How Can You Calculate FLA From LRA

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.

Hi,

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

FLA Mean, Full load Ampere

LRA Mean, Lock Rotor ampere

Thanks,

Ediri

are you sure about that.

for me 'LRA mean lock rotor ampere.thats all

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

hosain

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

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

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

If the rotor is locked, then the motor doesn't have any load. If the rotor were locked, then the motor will behave the same as though it were collected to either no load or a massive overload.

The important thing is how the circuit protection operates in such circumstances.

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

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

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

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

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

Thanks Mr. Marco Kes:

Bye

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

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

<

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!

Easy. Follow the electrical code applicable in the location; in the UK it would be British Standard 7671, for example.

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?

~10Amps

Double check from motor nameplate HP/KW.

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.

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

Supply voltage divided by resistance at the motor terminals would be one way, though it doesn't allow for the inductive reactance at the windings, which will reduce the calculated figure on an AC supply.

If the Original Poster is trying to size a cable then the job is best left to a qualified local Electrician: these people do these things routinely as part of their everyday business and have no need to post in a global anonymous Engineering forum.

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.

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

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."

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

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.

Thanks Sir.

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

Not without seeing the motor rating plate, no.

The equipment supplier might be able to do this over the telephone, though.