Generating Voltmeter (GVM)
Principle of Operation Generating Voltmeter (GVM) :
- The generating voltmeter comprises essentially of an insulated stator mounted facing the high voltage terminal.
- It is periodically shielded from this high voltage terminal by the rotation of a grounded sectored disk at a constant speed. The stator to terminal capacitance is thus caused to vary periodically which induces a stator current, which is the measure of the terminal voltage. It is rectified with a precision rectifier and is read on a digital meter. It is well known that the flow of current in a capacitors of takes place due to the rate of change of charge.
The current ‘i’ follows the equation :
i = Utx dC / dt ….(1)
Ut = terminal voltage
C = terminal – signal – plate capacitance
Mathematically, we can write I = -dQ / dt, where Q is the charge in a capacitor. Again we know that Q = CV, where
C is the value of the capacitor and V is the voltage applied to the capacitor. If C and V are both variable, then the expression of current is given by
I = – d / dt = – (Cx dV / dt + V x dC / dt) ….(2)
When a capacitor is connected to a dc source the capacitor can only be mites charged, but the current flow is not possible unless there is a rate of change of mobivit charge. If there is a rate of change of capacitance the current flow is possible in case of dc also, as is evident from Equation (3)
I = -V x dC / dt ….(3)
(in the case of dc, dV / dt = 0)
- This resultant current is a measure of the DC voltage under test.
- The dome is placed atop a epoxy made insulated sheet rested over cylindrical insulators. The GVM is placed in a circular groove in a earthed metallic sheet above dome top.
- The metallic sheet is supported by epoxy nuts over four numbers of on threaded insulating rods of epoxy mounted at the four corners of the sheet.
- The distance and the angle of the metallic sheet and hence the GVM with respect to the dome is varied by sliding the nut supports of the sheets mounted on the four threaded rods. The figures show the arrangement used.
Measurements :
- The measurements can be made with GVM rotated at a constant speed of 1000 rpm.
- Measurements at linear distance of 135 mm, 150 mm, 200 mm and 300 mm between the GVM and the Dome top can be taken with GVM kept parallel to the dome base plate.
- Generating voltmeter are widely used to measure high DC voltages up to 100 kV.