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 1.2.2 Multiple Winding Motors. The advantages of the multiple winding method
are that it is relatively simple and inexpensive. The disadvantage is that
the number of different available speeds is limited. The multiple winding
method is therefore unsuitable for applications that require a continuously
varying speed of rotation.
1.2.3 Variable Diameter Pulleys. A typical variable diameter pulley has
conical faces that can be adjusted so that the distance between the faces is
variable. If the drive belt for the pulley has edges that are tapered to
match the shape of the pulley faces, the belt can be made to contact the
pulley anywhere along the pulley faces, depending on the distance between the
pulley faces. This effectively changes the pulley diameter. At larger pulley
diameters, the shaft driven by the pulley rotates more slowly. At smaller
pulley diameters,the shaft driven by the pulley rotates more quickly. The
advantages of the variable diameter method are that it is relatively simple
and it is comparatively inexpensive. The disadvantages are that its use is
limited to applications requiring approximately 30 hp or less; the pulleys
require careful alignment; and the belts are sensitive to small misalignments
and can be damaged easily.
1.2.4 Geared Transmissions. A geared transmission can be used to vary the
speed of rotation of the drive shaft in discrete steps. The advantages of
geared transmissions are that they are relatively reliable and can be used for
higher power applications. The disadvantages are that they are expensive,
relatively inefficient, and difficult to install.
1.2.5 Fluid Couplings. Fluid couplings, or torque converters, can be used to
provide a continuously variable output to the shaft of a centrifugal pump.
The advantages of this type of pump are that fluid couplings are simpler than
geared transmissions,are easier to install, and are continuously variable.
The principal disadvantage of these pumps is that they are comparatively
inefficient.
which the intensity of the magnetic field, which couples the two major
rotating parts of the clutch, can be varied. An increase in field intensity
increases the amount of magnetic coupling, with a consequent increase in
torque. The advantages of the eddy current coupler are that it is relatively
simple and reliable. Its principal disadvantage is that it is inefficient.
The slippage developed in the clutch causes a loss of energy that is
dissipated in the form of heat. Eddy current couplers and other devices that
allow the motor to run at full speed but at reduced torque, have an adverse
effect on the power factor of the motor.
1.2.7 Synchronous Inverters. Synchronous inverters are electronic devices
that vary the input power frequency to the motor. Advantages of the
synchronous inverter as a means to change motor rpm are numerous. Typical
output frequencies of a synchronous inverter range from 0 to 90 hertz, which
produces a wide and continuous range of motor speeds. The synchronous
inverter may be mounted in a remote location not requiring mechanical retrofit
changes. Because the synchronous inverter is not a mechanical device,
maintenance is minimal.
5-14
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