Danfoss 9/96
Design
The operation of the pressure control is
conditional only on the differential pressure, i.e.
the difference in pressure between the two
counteracting bellows, whereas it is independent
of the absolute pressure acting on both bellows.
The MP 55 and 55A can be set for different
differential pressures by the setting disc (3). The
set differential pressure can be read from the
internal scale.
The MP 54 has a fixed differential and has no
pressure setting disc. The factory-set differential
pressure is stamped on the front plate of the
control.
MP 55
Terminology
Differential range
The pressure difference between LP and OIL
connections within which the control can be set
to operate.
Scale reading
The differential between the oil pump pressure
and the pressure in the crankcase that exists at
the moment the contact system cuts in current
to the time relay on falling oil pressure.
Operating range
The pressure range on the LP connection within
which the control can operate.
Contact differential
The pressure rise above the set differential
pressure (scale reading) necessary to cut off
current to the time relay.
Release time
The period for which the differential pressure
control allows the compressor to run with too low
an oil pressure during start-up and operation.
If there is no oil pressure on starting, or if the oil
pressure falls below the set pressure during
operation, the compressor will stop after the
release time has elapsed.
The electrical circuit is divided into two completely
separate circuits, a safety circuit and an
operational circuit.
Function
The timer (e) in the safety circuit is activated
when the effective lubricating oil pressure,
the oil
differential pressure (the difference between the
oil pump pressure and suction pressure), is lower
than the set value.
The timer is deactivated when the oil differential
pressure is more than the set value plus the
contact differential.
1. Connection to pressure side of
lubrication system, OIL
2. Connection to suction side of
refrigeration plant, LP
3. Setting disc
4. Reset buttom
5. Test device
Electrical diagram
Differential pressure controls, type MP 54, 55 and 55A
Catalogue RK.00.H5.02 - Page 314
