Van de Graff Generator:
The generator is usually enclosed in an earthed metallic (calcium, barium, radium, etc.) cylindrical vessel and is operated under pressure (or) in vacuum. Charge is sprayed onto an insulating moving belt from corona points at a potential of 10 to 100Kv above the earth and is removed and collected from the belt connected to the inside of a insulator metal electrode through which the belt moves.
The belt is driven by an electric motor at a speed N = 1000 to 2000 m/minute. The steady or dc potential as equilibrium condition attained by the HV electrode when the leakage and load currents equal to the charging currents. The potential of the high voltage electrode (HV) above the earth at any instants is V = Q/C.
Q - Charge Stored
C - Capacitance of high voltage electrode to earth
The potential of the high voltage electrode (HV) rises at a rate of :
dv/dt = 1/C dq/dt = I/C
I - Net charging current
I = ∂bv
∂ - Charge density
b - width of belt in 'm'
v - Belt speed in m/sec
Working:
The lower spray point may consists of a number of needles connected to the controllable D.C source so that the discharge between lower spray point and belt (positive charge) is maintained. The collector needle is placed near the upper spray point and this needle collects the charger from corona points from the belt and is transferred on to the HV electrode where the belt enters the HV terminal. The belt returns with the charge dropped, and neutralized and fresh charge is sprayed onto it and it passes through the lower corona point.
In order to made the charging more effective and to utilize the return path of the belt for charging purposes, a self-inducing arrangements or a second corona point system by the rectifier inside the HV terminal.
Therefore the rate of charging of the terminal is doubled. To obtain self-inducing charging system, the upper pulley is connected to the collector needle and maintained at a potential higher than that of HV terminal.
The upper spray point also connected to the inside of the HV terminal, therefore the belt portion between collector and upper spray point is neutralized and leaves excessive negative charges which travels downward with belt and are neutralized at the lower spray point.
The generator is normally worked in a high pressure gaseous medium ranging from 5 to 15 atm. The gas may be nitrogen, air, freon, ccl2 mixture, sf6. The van de graff generator are useful for very high voltage and low current applications.
The output o/p voltage is easily controlled by controlling the corona source voltage and the rate of charging.
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