If you are provided with two inductors having same
inductance , which one will you choose ?
Answer - You
should choose the one with high quality factor or Q-factor because it will have
lesser losses due to its smaller resistance.
But , do you know how to measure Q-factor of a coil ?
The best way is - by using a Q-meter.
Q-meter working principle
A q-meter works on the principle of series resonance
i.e at resonant condition of a series ac circuit, the voltage across the
capacitor is equal to the Q-factor times supply voltage.
Vc = QV
If the supply voltage V is kept canstant , then Q ∝
Vc and the voltmeter
connected across the capacitor can be calibrated to read Q-factor directly.
Q-meter circuit connections are done as shown in
diagram.
The circuit uses a wide range radio frequency
oscillator having frequency range from 50kHz to 50MHz as the voltage supply source. The oscillator
terminals are shorted by a resistor of the order of 0.02Ω . This resistor
introduces almost no resistance to the series RLC circuit and makes the voltage
source internal resistance almost negligible. This is done to ensure supply
voltage maintained constant.
Now, the series RLC circuit is brought to resonance
condition by varying oscillator frequency.
when resonance is achieved , supply voltage V and
capacitor voltage Vc are measured by thermo-couple and electronic voltmeters
respectively.
From
thermo-couple and electronic
voltmeter readings Quality-factor can be easily calculated as
Q = Vc / V
Q-meter limitations and errors
:-
At resonance ,
In q-meter circuit, L and C
are fixed , so series RLC circuit Q-factor becomes inversely proportional to
resistance R.
As the resistance R of the
circuit is greater than what it should have been without the addition of 0.02Ω
and therefore measured Q-factor is lower than actual. For this reason, it is
precisely called the Q-factor of the circuit and not the coil.
However , the readings are
fairly correct as long as series resistance becomes more and
more larger than 0.02 Ω so that R value remains almost constant even
after adding 0.02 Ω .
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