There are other features incorporated in the
motor-driven variable displacement pump that you
should know about. A thermal protector manual reset
button is installed on the end of the motor, which is
concealed by a cover plate. See figure 7-21. This
thermal protector is a safety device that protects the
motor from overheating. The reset button will open
and stop the motor when the temperature exceeds
380° ±10°F. If the motor does not restart after
cooling, the cover plate over the reset button should
be removed and the reset button reset manually. If the
motor still fails to start, the motor pump assembly
should be replaced.
The motor-driven variable displacement pump
suction line is connected from the reservoir to the
suction port of the pump assembly, where fluid is
ported into the center of a centrifugal pump scroll.
The scroll is located between the main pump case and
the motor reduction gearbox of the pump assembly.
See figure 7-21.
The scroll houses a centrifugal
booster pump, which is mounted directly on the main
pump shaft. The constant-speed motor turns the
pump shaft through reduction gears at 3,200 rpm,
which is sufficient to boost the fluid pressure about 15
to 20 psi above the existing reservoir pressure. The
output of this integral pump is directed to two points
on opposite sides of the scroll housing.
One delivery point provides a constant flow of
hydraulic fluid for motor cooling through an internal
Figure 7-21.Motor-driven variable displacement piston pump.