Environmental Engineering Reference
In-Depth Information
Figure 5.58 Schematic of the SyncRel machine
ratio is ~0.94 and the denominator ratio is typically 0.91 for an IM. This results in
the expression in (5.37) equalling 0.904. In other words, it would appear that the
SyncRel machine is only capable of 90% the torque of the IM for identical stator
currents; but there is more.
Notice by inspection of the rotor construction illustrated in Figure 5.58 that a
SyncRel machine has no rotor losses. In the SyncRel machine, the rotor is
constructed of axial laminations (rain-gutter geometry) that lie in the direction of
d
-axis flux. The
q
-axis inductance is very low because of the large saliency and also
because
q
-axis flux must cross the many lamination to lamination insulation layers.
The IM rotor losses are
3
2
i
r
R
r
P
r
¼
(W)
ð
5
:
38
Þ
This means that unlike the IM, the SyncRel has no rotor losses to contend with,
so only stator copper losses must be accounted for. The IM stator current consists of
both magnetizing and load current components, for which magnetizing current is
approximately 15% of the total. Taking this into account results in the SyncRel
machine having only 63% the losses of the IM. So now, if the ratio in (5.37) is
again computed but for equal losses, the result is
T
r
T
i
¼
1
:
26
ð
5
:
39
Þ
This is a completely different picture of the SyncRel in comparison to the IM,
but it does not account for the fact yet that inverter losses in the SyncRel are higher
that for an IM because the power factor is lower in the SyncRel. Typical power
factors for IMs are 0.85, whereas for the SyncRel it is 0.80.
The advantage of the SyncRel machine over the IM diminishes as rating
increases. For larger machines, generally
>
20 hp, the magnetizing reactance of the
