Environmental Engineering Reference
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illustration of what is meant by 'seat of the electromagnetic force', consider the
superconducting motors and generators now being developed by the American
Superconductor Corporation in ratings of up to 5 MW. In a superconducting M/G,
the rotor contains high temperature superconducting wire (BSCCO-2233, an acro-
nym for the alloy Bi (2 x ) Pb x Sr 2 Ca 2 Cu 3 O 10 high temperature superconductor (HTS)
multifilament wire in a silver matrix that superconducts up to 110 K). The stator is
conventional design of laminated steel with copper conductor coils. Because the
rotor flux is so high, the airgap flux density will typically be in the range 1.7-2.0 T,
negating the need for stator teeth. The resulting coreless design requires non-
magnetic wedges or teeth to provide a restraint for the stator reaction force acting
on the conductors during operation.
L
Magnet
Axis of rotation
N
B
F
X
r
+I
S
-I
T
Figure 5.7 Torque production mechanism in the basic electric machine
The illustration in Figure 5.7 is the basic dc brushed motor in which the con-
ductors carrying currents labelled รพ I and - I would be connected to commutator
segments upon which current carrying brushes would ride. In the process, the
brushes would ensure that current was always injected into the conductor at the top
and extracted from the conductor at the bottom in Figure 5.7. The brushes and
commutator therefore maintain the current in the armature conductors so that the
armature field produced by the current carrying loop is maintained orthogonal to
the magnet produced field.
In Figure 5.7 the magnet field is shown filling the gap between the arc shaped
pole pieces in the north to south direction. The plane of the conductor loop lies
within this flux field so that when current is flowing in the conductors as shown,
this armature winding produces a magnetic field with its north pole oriented to the
left and orthogonal to the magnet field lines. The Lorenz force equation applied to
this geometry results in a force, F , on both conductors that is tangential to the loop
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