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Q-meter:
The Q-factor for a series L–C–R circuit is the voltage magnification at resonance, i.e.
\(Q-factor = \frac{voltage\ across\ capacitor}{supply\ voltage}\)
\(=\frac{V_C}{V}\)
The simplified circuit of a Q-meter, used for measuring Q-factor. Current from a variable frequency oscillator flowing through a very low resistance r develops a variable frequency voltage, \(V_r\), which is applied to a series L–R–C circuit. The frequency is then varied until resonance causes voltage \(V_c\) to reach a maximum value. At resonance \(V_r\) and \(V_c\) are noted. Then
\(Q-factor=\frac{V_c}{V_r} = \frac{V_c}{I_r}\)
In a practical Q-meter, \(V_r\) is maintained constant and the electronic voltmeter can be calibrated to indicate the Q-factor directly. If a variable capacitor C is used and the oscillator is set to a given frequency, then C can be adjusted to give resonance. In this way inductance L may be calculated using
\(f_r= \frac{1}{2\pi\sqrt{LC}}\)
Since \(Q= \frac{2\pi fL }{R}\),
then R may be calculated
Q-meters operate at various frequencies and instruments exist with frequency ranges from 1 kHz to 50 MHz. Errors in measurement can exist with Q-meters since the coil has an effective parallel self-capacitance due to capacitance between turns. The accuracy of a Q-meter is approximately ±5%.
Megger:
The megger is an instrument used for the measurement of high resistance and insulation resistance.
Essentially the megger insulation tester consists of –
The series resistance R’ protects the current coil in case the test terminals are short circuited and also contrast the range of the instrument.
What are absolute & secondary instruments ? Why is damping required in a measuring instrument ?
What are the special features of a high voltage Schering bridge? What is meant by “Wagner’s Earth”?
State and prove the Parseval’s theorem for power.
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