What are the classifications of Electrical Measuring Instruments?
- Absolute Instruments
- Secondary Instruments
Differentiate Absolute Instruments from Secondary Instruments.
Absolute Instruments give the value of quantity measured in terms of physical constants of instruments and its deflection only.
Secondary Instruments are calibrated and directly give the value of quantity to be measured.
What are the three types of Secondary Instruments?
- Indicating Instruments
- Recording Instruments
- Integrating Instruments
Define Indicating Instruments.
These are instruments fitted with a pointer which moves over a calibrated scale, and indicates electric quantity to be measured directly on the scale.
Examples include AMMETERS, VOLTMETERS, OHMMETERS, and WATTMETERS.
Define Recording Instruments.
Instruments that record the electrical quantity to be measured on a GRAPH PAPER for a desired period.
It carries an inked pen which rests gently over the graph and moves at a low and uniform speed in a direction perpendicular to the movement of the pen.
Examples include GRAPHIC RECORDERS and GALVANOMETER RECORDERS.
Define Integrating Instruments.
Instruments that measure the TOTAL QUANTITY of electricity consumed in a circuit in a given time.
Examples include AMPERE-HOUR meters, WATT-HOUR meters, and KILOWATT-HOUR meters.
What are the different effects used in Measuring Instruments?
- Magnetic Effect
- Electrodynamic Effect
- Electromagnetic Induction Effect
- Electrostatic Effect
- Chemical Effect
- Heating Effect
These differ based on where they are used and what applications they are utilized in.
What devices are Magnetic Effect and Electrodynamic Effect used in?
Ammeters, voltmeters, and wattmeters.
Magnetic Effect is used in more than just these devices, though, these are the main ones.
Where is the Electromagnetic Induction Effect used?
AC ammeters, voltmeters, wattmeters, and integrating meters.
Where is the Electrostatic Effect used?
Used in Voltmeters.
Where is the Chemical Effect used?
Used in DC ampere-hour meters.
Where is the Heating Effect used?
Used in Ammeters and Voltmeters.
What are ammeters?
It measures electric current. Its scale may be calibrated in amperes, milliamperes, or microamperes.
To measure current, it is inserted in series with the circuit being tested.
What is a Voltmeter?
It is used to measure the potential difference or voltage of a circuit.
A simple DC voltmeter can be constructed by placing a resistor (multiplier) in series with the ammeter meter movement, and by marking the meter face to read voltage across resistor and voltmeter.
Thevenin’s Theorem
A method used to change a complex circuit into a simple equivalent circuit.
It states that any LINEAR NETWORK of voltage sources and resistances (viewed from any two points) can be replaced by an equivalent resistance with an equivalent source.
What are ohmmeters?
An instrument to determine resistance.
Consists of a battery, a meter movement calibrated to read ohms, and a resistor.
What are Multimeters?
It is a single instrument capable of measuring voltage, resistance, and current.
The most common is the VOLT-OHM-MILLIAMMETER (VOM).
One-meter movement is used to measure milliamperes, DC voltage, AC voltage, and ohms.
What are Alternating Current Meters?
This measures electrical quantities of current and voltage, which change in amplitude and direction periodically with time.
For frequencies greater than a few Hz, moving-coil assembly cannot follow rapid changes due to inertia and damping.
Responds on the basis of the average value of alternating quantities.
What are the two methods of measuring AC Current or Voltage?
- Half Wave Rectifier Circuit
- Full Wave Rectifier Circuit
Half Wave is the simplest; its diode permits current to flow in one direction during positive half-cycle, and presents a very high resistance to current in the other direction during the negative half-cycle.
What is a Wattmeter?
An instrument that measures DC Power or Real AC Power.
Uses fixed coils to indicate current in the circuit, while the movable coil indicates voltage.
It is rated in terms of its maximum current, voltage, and power.
