AC circuits
A filter with a high Q will have a sharper change of impedance with frequency than one with a low Q, and reduced losses.
AC is generated and distributed using three phases, three equal voltages being generated at 120° phase intervals. The phase voltage VP is measured with respect to a common star point or neutral point, and the line voltage VL is measured between the separate phases. In magnitude,
Any three-phase generator, transformer or load can be connected in a star or Y con- figuration, or in a delta configuration (Fig. 2.17).
Magnetic circuits
A reluctance in a magnetic circuit behaves in the same way as a resistance in a dc electric circuit. Reluctance in series add together
Transformers and power reactors may contain no air gap in the magnetic circuit. However, motors and generators always have a small air gap between the rotor and stator. Many reactors also have an air gap to reduce saturation of the ferromagnetic parts. The reluctance of the air gap is in series with the reluctance of the steel rotor and stator. The high relative permeability of steel means that the reluctance of even a small air gap can be much larger than the reluctance of the steel parts of the machine. For a total air gap g (m) in a magnetic circuit, the magnetic flux density B in the air gap is given approximately by
where Nf is the number of series turns on the field winding of the machine, and If is the current in the field winding (Fig. 2.18).
