Question

List the various errors occurred in dynamometer type wattmeter and describe the way of compensating any one type of error.

Answer 1

Dynamometer type wattmeters are widely used for measuring electrical power, but they can exhibit several types of errors. Here are some common errors and a brief description of how to compensate for one of them:

### Common Errors in Dynamometer Type Wattmeters

1. **Stray Magnetic Field Error**
   - **Description**: External magnetic fields can influence the moving coil of the wattmeter, causing errors in the measurement.
   - **Compensation**: Shielding the wattmeter with magnetic shields or positioning it away from strong magnetic fields can help reduce this error.

2. **Loading Error**
   - **Description**: The wattmeter itself introduces a load into the circuit, which can affect the accuracy of the measurement.
   - **Compensation**: Using a wattmeter with high impedance and ensuring that the wattmeter is properly rated for the circuit can minimize loading errors.

3. **Voltage Drop Error**
   - **Description**: Voltage drops across the connections and leads of the wattmeter can cause inaccuracies.
   - **Compensation**: Ensuring proper and low-resistance connections and using high-quality leads can reduce voltage drop errors.

4. **Temperature Error**
   - **Description**: Changes in temperature can affect the resistance of the wattmeter components, leading to measurement errors.
   - **Compensation**: Calibration at different temperatures and using temperature compensation techniques can help manage this error.

5. **Power Factor Error**
   - **Description**: The wattmeter may not accurately measure power if the power factor is not properly accounted for.
   - **Compensation**: Using a wattmeter that is designed to handle different power factors or correcting the power factor with additional equipment can help.

### Compensating for Loading Error

**Loading Error Compensation:**

1. **Use of High-Precision Wattmeters**: Choose wattmeters with high accuracy and precision that have minimal impact on the circuit's load.

2. **Calibration and Testing**: Regularly calibrate the wattmeter to ensure it provides accurate measurements and test it under various loading conditions.

3. **Minimize Wattmeter Influence**: Ensure that the wattmeter's internal resistance is significantly lower than the resistance of the circuit being measured. This can be achieved by using wattmeters designed for specific measurement ranges and applications.

4. **Use of Correct Ratings**: Always select a wattmeter with appropriate voltage and current ratings for the circuit to prevent excessive loading.

By carefully selecting and maintaining the wattmeter and implementing these practices, the impact of loading errors can be minimized, resulting in more accurate power measurements.

Answer 2

Dynamometer-type wattmeters are commonly used for measuring power in AC and DC circuits. They operate based on the principle of electromagnetic force, where the deflection of a moving coil or vane is proportional to the power being measured. However, several types of errors can affect the accuracy of these wattmeters. Here’s a detailed list of common errors and a description of how to compensate for one type:

### Common Errors in Dynamometer-Type Wattmeters

1. **Stray Magnetic Field Error**:
   - **Description**: Stray magnetic fields from nearby electrical equipment or other sources can influence the magnetic field in which the wattmeter operates, causing errors in the power measurement.
   - **Compensation**: To minimize this error, the wattmeter should be shielded using materials that absorb or redirect stray magnetic fields. Ensuring that the wattmeter is used in a controlled environment away from strong magnetic fields also helps.

2. **Load Error**:
   - **Description**: The wattmeter might not accurately measure power if the load connected to the wattmeter does not match the intended measurement conditions (e.g., reactive loads).
   - **Compensation**: To address load errors, ensure that the wattmeter is calibrated for the specific type of load it will measure. For highly reactive or complex loads, use a wattmeter with the appropriate range and accuracy or use additional instrumentation to correct for load characteristics.

3. **Temperature Error**:
   - **Description**: Changes in temperature can affect the resistance of the internal components of the wattmeter, leading to inaccuracies in measurement.
   - **Compensation**: Use wattmeters with temperature compensation features or perform measurements in a temperature-controlled environment. Regular calibration and adjustments based on temperature variations can also mitigate this error.

4. **Power Factor Error**:
   - **Description**: Inaccuracies arise if the power factor of the load is not taken into account. Dynamometer-type wattmeters may assume a power factor of unity unless explicitly corrected.
   - **Compensation**: To compensate for power factor error, use a wattmeter with built-in power factor correction or measure the power factor separately and apply corrections to the wattmeter readings.

5. **Hysteresis Error**:
   - **Description**: Hysteresis in the magnetic materials used in the wattmeter can cause lag in the response and affect accuracy.
   - **Compensation**: Use materials with low hysteresis for the moving parts and ensure regular maintenance and calibration to reduce the impact of hysteresis errors.

6. **Zero Error**:
   - **Description**: Zero error occurs when the wattmeter does not read zero when no power is being supplied. This can result from imperfections in the mechanical or electrical setup of the wattmeter.
   - **Compensation**: Before taking measurements, always perform a zero adjustment or calibration to ensure that the wattmeter reads zero when no power is applied.

### Compensating for Power Factor Error

**Power Factor Error Compensation**:

1. **Understanding Power Factor**: Power factor is the ratio of the real power (measured in watts) to the apparent power (measured in volt-amperes). For AC measurements, the wattmeter needs to account for both real and reactive power.

2. **Using a Power Factor Corrector**: Some advanced dynamometer-type wattmeters come with built-in power factor correction circuits. These circuits adjust the readings based on the power factor of the load. If the wattmeter has this feature, it will automatically correct the readings for power factor.

3. **Separate Measurement and Correction**:
   - **Measure Power Factor**: Use a separate power factor meter to measure the power factor of the load.
   - **Apply Correction**: Apply the correction factor to the wattmeter readings manually. For instance, if the wattmeter reads 100 watts and the power factor is 0.8, the actual power is \( \text{Real Power} = \text{Measured Power} \times \text{Power Factor} = 100 \times 0.8 = 80 \text{ watts} \).

4. **Regular Calibration**: Ensure that the wattmeter is regularly calibrated under known power factor conditions to maintain accuracy.

By addressing these errors and employing appropriate compensation techniques, the accuracy of dynamometer-type wattmeters can be significantly improved, ensuring reliable power measurements in various applications.