DC servomotors are
high-performance motors normally used as prime movers in computers,
numerically controlled machinery, or other applications where starts
and stops must be made quickly and accurately.
Servomotors have lightweight,
low-inertia armatures that respond quickly to excitation-voltage
changes. In addition, very low armature inductance in these motors
results in a low electrical time constant (typically 0.05 to 1.5 ms)
that further sharpens motor response to command signals.
Servomotors include permanent-magnet,
printed-circuit, and moving-coil (or shell) motors. The rotor of a
shell motor consists of a cylindrical shell of copper or aluminum
wire coils. The wire rotates in a magnetic field in the annular space
between magnetic pole pieces and a stationary iron core.
The field is provided by cast Alnico
magnets whose magnetic axis is radial. The motor may have 2, 4, or 6
poles.
Each of these basic types has its own
characteristics, such as inertia, physical shape, cost, shaft
resonance, shaft configuration, speed, and weight. Although these
motors have similar torque ratings, their physical and electrical
constants vary considerably.
The choice of a motor may be as simple
as fitting one into the space available. However, this is generally
not the case since most servosystems are very complex.
DC Traction Motors. These are dc
series motors typically rated 140 hp, 310 V, 2500 r/min. Four motors
are used in each transit car, two on each axle.
The power supply is 600 to 1000 V dc
from the third rail, which is powered by 2500- to 5000-kW rectifier
sets in rectifier substations located along the track. Starting and
speed control are by either a cam controller or a chopper controller
on board the transit car.
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