Civil Engineering Reference
In-Depth Information
7.4.1 Basic Principles and Operating Modes
of Electrical Machines
Electrical machines are traditionally classified as alternating current machines
(synchronous machines, asynchronous or induction machines, commutator
machines) and direct current machines (commutator machines) according to the
voltage of the feeding line. The stationary structure (stator) and the rotary structure
(rotor) are both generally provided with windings. One structure has to generate a
magnetic flux by means of a particular set of windings (field windings in commuta-
tor machines and synchronous machines) or permanent magnets, and the other
structure has the main windings (armature windings) through which power flows
from the electric line to the shaft or vice versa. Asynchronous machines (also called
induction machines) have polyphase stator windings (generally two or three phase)
which generate a rotating flux and carry the electric power fed by the line; rotor
windings are usually polyphase squirrel-cage windings.
A.c. (alternating current) machines can be fed directly from the line at medium
or low voltage depending on the machine power (medium voltage for high-power
machines, more than 500 kW). Synchronous machines must rotate at a synchronous
speed related to the line frequency (synchronous speed: in r/min
¼
60
f
/pair of poles;
in rad/s
f
/pair of poles where the frequency
f
is expressed in Hz) to effect the
energy transformation, so they have to rotate at a constant speed and are not self-
starting. All the other a.c. machines (except the synchronous ones) may work at any
speed and are self-starting.
D.c. (direct current) machines require a d.c. voltage line that is not generally
available in industrial applications. Power converters from a.c. to d.c. voltage are
therefore necessary.
¼
2
π
As a general rule, shaft torque can be expressed as the product
of magnetic flux (generated by a current flowing inside a field winding
or inside a polyphase winding or by a permanent magnet in one struc-
ture) multiplied by current flowing inside the winding in the other
structure (stator or rotor). This relationship is the basis of all torque
control methods.
At steady state, torque-speed characteristics can be used to represent the output
of the machines when operating as motors and the input when operating as
generators. The working point is the intersection between the torque-speed curve
of the machine and the load curve as shown in Fig.
7.6
where the characteristics of
the various classes of machine are represented.
If the machine is fed directly from the line at constant voltage and frequency,
there is only one torque-speed characteristic (that is, the locus of working points of
the machine) and therefore only one working point is allowed for each load. Thus,
at constant voltage and frequency, torque and speed values at the working point are
imposed by the load and may change only if one of the load parameters varies.
