5. CIRCUIT PARAMETER MEASUREMENT, CRO AND OTHER METERS (EE 3003)

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5. CIRCUIT PARAMETER MEASUREMENT, CRO AND OTHER METERS

5.1 Measurement of Resistance

5.1.1 Low Resistance: Kelvin’s Double Bridge

• Purpose: The Kelvin's Double Bridge is used to measure very low resistances with high accuracy. It eliminates the effect of lead and contact resistance, which can significantly affect the measurement of low resistances.

• Working Principle:

  • The bridge consists of four arms: two for the unknown resistance and two for the standard resistances.
  • Kelvin's Bridge uses two sets of connections: one for the measurement of the potential difference and one for the current supply.
  • The current is supplied through the outer arm (which carries the current), and the potential difference is measured through the inner arm, which eliminates the effect of lead resistance.

• Advantages:

  • Provides high accuracy for low resistances (in the milliohm range).
  • Minimizes the error caused by the resistance of the leads and contacts.

5.1.2 Medium Resistance: Voltmeter and Ammeter Method

• Purpose: This method is used for measuring resistances that are in the medium range (i.e., in the range from a few ohms to several kilohms).

• Working Principle:

  • In this method, a known current is passed through the resistor using an ammeter. The potential difference across the resistor is then measured using a voltmeter.
  • Ohm’s Law is applied to calculate the resistance: $$R = \frac{V}{I}$$ where $V$ is the voltage across the resistor, and $I$ is the current through it.

• Advantages:

  • Simple and easy to use.
  • Suitable for measuring resistances in a wide range, provided that the current source is stable and precise.

5.1.3 High Resistance: Megger and Ohm Meter (Series and Shunt)

• Purpose: This method is used to measure very high resistances, typically in the range of megaohms.

• Megger:

  • Megger (short for insulation resistance tester) is used to measure high insulation resistance, typically for electric cables, motors, transformers, etc.
  • It applies a high voltage (usually in the range of 500V to 5000V) to the insulation and measures the resistance.
  • The Megger has a hand-cranked or battery-operated mechanism to generate the high testing voltage.

• Ohm Meter (Series and Shunt):

  • Series Type: In this arrangement, the unknown resistance is placed in series with a known resistor, and the total resistance is measured using an ohmmeter. This setup is usually used for moderate resistance ranges.
  • Shunt Type: In this arrangement, the known resistor is placed in parallel with the unknown resistance. This type is often used when measuring very high resistances, where the unknown resistance is much larger than the shunt resistor.

• Advantages:

  • Megger is particularly useful for insulation testing.
  • Ohmmeter methods can measure high resistances but require proper precautions to ensure accuracy.

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5.2 Measurement of Inductance Using Anderson Bridge (No Derivation and Phasor Diagram)

• Purpose: The Anderson Bridge is used to measure the inductance of an inductor (L) with high accuracy.

• Working Principle:

  • The Anderson Bridge is a type of AC bridge used for measuring the inductance of a coil.
  • The bridge consists of resistors and capacitors, along with the unknown inductor. By balancing the bridge, the value of the inductance can be determined.
  • In its balanced state, the bridge gives a precise value of the inductance without the need for complex calculations or measurements.

• Advantages:

  • Provides an accurate measurement of inductance in the presence of complex reactances.
  • Highly suitable for precise lab measurements of inductors.

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5.3 Measurement of Capacitance Using Schering Bridge (No Derivation and Phasor Diagram)

• Purpose: The Schering Bridge is used to measure capacitance and the loss angle (or loss tangent) of capacitors in AC circuits.

• Working Principle:

  • The Schering Bridge works similarly to other AC bridges but is designed for measuring capacitive reactance.
  • It consists of a standard capacitor, the unknown capacitor, and a series of known resistors and capacitors. The bridge is balanced by adjusting the components, and once balanced, the value of the unknown capacitance can be determined.
  • It is also used to measure the loss factor (or dissipation factor) of capacitors, which indicates the energy lost as heat in the capacitor.

• Advantages:

  • Accurate and reliable for measuring both small and large capacitors.
  • Can measure both the capacitance and the loss angle, which is crucial for evaluating the performance of capacitors.

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5.4 Other Meters

5.4.1 Earth Tester

• Purpose: An earth tester is used to measure the earth resistance or earth grounding system resistance.

• Working Principle:

  • The tester applies a small current through the earth electrode and measures the potential difference to calculate the earth resistance.
  • Common methods include the fall of potential method, where two auxiliary electrodes are used to measure the voltage drop between them and the ground.

• Applications:

  • Used in electrical installations to ensure the effectiveness of grounding systems.

5.4.2 Digital Multimeter

• Purpose: A digital multimeter (DMM) is a versatile instrument used to measure a variety of electrical parameters, including voltage, current, and resistance.

• Working Principle:

  • A DMM works by converting the analog signal (voltage or current) to a digital value using an analog-to-digital converter (ADC). It displays the measured value on a digital screen.

• Applications:

  • Used in labs, industries, and electrical installations for troubleshooting and testing.
  • Provides high accuracy for a wide range of measurements (DC/AC voltage, DC current, resistance, etc.).

5.4.3 L-C-R Meter

• Purpose: An L-C-R meter is used for measuring inductance (L), capacitance (C), and resistance (R) of electronic components.

• Working Principle:

  • The meter applies an AC signal to the component under test and measures its impedance. Based on the impedance, the meter calculates the values of L, C, and R.

• Applications:

  • Used for testing inductors, capacitors, and resistors in circuit design and maintenance.

5.4.4 Phase Sequence Indicator

• Purpose: A phase sequence indicator is used to determine the phase sequence (direction of rotation) of a three-phase system.

• Working Principle:

  • The device typically uses a rotating field to indicate the phase sequence. The indicator will display whether the phase sequence is correct (clockwise) or incorrect (counterclockwise).

• Applications:

  • Essential in motors and other equipment that require a specific phase sequence for correct operation.

5.4.5 Power Factor Meter (Single Phase and Three Phase Dynamometer Type)

• Purpose: A power factor meter measures the power factor of an AC system, which indicates the phase difference between voltage and current.

• Single Phase Power Factor Meter:

  • This meter uses a dynamometer type mechanism, where the power factor is determined by the phase difference between the voltage and current in a single-phase system.

• Three Phase Power Factor Meter:

  • Similar to the single-phase power factor meter but used for three-phase systems. It uses multiple dynamometer elements or uses a three-wattmeter method to determine the power factor.

• Applications:

  • Used to monitor the efficiency of AC electrical systems and to ensure power factor correction in industrial setups.