Working Principle of Moving Iron Instruments

Moving Iron Instruments are the most common type of ammeter and voltmeter used at power frequencies in laboratories. These instruments are very accurate, cheap and rugged as compared to other AC instruments. Working Principle of Moving Iron Instruments: In Moving Iron Instruments, a plate or van of soft iron or of high permeability steel forms … Read more

Why Moving Iron Instrument Scale is Non-uniform?

The deflection of needle of moving iron instrument is given in terms of rms value of voltage or current. As the angular deflection of Moving Iron Instrument is proportional to square of operating current, therefore the instrument has basically square law response. Due to this square law response, the Moving Iron Instrument Scale is non-uniform. … Read more

What Happens if Ammeter Connected in Parallel and Voltmeter Connected in Series?

It is a well known fact that Ammeter is always connected in series with the element through which current is to be measured. Figure below shows two cases, in one case ammeter connected in series while in other case, ammeter is connected in parallel. As an Ammeter has very low internal resistance therefore ammeter connected … Read more

Working Principle of Residual Voltage Transformer

A Residual Voltage Transformer is used to measure the residual voltage of three phase system during single phase fault. During normal operating condition, the summation of three phase voltage is zero but in case of single phase fault, the scenario changes and there exists a residual voltage. Let us first discuss residual voltage in case … Read more

PMMC Instruments – Construction and Working Principle

Permanent Magnet Moving Coil or PMMC Instruments are the most accurate type for the measurement of DC current or voltage. The working principle of PMMC instruments are same as that of d’Arsonaval type of Galvanometer, difference being that a direct reading instrument is provided with pointer and scale. Construction of PMMC Instruments Permanent Magnet Moving … Read more

Electrodynamic Type Instrument – Construction and Working Principle

Electrodynamic type instrument is used for the measurement of AC and DC quantities unlike PMMC instrument which can only be used for the measurement of DC quantity. This type of instrument is known as Elctrodynamometer type instrument. Construction of Electrodynamic Type Instrument An electrodynamic type instrument consists of Two Fixed Coil, a Moving Coil, Control … Read more

Torque Equation in Electrodynamometer Instruments

Electrodynamometer Instruments are widely used as an ammeter, voltmeter and wattmeter. This instrument can measure AC as well as DC quantities. This means that the torque in electrodynamometer instruments must have a finite value and unidirectional for AC and DC quantity unlike PMMC instruments. In this post we will be discussing the torque equation in … Read more

Non-uniform Scale of Electrodynamometer Type Instrument

Unlike a PMMC instrument, the electrodynamometer type instrument has non-uniform scale. The non-uniform scale of this instrument can be explained by the equation of deflection. As we know that the deflection in electrodynamometer type ammeter and voltmeter is given as Ɵ = KI2dM/dƟ for ammeter and Ɵ = KV2dM/dƟ for voltmeter Therefore, if dM/dƟ is … Read more