Environmental Engineering Reference
In-Depth Information
4.3.3 Further Details on Transformers
The manufacturer of an electrical machine such as a transformer will indicate on the name-
plate the normal operating conditions, e.g.: ' 11 000 : 415 V, 50 Hz, 500 kVA ' . The
rated output
of 500 kVA can be maintained continuously without excessive heating and the consequential
deterioration of the winding insulation. Because the heating is dependent on the square of
the current, the output is rated in apparent power (kVA) rather than active power (kW). When
supplying a zero power factor load, a transformer can be operating at rated temperature while
delivering zero active power.
On large transformers, taps on the windings allow small adjustments on the turns ratio.
Often these taps are operated by an automatic tap changer that maintains the voltage, usually
on the secondary, at a fi xed value irrespective of the load on the transformer.
It can be shown [1] that the size and therefore, weight and cost of a transformer are inti-
mately related to the frequency of operation. The higher the frequency the lower are the
weight and cost. For these reasons, in power electronic systems whenever a transformer is
to be used, higher frequencies are employed, a topic to be revisited later.
4.4 The Asynchronous Generator
4.4.1 Construction and Properties
Asynchronous
or
induction machines
operating as motors are the most widely used electro-
mechanical converters. In an induction machine the stator is identical to the one for synchro-
nous machines shown in Figure 4.3 in which three-phase currents supplied to the stator
produce a rotating magnetic fi eld (RMF). The rotor, however, is radically different and it has
neither an external magnetizing source nor permanent magnets. Instead, alternating currents
are injected in the rotor from the stator through induction or transformer action - hence the
useful parallel with the operation of a transformer. It is the interaction between these induced
rotor currents and the stator RMF that results in torque production.
In its most common form, the rotor consists of axial conductors shorted at the ends by
circular rings to form a
squirrel - cage
or just
cage
, as shown in Figure 4.12. Although for
the purposes of renewable energy sources there is interest in the generation mode, it is
easier initially to understand the operation of the induction machine from the motoring
perspective.
As the stator RMF moves at
s
(given by Equation (4.4)) past the stationary rotor conduc-
tors, three-phase electromotive forces (EMFs) are induced in the spatially shifted rotor con-
ductors by a fl ux cutting action. The resulting rotor currents, according to Lenz's law [1], are
of such magnitude and direction as to generate a torque that speeds up the rotor. If the rotor
were to achieve speed
ω
s
, there would be no change in fl ux linkage, no induced voltage, no
current in the rotor conductors and therefore no torque. For EMFs to be induced in the rotor
conductors they should possess some relative speed with respect to the stator RMF. For
motoring, the rotor therefore turns at a lower speed
ω
r
.
It can be shown that the rotor currents produce an RMF whose speed depends on the fre-
quency of these currents. For a constant torque interaction to take place, the rotor RMF must
rotate in synchronism with the stator RMF, as in the case of the synchronous machine. How
is this accomplished if the rotor rotates at a lower speed than
ω
ω
s
?
