23 Q-Meter

Vinay Gupta

epgp books

     Introduction

 

 

Quality factor is a very important factor in electrical/electronic circuits and components and is denoted by Qfactor. It is extremely significant factor for the individual components. For an ideal inductor having zero resistance, the quality factor is said to be infinite and is defined as:

 

 

 

 

 

 

 

which is again infinity.

 

Q measurement is relevant to the measurements on the tuned circuits. As is understood that power in an all permeating parameter. In all kinds of circuits such as AC, DC, tuned, non-tuned, AF,RF, power requirements needs to be estimated for various aspects including design, commercial viability, special application areas and so on. Power factor is defined as a parameter which is determines usable power and is different from the part that is reactive in nature.

 

Q-Meter

 

The quality of passive electrical elements and circuits is measured by Q-meter. The basic principle is to  roduce resonance with the element connected as required with a dual or complementary element and excite the scheme with variable frequency oscillator. The Q-factor of the element is defined as the ratio of its reactance at resonance to its loss parameter or resistance. The voltage drop across the element at resonance is Q times the applied voltage. Also, |XL|=|XC| at the resonance.

 

 

Fig.1. Circuit for Q-measurement with vector diagram

 

The circuit scheme of a meter shown in Fig. 1 consists of an ideal capacitor C for the measurement of the Q –
factor of a lossy inductor L, its loss resistance being indicated as R. If the current through the circuit at resonant condition is I when XL = 2πifrL and XC = -i/2πfrC, where fr = resonance frequency, then it follows that voltages across the capacitor and inductor are equal in magnitude and opposite in phase. Net output voltage is given by Vo = IR being the same as the input voltage Vi. If a voltmeter connected across the capacitor shows Vc = IXc, then Q of the inductor coil is:

 

Q = |XL|/ R =|XC|/ R = |IXC|/ IR = Vc/Vi

 

As Vi is known, the voltmeter indicating Vc is directly calibrated in Q-factor. Voltage Vc is obtained by varying frequency of the oscillator when the indication becomes maximum. It must be kept in mind that if the capacitor has a ‘leakage’ resistance and the oscillator output resistance is nonzero which should be negligibly small with respect to R, then the indicated Q is slightly different from the actual value. Further, the stray capacitance, especially at high frequencies, causes change in the effective L-value and hence the Q-factor. The
interwinding capacitance of the coil behaves similarly.

 

The frequency at which resonance occurs is given by the relation:

 

 

 

This is a well-known relation. However, in practical situations the same relation is adopted to suit the conditions of operation and measurement. Actually, the practical measuring schemes are slightly modified, mainly because of the measuring uncertainty in the value of Vi. A small shunt resistance is supplied by the oscillator which acts as the source of the circuit. The voltage across the shunt resistance Vsh becomes the multiplying factor then. For wide frequency ranges from 0.05 to 500MHz a thermocouple meter is used for measuring Vsh. The oscillator is provided now with a variable series resistance for adjustment of Vsh. Tuning is
done by oscillator frequency or by varying the capacitor.

 

Q Measurement by Measuring Impedence:

 

Two different schemes of Q measurement are proposed, depending upon the nature of the impedence measurement. These are the series connection scheme for low impedence and the parallel connection scheme
for high impedence.

 

Series Scheme:

 

Fig.2. Circuit for Q-measurement by measuring impedence – the series scheme

 

If the low impedence components are to be measured which have say low resistance, low inductance and high capacitance, the series scheme is used. Figure 2 shows the series scheme which consists of a low resistance Z1 placed in series with the work coil supplied with the instrument. The measurement is done in two steps (1) Z1 is shorted by a strap S and the circuit is tuned by varying C to a value C1. The Q1 indicated by the voltmeter becomes the reference. (2) Z1 is brought into the circuit by opening S, C is tuned to a new value C2 and Q2 is obtained in the voltmeter. If R is negligibly small, we get:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Parallel Connection Scheme:

 

Parallel connection scheme is a circuit used for high impedence measurement and is shown in Fig. 3.

 

Fig.3. Circuit when impedence is high – the parallel connection

 

Initially the high impedence is isolated by the switch S and capacitance C1 and Q value (Q1) noted for resonance with the essential working coil. Then, Zh is brought into the circuit parallel to C and the coil, and returning is done to obtain C2 and Q2. Here again, for the first measurement

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The shunt resistance Rsh across the oscillator produces appreciable error in the measured value of Q unless Rsh is negligible with respect to coil resistance. In direct connection shown in Fig. 1, if Rsh is percentage of R, changing R to R(1+a/100), then the Q-factor changes by

 

ΔQ = Qact – Qind

 

 

 

 

 

 

 

 

 

 

 

Meggers

 

The megger is an instrument used for the measurement of high resistance and insulation resistance. Essentially, the megger insulation tester consists of a hand driven DC generator and a direct reading true ohmmeter. A simplified diagram of electrical connections of the instrument is shown in fig. 3. Permanent magnets provide the field for both the generator and the ohmmeter. The moving element of the ohmmeter consists of three coils, known as current (or deflecting) coil, pressure (or control) coil and compensating coil, which are mounted rigidly to a pivoted central shaft and which are free to rotate over a stationary C-shaped iron core. The coils are connected to the circuit by means of flexible leads (or ligaments) that exert no restoring torque on the moving element. Hence, the moving element may take up any position over the scale when the generator handle is stationary.

 

The current (or deflecting) coil is connected in series with the resistance R between one generator terminal and the test terminal marked ‘L’. The series resistance R protects the current (or deflecting) coil in case the test terminals are short circuited and also controls the range of the instrument. The pressure (or control) in coil in series with a compensating coil and protection resistance R is connected across the generator terminals. Compensating coil is provided to obtain better scale proportions and to make the instrument astatic.

 

When the current from the generator flows through the pressure coil, the coil tends to set itself at right angles to the field of the permanent magnet. With the test terminals open, corresponding to infinite resistance, no current flows through the deflecting coil. The pressure coil thus governs the motion of the moving element, causing it to move to its extreme counter clockwise position. The point on the scale indicated by the pointer under this condition is marked infinite resistance.

 

Current coil is wound to produce clockwise torque on the moving element. With the test terminals marked L and E short-circuited, corresponding to zero external resistance, the current flowing through the current coil is large enough to produce enough torque to overcome the counter clockwise torque of pressure (or control) coil. This moves the pointer to its extreme clockwise position. The point on the scale indicated by the pointer under this condition is marked zero resistance.

 

When a resistance under test is connected between the test terminal L and E, the opposing torques of the coils balance each other so that pointer comes to rest at some intermediate point on the scale. The scale is calibrated in mega ohms and thousands of ohms (fig. 3) so that pointer indicates directly the value of resistance under test.

 

Figure: 500V Megger Scale

 

The guard ring is provided to shunt the leakage current over the test terminals or within the testor itself to the negative terminal of the generator without passing through the current (or deflecting) coil of the instrument and thus eliminates errors due to it. Usually, a terminal known as guard terminal is provided by means of this guard ring may be connected to a guard wire on the insulation under test.

 

The test voltage is generated by the generator G which, in many portable sets, is driven by the means of hand operated crank. The higher test voltages are used in the instruments with the higher resistance ranges. Variations in generator voltage in a given instrument do not appreciably effect the readings unless the apparatus under test has considerable capacitance. To avoid the effect of charging and discharging currents, which are due to variation in applied voltage, special type of sleeping clutch is fitted to the handle so that the generator speed and output voltage remains constant even when the handle speed is variable. Since the same magnet system supplies for both instrument and the generator, the instrument indications are independent of the magnet strength.

 

Operation: The resistance under test is connected between the test terminal (L and E). The generator handle is then steadily turned at uniform speed. In the case of the tester fitted with a special type of slipping clutch to the handle the speed is controlled by the centrifugal device. The turning of the handle must be kept up until the pointer gives a steady reading. The larger the capacitance of the object under test longer it will take to charge and give a steady reading.

 

Continuity Tester

 

The continuity tester is an instrument used for checking the continuity of circuits, i.e., for determining, by resistance measurement, whether given circuit such as the inner conductor of a cable or a run of steel conduit, has good conducting capacity throughout its length without a complete or partial break. Alike megger this also consist of a hand driven DC generator and a low resistance reading ohmmeter housed in one case. It is usual to have continuity scale to read upto 100 ohm or so. The scale is open in the low range so that values upto 1 ohm are easily read, since this is the most important part of the scale from the point of view of continuity measurements.

 

If the resistance under measurement is very low, the voltage drop across the deflecting coil circuit is low, and very little current flows through it therefore the pointer is deflected in the same direction as infinity on the insulation scale, to the point marked ‘zero’ on the continuity scale. On the other hand, if the resistance under measurement is high the voltage drop across the deflecting coil is increased the current through the deflecting coil is increased therefore pointer is moved to the other end of the scale.

 

Questionnaire

  1. Define Q-factor.
  2. Describe the construction and working of Q-meter.
  3. Describe the series connection scheme for Q measurements.
  4. Describe the parallel connection scheme for Q measurements.
  5. What do you understand by Meggers?
you can view video on Q-Meter

    References:

  1. Electronic Measurements and Instrumentation by Bernard M. Oliver and John M. Cage.
  2. Measurement and Instrumentation Principles by Alan S. Morris.
  3. Instrumentation and Measurement in Electrical Engineering by Roman Malaric.
  4. Measurement and Instrumentation Systems by William Bolton.
  5. Engineering Measurements and Instrumentation by Leslie Frank Adams.
  6. Electrical Measurements and Instrumentation by U. A. Bakshi.
  7. Introduction to Measurements and Instrumentation by Arun K Ghosh.