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#127117 - 05/01/01 12:38 AM true power compared to apparent power
Anonymous
Unregistered


Scott has offered to help us understand the differences between true power and apparent power -- why a volt-amp doesn't have to mean a watt.

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#127118 - 05/01/01 04:35 AM Re: true power compared to apparent power
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
one is influenced by lead/lag.
anyone recall;
ELI the ICE man


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#127119 - 05/01/01 06:23 PM Re: true power compared to apparent power
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Steve hit the culprite correctly with the ELI the ICE man reference.

Notice with each type of reactance, there's a difference in time when the voltage and current waves cross the zero line [or when they peak]?? That's what creates "Power Factors", since the waves are not crossing zero equally in time [AKA "Not In Phase Time"].

With something that has almost no Reactance [such a small amount of Reactance, it's less than 0.0001% of the load], it has close to no phase lag - meaning that the Voltage [E] and the Current [I] waves cross zero at the same time [in phase with each other]. These draw completely True Power only.

True power loads do not draw any VARs [Volt-Amps Reactive power], which means all the load current & Voltage for that Resistive load can be applied to the total wattage developed at the load.
In that case, ExI=P, and the P equals True Power [Wattage, or simply Watts].

Looking at this power on a Power Graph, the complete power wave is above zero line and peaks in one wave. [That's kind of the EE way of viewing it].

All this cannot be said for any REACTIVE elements and the circuits connected to them.

The term: ELI the ICE man shows which wave will lag, as to the type of Reactance involved. In the case of series elements with both types of Reactance, the one with the highest level will determine the type Reactance - also is the 2nd part of the Impedance formula.

These are abbreviations for terms:

X = Reactance [total],
XL = Inductive Reactance,
XC = Capacitive Reactance,
Z = Impedance,
R = Resistance [total],
KW and/or Wattage = True Power,
KVA and/or VA = Apparent Power,
KVAR and/or VAR = Reactive Power.

In case someone might be unfamiliar to ELI the ICE man, I'll describe it's intent:

ELI means that the voltage wave [E] will lead the current wave [I] on Inductive Reactances [L].
The Current wave will lag behind the Voltage wave, between 0.1 and 90 degrees - depending on the Power Factor at that element.

ICE means that the Current Wave [I] will lead the Voltage Wave [E] on Capacitive Reactances [C].
The Voltage wave will lag behind the Current wave, between 0.1 and 90 degrees - depending on the Power Factor at that element.

These Reactive elements draw Volt-Amperes from the AC line, in which two [2] separate types of power are contained:

1: True Power - this is the actual wattage being applied to the load element,

2: Volt-Amps Reactive - this is power that doesn't do anything as to the work [or kinetic energy] dissipated at the load is conserned. Simply, it does almost nothing usefull, but is a result of AC across something other than pure Resistance.

VARs are also termed "Scuttle / Shuttle Power", because they only flow between the power supply [transformer or generator] and the load element, but contribute nothing to the output KW at the load.
It does increase levels of Current [XL] which heats conductors, windings, and such - further aggrevating the Voltage Drops.
XC increases Voltages, so it too can result in an increased Current drawn on loads, due to the elevated E on the KW portions.

Power Factor [PF] is the relationship between the level of Apparent Power being drawn [KVA] and the level of True Power being used [KW] on a load. Levels run from 0.1% to 99.99999%
Nominal PF is 50% [this is a low power factor], whereas high PF is >95%. Corrected PF is anywhere from 70% to 90%.

The reciprocal power is the VARs.

VARs are magnetizing currents [XL], or Capacitive Charging Fields [XC], either way they do some type of charging.

Since there's all this lagging going on, the power cannot be correctly figured using ExI=P.
This will only result in Apparent Power [VA].
True Power is figured by adding the power factor to the formula:
ExIxPF=P [Volts x Amps x Power Factor = KW].

KVA / VA power waves on a power graph do not fall completely above the zero line. There's several peaks and the waves dip below zero, then rise above.
A phase angle [lag] of 90 degrees [the result of complete XL or XC only] will have equal peaks above and below the zero line. These cancel each other out, so there's no True Power being used at that element what so ever - even though there's plenty of KVA flowing.
This situation is not desirable for the complete load, as it does nothing!! It's something that a Ballast would {hopefully} have.

With phase lags less than 90 degrees, the level of power on the wave above the zero line becomes higher than below the zero line. Now there's something left over when the math is done - which results in True Power being included in the wave.


Let me know how this is and if I left something out.

Scott SET
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#127120 - 05/01/01 07:37 PM Re: true power compared to apparent power
Anonymous
Unregistered


Thank you, Scott.
 Quote:
It does increase levels of Current [XL] which heats conductors, windings, and such - further aggravating the Voltage Drops.
XC increases Voltages, so it too can result in an increased Current drawn on loads, due to the elevated E on the KW portions.

And would that energy dissipated in heating the conductors be metered as part of true power being drawn?

If the conductors become an unintentional part of the load element through heating, regardless of who is paying, is it then preferable to apply PF correction as close to the XL (or XC) source as practical, or is this something that can be just as well balanced once for the service in total?

For the next installment, perhaps you (or anyone else who wishes - it just looks like you are usually the one) could explain something of how an electric meter measures energy consumption.


[This message has been edited by Dspark (edited 05-01-2001).]

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#127121 - 05/01/01 08:48 PM Re: true power compared to apparent power
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Good Call!!

In this case, the total wattage on the circuit would be what's used by the load, plus the amount of power dissipated as heat.
Although that wattage from the heat will be low power, it might add upto as high as 3% of the total consumed KW. Run over time, that's an increase in KWH by 3%, while doing nothing productive.

Usually, the resulting heat causes a voltage drop, so the load's not going to be able to draw as much True Power as before, but the entire circuit might have the same total draw of True Power as it should - meaning the "hot Spots" on the conductors and windings will be the points of voltage drop[s], and also where the remaining wattage gets "Burned Up".

Swings in Voltage can really drum up some weird losses, especially on Reactive loads.
Something drawing KVA from the AC line will draw ever changing currents when the applied voltage changes. They will try to correct everything, but can only go so much!!

With the "Hot Spots", every one of them is a Resistor, in series with the load. As the current increases, so does the heat and eventually the resistance will increase.
Now there's not only a voltage drop, which effects the ExI=P at the load, but there's a level of current limitations at each Resistor.
That lowers the level of current flowing in the circuit, until things cool down, then more current can flow.
Meanwhile, all this baloney creates havoc with the load. If the desired True Power output at the load is non-linear [slowing a motor's speed down while cutting through a board], then it cannot continue developing that power and will end up stalling.

Fans and other types of pumps can deal with the reduction of available true power. They just spin slower, which moves less mass - which requires less power.
Ceiling fans have been purposely made to do this. You might have seen this result when selecting a slower speed!!
The fan spins slower, due to the limited amount of power available, which moves less air.
That ceiling fan analogy is one of the best ways to grasp the required power VS moved mass idea. I'll add more about it later.

Also, I'll add some stuff about KWH meters and maybe others - let me know if this sounds OK [Okee-Dokeee..eee..eee]

As to placing the PF correction elements, it's all in what level and how many machines are involved.

If only a few machines have low PF, then placing the correction near them is usually best - more accurately, on the LOAD side of their controllers [like mag starters]. Then everything gets protection as they should.

When there's random points across the location, then correction at the service feeders is better. This is when a steady level has been determined.

When the location has a varying PF, then what's known as a "Synchronous Condenser" [seems like the term??] is used.
This is a Synchronous motor run with no load connected to the shaft. It's soul purpose is to create XC or XL as needed, and at levels needed.
Kind of hard to explain how this gets done, but it's all automatically corrected by the motor.

I can add more on that too.

Got to go now!!

Scott SET
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#127122 - 05/01/01 10:13 PM Re: true power compared to apparent power
Anonymous
Unregistered


 Quote:
so the load's not going to be able to draw as much True Power as before

- which means that work will get done more slowly, so in the end, more energy is indeed consumed.

 Quote:
I'll add some stuff about KWH meters and maybe others - let me know if this sounds OK

Yes, I look forward to it!

 Quote:
When the location has a varying PF, then what's known as a "Synchronous Condenser" ... is used.
This is a Synchronous motor run with no load connected to the shaft. Its sole purpose is to create XC or XL as needed, and at levels needed.
Kind of hard to explain how this gets done, but it's all automatically corrected by the motor.

That seems right intuitively.

How much mass is there in the shaft?
What I am trying to figure out is whether this motor is storing/releasing inertial (kinetic) energy as with a flywheel (net no load) or whether it responds better with a lower mass shaft, ideally massless (true no load).

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