Introduction
The current Copeland® Sentronic® electronic oil pressure
safety control (see Figure 1) replaces two older Sentronics;
the four terminal, triac output style, and the five terminal,
relay output style (see Figure 2). The current Sentronic has
all the features of previous Sentronic, with valuable
additions.
All Sentronics utilize a pressure sensor and an electronic
control module to precisely measure oil pump differential
pressure. The main advantage of Sentronic is the
elimination of the traditional capillary tubes, bellows, and
pressure connections that mechanical pressure switches
require to measure differential oil pressure. These require
careful handling and are known to be a source of leaks in
refrigeration systems.
A second advantage of Sentronic is in the use of a precise
electronic clock for the two-minute time out circuit.
Traditional mechanical controls use resistance heaters to
provide the time to trip in the event of low oil pressure. 208-
volt systems, low ambient temperatures or brown-out type
conditions cause the heater output to be reduced, thus
increasing the time out period from two minutes to three or
four minutes when low oil pressure conditions exist. With
the electronic clock, the time out will always be the same.
As a result of the elimination of the capillary tube
measuring system and a more precise timing circuit,
Sentronic will improve the overall reliability of the
refrigeration system.
Any Copeland® compressor with the new oil pump design
(see AE4-1166) can utilize Sentronic. Any Sentronic can
replace existing capillary tube controls in the field.
As in the past, all new and replacement Copelametic®
compressors equipped with oil pumps require the use of
an Emerson approved oil pressure safety switch. Failure to
use an approved oil pressure safety switch will be
considered as misuse of the compressor and can
adversely affect warranty replacement of the compressor
should a lubrication connected failure occur.
To meet Emerson specifications, an oil pressure safety
switch must meet many requirements. These include
maintaining its pressure setting and time delay calibration
within close limits over the widest variation in operating
conditions to be expected and must successfully pass a life
test with a minimum of 200,000 cycles. Controls must be
of the non-adjustable, manual reset type with a 120 second
nominal time delay at rated voltage, have a cut-out
pressure setting of 7 to 9 PSID (Pounds per Square Inch
Differential, the difference between crankcase pressure
and oil-pump outlet pressure) and a cut-in (timer delay off)
pressure of 12-14 PSID.
Basic Control Operation
The Sentronic Sensor
The same oil pressure sensor is used for all Sentronics. It
mounts directly into the oil pump. The Sentronic sensor
measures oil pump differential pressure. It has an internal
contact that opens on low oil pressure and signals the
Sentronic electronic control module to begin time out. The
same contact closes when proper oil pressure is present
and stops the module time out.
Should oil pressure fall below 7-9 PSID for a period of two
minutes, the Sentronic module will open the control circuit,
using its Normally Closed (N.C.) contact, and shut the
compressor off.
Approximate oil pressure can be measured in the field. Oil
pumps are furnished with a Schrader valve mounted on the
oil pump discharge port. To measure oil pressure, subtract
crankcase pressure from discharge oil pressure.
Tripping of the oil pressure safety switch is a warning that
the system has been without proper lubrication for too long
a time. Repeated trips of the oil pressure safety control are
a clear indication that something in the system requires
immediate remedial action. On a well-designed system
there should be no trips of the oil pressure safety control,
and repeated trips should never be accepted as a normal
part of the system operation.
Caution: An electronic timer may be placed in series with
the compressor contactor to force a delay before each start
and prevent possible short cycling. The timer must be
located so it also prevents the Sentronic from energizing
during the timing period. SOME INEXPENSIVE TIMERS
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