Environmental Engineering Reference
In-Depth Information
which
x
is a number greater than 1 are prone to UCG when the rotor speed is above
some threshold that is less than the speed for which
U
oc
ΒΌ
U
dc
, suggesting that
some chaotic behaviour has set in. By chaotic behaviour, it is meant that the IPM
can either generate in the UCG mode a current given by (5.22) or not generate any
current at all should the inverter switches be all gated OFF.
Figure 5.23 is a reconstruction of a figure used in Reference 9 to explain this
effect. During the UCG mode of operation, the inverter active switches are gated
OFF so that only the uncontrolled rectifiers, the switch inverse diodes, are able to
function in a normal Graetz bridge fashion. This means that the IPM stator current
and its terminal voltage are operating at unity power factor the same as, for
example, in the Lundel automotive alternator fitted with a diode bridge. The
exception in the case of IPM, however, is that rather than balanced
d
- and
q
-axis
inductances the IPM has a saliency ratio, and therein lies the difference.
Depending on the IPM speed and loading, the vectors in Figure 5.23 assume
different proportions so that above a threshold speed it is possible to enter into
UCG mode.
jX
q
I
rq
q
-axis
wy
m
jX
s
I
r
jX
d
I
rd
V
s
-
I
rd
d
-axis
I
rq
I
r
Figure 5.23 IPM machine phasor diagram during UCG mode (derived from
Figure 4 in Reference 8)
As can be seen in Figure 5.23, the IPM internal voltage due to magnets can be
lower than the terminal voltage,
V
s
, during UCG mode of operation. This is pos-
sible in this situation because of the large
q
-axis inductance and low
d
-axis
inductance. The stator current through the inverter diodes,
I
r
, is, of course, 180
out
of phase with the terminal voltage during UCG mode, just as it is in a synchronous
alternator.
An important consideration for hybrid propulsion when using the buried
magnet variety of IPM and for which a CPSR of not greater than 4-5 is desired
