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AIR CONDITIONING & HEAT PUMP SYSTEMS
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BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
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BOOKSTORE - Air Conditioning "How To" Books
CAPACITORS for HARD STARTING MOTORS
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
CONDENSATION or SWEATING PIPES, TANKS
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
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DUCT INSULATION, Asbestos Paper
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DUCT SYSTEM & DUCT DEFECTS
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DUCTS, Asbestos Transite Pipe
DUST, HVAC CONTAMINATION STUDY
ELECTRIC MOTOR OVERLOAD RESET SWITCH
EVAPORATIVE COOLING SYSTEMS
FAN LIMIT SWITCH
GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION INSTRUMENTS
HEAT LOSS (or GAIN) in buildings
HEAT LOSS (or GAIN) INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HEATING SMALL LOADS
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LEED GREEN BUILDING CERTIFICATION
LOST COOLING CAPACITY
LOW VOLTAGE TRANSFORMER TEST
MOTOR OVERLOAD RESET SWITCH
MOLD in AIR HANDLERS & DUCT WORK
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
REPAIR GUIDES A/C / HEAT PUMP
REPAIR & DIAGNOSTIC FAQs for A/C
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
Seized air conditioner or heat pump compressor motor diagnosis: This air conditioning repair article discusses the Diagnosing Tight or Seized Air Conditioning Compressors and how to re-start a tight air conditioning compressor. Advice for measuring amps or current draw to check for a seized or tight or failing A/C or heat pump compressor motor. See the basic diagnostic steps suggested BURNED-OUT COMPRESSOR for A/C and heat pump damage indications and causes. Here we continue with details about examining and measuring A/C and heat pump compressors to continue compressor condition diagnosis.
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Sometimes a compressor may be internally damaged or worn in a way that makes the piston(s) run tight in the cylinder(s) rather than loose and worn (loose or worn air conditioner compressor diagnosis is described at LOST COOLING CAPACITY).
A tight or seized air conditioner compressor might occur, for example, if the compressor ran while low on oil. Or perhaps if the compressor is old and has not been run in some time. The A/C compressor may refuse to start at all, or may be hard-starting depending on when it last ran and what it's temperature is.
A "hard starting" "tight" air conditioner compressor will probably draw excessive current (Amps) during startup, so that measurement may be used as part of the diagnosis of this problem.
Re-Starting a Tight or Seized Air Conditioner Compressor
The service technician may try re-starting a hard-starting or tight air conditioning compressor by trying to move the motor backwards and forwards.
If the compressor can be freed up enough to start at all and the oil level is made correct, the technician may install a "hard start kit" such as we introduced at HARD STARTING and see CAPACITORS for HARD STARTING MOTORS for photos, wiring diagram, and installation instructions for air conditioner compressor, fan, blower, refrigerator motor, freezer motor, or other electrical motor starting booster capacitors.
If these steps work the compressor may continue to be used. If these steps do not get the tight or seized compressor running, it needs to be replaced. But even if these steps do work, the compressor has probably been damaged and you should not count on its long future life before it needs to be replaced.
Amp Draw Testing for A/C or Heat Pump Compressor Motors
Also see Basic Electrical Tests on Air Conditioner or Heat Pump Compressor Motors found at BURNED-OUT COMPRESSOR. Links there also return here.
How We Measure Amps or Current Draw at the Air Conditioner Compressor
Basic air conditioning compressor operation check: after the air conditioning system compressor/condenser unit has been running long enough to stabilize at its normal operating temperatures and pressures (typically 10 to 20 minutes), the service technician may measure the compressor's current draw in Amps using an ammeter or amp meter or multi-tester and appropriate connections.
Watch out: the nameplate data giving FLA/RLA or LRA for an air conditioning system may include multiple electrical components combined, and even if it pertains to just the compressor motor, according to some industry experts these figures may not be an accurate diagnostic number for checking on hermetically-sealed A/C or heat pump compressor motors. Joe Marchese pointed out back in 2003:
The current draw in AMPS is compared with the unit's specified FLA or "full load amps" found on the data tag or in the service manual for the unit. Remember that the total system FLA number includes the current draw of not only the compressor motor itself but also the condenser fan motor as well as the compressor motor base heater if a heater element is installed and operating. But in our table below we give example RLA/FLA (rated load amps / full load amps) and also LRA (locked rotor amps) data for both air conditioner compressor motors and heat pump compressor motors.
Tables of Typical RLA / FLA and LRA Amp Draw for A/C and Heat Pump Compressor Motors
Typical FLA current draw numbers for air conditioner compressors and heat pump compressor motors (second table below) using Whirlpool data as an example, include these data:
The apparent inconsistency of Amp draw increase in these tables is most likely due to variations in compressor motor designs among the different BTUH-rated units.
Locked Rotor Amps - LRA compressor motor data
The current draw in AMPS during compressor motor start-up can also be compared to the unit's specified LRA or "lock rotor amps" found also on the data tag we describe above. However most HVAC repair technicians look at the compressor motor FLA reading since that's the running-condition of the unit. LRA in some references is also referred to as "starting current inrush" - the amount of current drawn, in amps, when full voltage is applied to start up the electric motor, in this case in the air conditioner or heat pump compressor.
Example LRA data is found for both air conditioning compressors and heat pump compressors in the two tables above. Other sources give typical LRA numbers for typical 240V A/C compressors will typically be about 33 Amps per ton or 33 Amps per 12,000 BTUH for up to 3 ton units. So a two ton unit will draw 66-67A, a 3-ton unit 100A during startup only. Larger units do not continue this almost straight line increase in current draw. So a 4-ton A/C compressor may draw a bit under 120A, and a 5-ton unit 145A. All of these current draws are only during motor start-up and are at 240V AC. 
What's the difference between FLA and RLA Amperage Current Draw Ratings?
The reason you see us using "RLA/FLA" in the center table column is that these are about the same thing. The term "Full Load Amps" or "FLA" was officially changed to "Rated Load Amps" or "RLA" back in 1976. Regardless of calling it FLA or RLA, this is the maximum current the motor should draw during any running conditions. Note that this excludes high amp load during motor start-up. On modern air conditioners and heat pumps you should see RLA marked on the motor data plate. If you are reading an older article referring to "FLA", think RLA.
Some technicians and electricians apply RLA to compressor motors and still use FLA for other motors. RLA is used in circuit wire sizing.
Watch out: don't mix-up RLA (running load amps) with LRA (locked rotor amps). LRA is discussed above as well.
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