air con electrical

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Heat Pump Electrical Component Checks

Posted: January 9, 2005

In performing service and troubleshooting on heat pump systems, there are variouselectrical components to be checked. The following provides information to aid in inspectingand servicing these components.

Capacitor Electrical Check

Caution: Any capacitor showing signs of leaks or bulgingmust be replaced immediately. Do not operate thecompressor with a capacitor showing these signs.

Single-phase compressors require a run capacitor connectedto the run and start winding. This puts the two windings outof phase from one another and allows the compressor motorto start.

The best method of checking a capacitors integrity andcapacitance is to use a quality capacitor analyzer. Acapacitor analyzer can measure the capacitor dielectric anddetermine if the MFD value is proper or has deteriorated. If a

capacitor analyzer is not available, use a quality analogohmmeter to check the resistance between the capacitor plates.

1. Warning: Disconnect electrical power before performing this test. Failure to do so canresult in electrical shock causing personal injury or death.

2. Set the ohmmeter to the highest scale available.

3. Discharge the capacitor by shorting the terminals with a 2-watt 20,000-ohm resistor.

4. Place the ohmmeter probes on the capacitor terminals and read:

A. The meter pointer moves toward zero, then gradually moves to higher ohm values. Thecapacitor is good.

B. The meter pointer does not move. It is an open capacitor replace.

C. The meter pointer goes toward zero ohms and stays. It is a shorted capacitor replace.

5. Replace any failed capacitor with a capacitor having the same MFD and voltage rating.

F ig u r e 1 . Combination runcapacitor.

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Compressor Start Kits

Some compressors require the assistance of an additional capacitor to start. This capacitoris taken out of the circuit as soon as the compressor reaches about 75 percent of itsrunning rpm. The potential relay opens a circuit to the start capacitor.

Important: Only use factory specified start kits. Using an improper start kit can causepremature compressor failure.

The potential relay uses three terminals: 1, 2, and 5. The normally closed contacts arebetween terminals 1 and 2. The relay coil that opens them is between terminals 2 and 5.

Start Kit Connections1. One terminal of the start capacitor connects to line power going to the compressor Runterminal.

2. Relay terminal 1 connects to the other start capacitor terminal.

3. Relay terminal 2 connects to the compressor Start terminal.

4. Relay terminal 5 connects to the load side of the compressor contactor going to thecompressor Common terminal.

Start Kit OperationImmediately on starting, the start capacitor assists the run capacitor to turn thecompressor motor. As the motor comes up to speed a voltage builds in the compressorstart winding. When the motor reaches approximately 75 percent of its running rpm, thestart winding voltage energizes the coil between potential relay terminals 2 and 5. This coilthen opens its normally closed contacts 1 and 2. The start capacitor is now isolated fromthe compressor. A small bleed resistor between the start capacitor terminals discharges the

capacitor making it ready for the next start.

Start Kit Electrical Checks

F ig u r e 2 . Potential relay diagram.

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Warning: Disconnect all power to the outdoor unit before beginning this test. Servicing theunit with electrical power on can result in electrical shock, serious injury, or death.

1. Check the start capacitor using the same method as with a run capacitor. (First removethe bleed resistor.)

2. Measure the resistance between potential relay terminals 1 and 2. There should be zeroohms.

This indicates the contacts are properly closed. If there is infinite resistance, the contactsare open and the relay must be replaced.

3. Measure the resistance between the potential relay holding coil terminals 2 and 5. Use ahigher ohm scale. There should be several thousand ohms resistance. If there is infiniteresistance, the coil is open and must be replaced. Less than 1,000 ohms indicates a coilshort.

Reversing Valves

Electrical OperationThere are two valves in the reversing valve assembly. The solenoid or pilot valve isenergized by 24 volts when the thermostat mode is set to heating operation. The slidevalve is positioned by the pilot valve. Use the ohmmeter to check the 24-volt pilot solenoid.Do not energize the solenoid without the reversing valve pilot stem in place in the coil.Excessive current results in causing coil failure.

Electrical Check1. Put the thermostat into the cooling mode (into the heating mode for Rheem/Ruudproducts).

2. Measure the voltage at the solenoid wiring connections. There should be approximately24 volts.

F ig u r e 3 . Reversing valve solenoid.

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A. If there is no voltage, check for 24-volt power at the indoor transformer.

B. If there is 24 volts at the transformer, check the thermostat and the wiring circuit goingto the reversing valve.

3. If there is 24 volts present and the solenoid is intact, place a knife blade orunmagnetized screwdriver blade at the end of the solenoid. The solenoid shouldmagnetically pull at the blade.

A. If there is no magnetic pull, use an ohmmeter to determine if there is electricalresistance in the solenoid coil.

B. Remove the wires attached to the solenoid and measure the resistance.

C. Normal solenoid resistance should be 10-60 ohms.

D. If the solenoid has failed, remove the nut at the end, slide the solenoid from the shaft,and replace with another solenoid.

Mechanical OperationThe second part of the reversing valve assembly is a pressure operated slide valve. Thepilot solenoid valve directs refrigerant pressure to move the slide assembly in the valvebody. The position of the slide valve directs system refrigerant flow to produce heat orcooling at the indoor coil.

A sticking slide valve is the most common problem in reversing valves. This is most oftencaused by contaminants in the valve body caused by using unclean refrigerant lines, orexcessive flux and solder material.

Reversing valves that internally leak also cause problems but are rare. A temperature checkat the tubes with an electronic temperature tester usually indicates this fault.

F ig u r e 4 . Disconnect wiring plug to test the reversingvalve solenoid.

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Determine the two tubes on the valve carrying cold suction gas. In cooling they are thecenter and the one connected to the indoor coil. In heating they are the center and the oneconnected to the outdoor coil.

Mechanical Check1. Locate the correct refrigerant tubes leaving the reversing valve.

2. Measure the tube temperatures about 2 inches from the valve body.

A. If the temperature difference is within 4 degrees, the valve is not leaking.

B. If the reading is 10 degrees or more, there is a good chance the valve is leakinginternally.

3. If the valve has failed, remove the solenoid, recover the refrigerant, and unsolder thevalve body.

4. Wrap the new valve body with wet rags or heat sink material, and carefully braze thenew valve body in place.

5. Replace the solenoid onto the new valve body.

Motors

ElectricalThe motors in both the outdoor and indoor units are similar. Outdoor motors have onespeed while indoor motors usually are multispeed. Most fan motors require a capacitor forthe best torque and efficiency. If the motor is replaced, replace the capacitor.

The fan motor has both Run and Start windings. Extended run windings furnish a choiceof speeds with indoor products. Use an ohmmeter to troubleshoot motors.

Electrical Checks1. Be sure power is off to the motor.

F ig u r e 5 . Proper motor rotation indicator.

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2. Disconnect the motor wires.

3. Place one ohmmeter lead on Common and the other on Start. The meter should showresistance.

4. Place one ohmmeter lead on Common and the other lead to Run. The meter should showless resistance.

5. Leave ohmmeter lead on Common and touch each remaining lead. Each should indicateresistance.

6. If a winding does not show resistance, the motor has failed and must be replaced.

7. If the motor needs replacing, replace the capacitor as well.

MechanicalFan motors can suffer a bearing loss due to a lack of lubrication. This may be caused byexcessive water spilling from a roof above the unit. Lubrication can be washed out ofpermanently lubricated motor bearings. Over-lubrication causes dirt to stick to oiling portsand can contaminate the bearings. If a motor fails, replace it with a motor having the samerotation and electrical characteristics.

Note: When parts combination results in motor/blade interference, the fan blade should belocated to provide 1/8-inch motor/blade clearance.

Relays

Electrical OperationRelay coils are wires wound around an iron core. Whenenergized a coil becomes a magnet opening or closingcontacts that make or break electrical circuits. Electricalcontinuity indicates a good coil. Relay coils use specificvoltages. Using the wrong voltage either destroys the relay orit just doesnt operate. Avoid this by using the proper

replacement part for a failed component.

Relay contacts are switches and can become pitted orcorroded, creating resistance. Corrosion can generate enoughheat to weld the contacts together. Physical examination ofthe contacts shows this problem. With sealed relays, use anohmmeter to show open or closed contacts.

Electrical Checks1. Be sure all electrical power is off to the component

being checked.

2. Remove at least one wire from the component being checked.

F ig u r e 6 . Contactor coilresistance.

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3. Measure the resistance of the holding coil with an ohmmeter.

A. A specific resistance indicates a good holding coil.

B. Infinite resistance indicates a relay with an open holding coil. Replace this relay with anidentical part.

4. Measure the resistance across the normally closed (NC) relay contacts.

A. The resistance should be zero ohms.

B. If the resistance is more than 0.2 ohms the contacts are pitted. Replace this relay withan identical part.

5. Measure the resistance across the normally open (NO) relay contacts.

A. The resistance should be infinite.

B. If the resistance is less than infinite, replace with an identical part.

Mechanical ChecksIn relays, mechanical faults are few, with the exception of physical damage.

High-Pressure Control (HPC) Switch

The high-pressure switch shuts down the outdoor unit if excessively high head pressuresoccur. The switch is a 24-volt control and is wired in series with the compressor contactor.If it opens, it breaks power to the contactor and stops the outdoor unit.

Important: If the high-pressure switch is open, determine the cause, when possible,before resetting the control.

High-Pressure Switch ChecksIf the high-pressure switch opens the probable cause is an overcharge, dirty condenser coil,faulty or wrong size condenser fan motor, faulty fan blade or wrong rotation, recirculationof condenser air, additional heat source, noncondensibles, or equipment not matched.

Hot Gas Sensor Control (HGS)

The hot gas sensor shuts down the outdoor unit if excessively high discharge gastemperatures occur. The switch is a 24-volt control and is wired in series with thecompressor contactor. If it opens it breaks power to the contactor and stops the outdoorunit.

Its function is to detect loss of refrigerant charge in the system. If the system is low oncharge, the remaining suction gas must still cool a large compressor motor. The remaining

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gas temperature rises as it attempts to cool the large compressor motor.

Hot Gas Sensor ChecksIf the hot gas sensor switch opens the probable cause is a low charge or refrigerant leak.

Reprinted with permission from the Residential Split System Heat Pumps service manual

from Rheem Air Conditioning Division, Fort Smith, Ark. For more information, visitwww.rheemac.com.

Publication date: 01/10/2005

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