CR4® - The Engineer's Place for News and Discussion®

 Previous in Forum: Multisource Ground Fault for MTTTTTTM Switchgear Next in Forum: 3Phase Transformer Connection Selection
Commentator

Join Date: Jan 2017
Posts: 57

# Is My Memory Shot? 3Phase Calc Check?

05/16/2017 7:06 PM

So im told 480V 3P motor

And i also know its .5HP

So i should have enough to find the Power factor no?

I get

914VA = Sqrt(3)*480*1.1

746Watts/.5 = 373 Watts

so

914VA = Sqrt( 373,3^2 + Vars ^ 2)

Solve

means 834.9 Vars

Tan inverse ( 834.9/373) is about 65 degrees

Im being very very loose here assume ideal conditions and the math is just approximate before i get torn to shreds.

Just looking for a logical check here. Thanks

Interested in this topic? By joining CR4 you can "subscribe" to

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 19999
#1

### Re: Is my memory shot? 3Phase Calc Check?

05/16/2017 7:37 PM

PF = P / [(3)1/2 U I]

where

PF = power factor

P = power applied (W, watts)

U = voltage (V)

I = current (A, amps)

http://www.engineeringtoolbox.com/power-factor-electrical-motor-d_654.html

__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Commentator

Join Date: Jan 2017
Posts: 57
#2

### Re: Is my memory shot? 3Phase Calc Check?

05/16/2017 8:19 PM

Ah well then by your equation I get .407

My math gave me .42

Ill call that a success.

Thanks for that great website. Never seen that before

Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 19999
#3

### Re: Is my memory shot? 3Phase Calc Check?

05/16/2017 10:14 PM

I think you should take the amp reading under load for a more accurate assessment....

__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Commentator

Join Date: Jan 2017
Posts: 57
#8

### Re: Is my memory shot? 3Phase Calc Check?

05/17/2017 10:33 AM

Im getting the 1.1A rating from the standard that says

.5HP motor at 480LL is 1.1A

Right from the IEEE

Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 3947
#7

### Re: Is my memory shot? 3Phase Calc Check?

05/17/2017 10:01 AM

OK as long as he realises p is the electrical power, not the 0.5hp shaft power (I know it says applied power, which should give him a clue). He has to divide shaft power by efficiency to get p.

__________________
Give masochists a fair crack of the whip
Commentator

Join Date: Jan 2017
Posts: 57
#9

### Re: Is my memory shot? 3Phase Calc Check?

05/17/2017 10:38 AM

hu?

I have never in any time or place in lots of engineering seen or been told that a 1hp motor produces anything other then 746 watts of electrical power. Apparently im missing something and eager to learn what you mean

I should preface this by saying i dont normally work in this industry.

Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14345
#10

### Re: Is my memory shot? 3Phase Calc Check?

05/17/2017 10:47 AM

Motors do not always produce the nameplate rating of power, nor do they consume the same in electrical power. I actually depends on what mechanical load the motor is being required to support at the output shaft.

A motor spinning at full rpm's with nothing connected to the shaft will represent minimum input current (other than the current in-rush during start up).

A motor may have load added to the output shaft progressively, so that one will observe a slight decrease in shaft rpm (known as slippage). Eventually, the load becomes so great, it is possible to stall the output shaft to zero rotation, then all the electric power going in (the maximum the supply can inject in terms of current) is converted to heat, since the work output is now zero.

__________________
If it ain't broke, don't fix it. Just build a better one.
Commentator

Join Date: Jan 2017
Posts: 57
#11

### Re: Is my memory shot? 3Phase Calc Check?

05/17/2017 10:53 AM

Yes i understand that

So what you are saying is, that if a motor is rated at 1hp or 746 watts but is only 50% efficient in transferring the electrical power to mechanical power then we should say it only produces 373 Watts of mechanical power? The rest is consumed as heat.

However as far as the power factor is concerned it is still consuming 746 watts of electrical power. So why would we use anything else but 746 in the case of this hypothetical motor for a power factor calculation

Score 1 for Off Topic
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14345
#12

### Re: Is my memory shot? 3Phase Calc Check?

05/17/2017 10:58 AM

Hopeless. The phases result in current lagging the voltage in an inductive system.

As load increases, the phase lag must increase. How in heck do you know what the current and voltage are (at any given point in time) if you are not measuring at least something?

__________________
If it ain't broke, don't fix it. Just build a better one.
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14345
#18

### Re: Is my memory shot? 3Phase Calc Check?

05/17/2017 12:04 PM

So your degree justifies you abusive language. I see now.

Nobody said you were hopeless, although you could be.

I was having a hopeless time attempting to explain this in terms an engineer might understand.

__________________
If it ain't broke, don't fix it. Just build a better one.
Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 3947
#22

### Re: Is my memory shot? 3Phase Calc Check?

05/18/2017 12:14 PM

That's not right, and don't want to cause more confusion! Power factor cos(φ) is maximum at full load (φ is minimum) and is lower at part load. It's minimum at no-load, which is why no-load current is higher than you might expect, 25 - 30% FLC if memory serves, even though no-load power is low.

__________________
Give masochists a fair crack of the whip
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14345
#23

### Re: Is my memory shot? 3Phase Calc Check?

05/18/2017 12:21 PM

I apologize. Motors are not my wheel-house, as you all know. 3-phase power is hard enough for me when I "know" the hook-up, let alone any other time.

__________________
If it ain't broke, don't fix it. Just build a better one.
Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 3947
#26

### Re: Is my memory shot? 3Phase Calc Check?

05/18/2017 1:48 PM

No problem

__________________
Give masochists a fair crack of the whip
Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 3947
#14

### Re: Is my memory shot? 3Phase Calc Check?

05/17/2017 11:22 AM

Posted #14 before seeing yours. You've been told wrong I'm afraid. A motor doesn't produce electrical power, it consumes it, and delivers shaft power. The motor rating is the shaft output power. Somebody buying a motor to drive a piece of kit eg a pump where he knows the power requirement, needs to know the motor will provide it. He's not concerned with the electrical power, apart from wanting it to be as low as possible.

__________________
Give masochists a fair crack of the whip
Power-User

Join Date: Apr 2017
Location: Southern Illinois
Posts: 223
#24

### Re: Is my memory shot? 3Phase Calc Check?

05/18/2017 12:33 PM

He means that if the bearings of the motor become rusted and don't turn as freely, or if some other factor changes, then any rule-of-thumb conversion is not adequate and you need some real-life measurements instead of name-plate data or guess-factors.

__________________
Science is the "cookbook" for making things.
Guru

Join Date: Oct 2012
Location: 100 miles North from the World Center
Posts: 764
#4

### Re: Is my memory shot? 3Phase Calc Check?

05/17/2017 3:12 AM

Such a motor presents efficiency 64% and power factor 0.64.Then the rated current:

I=0.5HP*746/SQRT(3)/480V/.64eff/.64pf=1.095 A

__________________
Julius
Guru

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain, and now disconnecting ["brexit" - ugh] as Little England and Wales (not too sure about Wales bit, either). Kettle's on.
Posts: 27036
#5

### Re: Is my memory shot? 3Phase Calc Check?

05/17/2017 4:01 AM

Bear in mind that power factor will vary with mechanical load on the motor.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14345
#6

### Re: Is my memory shot? 3Phase Calc Check?

05/17/2017 9:29 AM

Sounds like your motor is going to get really hot. Just MHO.

__________________
If it ain't broke, don't fix it. Just build a better one.
Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 3947
#13

### Re: Is My Memory Shot? 3Phase Calc Check?

05/17/2017 11:14 AM

No, that's wrong. In the formula p is electrical power. See #7. There's various other mistakes in the original post. My Mathcad calc is below.

__________________
Give masochists a fair crack of the whip
Commentator

Join Date: Jan 2017
Posts: 57
#15

### Re: Is My Memory Shot? 3Phase Calc Check?

05/17/2017 11:25 AM

No where did i say the motor was 64% efficient

In fact we dont know motor efficiency

Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 3947
#16

### Re: Is My Memory Shot? 3Phase Calc Check?

05/17/2017 11:39 AM

You didn't but we have to start somewhere, and 64% is a reasonable figure for a small motor, also mentioned in #4. I said assumed in the calcs. You can check the relation between output power, amps, PF and efficiency in a motor catalogue.

BTW don't worry about the apparent lack of conversion factors, Mathcad takes care of that for you (in case you didn't know)

__________________
Give masochists a fair crack of the whip
Commentator

Join Date: Jan 2017
Posts: 57
#17

### Re: Is My Memory Shot? 3Phase Calc Check?

05/17/2017 11:49 AM

Yes sir

And thank you

I do love mathcad as well.

Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 3947
#19

### Re: Is My Memory Shot? 3Phase Calc Check?

05/17/2017 2:13 PM

No problem! Glad to help. If you want you could put my calcs back into Mathcad and use any value you like for efficiency, to see what effect it has.

__________________
Give masochists a fair crack of the whip
Power-User

Join Date: Apr 2017
Location: Southern Illinois
Posts: 223
#20

### Re: Is My Memory Shot? 3Phase Calc Check?

05/18/2017 10:59 AM

Sorry, but i don't remember any "Vars". Maybe my memory is shot or you need to convert to proper units. What are "Vars", anyway?

__________________
Science is the "cookbook" for making things.
Power-User

Join Date: Jul 2016
Posts: 392
#21

### Re: Is My Memory Shot? 3Phase Calc Check?

05/18/2017 12:11 PM

Volt amperes reactive

Power-User

Join Date: Apr 2017
Location: Southern Illinois
Posts: 223
#25

### Re: Is My Memory Shot? 3Phase Calc Check?

05/18/2017 12:48 PM

Oh,yeah. I remember the concept, but I never used them. Maybe that's why I forgot. Thanks.

__________________
Science is the "cookbook" for making things.
Guru

Join Date: Jan 2006
Location: Tamworth, UK.
Posts: 1573
#27

### Re: Is My Memory Shot? 3Phase Calc Check?

05/18/2017 6:48 PM

Is .5hp the nameplate rating? or is it shaft power output? If so, and if a 'typical' motor, then the numbers hint at it being a bigger motor on light load.

__________________
When arguing, remember mud-slinging = lost ground.
Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 3947
#28

### Re: Is My Memory Shot? 3Phase Calc Check?

05/18/2017 7:15 PM

I think it's fairly clear the 0.5hp is the nameplate rating, there's no mention of actual output being measured, and if it had been it's unlikely to be spot on 0.5hp, which is a standard rating.

But looking at it again, the wording "The sheet says its load is about 1.1 A" does look a little odd. What sheet, and why "about"? Though 1.1A is a reasonable FLC for 0.5hp rating, with expected PF and efficiency, as per earlier posts. In any case, if it were a bigger motor running light the figure used for efficiency, and hence calculated PF, which is what the OP was asking, would still be reasonable.

__________________
Give masochists a fair crack of the whip
Guru

Join Date: Sep 2006
Location: California, USA, where the Godless live next door to God.
Posts: 4444
#29

### Re: Is My Memory Shot? 3Phase Calc Check?

05/18/2017 10:14 PM

Just to summarize:

1. The nameplate HP rating of the motor is the rated MECHANICAL shaft power it provides, defined as Torque (ft. lbs.) x RPM / 5250. Nothing about watts, vars, etc, yet. So the nameplate HP is NOT the electrical power it consumes.
2. The electrical power consumed (aka "Absorbed Power') by the motor is only indirectly related to the nameplate HP rating, in that WHEN the motor is producing the nameplate rated HP at the shaft (as defined above), and the voltage is exactly as shown in the nameplate rating, the FLA will be as shown on that nameplate.
3. You cannot "calculate" the actual energy consumed by a motor, you can only measure it. The energy consumed by the motor is totally dependent upon the actual connected load, i.e. the real work it is doing, plus whatever losses are associated with making into a motor instead of a boat anchor.
4. The closest you can get without actual measurements in as ESTIMATE of the worst case scenario. So that worst case is that the motor is running at full output HP and you ASSume the line voltage is dead on exactly the same as the nameplate voltage design rating, and your nameplate provides the Efficiency and Power Factor at that rated output, then you apply the following formula:
kW = 1.732 x E x I x Eff. x PF / 1000
5. If you are missing even ONE of those elements or the conditions are not EXACTLY as per the nameplate, then you are guessing.
__________________
** All I every really wanted to be, was... A LUMBERJACK!.**
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14345
#30

### Re: Is My Memory Shot? 3Phase Calc Check?

05/19/2017 10:34 AM

Thank you, and that is about what I thought. I am just not as sharp as you and some others on electric motors.

I need to take a refresher on this topic, so now I know which Odesie topics to click on this afternoon.

Under full load, would that not be the condition where the largest amount of heat dissipation is necessary also? Does the efficiency increase as the motor load on shaft decreases to near nil?

__________________
If it ain't broke, don't fix it. Just build a better one.
Power-User

Join Date: Apr 2017
Location: Southern Illinois
Posts: 223
#31

### Re: Is My Memory Shot? 3Phase Calc Check?

05/19/2017 10:59 AM

"Does the efficiency increase as the motor load on shaft decreases to near nil?"

No way. A unloaded running motor has all losses and no output. When you load down a motor, the losses don't go up as fast as the output. Therefore a loaded motor has a higher output/loss ratio (efficiency).

__________________
Science is the "cookbook" for making things.
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14345
#32

### Re: Is My Memory Shot? 3Phase Calc Check?

05/19/2017 11:04 AM

Alright then, turn off the derned motor when you are done with it then. Thanks.

__________________
If it ain't broke, don't fix it. Just build a better one.
Interested in this topic? By joining CR4 you can "subscribe" to