PROTECTIVE CONTROLS
Each McQuay centrifugal is equipped with a complement of
safety controls to prevent the compressor from starting under
adverse conditions or to take the machine off-line when ab-
normal or unsafe conditions develop during operation.
In addition, there are a number of relay type devices which
operate to assure that the starting, operating and stopping
functions are carried out in proper sequence for the protec-
tion of the equipment. These relays can be identified on the
unit control diagram as
R1
R2,. etc.
The control panel is equipped with several indicator lights
to provide the operator with a quick status condition when
the compressor is shut down. With the reference to Figure
3 it will be noted that there are eight safety signal lights to
describe some condition which has caused the compressor
to be shut down. In a fault condition only one light can be
energized at one time.
IGURE
3.CONTROL
PANELLIGHTS
Most of these indicator lights are operated when some ab-
normal condition exists with the chiller unit. Conditions such
as low suction pressure, high discharge pressure, low oil
pressure or temperature, high motor winding temperature and
surge condition may cause some damage to the equipment
if allowed to continue uncorrected.
The troubleshooting guide in Figure 13, page 20, gives the
operator a comprehensive list of fault conditions and the pro-
bable causes. This guide should be consulted whenever the
compressor is shutdown by a protective control.
Each unit is equipped with an “External Failure” light on
the control panel. When energized, this signal indicates the
compressor cannot run due to a control problem external to
the control panel. Faults in the condenser water/chilled water
circuits or in the compressor starter could trigger such an ex-
ternal failure signal. This feature helps the operator locate
control system problems. Causes and corrective actions are
further described in the troubleshooting guide.
The two most severe failures which can occur in a hermetic
centrifugal compressor are surge conditions and motor
failure. McQuay has developed highly reliable protective
systems to guard against both of these failures. An understan-
ding of each will be helpful to the operator in analyzing the
unit performance. Both Guardistor and Surgegard are safe-
ty protective systems developed by McQuay.
Guardistor Motor Protection-Positive protection against
motor overheating is provided by the Guardistor system. The
heart of the Guardistor protective circuit are the thermistors
embedded in the motor windings to sense motor winding
temperature. When the motor temperatures are normal, the
thermistors have low resistance which remains nearly cons-
tant up to a predetermined critical temperature. At this
temperature, a sharp increase in resistance causes the
Guar-
distor relay to drop out and cause the control circuit to stop
the compressor. Such failure causes a lockout mode requir-
ing manual reset.
Surgegard-McQuay
uses Surgegard to sense the occur-
rence of surge condition and to stop the compressor before
the machine is damaged. The Surgegard relay will also pre-
vent the compressor from restarting until the cause of the
malfunction has been corrected. Possible causes for surge,
or rotating stall condition, may be dirty condenser tubes or
cooling tower or pump malfunction, which acts to elevate the
system’s head. The Surgegard relay is the safety control for
the PEH/PHH 063 and larger chillers. In the
PEH/PHH050
the high temperature thermostat provides this safety func-
tion. These controls are factory set and require no field
adjustment.
SAFETY CONTROL RESET
Certain safety controls in the
PEH/PHH
control panel will lock
out automatically when operating conditions exceed trip set-
tings. As an example, if the refrigerant discharge pressure
leaving the compressor exceeds the trip setting of the high
pressure cutout switch, the compressor will be taken off-line
and cannot be restarted until the pressure has returned to
normal and the high pressure cutout has been manually reset.
In the standard control center several other safety func-
tions also lock out under abnormal operating conditions.
These controls include the low pressure cutout, high
discharge temperature thermostat, high oil temperature
switch, low oil pressure, high suction temperature
(PEH/PHH050 only), Surgegard
(PEH/PHH063-126)
and ex-
ternal system failure.
Each of these fault signals can be identified by the control
panel lights and all can be reset with the single RESET but-
ton on the front of the control panel when the fault has been
corrected.
In the event of electrical system power interruption, the
standard
PEH/PHH units will lock out and will not restart
automatically. The EXT. FAIL light will go on when power is
restored and the RESET button must be manually reset to
restore automatic chiller operation.
IM 307 I Page 7
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