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Power System Terminology

09/25/2017 1:26 PM

Hello

I am confused about some terminology for small power systems

Mainly about the different between installed power and useful power.

Lets take a hypothetical small 110VAC bus and also for the sake of it, say a small 480V 3 phase bus

When looking at a drawing package for these small buses i always see on the first page two ampacities. "Installed" and "useful power".

I have found just by way of google that,

Installed power is = the sum of the nominal powers of all power-consuming devices in the installation

and i have been told that "useful power has to do with 2 main things

Efficiency and power factor essentially.

I was told that the useful power is the value to protect for and to size cable for.

And it is essentially the installed power with a .8 PF applied or simply 80% of the installed power

This seems strange to me.

So back to the installed power.

When summing up installed powers in the case of a motor. Is the number you add for the motor the nameplate power? Because i always was under the impression the nameplate power was mechanical power. And we all know the electric power consumption is more then the mechanical power do to losses.

So wouldn't you want to protect for MORE then that not less?

In a different scenario say you look up in the IEEE book that a 3/4 HP motor has a 1.8A draw

Is that 1.8A useful or installed?

Now back to my 110VAC bus

Say you installed a small 24 VDC power supply on a little 110vac bus

The cut sheet says it draws 6 amps 110 VAC and produces 20A DC

Is the 6A added to the value of "installed power" or useful power?

Thanks in advance for helping out

I think i am just confused about the wording

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#1

Re: Power System Terminology

09/25/2017 2:39 PM

The power demand is going to depend on the nature of the load....

http://www.electrical-installation.org/enwiki/Installed_power_loads_-_Characteristics

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#2

Re: Power System Terminology

09/25/2017 3:59 PM

The main point of my question is to understand the difference between installed and useful power

And how generally speaking those numbers are considered.

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#3
In reply to #2

Re: Power System Terminology

09/25/2017 4:07 PM

..."POWER FACTOR is the ratio between the useful (true) power (kW) to the total (apparent) power (kVA) consumed by an item of a.c. electrical equipment or a complete electrical installation. It is a measure of how efficiently electrical power is converted into useful work output. The ideal power factor is unity, or one."...

...all of this is explained in detail, with examples, on the website I linked to...You must know the nature of the load to determine power requirement of your supply...You seem to be trying to make this complicated when it is quite simple...

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#4
In reply to #3

Re: Power System Terminology

09/25/2017 4:18 PM

So are you trying to tell me that the difference in installed power and useful power is like the difference between VA and Watts but in a measure of current?

So VA obviously is the total complex power

where watts is the useful power effectively converted or real power of course

So installed power is the total current but includes both reactive current and real current

And useful power is just the real current?

Is that what your trying to say?

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#6
In reply to #4

Re: Power System Terminology

09/25/2017 4:25 PM

No that's not the whole picture, that's just the starting point, a lot of other factors come into play....Read start at A2

https://www.schneider-electric.pl/documents/Polish_Catalogue/Technical/eig.pdf

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#5
In reply to #3

Re: Power System Terminology

09/25/2017 4:22 PM

Im pretty sure you made a type here.

Power factor is indeed not the ratio of Apparent / Real

The power factor is indeed the ration to reactive vars / Real Watts

Taking the tangent inverse of that will tell you the angle between the two vectors

Taking the Cosin of that angle yields the power factor in a percentage from 0 - 1

Aka as the percentage

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#7
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Re: Power System Terminology

09/25/2017 5:57 PM

..." Because true power and apparent power form the adjacent and hypotenuse sides of a right triangle, respectively, the power factor ratio is also equal to the cosine of that phase angle. Using values from the last example circuit:

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-11/calculating-power-factor/

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#8

Re: Power System Terminology

09/25/2017 6:31 PM

I'm thinking you're getting dragged down by definitions. It's more important to understand the principles.

Total Power (KVA) = Power (KW) / PF. Lower power factor (PF) requires more current for a given amount of power.

Efficiency = Power Out/Power In. Your mechanical output from motors will always be less than electrical power input. Efficiency < 1.

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#9

Re: Power System Terminology

09/25/2017 10:32 PM

Im just going to leave this here :

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#10
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Re: Power System Terminology

09/25/2017 10:43 PM

This requires some hands-on research...

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#11
In reply to #10

Re: Power System Terminology

09/25/2017 10:52 PM

I concur. :)

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#12

Re: Power System Terminology

09/26/2017 9:57 AM

Ok, so i see your point here

And i understand all that. But what i said is not incorrect either.

We are both saying the same thing. We are just doing it with different trig identities.

But that is less concerning really.

One of my main questions i want to know is the following.

When we use a table like the IEEE standard to look up the current draw of a 3/4 hp induction motor and it says 1.8A or whatever it is.

Is that number taking into account all the corrections? Aka it makes an assumption of power factor and also efficiency based on generally available industrial machines.

Then the other question im interested in. How does one consider nominal power for DC items connected to the bus?

Say a 24VDC power supply? So say you have a 110 VAC bus in cabinet. And you want to determine the trasnformer sizing for the 110VAC bus.

On the bus you have some lights, maybe a few PLC output cars that draw very little because all they are doing is closing a relay and and a 24VDC power supply for some field devices.

The power supply says it draws 6A and outputs 20A DC

For DC items like that is it practice to just use the 6A number as a nominal load?

I wish someone would make a good book on low voltage design. In school we are taught all about how things work and yada yada but very little about practical design

Some of the books i have found thus far that claim to be "design books" are just another book on energy conversion and how induction machines work. If anyone knows of a good book for design methodology please let me know.

That wiki page you linked me to has some information on making estimates based on coordination and how much equipment is used. But its not the complete picture.

Thanks in advance.

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#13

Re: Power System Terminology

09/26/2017 4:36 PM

I am not in USA [ 110 & 480V suggests US practice] & my contact with National Electrical Code NEC is long ago but my approach would be to look at a copy in a library and look at the "Definitions" chapter.

Tabulating amp capacities as" installed" and "useful power" seems as if it were written by someone with little grip on the difference between current and power. Could there be an error in translation to english involved with the word "useful"? Your description of "useful power" suggests it is an essential power exceeding the "useful" installed output power - which is a contradiction.

What "drawing package" are you looking at? Could you scan the page and put it in a post so others know what you see?

As you state, the output power must be divided by efficiency and power factor to get a volts x amps input which is a bigger number - needing more amps at a standard voltage.

Wherever possible, use the current draw given for the equipment rather than an estimate from a required output, say a horsepower - size and speed of a motor have significant effect on efficiency and power factor. It is normal to take values from a manufacturers catalog for standard motors at the planning stage before actual types are known and get a speed if possible.

Motor rating plates or data tables often give efficiency, power factor, rated volts & amps at rated power as well as a starting current as amps or code letter. These are given because they are all necessary, not so you can calculate one from others...

Rated volts gives the required supply.

Rated current and starting current [and duration] decide cable size and protection (voltage drop during start may dictate thicker cable, transformers have a turn-on current which affects protection).

Efficiency with rated output gives losses which affect enclosure or ventilation requirements.

Power factor, especially summed up for a whole supply, may indicate increased voltage regulation or need for power factor correction.

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