Relationship: Locked rotor amp and full load amp 2

[place1234]

What's the relationshop between Locked rotor amp and full load amp?

 

[maypot]

Hi,

The locked rotor current is the measured current with the rotor locked and with rated voltage and frequency applied to the motor.
The amount of current a motor can be expected to draw under full load (torque) conditions is called Full Load Amps.
I hope this helps.

Bob

 

[electricpete]

Typically locked rotor amps is approx 5 * full load amps but it varies.

LRC at rated voltage can be determined from KVA code and horsepower (both on the nameplate).

FLA is usually on the nameplate.

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[TheBlacksmith]

Locked rotor is almost purely the impedance of the stator and the applied voltage, full load includes the counter EMF (that opposes applied voltage) caused by the rotor spinning.

Blacksmith

 

[rbulsara]

The relationship between the LRA and FLA depends on the design of the motor. In the USA motors are built to NEMA standards which have assinged code letters such as A,B,....G, H etc. to motor designs and are noted on motor nameplate. Each code letter relates to a multiplier which when mulitplied with the FLA gives the LRA.

These code letters and multipliers are also listed in NEC Article 430. I would suspect there is such published relationship for IEC motors as well.

 

[aolalde]

Hi place1234;

For standard motors similar to NEMA DESIGN B the ratio LRC/FLC is 5.0 to 5.6, this ratio range corresponds to the code letter "F".

For different type of design, the Nameplate has the "code letter designation".

 

[Marke]

The LRC (LRA) on high efficiency motors is commonly in the region of 700% - 850% of the rated current of the motor so it is significant. The LRC is primarily a function of the rotor bar design.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[stardelta]

I am not aware of any requirement relating to IEC standards that mentions the need for LRC to be mentioned on motor nameplates, however I may have missed something and would appreciate anyone providing me with further information on the subject.

 

[machmech]

I may get beat up badly here but I was under the impression that locked rotor amp or current relates to starting or inrush current at start of motor, and full load current is after start and running steady while under load. Jraef please advise!

 

[itsmoked]

machmech that is correct! Other than to say full load current isn't "running and steady" but is the MAXIMUM ALLOWED "running and steady". Most of the time motors don't run exactly at FLA.

 

[jraef]

LOL, machmech,
I read your post and was thinking "Didn't I already post something?". I started to get worked up because I thought maybe someone red flagged my post. Then I realized it was a very similar posting in a completely different forum!

You are essentially right, with the caviat that itsmoked brought up, but I think the OP was trying to get a formula for determining the relationship between the two, and the other posts explain it well.

Bottom line, there is no grand unifying formula because it is different on different motor designs, but LRC is going to be "somewhere" between 500% and 850% of FLC in a Squirrel Cage Induction Motor. In Wound Rotor Induction Motors it is totally dependant on the rotor resistance during start. Synchronous Motors can be very different, again depending on design. I am working on one right now that is only 280% so we have to custom design an overload curve to rotect it.





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[machmech]

Hello Jraef

The OP posted the word relationship and I misunderstood since I thought of metering under two entirely different conditions. Then Maypot posted "The locked rotor current is the measured current with the rotor locked and with rated voltage and frequency applied to the motor." I wondered if anyone in the field takes a reading like this, if so only reason I can think of is lack of in rush current capture on meter or extreme testing.

I have a handy Little Lewies manual and it gets me ballpark. And yes the other guy's responded well, But your use of a few more fonts makes things crystal clear.
Once again I Stand corrected.

Thank you Itsmoked and Jraef

 

[electricpete]

Not measured in the field like that but sometimes measured in the factory like that.

What fonts?

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[itsmoked]

electricpete; jraef must have 6 fonts!! and 2 colors! Dressed up like a par-tee gal.

Now I think electricpete is using a PC XT with a mono screen and one font loaded.

 

[electricpete]

I guess fonts was referring to another post. I have noticed that from jraef. But he's got some competition over in the power engineering forum from cuky who produces some real flashy posts (and also full of good info like jr).

My computer? Not quite an XT but close. My wife gets the superwhambeeydyne computer and I get the dog computer. I guess we know who wears the pants around here. Here comes my flashee smiley face. Get ready.

;-)


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[itsmoked]

Hahah...hmmm.. not even an animated one.. tisk tisk.

 

[davidbeach]

So, itsmoked, where do you get all those smiley faces?

 

[machmech]

Hi Folks

LOL Seek and ye shall find, I had a feeling I'd be taken to the wood shed for that whoopin! Have a good weekend.

Thanks

 

[itsmoked]

davidbeach; somebody noticed!?

Oddly it took a while for me to stumble across them.

Directly below the window you type in to post here, there is a box that sez "Emoticons/Smileys".

You should have a check in its box [default setting I think], this lets a reader see Smileys and Emoticons.

Click on the "Emoticons/Smileys" words.

A window will open that shows a zillion Smileys(non-animated). At the top of this column there are the titles "Animals" "Animated" etc., etc., click these and look around.

If you want to use one, just highlight the [xxxxx] in the left column including the [ and ] and drag it into your response, or copy&paste. That's all there is to it.

Now for colors. Very oddly the only instructions for this are in the "Preview Post" window.. {no sense at all} There you can read that for color, Example:

{color red}words to be colored{/color} would make the words in between the bracketed stuff the color specified. IF THE "{" brackets are actually "[" type brackets. I couldn't use them or they would've been processed causing words to be colored

Have fun!

 

[davidbeach]

itsmoked - thanks. I had long ago found the basic smileys, but hadn't ever noticed the links for more.