Capacitors - Bryant R-22 Application Manual And Service Manual

Fig. 11—Contactor
III. CAPACITORS
CAUTION: Capacitors can store electrical energy when
power is off. Electrical shock can result if you touch the
capacitor terminals 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.)
CAUTION: 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 inter-
nally shorted.
2. Capacitance testers are available which read value of
capacitor. 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.
WARNING: Exercise extreme caution when taking
readings while power is on. Electrical shock can cause
personal injury or death.
A88350
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)
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.
—14—
Fig. 12—Capacitors
12.5-22.5 OHMS
12.5 OHM
(BEIGE COLOR)
25-45 OHMS
25 OHM
(BLUE COLOR)
Fig. 13—PTC Devices
A91455
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