Civil Engineering Reference
In-Depth Information
FIGURE 7-8
Pole-changing stator winding for speed ratio 2:1.
consideration in such windings is to limit the space harmonics, which may
decrease the efficiency as the generator, and may also produce a tendency
to crawl when using the machine as the motor during the start up operation.
The coil pitch of the stator winding is fixed once wound, but its electrical
span depends on the number of poles. A coil pitch one-eighth of the circum-
ference provides full-pitch coils for an eight-pole connection, two-thirds for
a six-pole, and one-half for a four-pole connection. Too narrow a coil span
must be avoided. For a 2:1 speed-ratio generator, a possible coil span is
1.33 pole-pitch for the larger and 0.67 for the smaller pole number. In each
case, the coil span factor would be 0.87. Using the spans near 1 and 0.5, with
the span factor of 1.0 and 0.71, one can avoid excessive leakage reactance in
the lower-speed operation.
7.2.3
Variable-Speed Using Gear Drive
The variable-speed operation using variable-gear ratio has been considered
in the past, but have been found to add more problems than the benefits.
Therefore, such drives are not generally used at present.
7.2.4
Variable-Speed Using Power Electronics
The modern variable-speed drive uses power electronics to convert variable
voltage, variable frequency output of the generator into the fixed voltage, fixed
frequency output. The technology is similar to that used in the aircraft power
system. The trend of using such a system is being propelled by the declining
cost of the power semiconductors. Conventional silicon controlled rectifiers
