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Old 01-23-2012, 04:54 AM   #1
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how to calculate No Load Ampere


Please suggest me
1) the calculations and thumb rule(if any) to calculate NLA (No Load Ampere) of electric motor.
2) what is the effect of motor re-winding on NLA.?
3) parameters to consider for motor selection.

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Old 01-23-2012, 06:23 AM   #2
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how to calculate No Load Ampere


1. if you mean a running motor with nothing attached to it, depends upon the size of the motor
2. unknown
3. voltage, phase, horsepower, rpms, torque, duty cycle and others

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Old 01-23-2012, 07:08 AM   #3
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how to calculate No Load Ampere


What are you trying to run with the motor or is it just a general question?
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Old 01-23-2012, 01:33 PM   #4
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how to calculate No Load Ampere


i am running a coal feeder with a 2 HP motor. the motor is connected to a VFD (Variable Frequency Drive). the motor mostly operates at 525 rpm (17.50 Hz) while feeding coal. it is a 3 phase supply. motor goes at no load when the coal feeding stops. how much current would a 2 HP motor draw at no load.?

Also, how much amperes would a 1 HP, 5 HP & 10 HP motor draw at no load.?
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Old 01-23-2012, 01:46 PM   #5
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how to calculate No Load Ampere


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Originally Posted by neomech2012 View Post
i am running a coal feeder with a 2 HP motor. the motor is connected to a VFD (Variable Frequency Drive). the motor mostly operates at 525 rpm (17.50 Hz) while feeding coal. it is a 3 phase supply. motor goes at no load when the coal feeding stops. how much current would a 2 HP motor draw at no load.?

Also, how much amperes would a 1 HP, 5 HP & 10 HP motor draw at no load.?
Why are you coming to a "DIY Chatroom - DIY Home Improvement Forum > Home Improvement > Electrical" message board to ask this??

Ask your foreman or supervisor. If there is no one on site that knows this then hire someone who does.
This is NOT DIY work!
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Answers based on the 2008 & 2011 NEC.
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Old 01-23-2012, 02:04 PM   #6
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Originally Posted by Speedy Petey View Post
Why are you coming to a "DIY Chatroom - DIY Home Improvement Forum > Home Improvement > Electrical" message board to ask this??

Ask your foreman or supervisor. If there is no one on site that knows this then hire someone who does.
This is NOT DIY work!

Sir, actually i am a engineer trainee in a small textile company.. Pls help.
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Old 01-23-2012, 02:24 PM   #7
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how to calculate No Load Ampere


Post it on the contractor forum theres a link on bottem of page.
Normally a motor will have its specs on the plate
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Old 01-23-2012, 02:41 PM   #8
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how to calculate No Load Ampere


Just wondering why you would want to know the no load amperage.

Is the VFD output at 17.5 Hz while there is no load on the blower (assuming you are conveying pulverized coal)?

Usually the VFD will tell you what the amperes are, just have to scroll through the display to find that parameter.

If you can't find that on the VFD there you would have to know what the other parameters of the system are

Bhp = (volume x static)/(6356 x mechanical efficiency)

Volume of Blower
Total Static Pressure in inches WC
6356 - Constant
Mechanical Efficiency for fans I have found .7

Once Bhp is known amperes can be calculated by

Horse Power x 746
1.73 x Volts x Efficiency x Power Factor


These are just calculations, the ablsolute best way is put a clamp on amp probe on each leg of the 480 V motor feeds and measure the amperage.

This system is 480 V right? if not eliminate the 1.73 from the amperage calc. Efficiency is motor efficiency and should be on the name plate of the motor if unknown use .9 for a power factor.

Mark
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Last edited by Jackofall1; 01-23-2012 at 02:43 PM.
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Old 01-23-2012, 02:51 PM   #9
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Originally Posted by neomech2012 View Post
Sir, actually i am a engineer trainee in a small textile company.. Pls help.
For question 1, one way is to find similar motors in the online Grainger catalog, write down the item numbers, call their 800 number and ask to be transferred to "tech. help". They are pretty knowledgeable.

A less precise way is to finds the efficiency or NLA for several similar motors online, put the data into Excel, use the stat functions and come up with your own rules of thumb. You're looking for means, quartiles, slope/intercept and r, the correlation coefficient.

I recommend doing both, the first method as a check on the second.

BTW, especially for engineers I recommend doing consulting as soon as you can or have a second job totally unrelated to the first.

Most employers want to know
can you do the work?
will you do the work?
are you desperate? (and the second job solves this problem).

It also wouldn't hurt if you boned up on administrative law. The books on this will tell you that employers hold almost all the cards and you should know what cards they are holding.

Good luck.
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Old 01-24-2012, 12:39 AM   #10
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how to calculate No Load Ampere


The no-load current is specified at normal line voltage and 60Hz. That parameter is irrelevant to an installation with a VFD, if you are running at anything other than 60Hz and normal line voltage. The no-load current at any other voltage/frequency combination will be very different. You'll probably have to figure it out in the field by measuring it. Doesn't the VFD have a current display?
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Old 01-24-2012, 07:26 AM   #11
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For question 1, one way is to find similar motors in the online Grainger catalog, write down the item numbers, call their 800 number and ask to be transferred to "tech. help". They are pretty knowledgeable.

A less precise way is to finds the efficiency or NLA for several similar motors online, put the data into Excel, use the stat functions and come up with your own rules of thumb. You're looking for means, quartiles, slope/intercept and r, the correlation coefficient.

I recommend doing both, the first method as a check on the second.

BTW, especially for engineers I recommend doing consulting as soon as you can or have a second job totally unrelated to the first.

Most employers want to know
can you do the work?
will you do the work?
are you desperate? (and the second job solves this problem).

It also wouldn't hurt if you boned up on administrative law. The books on this will tell you that employers hold almost all the cards and you should know what cards they are holding.

Good luck.
Thats a bit dark and stormy, sounds like you were left with a bitter taste in your mouth. But its always good to have a couple of areas of expertice in your pouch.
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Old 01-24-2012, 09:33 AM   #12
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how to calculate No Load Ampere


I would consider this more complicated than a homework assignment in an electrical engineering undergraduate course although somewhat less complicated than a term project.

Maybe a two week undergraduate "project" over and above the usual daily homework or a one week graduate student project, or a half week assignment for an EE in the field.

This includes gathering published data for various motors and making your own formula and then computing your answer for the subject motor using your formula. This is part of what "engineering" is.

Note that almost none of us who frequent this forum are EE's.

(I noticed that your original question used the word "calculate" as opposed to "measure".)
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Last edited by AllanJ; 01-24-2012 at 09:35 AM.
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Old 01-27-2012, 02:22 PM   #13
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Thats a bit dark and stormy, sounds like you were left with a bitter taste in your mouth. But its always good to have a couple of areas of expertice in your pouch.
Did better than 87% of the people who took the LSAT before me but still couldn't get into law school. Now I don't have the bucks but I'm no less angry than back then.
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Old 01-27-2012, 08:03 PM   #14
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how to calculate No Load Ampere


A Leeson 2HP 3 high-efficiency TEFC 4 pole (1800 RPM) motor operated at 480 volts will draw 1.7 amps at no load, and 3.1 amps at full load.

The difference in the two currents at reduced frequency is very likely less.

Rob

P.S. In my experience;

1) The smaller the motor, the less difference in NLA vs. FLA

2) The lower efficiency models are usually less difference.

3) The more poles (more poles = higher RPM), the more the difference.

As far as I know, there's no way to accurately calculate the difference, it must be measured.

A lot of this difference is due to the basic design of the motor, there are many factors to consider.

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Last edited by micromind; 01-27-2012 at 08:13 PM. Reason: Added P.S.
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