Photograph of  This new compressor was placed directly against
a brick wall. One third of its condenser coil cooling ability was blocked. How to Diagnose & Repair Tight or Seized Air Conditioning Compressors
     

  • TIGHT or SEIZED AC COMPRESSORS - CONTENTS: How to diagnose tight or seized air conditioner compressors. How to re-start a tight or seized air conditioning compressor. Air conditioning compressor hard-starting. Tables of typical FLA, RLA, and LRA ampacity data for air conditioner and heat pump compressor motors
  • POST a QUESTION or READ FAQs about air conditioning compressor failures: tight, hard-starting, or seized A/C compressor motors
  • REFERENCES

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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.

Green links show where you are. © Copyright 2015 InspectApedia.com, All Rights Reserved.

Diagnosing Tight or Seized Air Conditioning Compressors

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 compressor chart will list the correct amp draw for the compressor under its various operating conditions. To use the chart, the technician must know the evaporating temperature, condensing temperature, and voltage applied to the compressor. ... It is important to measure the amp draw of the compressor only when you are trying to determine if it is operating properly. [6]

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:

A/C Compressor Motor BTUH [1]
A/C Compressor Motor RLA/FLA
(rated load Amps)
A/C Compressor Motor LRA
(locked rotor Amps)
24,600
7.9
41
28,400
9.8
55
35,600
16.7
79
40,500
17.9
112
46,500
19.9
109
57,500
26.4
134

[1] using Whirlpool[4] WPC44H series packaged air conditioning systems as an example. These units operate at 208-240V single phase electrical supply.

Note that these amperage figures are NOT the total amperage draw for the unit, they are specifically compressor motor amp draw data.

These FLA and LRA numbers are for example purposes only. Other compressor motor models and brands will have different, if generally similar by order of magnitude amperage draw numbers.

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.

Heat Pump Compressor Motor BTUH [2]
Heat Pump Compressor Motor RLA/FLA
(rated load Amps)
Heat Pump Compressor Motor LRA
(locked rotor Amps)
24,000
12.8
58
29,000
14.1
73
35,400
16.7
79
40,000
17.9
112
48,000
21.2
96
55,600
25.6
118

[1] using Whirlpool[4] WGPH45 series packaged heat pump systems as an example. These units operate at 208-240V single phase electrical supply.

Note that these amperage figures are NOT the total amperage draw for the unit, they are specifically compressor motor amp draw data.

These FLA and LRA numbers are for example purposes only. Other compressor motor models and brands will have different, if generally similar by order of magnitude amperage draw numbers.

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 compressor motor 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. [2]

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.

 

Continue reading at REPLACING A COMPRESSOR or select a topic from the More Reading links shown below.

Or see CAPACITORS for HARD STARTING MOTORS

Suggested citation for this web page

TIGHT or SEIZED AC COMPRESSORS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

More Reading

Green link shows where you are in this article series.

...




Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Questions & answers or comments about air conditioning compressor failures: tight, hard-starting, or seized A/C compressor motors

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Publisher's Google+ Page by Daniel Friedman

Click to Show or Hide Citations & References