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Copeland 21-1222
22-1230
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22-1230
Application Engineering Bulletin
AE-1230-R5
Revised October 1, 1984
OFF CYCLE MOTOR HEAT FOR LIQUID
REFRIGERANT MIGRATION CONTROL
The use of off cycle motor heat in lieu of a crankcase
heater is being considered more frequently by
users of single phase welded compressors, since it
does offer some economies and advantages. Off
cycle heat essentially performs the same function as
a crankcase heater. Either can offer some significant
advantages in system design.
It is almost impossible to entirely avoid some start
up buzz on any unit where liquid can migrate to the
compressor during a long off cycle. Maintaining a
warm compressor can eliminate noise problems of
this type.
Undoubtedly the life expectancy of any compressor
is affected by the number of times it is exposed
to large amounts of liquid refrigerant on start up.
The compressor may be able to clear the crankcase
without damage under repeated test conditions, but
certainly repeated stress of this nature can eventually
reach the critical stage. Although it is difficult to
obtain detailed long term experiences data, field
experience strongly indicates that over an extended
life, some type of crankcase heat can be a major
factor in compressor survival on all split systems.
Basically, off cycle heating is accomplished by
connecting the run capacitor used for the motor heat
cycle directly to the line beyond the unit disconnect
rather than to the run terminal. Thus when the unit is
cycled off by the thermostat, the start winding remains
connected across the line through the run
capacitor.
A schematic diagram of an off cycle heat connection
is shown in Figure 1. Note that the run capacitor
has been split. Assume that the standard run
capacitor is 35 MFD. A 15 MFD capacitor is connected
in the normal fashion while a 20 MFD run
capacitor is connected for off cycle heat. Since the
capacitors are in parallel in a normal operating condition,
the compressor then sees the normal 35
MFD.
FIGURE 1
TYPICAL SCHEMATIC CONNECTION
FOR OFF CYCLE HEAT
Standard run capacitor 35 MFD
Heating run capacitor 20 MFD
© 1972 Copeland Corporation

Copeland 21-1222 22-1230
The control may be contactor or any other device
that breaks the line current when the compressor
is not operating.
The same sort of electrical connection may be
accomplished by means of a split run capacitor,
which in effect is actually two capacitors in a single
casing.
Off cycle heat has a number of advantages and
some disadvantages.
1) It is not as susceptible to damage as an external
crankcase heater and there is no heater element to
burn out. the life expectancy of external crankcase
heaters is very probably not as good as we would
like to see.
2) Since it is economical and compact, off cycle
heat may be attractive on small package units.
3) One of the side benefits of off cycle heat may be
better low voltage starting characteristics. A warm
stator and rotor develops more starting torque than
a cold motor, and this can materially improve the
compressors ability to start under marginal conditions.
4) Since the off cycle heat circuit is mad through
the motor protector, in the event of a protector trip,
no heat is added until the protector resets. External
crankcase heaters are frequently connected across
the line so that they are energized continuously. On
split systems where the compressor is located in an
insulated compartment to reduce noise to a
minimum, the reset time after a protector trip may
be excessive unless the motor temperature is reduced
fairly quickly. Off cycle heat would have a
definite advantage over a crankcase heater in this
type of situation.
A major disadvantage is that the compressor is
electrically energized at all times, even when the
unit is off. Maintenance personnel or users may run
the risk of a shock if attempting repairs without
being aware of the hazard.
There are at least three limiting factors that must
be considered in the application of off cycle heat.
1. Start winding temperatures during the off cycle
heating phase must be held within safe limitations.
2. Since there is no refrigerant vapor flowing to cool
the motor or protector during the off cycle, the protector
is sensing a different condition than that encountered
during either an operating cycle or a
locked rotor condition. Unless the capacitance is
limited, it is possible that the protector may trip on
start up due to temperature, even though the motor
windings are still within safe limits.
3. If start components are used, or if there is any
possibility that supplementary start components
may be applied in the field, the capacitance used for
off cycle heat must be limited to avoid impressing
excessive voltage on the start capacitor. Most manufacturers
limit continuous voltage on the start
capacitor to 10% of its nominal rating, with the exception
of General Electric who has approved a
continuous voltage of 20% of their nominal rating.
In order to avoid the possibility of a dangerous
electrical shock to maintenance personnel even
when disconnected from power, a 220,000 ohm
bleed resister must be mounted across the run
capacitor used for off cycle heat.
In the event of a short in the heating run capacitor,
the motor protector could not prevent a start winding
burn in the motor, and a fuse of the proper size
must be mounted in the run capacitor for motor protection,
or a fused run capacitor must be used. The
necessary fuse size may be calculated as
follows:
l = C(E 1.2) divided by 2650
l = Fuse size in amps
(use next larger standard size)
C = Capacitance in MFD
E = Rated voltage of capacitor
In the case of the example in Figure 1, assume the
standard capacitor voltage rating was 370 volts.
l = 20 (370 x 1.2) = 3.35 amps Use 4 amp fuse
2650
Copeland has approved the use of off cycle heat
by several manufacturers on an individual basis.
Laboratory test have indicated that off cycle heat
properly applied can effectively raise the oil temperature
without creating dangerous temperatures in
the motor windings, and we have not encountered
any field problems traceable to this type of application.
2

Obviously, electrical connections are simpler if
the standard run capacitor can be used for off cycle
heat, without the necessity for splitting the
capacitor. Extensive tests have been run in the Copeland
Application Engineering Laboratory to establish
acceptable limits for the application of heating
run capacitors.
Copeland 21-1222
22-1230
A minimum temperature difference of 15°F between
ambient temperature and oil temperature was
established as the criteria as to whether off cycle
heat could be effectively applied. On some smaller
compressors operating on 115 volt power, insufficient
heat was generated to make off cycle heating
practical.
In order to prevent the possibility of nuisances trips
on start up in high ambient conditions, tests were
run at high ambient conditions to determine
stabilized compressor temperatures with various
sized run capacitors. Capacitor selection has been
limited to provide a maximum of 50°F temperature
difference between the compressor shell temperature
and ambient temperature. In general it was
found that power input should be limited depending
on the size of the compressor, with acceptable values
varying from 25 to 75 watts.
For those customers applications where tightly
enclosed compressor compartments can result in
temperatures surrounding the compressor remaining
above 110°F for extended periods, it is essential
that the manufacturer run adequate development
tests to insure that a nuisance protector trip does not
occur on start-up after long off periods if a
maximum size run capacitor is used for off cycle
heat.
If the fan motor is wired across the line and controlled
by the same single pole contactor that controls
current flow to the compressor motor, a trip of
FIGURE 2
the motor protector can create an unexpected circuit
that may allow sufficient current to flow to keep the
fan motor operating. The current path is shown by
the dotted line in Figure 2, through the fan motor,
through both motor windings, through the off cycle
heat run capacitor, and back to the line beyond the
contactor contacts.
It is doubtful if the current flow under such conditions
is sufficient to harm the compressor motor
windings, but it is possible the fan motor windings
might be adversely affected, and the current flowing
through the compressor motor could delay the reset
of the motor protector. The only solution to this
problem that has been found is the use of a two
pole contactor to break the fan motor circuit as well
as the compressor motor circuit.
3

Copeland 21-1222 22-1230
Following is a listing of Copelaweld compressors
with recommendation for capacitor sizing for off
cycle heat. The compressors indicated have been
listed with U.L. for off cycle heat application, the U.L.
listing covering the standard run capacitor as a
maximum.
Std. Run
Capacitor
MFD
Maximum Run Capacitor For
Off Cycle Motor Heat
Model
CRA1-0150-PFV 25 25
CRB1-0175-PFV 25 25
CRC1-0175-PFV 30 30
CRD1-0200-PFV 35 35
CRE1-0225-PFV 35 20
CRG1-0250-PFJ 35 25
CRH1-0275-PFV 40 25
CRJ1-0300-PFV 35 35
CRK1-0325-PFV 40 25
CRL1-0350-PFV 40 25
CRN1-0500-PFV 55 40
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Copeland Corporation
Sidney, OH 45365-0669
Printed in U.S.A.