Generators - Wind Energy Systems for Electric Power Generation - page 27

Chapter 3
Generators
3.1 General
Electrical machines are usually divided into three groups, namely d.c. machines,
asynchronous a.c. machines and synchronous a.c. machines. Of these machine types
the d.c. machines are no longer of practical interest as generators because of several
drawbacks; they require more maintenance effort, have an unfavourable power to
mass ratio and are not suitable for high voltage windings. Of the a.c. machines, both
asynchronous and synchronous types are in use. Included are induction machines,
denoting asynchronous machines of which only one winding is energized. In most
cases the load winding consists of three phases.
The machines discussed in the following belong to the electro-magnetic ma-
chines; electrostatic power conversion is outside the scope. The conventional a.c.
machine types have an outer stator carrying the main (primary, armature) winding;
the rotor is arranged inside the stator, with the air gap separating the inner stator
and outer rotor surfaces, The interacting magnetic field crossing the air gap in ra-
dial direction couples stator and rotor members. Other constructions, e.g. radial field
machines with outer rotor, and axial field machines may be interesting for special
designs in the field of wind power conversion.
Since every electrical machine is capable of working as a generator as well as
a motor, generators may also run up from electrical supply. The machines can also
serve for electrical braking.
A selection of topic literature on electrical machines is in [Chap04, Kra02].
The IEC provides a series of international standards applicable to electric ma-
chinery in [IEC60034]. Relevant IEEE standards are also mentioned [IEEE112,
IEEE115].
In the following asynchronous machines (AM) and synchronous machines
(SM) are described regarding their terminal properties and steady-state operation,
using equivalent circuit models. Here steady-state means that voltages and cur-
rents are sinusoidal functions of a single (fundamental) frequency and the ro-
tational speed is constant. Transient behaviour will be considered in a later
chapter.