ELECTROSTATIC GENERATOR

In electromagnetic generators, current carrying conductors are moved against the electromagnetic forces acting upon them. In contrast to the generator, electrostatic generators convert mechanical energy into electric energy directly. The electric charges are moved against the force of electric fields, thereby higher potential energy is gained at the cost of mechanical energy . The basic principle of operation is explained with the help of Fig. 2.7. An insulated belt is moving with uniform velocity ν  in an electric field of strength  E  (x ). Suppose the width of the belt is b  and the charge density  σ  consider a length dx  of the belt, the charge dq = σ  bdx.

The force experienced by this charge (or the force experienced by the belt). 

Assuming no losses, the power output is also equal to VI.Fig. 2.8 shows belt driven electrostatic generator developed by Van deGraaf in 1931. An insulating belt is run over pulleys. The belt, the width of which may vary from a few cms to metres is driven at a speed of about 15 to 30 m/sec, by means of a motor connected to the lower pulley. The belt near the lower pully is charged electrostatically by an excitation arrangement. The lower charge spray unit consists of a number of needles connected to the controllable d.c source (10 kV–100 kV) so that the discharge between the points and the belt is maintained. The charge is conveyed to the upper end where it is collected from the belt by discharging points connected to the inside of an insulated metal electrode through which the belt passes. The entire equipment is enclosed in an earthed metal tank filled with insulating gases of good dielectric strength viz. SF 6  etc. So that the potential of the electrode could be