Bacharach QV2 Manuel d'utilisateur télécharger pdf (Page 1)

that the oil failure switch was the culprit.

After relieving the liquid refrigerant

from the crankcase you checked the oil

level. It was a little on the low side at 1/4

of a glass, but within the manufacturer’s

recommended limits. You carefully

restarted the compressor and proceeded

to check the net oil pressure. It was

measured to be 26 psi; again well within

the manufacturer’s speciﬁcation.

Being a thorough technician, you

went the extra mile and veriﬁed the cor-

rect operation of the oil failure control.

You found the control’s switch opened

45 seconds after the net oil pressure had

fallen below 6.5 psi; perfectly within the

specification for the make/model of the

compressor. No visible cause was identi-

fied. This is a nuisance oil failure trip.

How inconvenient.

Let’s be clear though. The only nui-

sance regarding this scenario is that a

reason for the oil failure trip cannot be

easily determined. To imply that an oil

failure trip happens intermittently for no

reason implies that oil failure controls

have a mind of their own, and periodi-

cally decide to shut off a compressor

simply to add spice to the technician’s

life. Nothing could be further from the

truth. Unless an oil failure control is

defective, the only time it will trip is

when the compressor’s net oil pressure

falls below the control’s cut-in point for

the specified amount of time necessary

for the control to lockout.

REAL OIL PRESSURE

Understanding component operation

always makes troubleshooting system

problems easier; but first a brief explana-

tion of the term “net oil pressure” is

required. The oil pump inlet is fed

through a screen located in the compres-

sor sump, so crankcase pressure would be

the same as oil pump inlet pressure. The

pump takes this oil at crankcase pressure

and increases its pressure to some value

above crankcase pressure. Unless the

crankcase pressure is 0 psi, the oil pump

outlet pressure does not tell the complete

story. The real oil pressure, or “net oil

pressure,” is the difference between

pump inlet pressure (crankcase pressure)

and pump outlet pressure (discharge

pressure).

A brief review of how oil failure con-

trols operate is in order. Unlike the typi-

cal low, high, or dual pressure control,

which has a single switch

in it, the oil failure control

requires two separate cir-

cuits to cause a lockout

after experiencing low net

oil pressure for a pre-

determined time (see

Figure 1).

The control circuit

switch

SC1 is a time delay switch

which is wired in series (at

terminals L and M) with

the other required controls

in the holding coil circuit

for the compressor motor

starter. It is considered the

slave in this arrangement,

and only opens when acted

upon by the time delay heater circuit. In

most applications SC1 would be a manu-

al reset switch. However; if a start-stop sta-

tion were used, an oil failure control with

an automatic reset switch could be used.

With this configuration an oil failure

control trip would cause the start-stop

THE GOODS ON OIL FAILURE TRIPS

refrigeration

BY DAVE DEMMA

Do you remember that service call you were dispatched to where the frozen food was melting

in the display cases? Upon arrival you found the compressor off; apparently for quite some

time as its body was stone cold. A quick check of the control circuit with a voltmeter revealed

station to open, locking out the compres-

sor starter’s holding coil circuit. While

this provides foolproof lockout from low

oil pressure, oil failures with this control

arrangement are more difficult to trou-

bleshoot. An automatic reset oil failure

control that trips the start-stop station

when low oil pressure occurs does not

leave any evidence that low oil pressure

was the problem for compressor lock-out.

The time delay heater circuit

This circuit requires a 208/240 volt sup-

ply to be applied between terminals V1

and 2 (or 120V between terminals V and

2). P1 is a normally closed differential

pressure switch which is connected to the

compressor crankcase and the oil pump

discharge. It measures net oil pressure.

P1 opens when the net oil pressure is

above the cut-in point for the control,

however the cut-in point will vary

depending on the compressor manufac-

turer’s specification. Some controls allow

for adjustment to the cut-in point, and

others are preset. H is a resistance heater;

the device which trips switch SC1 when

CONTINUED ON PAGE 90

86 HPAC • 80 years MARCH 2006

FIGURE 1

OIL FAILURE CONTROL

HPAC02_086,090,092 3/1/06 10:45 AM Page 86

1 2 3 4 5 6 7 8

Résumé du contenu

Page 1 - 80 years MARCH 2006

that the oil failure switch was the culprit.After relieving the liquid refrigerantfrom the crankcase you checked the oillevel. It was a little on the

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Bacharach QV2 Manuel d'utilisateur Page 1

Naviguer en ligne ou télécharger Manuel d'utilisateur pour Pompes à eau Bacharach QV2. refrigeration BY DAVE DEMMA Manuel d'utilisatio

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Bacharach QV2 Manuel d'utilisateur Page 1

Naviguer en ligne ou télécharger Manuel d'utilisateur pour Pompes à eau Bacharach QV2. refrigeration BY DAVE DEMMA Manuel d'utilisatio

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