may be desirable to prevent a possible nuisance trip by
means of a current sensing relay.
The Penn R10A (Johnson Controls) current sensing relay
has been developed for this purpose. It is mounted on the
load side of the contactor, senses by induction the full
operating current of one phase of the motor, closes on a
rise in current above 14 amps, and opens if the load
current falls below 4 amps.
The use of the current sensing relay allows the compressor
to cycle on the internal inherent protector without affecting
the operation of the oil pressure safety control.
Both Diagrams 5A and 5B use a current relay (C.S.). When
the current relay is not energized by motor current, its
Normally Open (N.O.) contact opens the circuit that
powers the Sentronic to avoid a nuisance trip.
Diagram 5B shows the circuit used with the older model
Sentronic. An external control relay, "R", is required to
maintain power to the module in the event of an oil
pressure safety trip since the module requires power to
reset. When the module is tripped on low oil pressure, relay
"R" is not energized, and the relay "R" Normally Closed
(N.C.) contact provides a voltage path to the module.
The circuit of Diagram 5A uses the new Sentronic. The
current relay operates in the same manner as in Diagram
5B, but the oil pressure switch requires no power to reset,
so it needs no external relay to provide a reset power path.
NOTE: On some 550-volt motor-compressors, it may be
necessary to loop the current carrying wire so that it
passes through the current sensing relay twice in order to
increase the metered amperage to close the relay
contacts.
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