2. MEASUREMENT OF VOLTAGE AND CURRENT (EE 3003)

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2. MEASUREMENT OF VOLTAGE AND CURRENT

2.1 Analog Meters: Construction, Working, Salient Features, Merits, and Demerits

2.1.1 Permanent Magnet Moving Coil (PMMC) Meter

• Construction:
  • The PMMC meter consists of a coil placed in the magnetic field produced by a permanent magnet. The coil is suspended in such a way that it can rotate in the plane perpendicular to the magnetic field. The coil is connected to a pointer, which moves over a calibrated scale.
• Working:
  • When current flows through the coil, it produces a magnetic field around the coil, which interacts with the permanent magnetic field. This interaction causes the coil to rotate, and the pointer moves over the scale to indicate the current or voltage.
  • The coil rotates because of the torque produced by the interaction between the current in the coil and the magnetic field.
• Salient Features:
  • Provides accurate and linear scale readings.
  • Mainly used for DC measurements.
  • Requires low power consumption.
• Merits:
  • High accuracy and sensitivity.
  • Linear scale, making it easy to read.
  • No errors due to hysteresis or eddy currents.
• Demerits:
  • Cannot be used for AC measurements directly.
  • Fragile, susceptible to mechanical shocks.
  • Requires a constant magnetic field for accuracy.

2.1.2 Permanent Magnet Moving Iron (PMMI) Meter

• Construction:
  • The PMMI meter has a soft iron vane or a soft iron needle placed in the magnetic field created by a permanent magnet. The moving iron is displaced under the influence of the magnetic field when current flows through the coil wound around it.
• Working:
  • When current flows through the coil, it generates a magnetic field. This magnetic field magnetizes the iron vane and causes it to move, which is reflected on the scale.
  • The amount of movement is proportional to the RMS value of the current or voltage, making it suitable for both AC and DC measurements.
• Salient Features:
  • Can be used for both AC and DC measurements.
  • Non-linear scale.
  • Can be designed for a wide range of currents.
• Merits:
  • Can measure both AC and DC quantities.
  • Less affected by mechanical shocks than PMMC meters.
• Demerits:
  • Non-linear scale.
  • Less accurate than PMMC meters.
  • More power loss due to hysteresis and eddy currents.

2.2 DC Ammeter: Basic, Multi-Range, Universal Shunt

• Basic DC Ammeter:
  • A basic DC ammeter is used to measure the current in a circuit. It is designed with a low resistance (shunt) and a moving coil meter to measure current in the range of the instrument.
  • The meter's scale is calibrated to read directly in amperes.
• Multi-Range Ammeter:
  • A multi-range ammeter is used to measure currents in multiple ranges. It is designed by placing a set of shunts in parallel with the meter. By selecting different shunts using a range switch, the meter can measure a wide range of currents.
• Universal Shunt:
  • A universal shunt is a specific type of shunt used in multi-range ammeters. It is a precision resistor, and the value of the shunt is selected to give different current ranges based on the measurement conditions.
  • The ammeter's current range can be extended by changing the shunt to measure higher currents or using a scale that shows multiple ranges.

2.3 DC Voltmeter: Basic, Multi-Range, Concept of Loading Effect and Sensitivity

• Basic DC Voltmeter:
  • A basic DC voltmeter consists of a high resistance in series with the PMMC meter. This ensures that the meter draws as little current as possible from the circuit under measurement.
  • The scale is calibrated in volts to indicate the potential difference across two points.
• Multi-Range DC Voltmeter:
  • A multi-range voltmeter uses different series resistances to extend the range of the voltmeter. A range switch is used to select the appropriate resistance for different voltage measurements.
• Loading Effect:
  • Loading effect refers to the influence that a measuring instrument has on the circuit being measured. A voltmeter, for example, draws some current when measuring the voltage, which can alter the voltage across the load.
  • To minimize the loading effect, voltmeters are designed with very high internal resistance so that they draw minimal current.
• Sensitivity:
  • Sensitivity of a voltmeter refers to the smallest change in the measured voltage that can be detected by the instrument. A highly sensitive voltmeter is capable of detecting very small voltage differences.
  • Sensitivity is generally expressed in terms of the internal resistance and the full-scale deflection of the meter.

2.4 AC Voltmeter: Rectifier Type (Half Wave and Full Wave)

• Rectifier Type Voltmeter:
  • An AC voltmeter uses a rectifier (diode) to convert the AC voltage into a DC signal, which is then measured by a standard DC voltmeter (such as a PMMC meter).
• Half-Wave Rectifier Voltmeter:
  • In a half-wave rectifier AC voltmeter, only one half of the AC waveform is passed through, and the other half is blocked. The rectified signal is then measured as DC voltage.
  • This type of rectifier introduces errors because only part of the waveform is measured.
• Full-Wave Rectifier Voltmeter:
  • A full-wave rectifier AC voltmeter allows both halves of the AC waveform to pass, resulting in a more accurate measurement of the RMS value of the AC voltage.
  • This type is more accurate than the half-wave rectifier because it utilizes the entire AC waveform.
  • However, it still measures the average rectified value, so additional calibration is required for accurate RMS readings.

2.5 Current Transformer (CT) and Potential Transformer (PT): Construction, Working, and Applications

Current Transformer (CT)

• Construction:
  • A CT consists of a primary winding (usually a single-turn conductor) that carries the current to be measured. The secondary winding is wound around the core and produces a proportional output current.
  • The core is typically made of laminated iron to reduce eddy currents.
• Working:
  • The primary current induces a magnetic flux in the core, which is transferred to the secondary winding. The secondary current is proportional to the primary current, allowing for accurate current measurement.
  • The ratio of primary to secondary current is defined by the CT’s turns ratio.
• Applications:
  • CTs are widely used in measuring high currents and for protective relaying in electrical circuits, especially in power transmission and distribution systems.

Potential Transformer (PT)

• Construction:
  • PTs are similar to step-down transformers with a primary winding connected to the high-voltage side and a secondary winding providing a reduced voltage.
  • The core is designed to operate efficiently under the high-voltage conditions.
• Working:
  • PTs step down the high voltage to a lower value, proportional to the turns ratio, which can then be measured by standard voltmeters.
• Applications:
  • Used in high-voltage measurement systems for voltage measurement and protection purposes in power systems. PTs are found in both industrial and utility applications where voltage measurement is required.