Types of Ammeters, Voltmeters Electrical Instruments

Introduction – Ammeter & Voltmeter:

                  Direct measurement of voltage and current is done with the use of voltmeter and ammeter respectively. In construction and principle of operation, both the instruments are very much same.

Connecting voltmeter and ammeter in a Circuit:

Voltmeter connection

  • Voltmeter is used to measure the potential difference across a load.
  • So the resistance of the voltmeter shall be very high compared to circuit resistance.
  • In contrast to this, if the voltmeter resistance is small, and connecting the voltmeter across load will be equal to parallel connecting another load.
  • Then the circuit conditions are changed very much and no worth-while reading will be measured.
  • So the resistance of voltmeter must be very high.
  • If the meter’s moving system has only a small resistance, additional resistance will be included in series with the moving coil.
  • This additional resistance is known as swamping resistance.

Ammeter_Connection

  • An ammeter is used to measure the current flows through the circuit.
  • So the resistance must be very small.
  • If the ammeter resistance is very high, it will introduce high resistance in series with the existing circuit an disturb the circuit conditions.
  • Ammeter should never connect across a load or voltage source.

Classification of Ammeters and Voltmeters

Types of Ammeters, Voltmeters are discussed in the below section:

Ammeters and voltmeters are classified as follows:

  1. Moving Iron Type
    (a) Attraction type
    (b) Repulsion type
  2. Moving Coil
    (a) Permanent magnet type
    (b) Dynamometer type
  3. Hot wire
  4. Induction type
  5. Electrostatic (voltmeter only)

Some of the meters may be used for DC as well as AC measurements. Some may be used for DC only and some for ac measurements only.

DC and AC instruments:
                   Those instruments whose deflections are proportional to the square of the current or voltage to be measured may be used for DC as well as ac measurements.
The following types of instruments are used for dual purpose.

  • Moving Iron attraction type
  • Moving Iron repulsion type
  • Moving coil dynamometer type
  • Electro static type
  • Hot wire type

One more type called as rectifier instruments. It is a permanent magnet moving coil instrument plus a rectifier is also suitable for ac/dc measurements. 

In DC,  only the coil resistance is the opposition to current flow. But in ac, the coil resistance and reactance will oppose the flow of current. So one scale may not suit for both ac and dc readings. Some meters (like rectifier type meters), will have separate scale for dc and ac measurements.

Only DC instruments
              In any instrument, if the deflection is directly proportional to the voltage or current to be measured, then they are suitable for dc measurement only. If we use such kind instrument in ac circuit, the moving system will try to move forward and backward several times within a second. So it will simply vibrate and no reading will be shown. The permanent Magnet Moving Coil meter (PMMC) is the one suited for dc measurement only.

Only AC Instruments
               The instruments which utilize the electromagnetic induced current for producing deflection are used for ac measurement only. We know that electromagnetic induction is not possible with DC current. Therefore, induction type instruments (Voltmeter,  ammeter and wattmeter) are used only for AC measurements.

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