Electrostatic voltmeters are special types of electrical measuring instruments used for measuring very high voltages. These instruments work on the principle of electrostatic force that exists between charged plates. They are highly useful for both AC and DC voltage measurements because they consume negligible power and provide accurate readings without affecting the circuit conditions.
In this article, we will learn about electrostatic voltmeters, their construction, working principle, types, characteristics, advantages, disadvantages, applications, and range extension methods in detail.
What is an Electrostatic Voltmeter?
An Electrostatic Voltmeter is an electrical measuring instrument that measures voltage by utilizing the electrostatic force of attraction or repulsion between charged plates.
It is basically an air capacitor in which:
- One plate is fixed.
- The other plate is movable and connected to a pointer.
- When voltage is applied across the plates, electrostatic force is produced.
- This force creates a deflecting torque that moves the pointer over the calibrated scale.
Electrostatic voltmeters can be used for measuring both direct voltage (DC) and alternating voltage (AC).
Construction of Electrostatic Voltmeter
The main parts of an electrostatic voltmeter are:
- Fixed plate
- Movable plate
- Pointer and scale
- Jewelled bearings
- Control mechanism
The movable plate is mounted on jewelled bearings to reduce friction. When voltage is applied between the fixed and movable plates, electrostatic force acts between them and causes the movable plate to rotate. This movement is indicated by the pointer on the scale.
Working Principle of Electrostatic Voltmeter
The working principle of an electrostatic voltmeter is based on the electrostatic force between charged plates.
When a potential difference is applied across the plates:
- The plates become electrically charged.
- An electrostatic force of attraction or repulsion is produced.
- This force generates deflecting torque.
- The movable plate rotates.
- The pointer attached to the movable system indicates the voltage value on the scale.
Since the force depends upon the applied voltage, the instrument directly measures the potential difference.
Types of Electrostatic Voltmeters
Electrostatic voltmeters are mainly classified into three types:
1. Attracted Disc Type Electrostatic Voltmeter
- Usual voltage range: 500 V to 500 kV
- Works on electrostatic attraction between discs.
- Suitable for very high voltage measurements.
2. Quadrant Type Electrostatic Voltmeter
- Usual voltage range: 250 V to 10 kV
- Contains quadrant-shaped plates.
- Provides better sensitivity compared to attracted disc type.
3. Multicellular Type Electrostatic Voltmeter
- Usual voltage range: 30 V to 300 V
- Contains multiple cells to increase operating force.
- Used for comparatively lower voltage measurements.
Characteristics of Electrostatic Voltmeters
Electrostatic voltmeters possess several unique characteristics which make them suitable for high-voltage measurements.
Important Characteristics
- Deflecting torque is very small at low voltages.
- They are mainly used for voltage measurement only.
- Cannot be used as an ammeter.
- Suitable for both AC and DC measurements.
- Insensitive to magnetic fields.
Advantages of Electrostatic Voltmeters
- They can measure both AC and DC voltages.
- They draw negligible power from the circuit, so circuit conditions remain unaffected.
- They are free from hysteresis and eddy current losses because no iron parts are used.
- The readings are independent of waveform and frequency.
- They are not affected by stray magnetic fields.
- Suitable for very high voltage measurements.
Disadvantages of Electrostatic Voltmeters
- The operating force is very small at low voltages.
- Errors due to friction are difficult to avoid.
- These instruments are expensive.
- They are large in size and not mechanically robust.
- The scale is non-uniform and crowded at the beginning.
Applications of Electrostatic Voltmeters
Electrostatic voltmeters are widely used in high-voltage applications due to their low power consumption and high accuracy.
Main Applications
- Measurement of very high DC voltages.
- Measurement of very high AC voltages.
- Used when transformer-based measurement methods must be avoided.
- Used in laboratories and high-voltage testing stations.
- Used for measuring low voltages where an open circuit condition must be preserved.
Range Extension of Electrostatic Voltmeters
The range of electrostatic voltmeters can be extended by using multipliers. Two methods are commonly used:
1. Resistance Potential Divider
- Suitable for voltage ranges up to 40 kV.
- Can be used for both AC and DC voltages.
2. Capacitance Potential Divider
- Suitable for voltage ranges up to 1000 kV.
- Used only for AC voltage measurements.
Why Electrostatic Voltmeters are Used for High Voltage Measurement?
Electrostatic voltmeters are preferred for high voltage measurement because:
- They consume extremely low power.
- They provide insulation from the measuring circuit.
- They are unaffected by frequency variations.
- They can measure very high voltages safely and accurately.
Comparison Between Electrostatic Voltmeter and PMMC Voltmeter
| Parameter | Electrostatic Voltmeter | PMMC Voltmeter |
|---|---|---|
| Working Principle | Electrostatic force | Magnetic effect of current |
| Voltage Type | AC and DC | Mainly DC |
| Power Consumption | Very low | Comparatively higher |
| Best Suitable For | High voltages | Low and medium voltages |
| Scale | Non-uniform | Uniform |
Frequently Asked Questions (FAQs)
What is the principle of electrostatic voltmeter?
Electrostatic voltmeter works on the principle of electrostatic force between charged plates.
Can electrostatic voltmeters measure AC voltage?
Yes, electrostatic voltmeters can measure both AC and DC voltages.
Why are electrostatic voltmeters used for high voltage measurement?
Because they consume negligible power and can safely measure very high voltages accurately.
What is the main disadvantage of electrostatic voltmeter?
The operating force is very small at low voltages, making the instrument less sensitive.
Can electrostatic voltmeters be used as ammeters?
No, electrostatic instruments are used only for voltage measurement.
Conclusion
Electrostatic voltmeters are important measuring instruments specially designed for high voltage measurements. They operate using electrostatic force between charged plates and are suitable for both AC and DC measurements. These instruments offer advantages like negligible power consumption, freedom from hysteresis losses, and independence from frequency and waveform. However, they are expensive and less suitable for low-voltage applications due to their small operating force.
Because of their excellent performance in high-voltage systems, electrostatic voltmeters are widely used in laboratories, testing stations, and power system applications.