Carrier 38E - Manual (Page 6)

CAPACITORS — (See Fig. 5.)
A CAUTION
Capacitors can store electrical energy when power
is off. Electrical shock can result if you touch the
capacitor terminals and discharge this stored energy.
Exercise extreme caution when working near
capacitors. With power off, discharge stored energy
by shorting across the capacitor terminals with a
15,000-ohm, 2-watt resistor, or a screwdriver blade
with insulated handle.
RUN CAPACITOR START CAPACITOR
Fig. 5 — Capacitors
Capacitors are used as a phase shifting device to aid in
starting certain single-phase motors. Check capacitors
as follows:
1. Always check capacitors with power off. Attempting
to troubleshoot a capacitor with power on can be
dangerous. Defective capacitors may explode when
power is applied. Insulating fluid inside is combustible
and may ignite, causing burns. After power is off,
discharge capacitors as outlined above. Disconnect
capacitor from circuit. Use ohmmeter, check each
terminal to ground (use capacitor case). Discard any
capacitor that shows resistance. Place ohmmeter leads
across capacitor and place on R x 10k scale. Meter
should jump to a low resistance value and slowly climb
to higher value. Eailure of meter to do this indicates
an open capacitor. If resistance stays at zero or a low
value, capacitor is shorted.
2. Capacitance testers are available which will read value
of capacitor. If value is not within ± 10% value stated
on capacitor, it should be changed. If capacitor is
not open or shorted, its capacitance value is calcu­
lated by measuring voltage across capacitor and
current it draws.
A WARNING
Exercise extreme caution when taking readings
while power is on. Use following formula to
calculate capacitance:
^ . , r , . 2650
X
amps
Capacitance (mfd) =
volts
3. Remove any capacitor that shows signs of bulging,
dents or leaking. Do not apply power to a defective
capacitor as it may explode.
Hard-Start Capacitors and PTC Devices — Sometimes,
under adverse conditions, a standard run capacitor in a
system is inadequate to start compressor. In these
instances, a start-assist device is used to provide an extra
starting boost to compressor motor. The first device is
called a PTC (positive temperature coefficient) or ther­
mistor (see Fig. 6). It is a resistor wired in parallel with run
capacitor. As current flows through it at start-up, it heats
up. As it heats up, its resistance increases greatly, until
it effectively lowers current through it to an extremely
low value. This, in effect, removes it from the circuit.
After system shuts down, resistor cools and resistance
value returns to normal, until next time system starts.
Thermistor device is adequate for most conditions,
however, in systems where off cycle is short, device
cannot cool fully and becomes less effective as a start
device. It is an easy device to troubleshoot. Turn off all
power to system.
Check thermistor with ohmmeter as described below.
If indoor coil does not have a bleed-type expansion
device, it may be necessary to remove start thermistor
and replace with accessory start capacitor and relay.
Shut off all power to unit. Remove PTC from unit.
Wait at least 10 minutes for PTC to cool to ambient
temperature.
Measure resistance of PTC with ohmmeter. Resistance
of 25-ohm PTC is measured between center tab and
end tab with jumper across 2 end terminals.
Fig. 6 — PTC Devices
The cold resistance (Rj) of any PTC device should be
approximately 100 - 180% of device ohm rating.
50-ohm PTC = 50 - 90 ohm resistance
25-ohm PTC = 25 - 45 ohm resistance
If PTC resistance is appreciably lower or more than
200% higher than rating, device is defective.
If thermistor is good and compressor does not start,
disconnect thermistor from starting circuit. Give com­
pressor a temporary capacitance boost. Run compressor
for 10 minutes, shut off, allow system pressure to equal­
ize. Reconnect start thermistor. Try restarting com­
pressor without boost capacitor. If after 2 attempts,
compressor does not start, remove thermistor. Add an
accessory start capacitor relay package.
Temporary Capacitance Boost — (See Fig. 7.) There are
times when a temporary capacitance boost is needed to
get compressor started. Do not under any circumstances
attach temporary boost capacitor directly across com­
pressor terminals. Serious personal injury can result.
Exercise extreme caution with this procedure when high-
voltage power is on. If compressor motor does not start,
it may be due to low-line voltage, improper pressure
equalization or weak run capacitor. Check each possi­
bility, attempt capacitance boosting before adding
auxiliary start capacitor and relay.