Electrostatic Voltmeter MCQ || Electrostatic Voltmeter Questions and Answers

Answer.3. Voltmeters

Explanation: 

• Electrostatic Voltmeters can effectively measure voltages without any charge transfer.
• One of the important features of an Electrostatic voltmeter is that it can precisely measure surface potential (voltage) on any type of material without the need for physical contact.
• As such, there is no occurrence of charge transfer as well as loading of the voltage source.
• Electrostatic voltmeters are mostly suitable in cases where voltage measurements cannot be performed using conventional contacting voltmeters.
• While conventional contacting Voltmeters need charge transfer to the voltmeter leading to loading and adjustment of the source voltage, an electrostatic voltmeter can be used without bothering about these issues.

Answer.3. Both AC and DC voltage

Explanation: 

•  An electrostatic instrument is basically a voltmeter that works on the principle of the static electric field.
• They are used for voltage measurement (especially for high voltages) but can also be used for measuring current and power with the additional arrangement.
• An electrostatic voltmeter is essentially an air capacitor; one plate is fixed while the other, which is coupled to the pointer, is free to rotate on jeweled bearings.
• When the voltage to be measured is applied across the plates, the electric force between the plates gives rise to a deflecting torque.
• Under the action of deflecting torque, the movable plate moves and causes the deflection of the pointer to indicate the voltage being measured.
• It can be used to measure direct as well as alternating voltages.

Answer.2. Both Statement (I) and Statement (II) are individually true but Statement (II) is NOT the correct explanation of Statement (I)

Explanation: 

• The electrostatic wattmeter is an electronic instrument mainly used for measuring small power at high voltages and low power factors.
• Generally, an electrostatic wattmeter is used for measuring of dielectric loss of a power cable.
• Electrostatic wattmeters can also be used for calibration purposes of other wattmeters as well as energy meters.
• The electrostatic wattmeter is generally a high-precision instrument, so it is also used in laboratories for small power measurements.
• And Electrostatic wattmeter is not a commercial instrument but it is a precision instrument. And also this instrument is not able to measure the power of high value.
• It does not have any error clue to waveform and frequency of the electrical signal.
• It is also free from eddy current error because no magnetic induction takes place in the instrument.
• The deflecting torque of the instrument is also very small.

Therefore, both statement (I) and statement (II) are individually correct but statement (II) is not the correct explanation of statement (I).

Answer.1. Voltmeter operating current has to be very small

Explanation: 

• The loading effect is the degree to which a measurement instrument impacts electrical properties like the voltage, current, and resistance of a circuit.
• In general, the resistance of an ideal voltmeter is infinite so that the voltmeter does not alter the circuit current.
• But, in the practical world, the value of resistance of the voltmeter is not infinity it is a high value only.
• If we use a voltmeter in small resistors circuit then there is not much change in the quantities of the circuit.
• If we use a voltmeter in high resistor circuits then the resistance of the voltmeter is less when compared to the circuit resistance, this will act as a shunt path for the current and therefore the voltage drop across the resistor where we want to measure the voltage will be less.
• Because of this, the reading of the voltmeter will not be the actual voltage drop rather it will be lower than actually existed before the connection of the voltmeter. This is called the Loading effect of the voltmeter.
• ​As voltmeter produces deflection by measuring a portion of load current drawn from the circuit under test.
• To minimize voltmeter loading, this current (required for the operation of the voltmeter) drawn should be very small.

Answer.4. All of the above

Explanation: 

Electrostatic voltmeters are used for high voltage measurement.

There are three types of electrostatic voltmeters, namely

(a) Attracted disc type electrostatic voltmeter

(b) Quadrant type electrostatic voltmeter

(c) Multicellular type electrostatic voltmeter

Answer.4. 20 kV

Explanation: 

Attracted type electrostatic voltmeter is used for the measurement of very high voltage, generally above 20 kV.

• The instrument is made up of two discs, one stationary and the other movable.
• The moving disc is placed exactly above the center of the fixed disc. It can move freely inside the fixed disc as the discs are separated by a small air gap.
• The moving disc is controlled by a spring. The voltage to be measured is connected across the two discs.
• The voltage applied produces an electrostatic field between the plates. Due to the force of attraction, the movable plate develops a deflecting torque which is proportional to the square of the voltage.

Answer.2. Four Non-metal

Explanation: 

The quadrant electrometer is made of four metal double quadrants with a small air gap between the quadrants.

• These quadrants are fixed. Inside the quadrants is a double sectored vane which is freely suspended by means of a thread made of phosphor bronze.
• This suspended vane can move freely inside the metal quadrants when a deflecting torque is developed.
• The vane can work as a needle to provide readings on a graduated scale.
• The voltage to be measured is connected between the fixed quadrants and the moving vane or needle.
• The voltmeter is suitable for the measurement of voltage of 10 kV and above, both DC and AC.

Answer.1. 10 kV

Explanation: 

The quadrant electrometer is made of four metal double quadrants with a small air gap between the quadrants. The voltmeter is suitable for the measurement of voltage of 10 kV and above, both DC and AC.

• These quadrants are fixed. Inside the quadrants is a double sectored vane which is freely suspended by means of a thread made of phosphor bronze.
• This suspended vane can move freely inside the metal quadrants when a deflecting torque is developed.
• The vane can work as a needle to provide readings on a graduated scale.
• The voltage to be measured is connected between the fixed quadrants and the moving vane or needle.

Answer.3. Both 1 and 2

Explanation: 

There are two types of connections in a quadrant electrometer. They are,

• Heterostatic connection
• Idiostatic connection

Heterostatic Connection: In this type of connection voltage to be measured is applied across the fixed quadrants. A high tension (HT) battery is used to charge the moving needle more positively than the fixed quadrants (i.e., voltage to be measured).

Idiostatic Connection: In this type of connection, there is no additional external voltage. The moving needle is connected to any pair of quadrants directly.

Answer.2. Kelvin Voltmeter

Explanation: 

Multicellular type electrostatic voltmeter is also called as Kelvin Voltmeter. It is one of the quadrant-type electrostatic voltmeters. It consists of a number of moving needles (vanes) with one quadrant as shown below. The measuring range of this type of instrument is 100 to 1000 volts.

Answer.4. All of the above

Explanation: 

Advantages of Electrostatic Voltmeter

1. They can be used for the measurement of both DC and AC voltages.
2. They are suitable for the measurement of high voltage, as high as 30 kV.
3. For DC measurement they only draw a small amount of initial charging current. For AC application, the current drawn by the voltmeter is negligible. So, the loading effect is very small and is an advantage over other types of voltmeters.
4. There is hardly any power loss in this instrument.

Answer.1. Not suitable for low voltage

Explanation: 

Disadvantages of Electrostatic Voltmeter

1. They are not suitable for low voltage measurement.
2. They are of large size.
3. They are comparatively costly.
4. Their graduated scale is non-uniform as deflection is proportional to square of the voltage.