What is lra of compressor?
We always recommend finding a team of trusted compressor engineers to assist with maintenance and help you get the most value from your compressors. With the right maintenance, you can expect many years of reliable service from your compressor.
When compressors fail suddenly “out of the blue,” it almost always indicates that there has been a maintenance oversight somewhere down the line. More rarely, there may be a problem within the operating environment, such as electrical issues or environmental contamination.
Whatever the case, you will see the most value from your compressor when you stay informed.
But it can be tough to get the answers you want, and you might not always have time to crawl through dozens of forum posts. Likewise, the information you find online may not apply directly to your specific compressor or your application. That can easily lead you in the wrong direction.
With that in mind, we’ve developed this list of common compressor questions and answers.
These commercial compressor FAQs will help you get off to the right start when you diagnose your unit.
“LRA” stands for locked rotor amps. This is a measure of the current draw you can expect from the unit’s motor under starting conditions when you apply full voltage. LRA conditions last for only a few seconds during a unit’s startup phase. You will be able to find this information on a plate on the compressor unit itself. It can also be tested with a clamp meter. Compressors should not draw the full LRA after startup!
All in all, LRA is about five to seven times higher than the normal operating current of the compressor. LRA is stamped onto the compressor because it must remain consistent throughout the unit’s service lifetime, and you can use it to determine other facts about the compressor.
If the information plate on your compressor is incomplete, you may be able to estimate the LRA without it. For a single-phase unit, the following equation can get you there: LRA = 1000* (kVA/HP)/Voltage.
Not to be confused with LRA, “RLA” refers to rated load amps. It is also called running load amps or full load amps (FLA). You can deduce an estimated RLA in a compressor if you have the LRA figure. The rule of thumb is to divide the LRA by six. Some textbooks suggest the precise figure is 5.56.
Check the unit plate for the precise voltage and amps before bringing your compressor online.
Start with the model and serial number. Within the model number, the three-digit code refers to the BTU capacity. You can determine the compressor’s tonnage from here: Every 12,000 BTUs equals one ton. For example, a 3-ton compressor unit would have the number 036.
The “T” rating on a commercial compressor refers to the specific climate where it is intended to be used. T1 is a moderate climate, T2 is a cold climate, and T3 is a hot climate. They are designed to operate in the following temperature ranges:
A properly remanufactured compressor should have no difference in either performance or operating efficiency when deployed in real world conditions. The remanufacturing process should account for all damaged components, completely replace them, and fully test the unit.
From a business standpoint, a remanufactured commercial compressor is frequently around 30% less expensive than an all-new commercial compressor from an OEM-affiliated wholesaler. Likewise, the remanufactured unit can often be sourced several weeks sooner, which is essential in an emergency.
When discussing electricity for refrigeration and how it effects compressors, you must understand lock rotor amps (LRA).
If you check the data plate on any refrigeration compressor you'll notice the LRA.
What does it mean?
The LRA indicates how much amperage the motor will pull while trying to make the rotor spin inside the stator, if the rotor is locked up!
What could caused it to lock up?
On reciprocating compressors, usually the crankshaft or a piston has broken.
So, what should you do when you have a compressor pulling LRA?
If the compressor is three phase, try reversing any two of the compressor leads.
What do I mean by reversing leads?
Take any two of the three wires and interchange them.
Put the first wire where the second was and put the second wire where the first was.
Reversing any two leads on a three phase motor causes the motor to change rotation.
If the compressor is not "busted up" inside, but merely seized, it may start.
What could cause it to seize?
The most likely suspect is, lack of lubrication.
But that's another topic for discussion.
Also check the evaporator. Is it flooded with liquid refrigerant?
If the compressor still doesn't start after reversing leads?
You're going to have to inspect the crankcase and probably replace the compressor.
When your amp meter registers LRA, it's usually not good news for the customer!
What is the meaning of LRA in a compressor? “LRA” stands for locked rotor amps. This is a measure of the current draw you can expect from the unit's motor under starting conditions when you apply full voltage. LRA conditions last for only a few seconds during a unit's startup phase.
Reviewing and understanding the information on equipment nameplates is important in properly evaluating an air-conditioner or heat pump installation. It also simplifies the selection of the correct branch-circuit wire, overcurrent protection and disconnect sizes. Most all the information needed by the installer can be found by reference to the units data name-plate. The following is an example of the type of information found on the nameplate for a residential central air-conditioning unit.
Air-conditioning and heat pump equipment have a hermetic motor-compressor and are not treated the same as a conventional electric motor. The hermetic motor-compressor operates in the refrigerant environment and does not have a horsepower or full-load current rating such as a standard motor. Special terms are used to provide the necessary information to properly install wiring for this equipment.
One of the terms noted on the above nameplate is “Compressor R.L.A.” This is the Rated-Load Amperes or Rated-Load Current for the motor-compressor and is established by the manufacturer of the equipment. The rated-load current is the current resulting when the motor-compressor is operated at the rated load, rated voltage and rated frequency of the equipment it serves. This value is used in calculating the minimum circuit ampacity and overcurrent protective device ratings specified on the nameplate for this equipment. The RLA for our example is 18.0 amperes.
Another term noted on the air-conditioner nameplate is “Compressor L.R.A.” This term represents Locked-Rotor Amperes and is the maximum current flowing to the motor when it is in a locked, or not turning, condition. This value is necessary to ensure that the air-conditioner disconnecting means and controller have proper interrupting capacities. The LRA for our example is 96 amperes.
The term “Min. supply circuit ampacity,” or similar wording, is the minimum circuit ampacity required to select conductor size and switch rating for the unit. This value is determined by the following formula:
Minimum Circuit Ampacity = (RLA x 1.25) + Other Loads
The term “Max fuse or ckt brk size” indicates the maximum fuse or circuit breaker size permitted. The manufacturer has selected a rating that will permit the motor to start as well as provide overcurrent protection. The term “Min fuse or ckt brk fuse” rating indicates the smallest rating that will allow the motor to start.
The Rated Load Amperes (RLA) is used in calculating the maximum overcurrent protection permitted for the unit. Later in this chapter we will include a calculation for the unit covered by the above nameplate.
In some cases, the equipment is also marked with a “Branch-Circuit Selection Current” (BCSC). This is a value in amperes that must be used instead of the rated-load current for determining ratings of motor branch-circuit conductors, disconnecting means, controllers, and branch-circuit short-circuit and ground-fault protective devices whenever the running overload protective device permits a sustained current greater than the rated-load current. The value of branch-circuit selection current will always be equal to or greater than the marked rated-load current. Since the air-conditioning nameplate included in this chapter does not include a branch-circuit selection current, the rated-load current, or in this case RLA, is used for the calculations. See NEC Sections 440-2 and 440-4.
Note that the nameplate also references “HACR type breaker.” The marking “HACR” means Heating, Air Conditioning and Refrigeration. This marking means the circuit breaker has been tested and found suitable for use on this equipment including providing adequate protection for the smaller motor of the group, which is the outdoor fan. Circuit breakers used to supply this unit must be marked “HACR.”
Most circuit breakers manufactured today are HACR rated. This marking is common on most circuit breakers manufactured today but may not be present on older breakers located in existing equipment.
Remember that NEC Section 110-3(b) requires compliance with nameplate markings. If the nameplate specifies fuses only, substitution of even a HACR rated breaker marked only for fuses or if the breakers must be of the HACR type, failure to comply with the marking is a violation of this section. Generally the manufacturer offers a choice of either fuses or HACR rated circuit breakers. Check the nameplate information to make sure.
The required ampacity of branch-circuit conductors and rating of equipment for a hermetic refrigerant motor-compressor are based upon marking found on the unit nameplate giving the rated-load current. This information is necessary to determine the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection, and the separate motor overload protection. See NEC Section 440-6(b).
For a typical air-conditioning unit or heat pump outdoor unit which has a motor-compressor and additional load(s), such as a fan motor, the conductors supplying this equipment must have an ampacity not less than 125 percent of either the rated-load or branch-circuit selection current, whichever is larger, plus the full-load current of the fan motor. For the example nameplate, this value is:
(RLA x 1.25) + Fan Motor FLA
18.0 x 1.25 + 1.3 = 23.8 amperes
This value rounded up to 24 is the “MIN SUPPLY CIRCUIT AMPACITY” stated on the nameplate and is the required minimum ampacity of the branch-circuit conductors selected from NEC Table 310-16. Since the equipment nameplate includes this value, it is not necessary for the installer or inspector to perform this calculation.
NEC Table 310-16 shows that a No. 12 Type TW, THW or THWN copper conductor can safely carry 25 amperes continuously where operated in an ambient temperature not exceeding 86°F. Where the temperature in an attic or on the roof reaches temperatures in excess of this figure, ampacity correction factors listed in NEC Table 310-16 must be applied, which will make it necessary to increase the size of the conductors to compensate for the rise in temperature.
NEC Section 240-3(d) requires that the “overcurrent protection” must not exceed 20 amperes for a No. 12 conductor unless otherwise specifically permitted in Sections 240-3(e) through (g). Section 240-3(g) permits air conditioning circuit conductors to be protected in accord with Parts C and F of Article 440. Section 440-21 specifically states that the provisions of Part C are “in addition to or amendatory of the provisions of Article 240.” For example, the air conditioner nameplate marking indicates a “Maximum Fuse or Circuit Breaker Size” of 40 amperes. The minimum supply circuit ampacity is 24-amperes. In this case the nameplate on the air conditioner is marked for a Maximum Fuse or Circuit Breaker Size of 40 amperes, which will satisfy the requirements. Consequently, if no derating for ambient temperature is required, No. 12 conductors having an ampacity of 25 amperes are acceptable to supply this unit.
The concept for protection where a No. 12 copper wire with an ampacity of 25 amperes is permitted to have overcurrent protection of 40 amperes is as follows:
Paragraph 36.15 of the UL Standard 1995Heating and Cooling Equipment requires the maximum current rating of the branch-circuit overcurrent protection be based on:
(RLA x 2.25) + Other Loads (Fan Motor FLA)
18.0 x 2.25 + 1.3 = 41.8 amperes
If this calculated value is not a standard current rating of an overcurrent protective device, paragraph 36.16 of UL 1995 requires use of the next lower standard rating. In this example, the next lower standard device is 40 amperes as stated on the nameplate. Note here again it is not necessary to actually perform this calculation. It has been done, and the information is included on the units nameplate.
If the nameplate includes a “MINIMUM FUSE OR CIRCUIT BREAKER SIZE,” then NEC Section 110-3(b) would also require compliance with that marking in addition to the marking of the “MAXIMUM FUSE OR CIRCUIT BREAKER SIZE.”
Where the air-conditioning or heat pump compressor unit consists of a hermetic refrigerant motor-compressor(s) in combination with other loads, such as the fan motor, the horsepower rating of the disconnecting means is based on the summation of all currents at rated-load condition and also at locked-rotor condition.
In the example used in this chapter, the 18.0 ampere.0-ampere RLA of the compressor motor is added to the 1.3 ampere FLA of the fan motor. The total of 19.3 amperes is then considered to be the equivalent full-load current for the combined load. According to NEC Table 430-148, the full-load current rating of a 230-volt, single-phase, 3 horsepower motor is 17 amperes, while the full-load current rating of a 230-volt, single-phase, 5-horsepower motor is 28 amperes. Consequently, since the equivalent full-load current of the example A/C unit is 19.3 amperes, we must use the next higher rating, and the disconnect switch must have a minimum of a 5-horsepower, 230-volt, single-phase rating.
The ampere rating of the disconnecting means must also be a least 115 percent of the sum of all currents at rated-load condition. This minimum rating would then be 115 percent x
19.3 amperes = 22.3 amperes. If the disconnecting means includes or serves as the branch-circuit overcurrent protection for the unit, the rating required for the overcurrent device, rather than this minimum rating, would generally be the determining factor in sizing the disconnecting means. A fused disconnect switch containing either the maximum or minimum sizes of fuses listed on the nameplate would exceed this 115 percent minimum requirement. If an unfused disconnect switch is used as the disconnecting means, however, then this 115 percent rating and the horsepower rating would establish the minimum switch rating.
There is one other consideration in establishing the correct size of the disconnecting means serving the air- conditioning unit. The disconnecting means rating must also be based on currents at locked-rotor condition. Refer to NEC Table 430151A for the conversion of locked-rotor current to horsepower. In our example, the nameplate indicates that the motor-compressor LRA is 96 amperes. Since the nameplate does not give a LRA for the fan motor, we assume it to be six times the FLA or 6 x 1.3 amperes = 7.8 amperes. Adding this to the motor-compressor LRA of 96 amperes gives us an equivalent LRA for the combined load of 103.8 amperes. Again referring to NEC Table 430-151, we find that for a single-phase, 230-volt motor with a 103.8 amperes motor locked-rotor current, the disconnect switch should be based on a 5-horsepower rating. See NEC Section 440-12.
NEC Section 440-14 covers the required location of a disconnecting means for an air-conditioner or heat pump compressor. This rule specifically points out that the disconnecting means is required to be located within sight from and readily accessible from the air-conditioning equipment. The following two NEC Article 100 definitions must be clearly understood.
“In Sight From (Within Sight From, Within Sight)”: Where this Code specifies that one equipment shall be “in sight from,” “within sight from,” or “within sight,” etc., of another equipment, the specified equipment is to be visible and not more than 50 ft. distant from the other.”N
“Accessible, Readily: (Readily accessible.)”: Capable of being reached quickly for operation, renewal, or inspections, without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, chairs, etc.”N
This disconnecting means is permitted to be installed on or within the air-conditioning equipment. An air-conditioner or heat pump compressor is generally located on a concrete pad located at the outside of a one- or two-family dwelling. It may also be located under the house in a crawl space or on the roof. It is important to remember that the definitions of within sight and readily accessible have a significant meaning in this application.
The disconnecting means is usually located next to the air-conditioning unit and is, therefore, considered as being readily accessible since the proper working space should be provided in accordance with NEC Section 110-26.
However, where the disconnecting means is located “behind or above” the air conditioning unit, accessibility to the disconnect will be hindered and will be in violation of the working space requirements of NEC Section 110-26(a). This type of installation is incorrect and still remains as a very common problem in the field.
The purpose of the required disconnecting means is to provide a “ready and visible means” of disconnect for the person who will service or repair the equipment. The requirements of NEC Article 430, where a “locking type” of disconnecting means may be permitted out-of-sight of the motor, are not applicable for this equipment.
Note: A disconnecting means as described above is not required for cord- and plug-connected equipment such as room air conditioners.
Working space about electrical equipment that is likely to “require examination, adjustment, servicing, or maintenance while energized”N must be provided in accordance with Table 110-26(a). The working clearance must be in the direction of access to the equipment, or the part of the equipment, that is likely to be worked on while there are live exposed parts. It is very common for service persons to examine or test this equipment while it is energized.
Generally, this working space is 30 inches wide and 36 inches deep. Compliance with this working space rule requires that consideration be given to providing safe access at the time the equipment is being installed. Clear working space is required in front of access panels on this equipment.
The closest a/c unit appears to be in compliance with the workable space requirements of Section 110-26 while the further a/c unit disconnects are clearly in violation of that section
An indoor air-handling unit is installed for typical heat pump split systems. Refrigerant lines are run from the outdoor unit to the indoor unit that may be located in the crawl space, attic or indoors, such as in a utility closet or room. The indoor unit includes a fan for circulating air through the dwelling and controls, as well as the refrigeration coil. Where desired, resistance heating elements are added to the indoor unit to serve as backup heat in case of compressor failure and to provide extra heating capacity to provide faster heating recovery.
Many of the same rules apply to indoor units as apply to electric furnaces. A disconnecting means rated for the load to be served is required within sight of the indoor unit. In some cases, this disconnecting means consists of one or more circuit breakers that are located in the unit and are operational from outside the unit.
The minimum size of the branch circuit to the indoor unit is required to be not less that 125 percent of the resistance heat and motor load. See NEC Section 424-3(b).
A room air conditioner is considered to be an alternating-current appliance of the air-cooled window, console or in-wall type that is installed in the conditioned room and incorporates a hermetic motor-compressor. The following requirements cover equipment rated not over 250 volts, single phase, and such equipment may be cord- and plug-connected.
In determining branch-circuit requirements for a room air conditioner, a cord- and attachment plug-connected unit is considered as a single motor unit if its rating is not more than 40 amperes, 250 volts, single phase; total rated-load current is shown on the air-conditioner nameplate; and the rating of the branch-circuit protective device does not exceed the ampacity of the branch-circuit conductors or rating of the receptacle, whichever is less.
The total marked rating of a cord- and attachment plug-connected room air conditioner shall not exceed 80 percent of the branch-circuit rating where no other loads are supplied. If the branch circuit supplies lighting units or other appliances, the rating of the unit cannot exceed 50 percent of the rating of the branch circuit. See NEC Section 440-62.
An attachment plug and properly rated receptacle is permitted to serve as the required disconnecting means for a single-phase room air conditioner rated 250 volts or less if the following conditions are met:
Where a flexible cord is used to supply a room air conditioner, the length of such cord cannot exceed:
A room air conditioner that is fastened in place or connected by permanent wiring methods (fixed) requires that any exposed noncurrent-carrying metal parts likely to become energized are to be properly grounded under any of the following conditions:
See also NEC Article 440, Part G.
Section 110-3(b) of the National Electrical Code states: “Installation and Use. Listed or labeled equipment shall be installed, used, or both, in accordance with any instructions included in the listing or labeling.”N
Qualified laboratories typically list air-conditioning and heating equipment in accord with the following standards: This listing or labeling is performed by qualified electrical testing laboratories in accordance with the following UL Standards.
UL1995 — Heating and Cooling Equipment (This standard covers central heating, central air-conditioning, and heat pumps.) (UL1995 applies to central air-conditioning units and heat pumps.)
UL484 – Room Air Conditioners
These product safety standards detail the necessary safety tests and determine the required nameplate markings and instructions that are included by the manufacturer of the equipment. For example, paragraph 36.3(i) of UL 1995 specifies the equipment shall be marked with a “maximum overcurrent protective device size.” A typical nameplate will show the “MAXIMUM FUSE” and/or “MAXIMUM CIRCUIT BREAKER” size. If the nameplate specifies only fuses, then the unit is intended to be protected by fuses only. If the nameplate requires HACR (Heating, Air Conditioning and Refrigeration) circuit breakers, then the circuit breaker protecting the unit must be marked “HACR.”
If the nameplate includes both fuses and HACR circuit breakers, as is the case of our nameplate example in this chapter, then either is acceptable.
As noted above, NEC Section 110-3(b) would require that the branch-circuit overcurrent protective device comply with the type and size specified on the air- conditioner nameplate.
For room air conditioners, the nameplate will also be marked to show the type and maximum size overcurrent protection permitted for the unit. The comments previously discussed relative to use of fuses or HACR circuit breakers are applicable to room or window units as well as to central air conditioners and heat pumps. Where a room air conditioner is added to an existing dwelling and supplied from an existing panelboard or load center, it is important to verify the branch-circuit overcurrent device for the circuit supplying the air conditioner complies with the type and size overcurrent protection stated on the units nameplate.
It is recommended that the following electrical product safety guide card information and product safety standards be consulted for additional guidance on the proper installation, operation and use of electrical equipment covered in this chapter. The four letter code in parenthesis refers to the product category in the Underwriters Laboratories Directories.
