Model paper

Sub- Electronic Instruments

EL-209

Q.1 Explain working and application of Q meter?

Ans.

We know that every inductor coil has a certain amount of resistance and the coil should have
lowest possible resistance. The ratio of the inductive reactance to the effective resistance of the
coil is called the quality factor or Q-factor of the coil.

So Q = XL / R = ωL / R

A high value of Q is always desirable as it means high inductive reactance and low resistance. A
low value of Q indicates that the resistance component is relatively high and so there is a
comparatively large loss of power.

The effective resistance of the coil differs from its dc resistance because of eddy current and skin
effects and varies in a highly complex manner with the frequency. For this reason Q is rarely
computed by determination of R and L.

The Q-meter is an instrument designed for the measurement of Q-factor of the coil as well as for
the measurement of electrical properties of coils and capacitors. -This instrument operates on the
principle of series resonance i.e. at resonate condition of an ac series circuit voltage across the
capacitor is equal to the applied voltage times of Q of the circuit. If the voltage applied across the
circuit is kept-constant then voltmeter connected across the capacitor can be calibrated to
indicate Q directly.
Circuit diagram of a Q-meter is shown is figure. A wide-range oscillator with frequency range
from 50 kHz to 50 MHz is used as a power supply to the circuit. The output of the oscillator is
shorted by a low-value resistance, Rsh usually of the order of 0.02 ohm. So it introduces almost
no resistance into the oscillatory circuit and represents a voltage source with a very small or of
almost negligible internal resistance. The voltage across the low-value shunt resistance Rsh, V is
measured by a thermo-couple meter and the voltage across the capacitor, Vc is measured by an
electronic voltmeter.

For carrying out the measurement, the unknown coil is connected to the test terminals of the
instrument, and the circuit is tuned to resonance either by varying the frequency of the oscillator
or by varying the resonating capacitor C. Readings of voltages across capacitor C and shunt
resistance Rsh are obtained and Q-factor of the coil is determined as follows :

By definition Q-factor of the coil,

Q = XL / R

And when the circuit is under resonance condition

XL = XC

Or IXL = IXC = VC

And the voltage applied to the circuit.

V = IR

So, Q = XL / R = IXL / R = VC / V

This Q-factor is called the circuit Q because this measurement includes the losses of the
resonating capacitor, voltmeter and the shunt resistor Rsh. So, the actual Q-factor of the coil will
be somewhat greater than the calculated Q-factor. This difference is usually very small and
maybe neglected., except when the resistance of the coil under test is relatively small in
comparison to the shunt resistance Rsh.

Application of Q meter

 The Q meter can be used for many purposes. As the name implies, it can measure Q and
is generally used to check the Q factor of inductors. As the internal tuning capacitor has
an air dielectric its loss resistance is negligible compared to that of any inductor and
hence the Q measured is that of the inductor.
 Unknown impedance can be measured using Q-meter, either by series or shunt
substitution method. IF the impedance to be measured is small, the former is used and if
its large, the latter method is used
  Characteristic impedance of a transmission line can be measured using series substitution
method
 Measurement of quality factor involving comparison of inductances and capacitances

Q.2 Explain magger?

Ans. Insulation resistance IR quality of an electrical system degrades with time, environment
condition i.e. temperature, humidity, moisture and dust particles. It also get impacted negatively
due to the presence of electrical and mechanical stress, so it’s become very necessary to check
the IR (Insulation resistance) of equipment at a constant regular interval to avoid any measure
fatal or electrical shock.

The device enable us to measure electrical leakage in wire, results are very reliable as we shall be
passing electric current through device while we are testing. The equipment basically use for
verifying the electrical insulation level of any device such as motor, cable, generator winding,
etc. This is a very popular test being carried out since very long back. Not necessary it shows us
exact area of electrical puncture but shows the amount of leakage current and level of moisture
within electrical equipment/winding/system.

This can be separated into mainly two categories:-

1. Electronic Type (Battery Operated)
2. Manual Type (Hand Operated)
But there is another types of megger which is motor operated type which does not use battery to
produce voltage it requires external source to rotate a electrical motor which in turn rotates the
generator of the megger.

Electronic Type Megger

Important parts:-
1. Digital Display: - A digital display to show IR value in digital form.
2. Wire Leads :- Two nos of wire leads for connecting megger with electrical external system
to be tested.
3. Selection Switches: - Switches use to select electrical parameters ranges.
4. Indicators: - To indicates various parameters status i.e. on-Off. For Example Power, hold,
Warning, etc.

Hand Operated Megger

Important parts:-

Analog display:- Analog display provided on front face of tester for IR value recording.
Hand Crank:- Hand crank used to rotate helps to achieve desired RPM required generate
voltage which runs through electrical system.

Wire Leads:- Used same as in electronic tester i.e. For connecting tester with electrical system.

Q.3 list the advantages of digital multimeter?

Ans. 1. It offers automatic output display.

With this device, it will be easy for you to gauge readings, since its output is displayed
automatically in numbers through a seven-segment display, unlike the analog multimeter that
requires you to take a closer look into the scales to read values. The analog type also takes time
and risks human errors to occur, especially for those with poor eyesight.

2. It ensures accuracy.
Typically, a digital multimeter can show more accurate values than the analog and can provide
output of up to 4 decimal places that the analog could never show. With regards to fluctuations,
any value within the range of the fluctuations is mostly considered as valid.

3. It has auto polarity functions.

Multimeters can read negative values, especially in terms of voltage. When you place the probes
into the opposite polarity, you will get a negative output. This is also where a digital type is more
advantageous than the analog, as placing probes into the opposite polarity can cause the analog
type to break.