Capacitors - Carrier R-22 Application Manual And Service Manual

Step 3—Capacitors
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. 12.)
Fig. 12—Capacitors
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 zero or a low value, capacitor is internally shorted.
2. Capacitance testers are available which read value of capaci-
tor. If value is not within ± 10 percent value stated on
capacitor, it should be changed. 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 the following formula to calculate capacitance:
Capacitance (mfd) = (2650 X amps) divided by (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.
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 positive-temperature
coefficient (PTC) or thermistor. (See Fig. 13.) It is a resistor wired
in parallel with the run capacitor. As current flows through the
PTC at start-up, it heats up. As it heats up, its resistance increases
greatly until it effectively lowers the current through it to an
extremely low value. This, in effect, removes it from the circuit.
20-36 OHMS
BLUE
20 OHM
(BLUE COLOR)
A91455
After system shutdown, resistor cools and resistance value returns
to normal until next time system starts. 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.
Consult pre-sale literature for application of start kits. 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.
1. Shut off all power to system.
2. Check thermistor with ohmmeter as described below.
3. Shut off all power to unit.
4. Remove PTC from unit. Wait at least 10 minutes for PTC to
cool to ambient temperature.
5. Measure resistance of PTC with ohmmeter as shown in Fig.13.
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
25–ohm PTC = 25–45 ohm resistance — blue color
20–ohm PTC = 20–36 ohm resistance — blue color
If PTC resistance is appreciably less than rating or more than 200
percent higher than rating, device is defective.
If thermistor is good and compressor does not start:
1. Disconnect thermistor from starting circuit.
2. Give compressor a temporary capacitance boost (see next
section).
3. Run compressor for 10 minutes, shut off, and allow system
pressure to equalize.
4. Reconnect start thermistor.
14
12.5-22.5 OHMS
12.5 OHM
(BEIGE COLOR)
25-45 OHMS
25 OHM
(BLUE COLOR)
Fig. 13—PTC Devices
A88414