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 FIGURE 5-1. Automatic Start-Stop Governor
pressure rises to a preset level, the pressure switch contacts open,
deenergizing the starter and shutting down the motor. When receiver pressure
falls to another preset value, the pressure switch contacts close, starting
the driving motor. There are several methods of obtaining variable speed
control of electric motors.
1.2.1 Belt and Pulley Arrangement. The variable speed drive unit consists of
a constant speed motor and a belt and pulley arrangement to vary the output
speed. Speed change is accomplished by varying the distance between two disks
that form each pulley. Turning the speed control dial moves one of the disks
on the motor shaft toward its companion disk which forms a belt pulley. This
causes the belt to climb up on the tapered disks to a larger diameter.
Simultaneously, since the belt is of fixed length, the belt causes the two
disks of the driven pulley to separate and permit the belt to assume a smaller
diameter. This change of pulley diameters results in increased speed of the
driven shaft while the motor speed remains constant. Reverse movement of the
control dial results in a decrease in the speed of the driven shaft. The use
of this arrangement is limited to drivers of 30 hp or less.
1.2.2 Hydraulic or Magnetic Couplings. Although not drivers, these types of
couplings are sometimes a part of the driving mechanism and provide variable
speed output from a constant speed driver. Hydraulic or magnetic couplings
are also used to vary the speed of compressors driven by squirrel cage
induction or synchronous motors.
1.3 Steam Turbine Drives. Commonly used with centrifugal or axial
compressors, steam turbines can be fitted with variable speed governors which
allow the turbine speed to be set at any point within its operating speed
5-2
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