Environmental Engineering Reference
In-Depth Information
generators consist of circulating current and eddy current loss originating from
cross-slot leakage flux. A remedy has been to transpose the armature bars along the
length of the stator so that their position changes from top of the slot at one end, to
middle of the slot in the central section of the stator, and on to bottom of the slot at
the opposite end. The typical armature bar transposition is 540
so that end turn
coupling is also minimized. Even with such transposition of a conductor in very
large machines, it is common to still have a 20
C temperature difference between
conductors at the bottom of a slot and those at the top of a slot. The reason is that
cross-slot leakage is higher at the top coils so that higher eddy current losses are
experienced.
8.2
Inverter
Losses in the electronic power processor can be grouped into active component
(semiconductor) losses and passive component losses. Passive components expe-
riencing losses related to power throughput are the link capacitors, device snubbers
if used and current shunts if used. Active device losses are decomposed into con-
duction, switching and reverse recovery losses.
This section gives a brief introduction to inverter losses and some of the tra-
ditional methods used to quantify inverter losses.
8.2.1 Conduction
Conduction loss in a power inverter is due to the power dissipated in the semi-
conductor chip by the simultaneous current and voltage stress. During ON-state
conduction, a majority carrier device such as a MOSFET will experience a voltage
drop that is linearly proportional to the current through the device and the resis-
tance of the device. In the OFF-state the resistance increases by six orders of
magnitude or more. Minority carrier devices, on the other hand, experience con-
ductivity modulation during the ON-state and have a voltage drop across the device
terminals, which is a logarithmic function of the current through the device. IGBTs
are representative of minority carrier devices as are diodes, bipolar transistors and
thyristors.
The simplest device, the bipolar diode, has a voltage-current characteristic
given by the ideal diode (8.7), where
k
= 1.38
10
23
J/K (i.e. the Boltzmann
constant),
q
= 1.602
10
19
coulomb is the electronic charge,
K
= 298 is the
nominal temperature in Kelvin, and
I
0
is the diode saturation current ~10
14
A. At
room temperature the diode voltage coefficient is 0.026 and at a forward current of
10 A the diode voltage is 0.9 V. Power diodes at higher currents will have a dif-
ferent value of saturation current:
KT
q
I
I
0
V
D
¼
ln
ð
V
Þ
ð
8
:
7
Þ
