frequency meters.pptx

frequency meter

1. Mechanical frequency meter

2. Reed:- A reed is a thin strip of material which vibrates to produce a sound on musical Instrument. youtube.com/ watch?v =4t9E-YFnOWU

3. When the electromagnet is connected with the supply, its magnetism varies with the frequency of the supply all reeds experience attractive force and start vibrating. Only particular reed, whose natural frequency become equal to the supply frequency comes in mechanical resonance and vibrates with maximum amplitudes. The frequency meter is connected across the supply, whose frequency is to be measured.

4. The current flows through the coil of the electromagnet and the reeds feel force of attraction. The current flows through the coil of the electromagnet and the reeds feel force of attraction. The force of attraction is proportional to the square of the current, therefore it varies at twice the supply frequency and a force acts on the reeds at every half cycle.

5. The reeds being free at their top hence start vibrating at their natural frequency as mentioned above. But a particular reed vibrates rapidly as this reed attains the resonance. The frequency opposite to this reed which is vibrating with maximum amplitude is read on the scale.

6. Electrical frequency meter

7. The electrical resonance type frequency meter is an indicating type instrument. As the name suggests its action depends upon the electrical resonance. It mainly consists of a fixed coil and a moving coil. There is a laminated iron core of varying cross-section.

8. The electrical resonance type frequency meter measures the frequency of these supply mains. Now there is a moving coil which is so pivoted at its top end that it can move along the extended core of the fixed coil like a pendulum. The pointer of the instrument is so attached at the top end of the moving coil that its tip moves along the semicircular dial. Now, we connect on a capacitor across the two leads of the moving coil.

9. Working Principle of Electrical Resonance Type Frequency Meter Due to the current in the moving coil, the moving coil produces a flux in phase with the current. This flux flows along with the extended core of the fixed coil. Therefore the flux links the moving coil. Hence, the flux induces an emf across the moving coil. Obviously, this induced emf lags the flux by 90°. Since it is a coil; the moving coil will have some inductive reactance. Again, as it is connected across a capacitor, it will have some capacitive reactance also.

10. Torque Equation I1 is the supply current of the fixed coil I2 is the induced current of the moving coil. Now, we have already mentioned that the phase angle between the supply current I1 (current in the fixed coil) and the emf induced in the moving coil is 90°. Again there is a phase difference between the induced emf and the induced current I2 (current in the moving coil). Let us consider the angle of this phase difference is α. So, the actual phase difference between I1 and I2 will be (90°-α).

11. Therefore, we can write the expression of the torque (T) as

12. Resonance That can only be possible when inductive reactance of the moving coil becomes equal to its capacitive reactance. Again, the inductive reactance (2π fL ) depends upon the angular position of the moving coil on the extended core of the fixed coil. So, when we just switch on the supply, the fixed coil starts attracting the moving coil towards it. This attraction due to the torque acting on the moving system. Therefore, the moving coil starts rotating along with the pointer attached to it. As a result, the inductive reactance of the moving coils changes. Then after certain angular rotation of the moving coil the inductive reactance of this coil exactly becomes equal to the capacitive reactance of the coil. At that point of time, there will be no torque acting on the moving system of the electrical resonance type frequency meter.

13. Resonance Therefore the pointer of the instrument becomes stationary at that point. If somehow the supply frequency changes, the value of inductive reactance of the instrument also changes. Therefore the resonance of the circuit gets disturbed. Therefore again the deflecting torque appears on the moving system and tries to rotate it further. Hence, again the inductive reactance of the moving coils changes. And after a certain rotation again resonance occurs. So, here again, the torque becomes zero. Therefore the pointer rests on a new position. So, we have seen how the position of the pointer on the dial of electrical resonance type frequency meter changes with changing the supply frequency.