Environmental Engineering Reference
In-Depth Information
Asynchronous machines
Design and operating conditions
The
stator
of an asynchronous machine, i.e. the stationary part, is in principle
the same as a synchronous machine: three spatially distributed windings
generate the rotating field in the stator.
However, the
rotor
of an asynchronous machine is totally different from
that of a synchronous machine. The rotor of an asynchronous machine has
neither permanent magnets nor DC windings. There are two different types of
rotor for asynchronous machines: the squirrel-cage, or cage rotor, and the
slipring rotor.
The three-phase windings of a
cage rotor
are made of bars that are inside
slots in the rotor. Short-circuiting rings connect the ends of the bars.
The ends of the windings of a
slipring rotor
are only connected internally
on one side. The beginnings of the windings are connected via sliprings and
graphite brushes to the outside of the machine. There, they can be short-
circuited over rotor resistances. This can improve the behaviour of the
asynchronous machine during start-up.
The most important advantage of the asynchronous machine is its simple
and robust construction since a cage rotor has no sliprings, which could be
mechanically strained.
The synchronous speed
n
S
of the rotating field in the stator can again be
calculated from the mains frequency
f
1
and the pole pair number
p
:
(5.88)
When the asynchronous machine is at a standstill, the rotating field of the
stator passes the standing rotor. This induces a voltage in the conductors of
the rotor windings. Due to this voltage, bar-type currents emerge in the bars of
the winding in the rotor. The magnetic field of these currents creates a
tangential force and the rotor starts to move. In motor mode, the rotor speed
n
is always lower than the synchronous speed
n
S
of the stator since a speed
difference is needed to induce voltages in the rotor. The relatively small
difference between rotor speed
n
and synchronous speed
n
S
is called
slip
:
(5.89)
If an asynchronous machine works as a generator, the rotor moves faster than
the rotating field of the stator. The machine generates active power and feeds
it via the stator to the mains. Table 5.7 summarizes different operating
conditions of an asynchronous machine.
In contrast to the synchronous generator, the asynchronous generator
always needs reactive currents as shown in the next section. An overexcited
synchronous machine or power electronics can generate the necessary reactive
