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Active Contributor

Join Date: Sep 2007
Posts: 21

How Can You Calculate FLA From LRA

09/16/2007 5:15 AM

Hi Everybody,

If can anybody tell me how I can calculate FLA from LRA?

All I know is a Compressor motor/ 415V@50Hz/ 3Ph and LRA 59A. So I want to know FLA for this Motor.

RGS

Ediri

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#1

Re: how can calculate FLA from LRA

09/16/2007 5:38 AM

Can u please define the terms properly.I just wanted to check with u

By stating 1. FLA u mean Full Load Ampere

2 . LRA??

Is it Lowest rating Ampere of compressor or current of compressor at no load.

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Active Contributor

Join Date: Sep 2007
Posts: 21
#2
In reply to #1

Re: how can calculate FLA from LRA

09/16/2007 7:04 AM

Hi,

sorry for that,,,,,,,,,

FLA Mean, Full load Ampere

LRA Mean, Lock Rotor ampere

Thanks,

Ediri

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Anonymous Poster
#20
In reply to #1

Re: how can calculate FLA from LRA

03/06/2008 5:41 AM

are you sure about that.

for me 'LRA mean lock rotor ampere.thats all

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Anonymous Poster
#21
In reply to #1

Re: how can calculate FLA from LRA

07/05/2008 12:36 PM

LRA stands for locked rotor amper and it is usually 10 times FLA

hosain

Hosain2000@yahoo.com

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New Zealand - Member - New Member

Join Date: Aug 2007
Location: Christchurch, New Zealand
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#22
In reply to #21

Re: how can calculate FLA from LRA

07/05/2008 5:00 PM

Usually in the range of 5.5 x - 9 x FLA. Modern motors commonly 7.0 - 7.5 x FLA.

Rarely as high as 10 x FLA

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Power-User

Join Date: Nov 2005
Posts: 173
Good Answers: 3
#3

Re: how can calculate FLA from LRA

09/16/2007 8:45 AM

FLA is usually the current drawn from the zero start-up of any motor in question.

The genreal rule of thumb for FLA calculation is the LRA rating X 150% = FLA.

59A x 150% = 88.3A

So calculate your primary fuss box for 100A max load and you wire size will be #1 Awg. If your motor has any type of failure during opperations, your feeders will still be protected. Another rule of thumb, most electrical ratings should be calculated at an 80% max load. Since your FLA rating is 88A, we round up to the next size that gives us an 80% rating, so #1awg wire is rated for 100A continuose load factor. It will suffice for your motor.

Maximo

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Guru
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#11
In reply to #3

Re: how can calculate FLA from LRA

09/17/2007 8:50 AM

Not true. FLA is current at rated horsepower. LRA is current draw with locked rotor. Starting current is a function of motor AND load.

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Guru

Join Date: Jul 2006
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#16
In reply to #3

Re: how can calculate FLA from LRA

09/17/2007 6:40 PM

I fully agree with Bill--the answer given by Maximo is NOT TRUE. I normally wouldn't add a post here because the other answers give a fairly good range of answers to the original post. However, the score of 1 for "good answer" requires my rebuttal. Perhaps we need a place to rate posts for a "bad answer".

JMM

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Power-User

Join Date: Jul 2007
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#17
In reply to #16

Re: how can calculate FLA from LRA

09/17/2007 9:58 PM

You've got my support on that one - for whatever value it has

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Participant

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#4

Re: how can calculate FLA from LRA

09/16/2007 9:28 AM

hi RGS

The ratio of starting current (LRA as locked rotor amps) to running current (FLA as full load amps) varies with the size and type of motor and may be 600% or more.

LRA (starting current) is always higher than FLA (running current)

As a practical rule for domestic an industrial squirrel cage AC medium size motors, it is very common to consider the ratio LRA/FLA=6.

MFK

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Active Contributor

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#5
In reply to #4

Re: how can calculate FLA from LRA

09/16/2007 10:13 AM

Thanks Lot Mr Marco Kes:

Thanks Lot

LRA/ FLA=6 can this formula use for single phase also?

And what is the HP range for domestic an industrial squirrel cage AC medium size motors?

RGS.

Ediri

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Participant

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#6
In reply to #5

Re: how can calculate FLA from LRA

09/16/2007 11:22 AM

1)This formula also applies for single phase; you must consider that the typical maximum hp rating of single phase motors is about 10HP.

2)In fact this rule applies in all HP rating (with independence of the nominal voltage of the motor).

I wished to say that generally speaking, the larger the motor, the lower the LRA per HP.

It is very useful to refer to NEC Table 430-151 (locked rotor current conversion table)

MFK

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Active Contributor

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#7
In reply to #6

Re: how can calculate FLA from LRA

09/16/2007 12:13 PM

Thanks Mr. Marco Kes:

Bye

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#8
In reply to #7

Re: how can calculate FLA from LRA

09/16/2007 6:09 PM

Use the x 6 as a very rough rule of thumb only. The LRA is usually in the range of 550% to 900% of the FLA of the motor. Modern higher efficiency motors tend to have a higher LRA than older motors and a ratio of around 750% is very common.

I would not rely on a "calculated" value for protection settings or cable sizing. Get the right answers from the manufacturer.

Best regards,

Mark Empson
http://www.LMPhotonics.com | http://www.LMPForum.com

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Anonymous Poster
#9
In reply to #8

Re: how can calculate FLA from LRA

09/16/2007 11:27 PM

Siemens has some training available from the home page. Here's the motor page. They might be able to help you with a specific application.

http://www2.sea.siemens.com/Products/Electric-Motors/Electric-Motors.htm

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#12
In reply to #4

Re: how can calculate FLA from LRA

09/17/2007 10:21 AM

<

As a practical rule for domestic and industrial squirrel cage AC medium size motors, it is very common to consider the ratio LRA/FLA=6.>

That's it!

Fortunately, at starting(LRA condition) load is low-low. So rotor speeds up fast to MATCH the demands of the LOAD near (~95%)synchronous Speed.This can be 110% rated load--but watch out for overheating!

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Anonymous Poster
#28
In reply to #4

how can calculate the cable size?

09/22/2010 1:21 AM

Dear sir, how can i calculate the cable size for 1500rt chiller,1110kw load and FLA-72A,AND LRA-428A? how do i need to select the cable size, as per FLA rating or LRA rating? or simply flaX150%-for this? is it 3cx50 cu/awa/pvc? or 3cx185 cu/awa/pvc? or 3cx300 cu/awa/pvc?

even for all the motors how to select the cable size, as per fla or for 1.5 times FLA or 6 times of FLA(LRA)?

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Guru

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#10

Re: how can calculate FLA from LRA

09/17/2007 12:02 AM

Being a compressor motor it may be a NEMA D motor, so the LRA/FLA may be less.

The 6:1 rule is usually a conservative rule of thumb for calculating inrush LRA. But I would not use it for setting an overload for the running amps. Contact the manufacturer.

You did not mention the NP HP, what is it?

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#13

Re: how can calculate FLA from LRA

09/17/2007 10:24 AM

~10Amps

Double check from motor nameplate HP/KW.

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Power-User

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#14

Re: how can calculate FLA from LRA

09/17/2007 10:25 AM

You can't! There are too many variables involved. You can only guesstimate the FLA based on the information given. Post #4 is your best "guessing" answer so far. If you can't read the motor nameplate to get the FLA, contact the manufacturer. If you can't read the Manufacturer's name on the nameplate, post a photo of the motor. Maybe one of us can recognize it. If this motor has been in service driving this compressor, give us the compressor specs. We may be able to reverse engineer this application to get some decent number that you can use to calculate wire sizes, motor protection values, etc. You could then run the compressor and measure the current draw, voltage drop, motor temperature rise etc. and adjust the load accordingly. If time must be spent getting this project up and running, I'd spend the time trying to get the FLA information from the manufacturer.

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#15

Re: how can calculate FLA from LRA

09/17/2007 11:15 AM

I'm curious as to where did you obtain the LRA value from?

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#18

Re: how can calculate FLA from LRA

09/18/2007 12:28 AM

That is unfortunately a common practice on some HVAC chiller compressor motors from people like York, Carrier and Trane. I don't know exactly why, but I suspect that it's because the chiller manufacturers want to try to get users to use their own service people under contract. It's a pain in the rear end. Their contention is that the motor protection is much more complex than normal because of the complex power ratings they use, the fact that the refrigerant is used to cool the motors themselves and the consequences of not getting it right are catastrophic. So they contend it should not be left to basic electricians. I don't know how valid that is, but by not providing pubished information most users succumb to the tactic and sign the long term service contract.

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Member

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#19

Re: how can calculate FLA from LRA

09/18/2007 5:57 PM

Well, some motors have something called CODE LETTER stamped on the plate, this figure gives you a relation between locked rotor KVA and the HP rating of the machine, which in turn, will let you calculate the true LRA/FLA ratio, if the nameplate of the motor has this code letter, you use the following table, NEMA standard, to know this ratio, (I copied the following information, on bold characters, from a Gillete Generators Inc Web page) :

You can see this info at http://www.gillettegenerators.com/sizing/sizing03.html

Look the nameplate of the motor, to see if it has a Code Letter, if not you can safely assume it is a K letter motor, with a 8.0 to 8.00 KVA locked rotor/HP ratio. According to the data you gave, your motor is about 5 HP, with a FLA of more or less 6.5 A

Hope this could be useful

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Anonymous Poster
#23

Re: how can calculate FLA from LRA

03/29/2009 10:47 PM

According to the Canadian Electrical Code, I quote.

"Locked rotor current means a current rating marked on electrical equipment or, where not marked, shall be deemed to be equal to 6 times the full load current rating from the nameplate of the equipment."

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Member

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Posts: 7
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#24
In reply to #23

Re: how can calculate FLA from LRA

11/11/2009 7:21 AM

hi,

even if the LRC is 600% is the std but in eff1 motors it may be more than 600% if we are not agree on the point and state the rule it may affect the effeciency parameters. How we can tackle that?

Chaitanya

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Anonymous Poster
#25

Re: how can calculate FLA from LRA

06/01/2010 8:53 PM

Ediri

A motor is a rotating machine. Therefore what is available at the shaft is a certain value of Torque at specified rotor RPM (which is know as rotor speed, Nr). On the motor nameplate, it should state the following as a minimum, as reqired by regulation.

Voltage, frequency, No. of phase (3), Power, power factor, winding insulation class (A, F, H...). With this information the FLA can be calculated as follows.

Power=1.7(Voltage).(FLA).(power fcator)

It is not possible to calculate FLA from LRA using electrical theory.

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Active Contributor

Join Date: Sep 2007
Posts: 21
#26
In reply to #25

Re: how can calculate FLA from LRA

06/02/2010 8:14 AM

Thanks Sir.

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Anonymous Poster
#27

Re: How Can You Calculate FLA From LRA

06/10/2010 7:23 AM

CAN UU TELL ME THE RANGE OF EQUAL OF COMPRESSORE

91.1 LRA 7.5 HP COPLAND USA AND 72 LRA 7.5HP BRISTAL USA

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Users who posted comments:

Anonymous Poster (7); Bill (1); chaitanya1986 (1); dhilju82 (1); ediri69 (4); FKIA (2); GW (1); jmueller (1); joeconed (1); JRaef (1); LordMaximo (1); Marco Kes (2); marke (2); Mister (1); MUKULMAHANT (2)

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