28-06-2013, 12:10 PM
EI 34-ELECTRICAL MEASUREMENTS
ELECTRICAL.doc (Size: 102.5 KB / Downloads: 151)
1. Define Ballastic sensitivity?
Ballastic sensitivity sq =θ1/Q
2. State the advantage and disadvantage of rectifier type instruments?
Advantage
The frequency range extends from about 20 hz to high audio frequency With the introduction of germanium and silicon rectifier rectifier instrument may be built in half wave configuration with a sensitive PMMC instrument at frequency in the region of 3000 MHZ.
Disadvantage
i) Effect of waveform
ii) Effect of rectifier Resistance
3. Why is the MI meter has non uniform scale?
The deflection in a MI instrument is given by
θ=1/2 *I2 /K*dL/dθ
Thus the angular deflection is proportional to the square of the operating current and the instrument has a square law response. The deflection is in terms of rms value of current or voltage as the deflection is proportional to square of the current it is evident that the scale of such instrument is non-uniform.
4. What is residual error?
The happenings or disturbances about which we are unaware are lumped together and called Random or Residual .Here the errors caused by these happenings are called Random error .Since these errors remain even after the systematic errors have been taken care of we all these errors as residual.
5. Why electrostatic instruments cannot be used for the measurement of low voltage while electromagnetic instrument can be?
When the voltage being measured is small the two discs should be very near together in order to get an appreciable force. In such cases the measurement of distance between the plates is difficult to carry out. The solution lies in increasing the voltage between fixed and moving plates by using hetrostatic connection.
6. Define loading effect of an instrument?
The incapability of the system to faithfully measure record or control the input signal in undistorted form is called the loading effect.
7. Define the term current sensitivity and voltage sensitivity?
The current sensitivity of a galvanometer is defined as the deflection produced by unit current
Current sensitivity Si=Gi/K
Voltage sensitivity
Voltage sensitivity is the deflection in scale division per unit voltage impressed on the galvanometer.
Sv=d/i*Rg*106
8. What precautions are to be observed when using an ammeter?
Ammeter which is connected in series with the circuit carrying the current under measurement must be very low resistance. So that the voltage drop across the ammeter and power absorbed from the circuit are as low as possible.
9. State the advantage disadvantage of PMMC instrument?
Advantage
i) The scale is uniformly divided.
ii)The power consumption is very low as 25 μw to 200 μw.
Disadvantage
i) There instrument useful only for dc
ii) The cost of this instrument is higher than that of MI instrument.
10. Compare Ammeter and Voltmeter.
Ammeter
a. The ammeter carries the current to be measured or a definite fraction of it and the current or its definite fraction produces the deflecting torque.
b. Connected in series.
Voltmeter
a. Carries the current proportional to the voltage to be measured which produces the deflecting torque
b. Connected in parallel
11. What is the basic principle of PMMC instrument?
Working principle is same as the d’Arsonoval type of galvanometer the difference being that a direct reading instrument is provided with a pointer and scale.
12. Compare and contrast moving iron and Electro dynamometer type meters?
MI
a. Suitable for DC or AC
b. Inexpensive type used for rough indication of currents and voltages widely used in indicator type applications such as on panels
Electrodynamometer type instrument
a. Suitable for DC or AC
b. Widely used for precise ac current or voltage measurement current frequencies Used as standard meter for calibration and also as transfer instrument