What is an Electrostatic Instrument?
An electrostatic instrument, also known as an electrostatic voltmeter, uses the force of attraction or repulsion between two charged bodies to produce deflecting torque. This effect is used to measure the voltage of both AC and DC electrical circuits.
The instrument consists of two plates, a fixed plate and a movable plate. When a potential difference is applied across the plates, an electrostatic field is created. The force of attraction or repulsion between the plates causes a torque to be exerted on the movable plate, which in turn causes the pointer attached to the plate to deflect from its zero position.
The deflecting torque is directly proportional to the potential difference between the two plates and the area of the plates. Since the electrostatic effect is well suited for measuring very high voltages, these instruments are often used in applications requiring high voltage measurements.
It's important to note that electrostatic instruments are not suitable for measuring current, as the force of attraction or repulsion between the plates is dependent on the voltage, not the current.
There are two ways of constructions of electrostatic instruments:
- In the first type of construction, the electrostatic instrument uses charges stored between two plates to produce a force of attraction or repulsion. These charges are of opposite polarity, which creates an electrostatic field between the plates. The movement of the movable plate towards the fixed plate due to this force results in the production of deflecting torque, which moves the pointer attached to it.
- In the second type of construction, the electrostatic instrument uses a rotary plate instead of fixed and movable plates. The rotary plate has charges on it and as it rotates, it experiences forces of attraction or repulsion from the electrodes surrounding it. This creates the deflecting torque and results in the movement of the pointer attached to the rotary plate.
Both types of electrostatic instruments work based on the principle of electrostatic forces between charges of opposite polarity, and the movable element in the instrument is moved by the force of attraction or repulsion. The main difference is in their physical construction and the way the charges are stored and used to create the deflecting torque.
Working Principle of Electrostatic Instrument:
The principle of operation of an electrostatic instrument is based on the interaction of two electrodes that carry opposite electrical charges. A voltage is applied across the electrodes, which creates an electrostatic field between them. The force of attraction or repulsion between the electrodes produces a deflecting torque, which causes the pointer attached to the instrument to move. The movement of the pointer indicates the magnitude of the voltage being measured.
The electrostatic instrument can measure both AC and DC voltages. The voltage to be measured is converted into an electrostatic field, which produces the deflecting torque. The voltage can be either AC or DC, but it must be converted into an electrostatic field to produce the deflecting torque.
Electrostatic instruments are devices that measure voltage by using the force of attraction or repulsion between two plates that have opposite charges. The principle behind these instruments is that when a potential difference is applied to two plates, the plates become charged with opposite polarities and there is a force of attraction between them. If one of the plates is made to move, it will be attracted to the other plate, which will produce a deflecting torque. This torque will cause the pointer attached to the moving plate to move, indicating the voltage being measured.
One of the main challenges of using electrostatic instruments is that they are not very sensitive. In order to measure small voltages, the distance between the plates must be very small, but this can be difficult to achieve. Additionally, the force of attraction between the plates is very weak, which makes it difficult to measure small movements.
To overcome these limitations, one approach is to use a heterostatic connection, which requires an additional high voltage DC supply. This can increase the cost of the instrument. Another approach is to step up the voltage being measured, but this also increases the cost and is only applicable for measuring alternating voltages.
There are two types of electrostatics Instruments
1. Linear Type Electrostatic Instrument
2. Rotary-type Electrostatic Instrument
1. Linear Type Electrostatic Instrument:
In a linear type electrostatic instrument, there are two plates, plate A and plate B. Plate A is positively charged and plate B is negatively charged. The plate A is fixed and plate B is movable. The movable plate is connected to a spring which helps in controlling the movement of the plate.
When a voltage is applied to the plates, the force of attraction occurs between the positively charged plate A and negatively charged plate B. The movable plate B tries to move towards the fixed plate A until the force of attraction between them becomes maximum. The amount of energy stored between the plates is determined by the capacitance between the plates. The capacitance is represented in the unit of Farads (F).
The energy stored between the plates can be calculated by the following expression:
E = 1/2CV^2,
where C is the capacitance between the plates in Farads and V is the voltage applied to the plates.
The linear type electrostatic instrument is mainly used to measure very high voltages. Because of the weak force of electrostatic attraction, the distance between the plates has to be very small to measure small voltages. However, this makes the instrument bulky and hard to use for small voltage measurements. Therefore, these instruments are best suited for measuring very high voltages.
2. Rotary-type Electrostatic Instrument:
In a rotary type electrostatic instrument, the instrument has a rotating shaft or rotor that is connected to a set of plates. These plates have opposite charges, and as the rotor rotates, there is a force of attraction or repulsion that acts between the plates. This force causes the rotor to rotate, and the rotation is then used to move a pointer or indicator on the instrument's face.
The amount of rotation and the position of the pointer give a measure of the voltage being measured. The movement of the rotor is controlled by a spring, which provides the restoring force to keep the rotor in its zero position. This type of instrument is suitable for measuring high voltage and it can be used for both AC and DC voltage measurements.
Advantages of Electrostatic Instruments:
- They can measure both AC and DC voltages.
- They have low power consumption.
- They can measure high voltages.
- They can measure angular displacement in rotary type instruments.
- They have low waveform and frequency errors.
- They are not affected by stray magnetic fields.
- They are designed for large voltage measurements.
- They have no errors due to hysteresis or eddy currents.
- They can be used for precision measurements of high voltages.
- They can measure very high voltages with the use of capacitor multipliers.
Disadvantages of Electrostatic Instruments:
- They have a non-uniform scale.
- They have small operating forces.
- They are costly.
- They have a large size.
- They are not robust in construction.
- They can only measure voltages, not other quantities.
- They are only suitable for high voltage measurements.
- They are not suitable for low voltage measurements.
- They require careful operation.
- The scale is non-linear.