coils:
- Current Coil (CC): Connected in series with the load so it carries the load current.
- Potential Coil (PC): Connected across the supply voltage through a series resistance.
The deflection torque in a wattmeter is proportional to the product of the instantaneous values of current (through CC) and voltage (across PC), which represents the power.
Why is the Potential Coil Made Highly Resistive?
The potential coil is connected across the supply, so it should draw very little current. If the PC had low resistance, it would draw excessive current and affect the circuit conditions. Hence, a high resistance is connected in series with the PC to make it:
- Draw negligible current.
- Keep the current in phase with the applied voltage (minimizing phase error).
- Ensure accurate measurement of real power.
Why is the Current Coil Made Highly Inductive?
The current coil is connected in series with the load and should carry the load current without significant voltage drop. To achieve this:
- The CC is wound with thick wire of low resistance (to avoid power loss).
- It is designed with a large number of turns on an iron core, making it highly inductive.
- High inductance ensures the current coil produces the necessary magnetic field proportional to load current, without introducing resistance-related errors.
Phase Error Considerations
Ideally, the current in the potential coil should be in phase with the applied voltage, and the flux produced by the current coil should be in phase with the load current. In practice:
- The PC is made resistive → reduces phase lag between voltage and current.
- The CC is inductive → ensures strong flux production with minimal resistance.
These design choices minimize phase error and allow the wattmeter to measure true power accurately, even for inductive or capacitive loads.
Summary of Reasons
| Coil | Property | Reason |
|---|---|---|
| Potential Coil (PC) | Highly Resistive | Draws negligible current, keeps current in phase with voltage, reduces phase error. |
| Current Coil (CC) | Highly Inductive | Provides strong flux proportional to current, avoids resistance losses, minimizes heating. |
Conclusion
The potential coil of a wattmeter is made highly resistive to avoid current consumption and phase error, while the current coil is made highly inductive to create accurate magnetic flux proportional to load current. These design choices ensure that the wattmeter reads the true power in AC circuits.
FAQs on Wattmeter Coils
1. Why is a series resistance used with the potential coil?
To limit the current drawn by the potential coil and make it behave like a purely resistive load.
2. Why is the current coil made with thick wire?
Because it carries the load current, thick wire minimizes power loss and heating.
3. What will happen if the potential coil is not resistive?
It will cause phase error, increase current consumption, and give inaccurate power readings.
4. Can a wattmeter measure power in both AC and DC circuits?
Yes, it can measure power in both AC and DC systems. In AC, proper coil design ensures accuracy despite phase shifts.
5. What error occurs if the current coil is not inductive?
If the CC is not sufficiently inductive, it may behave resistively and distort power measurements due to increased losses.