kSZaRk-Module 7AV EASA GCAA CAT B Book 2

7.3 Tools (Level 3) Operation and Function of Electrical General Test Equipment

Key Electrical Values to Measure

  • Necessary values for electrical circuits:

    • Ampere (A): Measures current.

    • Volt (V): Measures voltage.

    • Ohm (Ω): Measures resistance.

    • Watt (W): Measures power.

Common Electrical Measuring Instruments

  • Instruments used in this module:

    • Ammeter: Measures current.

    • Voltmeter: Measures voltage.

    • Ohmmeter: Measures resistance.

    • Wattmeter: Measures power.

Principles of Electrical Measurements

  • Principle of Measurements:

    • Most measurements rely on electromagnetism.

    • Assumptions:

      • The strength of the electromagnetic field is proportional to the current.

      • Voltage, resistance, and power relate to current flow.

  • D’Arsonval Movement:

    • Widely used; pointer deflection proportional to current through the moving coil.

    • Reference magnetic field created by a horseshoe-shaped permanent magnet.

D'Arsonval Meter Components

  • Components and Operations:

    • Current Through Coil: Current enters via calibrated hairsprings; generates magnetic field.

    • Electromagnetic Damping: Minimizes oscillation of the pointer.

    • Coil Characteristics: Thin aluminum frame generates eddy currents opposing motion.

Meter Reading & Terms

Characteristics of D’Arsonval Meter Movement

  • Full-Scale Current: Amount of current for full scale deflection.

  • Ohms per Volt Sensitivity: Reciprocal of full-scale current; affects resistance needed for different voltages.

  • Meter Resistance: Total resistance includes moving coil, hairspring, and potential temperature compensating resistors.

  • Deflecting Force: Caused by current in the moving coil—it moves the pointer.

  • Controlling Force: Usually mechanical (spring) resisting deflection.

  • Damping Force: Opposes movement; ensures quick settling of pointer without overshoot.

Ammeters

  • Usage and Sensitivity:

    • Measures current without extra components if sensitivity is proper.

    • External shunt necessary for high current ranges to ensure correct measurement.

  • Shunt Calculation Example:

    • An external shunt with a known voltage drop (50mV) can calculate resistance using Ohm's Law.

Voltmeters

  • Design and Operation:

    • High resistance in series limits current for full-scale deflection.

    • Multiplier Resistors: Used to increase the voltage range of the meter.

    • Multi-range voltmeters employ a single meter movement with selector switch for varying ranges.

Ohmmeters

  • Types of Ohmmeters:

    1. Series Type: Uses batteries and resistors to measure.

    2. Potentiometer-Type: Utilizes a voltage divider circuit for better scale range.

    3. Shunt-Type: Suitable for low resistance measurements.

  • Series Type Operation: Adjust variable resistor for zeroing before measurement.

Multimeters

  • Types:

    1. Analogue Multimeter: Measures various ranges for voltage, current, and resistance.

    2. Digital Multimeter (DMM): Measures voltages, ohms, and amperes, often with more functionality.

  • Measurement Principles:

    • Voltage: Connected in parallel to components.

    • Current: Connected in series, requiring circuit disconnection.

AC Power Measurement Concepts

  • Basic Definitions:

    • True Power: Power consumed (watts) in resistive circuits.

    • Apparent Power: Product of voltage and current (measured in volt-amps).

    • Reactive Power: Power returned to the supply (measured in VARS).

    • Power Factor: Ratio of true power to apparent power; indicates system efficiency.

Electrodynamometer & Watt-Varmeter

  • Electrodynamometer Usage: Measures power in DC and AC circuits with magnetic field principles.

  • Watt-Varmeter: Designed for measuring load in aircraft electrical systems; capable of measuring real and reactive power.

Other Tools and Safety in Use

  • Other Meter Types: Including moving vane meters, D'Arsonval meters with rectification, thermocouple ammeters, etc.

  • Important Safety Precautions:

    • Always use suitable instruments; ensure correct range settings.

    • Power off circuits before resistance measurements.

    • Confirm serviceability and calibration of instruments.