Carrier 38TN Service Manual page 6

Step 3—Capacitor
Capacitors can store electrical energy when power is off.
Electrical shock can result if you touch the capacitor termi-
nals and discharge the 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.
NOTE: If bleed resistor is wired across start capacitor, it must be
disconnected to avoid erroneous readings when ohmmeter is
applied across capacitor. (See Fig. 6.)
Fig. 6—Capacitor
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.
Capacitors are used as a phase-shifting device to aid in starting
certain single-phase motors. Check capacitors as follows.
1. After power is off, discharge capacitors as outlined above.
Disconnect capacitor from circuit. Put ohmmeter on R X 10k
scale. Using ohmmeter, check each terminal to ground (use
capacitor case). Discard any capacitor which measures 1/2
scale deflection or less. 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. Failure of
meter to do this indicates an open capacitor. If resistance stays
at 0 or a low value, capacitor is internally shorted.
2. Capacitance testers are available which will read value of
capacitor. If value is not within ± 10 percent value stated on
capacitor, it should be replaced. If capacitor is not open or
shorted, the capacitance value is calculated by measuring
voltage across capacitor and current it draws.
Exercise extreme caution when taking readings while power
is on. Electrical shock can cause personal injury or death.
Use following formula to calculate capacitance:
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.
Step 4—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 compres-
sor motor. This device is called a positive temperature coefficient
(PTC) or thermistor. (See Fig. 7.) It is a resistor wired in parallel
with the run capacitor. As current flows through the PTC at
start-up, it heats up. As PTC heats up, its resistance increases
greatly until it effectively lowers the current through itself to an
extremely low value. This, in effect, removes the PTC from the
circuit.
After system shutdown, 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 fully cool and becomes less effective as a
start device. It is an easy device to troubleshoot. Shut off all power
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to system.
Check thermistor with ohmmeter as described below. 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 as shown in Fig. 7.
The cold resistance (RT) of any PTC device should be approxi-
mately 100-180 percent of device ohm rating.
12.5-ohm PTC = 12.5-22.5 ohm resistance - beige color
If PTC resistance is appreciably less than rating or more than 200
percent higher than rating, device is defective.
Use of start capacitor and relay IS NOT RECOMMENDED on
units equipped with a Millennium scroll compressor.
Step 5—Cycle Protector
Solid-state cycle protector protects unit compressor by preventing
short cycling. After a system shutdown, cycle protector provides
for a 5 ± 2-minute delay before compressor restarts. On normal
start-up, a 5-minute delay occurs before thermostat closes. After
thermostat closes, cycle protector device provides a 3-sec delay.
(See Fig. 8, 9, and 10.)
Cycle protector is simple to troubleshoot. Only a voltmeter capable
of reading 24v is needed. Device is in control circuit, therefore,
troubleshooting is safe with control power (24v) on and high-
voltage power off.
With high-voltage power off, attach voltmeter leads across T1 and
T3, and set thermostat so that Y terminal is energized. Make sure
all protective devices in series with Y terminal are closed.
Voltmeter should read 24v across T1 and T3. With 24v still
applied, move voltmeter lead from T1 terminal to T2 terminal
across T2 and T3. After 5 ± 2 minutes, voltmeter should read 24v,
indicating control is functioning normally. If no time delay is
encountered or device never times out, change control.
6
12.5-22.5 OHMS
12.5 OHM
(BEIGE COLOR)
Fig. 7—PTC Device
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