Electrostatic Generator

Electrostatic Generator

 Q. How does an electrostatic generator work?

 Q. What is a large electrostatic generator called?

 Q. Electrostatic generator working principle

The drawbacks of Van de Graaff generators are that these are essentially high voltage but low power devices, and their power rating seldom exceeds few tens of kilowatts.

Therefore, electrostatic generators which effectively convert mechanical energy into electrical energy using variable capacitor principle were developed . These are actually duals of the electromagnetic machines and are CONSTANT VOLTAGE VARIABLE CAPACITANCE MACHINES.

Construction and Working of Electrostatic Generator

An electrostatic generator consists of a stator with interleaved rotor vanes forming a variable capacitor and operate in vacuum. (Refer Figure) The rotor is insulated from the ground and maintained at a potential of + V. The rotor to stator capacitance varies from Cm to Co and stator is connected to a common point between two rectifiers across the dc output which is -E volts.

The current through a variable capacitor is given by

I = CdV / V + V dc / dt

Where, C = capacitor charged to a potential V.

The power input into the circuit at any instant is

P = VI

∴ ∥ P = CV (dV/dt) + V² (dC/dt) ∥

 If dc is negative, mechanical energy is converted into electrical energy.

With the capacitor charged with a de voltage V, C (dV/ dt) = 0 and the power output

∥ P = V² (dC/dt) ∥

When the capacitance of the rotor is maximum (C_m), the rectifier B does not conduct and the stator is at ground potential. The potential E is applied across the rectifier A and V is applied across Cm. As the rotor rotates, the capacitance C decreases and the voltage across C increases. Thus, the stator becomes more negative w.r.t. ground. When the stator reaches the line potential -E the rectifier A conducts, and further movement of the rotor causes the current to flow from the generator. Rectifier B will now have E across it and the charge left in the generator will be

Where,

Q₀ = C₀  (V + E) + (C_S + C_r)

C_S = Stator capacitance to earth.

C_r = Capacitance to rectifier B to earth

C₀ = Minimum capacitance value of C (Stator to rotor capacitance).