Question

how an induction type Instrument can be used as an AC Voltmeter

Answer 1

Question : How and induction type instrument can be used as an AC voltmeter ?

Answer : Generally for the AC measurement Induction type instruments are used Because we know that induction principle works only for AC.

Induction type instruments mainly consists of electromagnet, Poles and Aluminium disc.Electromagnet produces the required magnetic field and AC in the magnetic field produces the flux between the poles, We place the aluminum disc in the magnetic field therefore the changing flux will  links the aluminium disc.Due to which flux induces  eddy current in the disc and this eddy current interacts consequently with the flux as a result of these mechanical torque is acting on the disc and rotate it

Mechanical torque depends on the two factors mainly that is strength of magnetic field and value of eddy current , In turn the strength magnetic field depends on the current of electromagnet and eddy current also depend on the current of electromagnet .Therefore the Torque is directly proportional to the square of current if electromagnet

Equation of torque of induction type instruments is

Torque will be zero if is 90 degree

is phase angle between flux and eddy current

Therefore we much prevent the phase angle to not to be 90 degree

Which is achieved by two methods

Ferrari's type instrument and shaded pole instrument

will be discussing shaded pole instrument because in AC voltmeter we are using that type of construction

In shaded Pole method Flux is produced by providing a shaded pole in the electromagnet and instrument in which we use this method is known as shaded pole induction type instrument. What we do in this method is we just split the pole faces into two parts and copper band is fitted on the one of the either faces and we refer that face of the pole as shaded pole and the flux produce by the shaded and unshaded pole have phase differences due to which two different fluxes will induce and these fluxes produces two eddy currents on the aluminium disc due to which the torque produced will be deflecting torque.

Now will be using this type of construction to use this instrument as a voltmeter .Firstly we must know what is voltmeter ?

Voltmeter is a instrument use to measure the potential difference between to points in the electrical circuit

Shaded pole type induction instrument is used to measure the potential difference. A non inductive high resistance is inserted in series with the shunt coil and connected across the supply whose potential difference or voltage we are measuring As we are connecting the voltmeter across the supply the current flowing through the coil is very small . we are providing the spindle of aluminium disc with the pointer moving over a calibrated scale in terms of voltage and in order to provide the controlling torque spiral springs are provided on both the ends of the spindle. Here we are using C shape permanent magnet which provide us with a damping torque on the spindle . Since to provide controlling torque spiral springs are provided the scale of the instrument is uniform because in this type of instruments the torque is directly proportional to angle of deflection of pointer