Environmental Engineering Reference
In-Depth Information
carrying capacity of copper (aluminium for induction machines). Current carrying
capacity of copper wire is limited by its thermal dissipation, which in turn sets
bounds on current density,
J
cu
. In electric machine design practice these bounds are
<
=
;
!
<
=
2
6
20
:
3 min
30 s
Cont
J
cu
¼
ð
4
:
8
Þ
:
:
;
A
=
mm
2
Equation (4.8) contains the thermal constraints of the machine sizing design.
Higher current densities, up to 2
10
8
A/m
2
for copper, define its fusing current limit.
Conductors are placed in slots in the stator iron. The tooth surface to tooth-slot
pitch must maintain sufficient surface area in order to support the magnetic loading
for the materials used and the particular choice of machine technology. Table 4.8
summarizes magnetic loading for the four major classes of electric machines:
Table 4.8 Electric machine sizing: magnetic loading
B
(Wb/m
2
)
Type
Symbol Airgap (mm)
Surface permanent magnet machine
SPM
<
1.5
~0.82
Interior/inset permanent magnet machine
IPM
~1.0
0.7
Asynchronous, induction machine (also syncrel)
IM
~0.6
0.7
Variable/switched reluctance machine
VRM
<
0.5
0.8*
*Highly localized in gap between surfaces of opposing double saliencies.
The electric loading,
A
, for the various machine technologies listed in Table 4.8,
is determined by using the current density limitations (4.8), from which the bounds
on electric loading can be found as follows:
<
=
3
min
6
10
4
3
10
4
3
10
4
Technology
SPM
IPM
IM
=
VRM
30 s
8
10
4
4
:
5
10
4
4
:
5
10
4
A
¼
ð
A
rms
=
m
Þ
ð
4
:
9
Þ
:
;
Notice that the electric loading definitions in (4.9) are in A
rms
/m, not in peak-
amperes. This is well defined for sinusoidal machines, but somewhat limited for
non-sinusoidal machines such as the VRM.
The machine sizing procedure using (4.9), and supported by the definitions of
electric and magnetic loading, permits the first approximation to machine sizing to
be accomplished without resorting to finite element or detailed computer design
since the lamination design has not been fixed at this point, only the major
packaging dimensions. The process of working with electromagnetic surface trac-
tion as just described is akin to having a detailed lamination design, imposing the
electric loading, and then using a magnetics finite element solver to find the flux
