Air friction Damping: An air friction damping device is usually provided with moving iron instruments such as voltmeters and ammeters.
In this device, a light aluminum vane is attached to a spindle as shown in Fig.
and this vane is free to move in a fixed air chamber known as a sector. When the pointer deflects, the vane in the sector also moves. The air in the sector produces friction in the movement of the vane and thus necessary damping torque is obtained.
Eddy Current Damping: It is used in a strong operating field e.g., PMMC (Permanent Magnet Moving Coil instrument) type.
Fluid Friction Damping: Used in high voltage measurement. Used in the vertically mounted instrument. e.g., Electrostatic type instrument.
The maximum power transfer theorem states that the DC voltage source will deliver maximum power to the variable-load resistor only when the load resistance is equal to the source resistance. Similarly, this theorem states that the AC voltage source will deliver maximum power to the variable complex load only when the load impedance is equal to the complex conjugate of the source impedance.
Hence when Ri = RL the transfer of power will be maximum.
The slow but continuous rotation of the energy meter (Induction type) when only the pressure coil is excited and no current is flowing through the current coil is called creeping. This error may be due to excessive friction compensation, excessive voltage supply, stray magnetic field, etc.
In order to prevent creeping at no-load, two holes of the same radius are drilled in the disc on the opposite side of the spindle.
In the same way that we can have series, parallel or series-parallel electric circuits, we can also have equivalent series and series-parallel (but not parallel) magnetic circuits.
In practice, a magnetic circuit may consist of several parts in a series of different lengths, cross-sectional areas, and permeabilities. In such a series magnetic circuits, all the reluctances of several parts will get summed up together (as resistors in an electric circuit) to form the net reluctance of the circuit.
In series magnetic circuits the flux passing through each part will be the same (as current in the series electric circuits).