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Guru

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How to Calculate Motor Full Load Current

03/04/2009 9:42 AM

Should we consider the efficiency to calculate the motor full load current?

For example:

If we have the motor of the following specifications:

25 HP, 60 Hz, 480 V, 3 Phase, 0.8 p.f, 89% efficiency at full load

Which of the below formula should we use to calculate the Full Load Current?

Motor Full Load Current = (0.746 x 25 x 1000) / (1.732 x 460 x 0.8)

OR

Motor Full Load Current = (0.746 x 25 x 1000) / (1.732 x 460 x 0.8 x 0.89)

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Guru
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#1

Re: Motor Full Load Current calculation

03/04/2009 10:45 AM

Depends on what the 25HP is? formula 1 if 25HP is input power and formula 2 if its output power.

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#10
In reply to #1

Re: Motor Full Load Current calculation

03/06/2009 3:27 PM

See, the original post is telling that readings are based on name plate specifications. If that is so, then it is illlogical to ask whether 25 HP is input or output.

All the name plates are giving motor output powers only.

Hence FLA = KW / 1.732 x V x pf X effi. (if motor is 3 phase motor)

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Guru
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#12
In reply to #10

Re: Motor Full Load Current calculation

03/07/2009 2:18 AM

Where is the word name plate in original post?

And name plate is not a science it just describes certain data that is the requirements of certain standard and regimes.That does not mean there is no such thing like input power.

What if tomorrow world over convention changes and they start writing input power consumption on name plate.are you going to change your post?

Standard is a set of document that is made for our convenience but that doesn't means its the science.also its not necessary that standards are always logical. if that would be the case there would be no amendments in due course.

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#18
In reply to #12

Re: Motor Full Load Current calculation

03/24/2009 6:36 PM

Hai You I think you no need to get so anguish on name plate details/specs and why yu are further worrying about tomorrow's world conventions?

Pls refer the specifications for making name plates. They have a set of procedures. And the directions are not only for motors but also pumps, tvs, mobiles, computers, cables, pipes, stoves, shooes, etc. It is not like that any one can write any thing.

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Guru

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#2

Re: How to Calculate Motor Full Load Current

03/04/2009 11:22 AM

Best is to check the motor rating plate and don't calculate.

calculate only if it is a homework/project work.

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Guru
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#3

Re: How to Calculate Motor Full Load Current

03/04/2009 3:45 PM

Motor nameplate data is mechanical power. Electrical power takes efficiency and power factor into consideration. So the electrical power formula would be the one with both factored in, the second one in your post. The other one is essentially meaningless when determining current.

Your formula is a bit redundant however. No need to use .746 and then multiply by 1000, just use 746 W/ HP, so 25 x 746 / 1.732 x 460 x .8 x .89

But the Power Factor numer will vary my loading, so keep that in mind. In other words, if you want to know amps at any given amount of mechancial load on the motor, you will need to know the pf at that point, which gets worse as the load decreases. In your case here you were looking for Full Load current, so your formula is valid, I'm just pointing out that for determinig true load power consumption, you need a bit more than standard nameplate data.

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Guru

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#4
In reply to #3

Re: How to Calculate Motor Full Load Current

03/04/2009 8:59 PM

At our part of globe , motor name plate data do provide FLA in both Y and Δ

Alsorequired as per IEC adn NEC

http://ecmweb.com/mag/electric_understanding_induction_motor/

http://tristate.apogee.net/mnd/mfnrovr.asp

Hence I was surprised the OP is asking this question.

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#5
In reply to #3

Re: How to Calculate Motor Full Load Current

03/05/2009 4:12 AM

dear sir

i'm agree with you, as the data on motor name plate considering the output mechanical power, not the input electrical power so we should considering the efficiency for full load current calculation, and copmaring it with the rated current on name plate of the motor

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Guru

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#8
In reply to #5

Re: How to Calculate Motor Full Load Current

03/06/2009 7:48 AM

It is obviuos that you are not aware of the IEC/NEMA codes.. These codes, either of them are now adopted by most of the nations/ national codes are mede inline.

Excerpts from the first link that I referred

The NEC states that the motor nameplate must show the following information:

  • Rated voltage or voltages
  • Rated full-load amps for each voltage level
  • Frequency
  • Phase
  • Rated full-load speed
  • Insulation class and rated ambient temperature
  • Rated horsepower
  • Time rating
  • Locked-rotor code letter
  • Manufacturer's name and address

In addition to this required information, motor nameplates may also include data like frame size, NEMA design letter, service factor, full-load efficiency, and power factor.

Finally, some nameplates may even include data like bearing identification numbers, certification code, manufacturer serial number, and symbols and logos.

and copmaring it with the rated current on name plate of the motor

Since there is already the full load rated current on the name plate, which you are agreeing too, I don't see any fun in comparing with the calculation. As a regular user of the motors in various applications, from few KW to a couple of MWs, at least we do not take the pain.

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#9
In reply to #8

Re: How to Calculate Motor Full Load Current

03/06/2009 3:11 PM

Mr. sb

thanks for your clarification for motor name plate !!!!!!!!

but the question for Mr, msamad was

"Should we consider the efficiency to calculate the motor full load current?"

and any one can read the name plate data easily but the engineering wise is how to

learn more about it,

and if you think that the rated power on the name plate of any motor is for output

power, so????!!!!!!

Regards

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Guru

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#11
In reply to #9

Re: How to Calculate Motor Full Load Current

03/06/2009 10:46 PM

if you think that the rated power on the name plate of any motor is for output

power.

It is not what I think, it is..

Input = output/η.

KVA = KW / cosΦ

KVA = (√3) V I

where is the question (as per as the OP is concerned) ?

and if the question is whether the motor rating KW/ HP is input (electr) of output (mech) it is already covered in quite a few threads.

In the end still the question remains - if the motor name plate already has clearly the rated current (FLA) at the rated voltage why to calculate . Unless it is a class assignment. (Of course I didn't look from this angle).

And if the voltage/ load is not the rated , even η can not be taken.

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#22
In reply to #11

Re: How to Calculate Motor Full Load Current

06/08/2010 9:26 AM

i will do it with you soon

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#23
In reply to #11

Re: How to Calculate Motor Full Load Current

02/22/2012 8:14 AM

dear Mr sb

I hope that you realize that the motor efficiency will cause extra input power than mentioned on the name plate so the motor full load line current will be greater what is mentioned on the motor name plate as all data mentioned on motor name plate based on shaft power not input power, for this reason we have motor effieciency

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#6

Re: How to Calculate Motor Full Load Current

03/05/2009 5:28 AM

Dear

Motor Full Load Current = (0.746 x 25 ) / (1.732 x 460 x 0.8 x 0.89)

Above formula is correct

S.Pathan

India

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#7

Re: How to Calculate Motor Full Load Current

03/05/2009 1:37 PM

The formula to be used in your case is the following:

P = I * U * sqrt(3) * pf * eff, which is

I = P/(sqrt(3)*U*pf*eff), (for Y connection)

I* sqrt(3) = P/(U * pf * eff )(for delta connection)

where

I = full load current [Amperes] or (I * sqrt(3) for delta)

P = output power [Watts]

U = voltage (line-to-line) [Volts]

pf = power factor as decimal

eff = efficiency as decimal

Since you have the output power in HP (horse-power) you have to convert it to Watts, where 1 HP = 746 W

For your data:

P = 25 * 746 = 18650 W

U = 480 V

pf = 0.8

eff = 0.89

This way you get:

I = 18650/(sqrt(3)*480*0.8*0.89) = 31.5 A (for Y/wye/star)

and I = 54.56 A (for delta connection)

The frequency does not intervene directly in the formula. To get the result in the indicated measuring unit [A] all data have to be entered in measuring units as shown above [W,V,..]

I represents in this case the full load current. For other values for other parameters you get other currents at lower loads. If the power supply system is the same ( voltage, frequency), the values for P, pf and eff vary, as other participants also pointed out.

Be careful that the value of 31.5A is valid only for a motor designed to be connected in Y (wye/star). If the motor has been designed to be connected in delta, the current value is 54.56 A.

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Guru

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#13
In reply to #7

Re: How to Calculate Motor Full Load Current

03/07/2009 10:54 AM

Thanks everyone who participated in this discussion. I know it is a silly question but it is very important as I came through the problem in my design work.

I truly understand the NEC 2008 article 430 (especially 430.6(A), Table 430.250 etc.).

My problem here is:

In my design I have to use 350 HP, 4160 V motors. I don't have the motor vendor data (name plate data) at this stage of my design. It is needles to say that I have to find the motor full load current at this stage of my work. I know NEC Table 430.250 should work but Table 430.250 doesn't have any column for 4160 Volts (it has 2300 Volts column). NEC 430 doesn't say anything about how to determine the FLA for a motor of Voltages that is not available in this table. So, I have only 2 options:

(1) To convert FLA from 2300Volts to 4160 Volts with a multiplying factor (4160/2300). The FLA of the motor can be 83 x (4160/2300) = 150 A.

(2) To calculate FLA using the basic electrical formula that we are discussing.

There is nothing in NEC that we can do the option 1 (I think that is a flaw in NEC). I was trying to use the option 2 and then I came through this silly question.

I need valuable suggestions in this regards.

Hope it should now clear why it was posted.

Thanks,

- MS

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Guru

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#14
In reply to #13

Re: How to Calculate Motor Full Load Current

03/07/2009 1:06 PM

That's why you should put the cards down in the beginning .

Well now you have the answers. (Several of the threads but boils down to same)/

But what is your assumed η ?

And remember you have to take care of the starting current too.

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#15
In reply to #13

Re: How to Calculate Motor Full Load Current

03/09/2009 12:28 PM

MS,

When going from a lower voltage to a higher voltage the current goes down. Considering the efficiency, power factor nearly similar, for the same rated power, the full load current in your case should be 83 x 2300/4160 = 45.3 A.

The NEC is not a design tool, so it does not indicate any possible situation.

Item under 2 (basic formulas) is always valid if applied correctly.

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#16
In reply to #15

Re: How to Calculate Motor Full Load Current

03/09/2009 12:49 PM

Thanks so much. It is my mistake - realized after the post.

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#17
In reply to #16

Re: How to Calculate Motor Full Load Current

03/09/2009 10:01 PM

Getting clarifications always welcome (never mind our barbs)

The problem is with so menay homeworks going on (Is it homework season?) it becomes difficult for us to distinguish homework from actual doubt.

At least put your biodata in profile, since at least we can check and answer accordingly.

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#20
In reply to #13

Re: How to Calculate Motor Full Load Current

04/05/2010 12:36 PM

Just as a general rule-of-thumb, 4000V motors can be estimated at .14A/HP. This value is shown in most EASA motor application booklets, unfortunately you can no longer get those on-line for free, they sell them now. If you know of a good EASA member motor shop in your area, they may give them away to customers.

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#19

Re: How to Calculate Motor Full Load Current

04/05/2010 1:57 AM

Why need to calculate when there is name plate? How about if there is none? I think the point here is to be able to know how you derived the answer, what is the correct formula in getting the answer. Its just like how you get the 3.1416 on Phi? oh yeah because my teacher said so.

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#21
In reply to #19

Re: How to Calculate Motor Full Load Current

05/31/2010 5:58 PM

From Post #13...

"My problem here is:

In my design I have to use 350 HP, 4160 V motors. I don't have the motor vendor data (name plate data) at this stage of my design. ..."

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#24

Re: How to Calculate Motor Full Load Current

08/08/2014 10:52 AM

Dear Mr.msamd

For your Point " Should we consider the efficiency to calculate the motor full load current? " my answer is yes it is to be accounted. That is as you have stated "Motor Full Load Current = (0.746 x 25 x 1000) / (1.732 x 460 x 0.8 x 0.89)

DHAYANANDHAN.S

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