MTS Hydraulic Power Unit Model 506-92 - Manual (Page 42)

Theory of Operation 5-5
Stop control When the stop switch at PLC input 3 is pressed, the two pump motors are
shut down after a one second delay. The supercharge pump motor,
however, continues to run for approximately five seconds.
Pressing Emergency Stop turns off the HPU by de-energizing relay CRM.
CRM contacts open to cut off power to the motor starters. The
supercharge pump may continue to run briefly while the main pumps
wind down.
5 . 2 . 2 Interlocks
The HPU contains interlock circuitry to shut down the HPU when
various conditions occur—such as low level hydraulic fluid, over-
temperature, low pressure, dirty filters and motor overload.
Reset When the interlock condition has been corrected and the HPU is restarted,
the Reset switch at PLC input 8 must be pressed to reset the interlock
circuitry.
Low-level hydraulic
fluid Low fluid level switch FS1 protects the HPU if there is an appreciable loss
of hydraulic fluid. Whenever the level of hydraulic fluid in the HPU
reservoir drops below a preset limit, switch FS1 (at PLC input 19) opens to
turn off all pump motors, and disable the supercharge pump. PLC output
100 will go low to signal the low fluid fault through the remote
connector. PLC output 9 lights the Low Oil Level indicator on the HPU
front panel.
Fluid over-
temperature
condition Whenever the temperature of the hydraulic fluid in the HPU reservoir
exceeds a preset limit, switch TAS1 (at PLC input 22) opens to turn off the
HPU pump motors. PLC output 101 will go low to signal the over-
temperature fault through the remote connector. PLC output 8 lights the
Fluid Over-Temperature
indicator on the HPU front panel.
The supercharge pump can be operated to cool the fluid. Refer
Subsection 2.3 for a fluid cooling procedure.
Low-inlet pressure Pressure switch PS1 protects the main pumps if inlet pressure drops below
the minimum level required by the main pumps. Switch PS1 (at PLC input
20) closes whenever low-inlet pressure drops below a preset limit
(typically 30 psi). Opening PS1 while the pump motor is operating, turns
off the HPU.
Optional water
shut-off
solenoid valve On units equipped with this valve, switch TAS2 (at PLC input 23) opens to
energize the water-inlet solenoid SOL4 (at PLC output 7) whenever the
hydraulic fluid exceeds a preset temperature limit. This solenoid opens the
water shut-off valve, letting water flow through the heat exchanger to
cool the hydraulic fluid.
Dirty filter The low-pressure fine filter is monitored by
∆
P switch PS2 (at PLC input
21). When a dirty filter creates excessive differential pressure across the
filter, switch PS2 opens. PLC output 102 generates the dirty filter warning
through the remote connector. PLC output 11 lights the Dirty Filter
indicator on the HPU front panel. If the indicator is active when the HPU
shuts down, Reset must be pressed (after the filter is replaced) to restart the
HPU.