Bryant R-22 service manual III. Capacitors, Start Capacitors and PTC Devices

A88350

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.

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A91455

Fig. 12—Capacitors

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.

12.5-22.5 OHMS

			12.5 OHM
		(BEIGE COLOR)		25-45 OHMS
	20-36 OHMS
	BLUE
	20 OHM			25 OHM
(BLUE COLOR)
				(BLUE COLOR)

A88414

Fig. 13—PTC Devices

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.