Volt*Amps and Watts on my home power meter

A

Allie

Guest
I am in the UK. A home power meter I bought says this in its
Chinese-English instructions. Can anyone make sense of it?

Press 'WATT' display Watt meter, then press
key to display VA meter, and active power
for 1 second each automatically.

'WATT/VA' Key is a toggle function Key, the
LCD will show Watt is the active power,
and VA is the apparent Power. (VA = Vrms Arms)


There seems to be some difference between V multiplied by A and
Watts?

It mentions "apparent power" but what is it?

-------

This is the meter:
http://www.prodigit.com/img/product/2000mu-01.jpg

These are the instructions (see #5):
http://www.prodigit.com/pdf/2000mu-01.pdf

It looks quite like the US's "Kill-A-Watt" Electric Usage Monitor
http://store.servomagazine.com/image.php?type=T&id=16502
 
Allie <billgates@microsoft.com> wrote:

It mentions "apparent power" but what is it?
Click to expand...
How hard is it to type "apparent power" into google?
--
 
In article <Xns9B818BDD05858451E7A@216.151.153.22>,
Allie <billgates@microsoft.com> wrote:

I am in the UK. A home power meter I bought says this in its
Chinese-English instructions. Can anyone make sense of it?

Press 'WATT' display Watt meter, then press
key to display VA meter, and active power
for 1 second each automatically.

'WATT/VA' Key is a toggle function Key, the
LCD will show Watt is the active power,
and VA is the apparent Power. (VA = Vrms Arms)


There seems to be some difference between V multiplied by A and
Watts?
Click to expand...
Allie-

This isn't necessarily a problem translating Chinese. Its one of those
things where if you already knew the answer, it would be quite obvious!

Some equipment such as electric motors, have inductance (coils of wire
wound around a metal core). Inductance has reactance that delays
current, so the peak current does not coincide with peak voltage.

If you simply measure voltage and current, the VA product would be
apparent power. For equipment that has reactance, actual power (Watts)
is less than VA, and the ratio of Watts divided by VA, is called the
power factor (PF).

If there is no reactance, such as the case of a heating element in a
stove burner, VA = Watts and PF = 1.0.

Fred
 
On Sat 27 Dec08 15:21, nospam <nospam@please.invalid> wrote
Allie <billgates@microsoft.com> wrote:

It mentions "apparent power" but what is it?


How hard is it to type "apparent power" into google?
Click to expand...
I asked several connected questions ... (1) How to read the display
of the meter. (2) "There seems to be some difference between V
multiplied by A and Watts?" (3) "It mentions "apparent power" but
what is it?"

When I had keyed "apparent power" into Google I got gobbledegoogle
back. For example, the Wikipedia had said

"Apparent Power (|S|) ... is, the absolute value of complex power S -
unit: volt-ampere (VA)" and "Apparent power is conventionally
expressed in volt-amperes (VA) since it is the product of rms voltage
and rms current." and "While real power and reactive power are well
defined in any system, the definition of apparent power for
unbalanced polyphase systems is considered to be one of the most
controversial topics in power engineering"

Heh? I'm not a specialist, you know! I want to know in ordinary
English. :)
 
On Sat, 27 Dec 2008 22:27:31 GMT, Allie <billgates@microsoft.com> wrote:

On Sat 27 Dec08 15:21, nospam <nospam@please.invalid> wrote
Allie <billgates@microsoft.com> wrote:

It mentions "apparent power" but what is it?


How hard is it to type "apparent power" into google?


I asked several connected questions ... (1) How to read the display
of the meter. (2) "There seems to be some difference between V
multiplied by A and Watts?" (3) "It mentions "apparent power" but
what is it?"
Click to expand...
On AC circuits, V * A has units of watts but is conventionally called
"volt-amps" to distinguish it, apparent power, from "real power" which
is (also) in watts but gets to keep the name as well. For "real power"
(and for all DC circuits) the current and voltage is in phase. Pure
imaginary power has current and voltage 90-degrees out of phase; no work
gets done there.

Apparent power includes both real power, on the x-axis, and imaginary
power, on the y-axis. It's imaginary since it doesn't do any real work,
it just sloshes back and forth between the reactive elements (inductive,
capacitive) in the power system. Apparent power is the hypotenuse
between them (and the product of V and A). The "power factor" is the
cosine of the angle that the apparent power makes with the x-axis. Note
that's the same as the ratio of the real power (adjacent) to the
apparent power (hypotenuse).

Utilities prefer that things stay near a unity power factor, since even
though the "imaginary" power doesn't do real work in your house, it does
show up as resistive losses on the power lines, so you're kind of paying
for less power (at your end) than the utility has to pump into the lines
at their end.

Don't worry about "unbalanced polyphase systems," since you'll probably
mostly (entirely?) be working with single phase household power.

--
Rich Webb Norfolk, VA
 
Let me try to give a simple explanation :

when you measure voltage and current in a device, then you multiply
Volts by Amps, you get apparent power.

because the Current and voltage may not be exactly in phase.

for a heater, a resistor, they are in phase, no problem

for a transformer, the current is always "late" regarding to voltage,
this is due to the coil. The best example is a microwave (big
transformer inside) or a motor.

As the current is not at the maximum at the same time than voltage, if
you mesure maybe every millisecond the current and voltage and then
multiply, you measure real power.

Your device is able to mde both measurements, apparent power and real power.

The issue is that if your apparent power is too big, you will get higher
current in your wires, even if the real power is not so high !

at home, this is a minor problem, but in plants, when you play with
hundreds of kilowatts, it makes a sense to check both.

JJ

Allie a écrit :
On Sat 27 Dec08 15:21, nospam <nospam@please.invalid> wrote
Allie <billgates@microsoft.com> wrote:

It mentions "apparent power" but what is it?

How hard is it to type "apparent power" into google?


I asked several connected questions ... (1) How to read the display
of the meter. (2) "There seems to be some difference between V
multiplied by A and Watts?" (3) "It mentions "apparent power" but
what is it?"

When I had keyed "apparent power" into Google I got gobbledegoogle
back. For example, the Wikipedia had said

"Apparent Power (|S|) ... is, the absolute value of complex power S -
unit: volt-ampere (VA)" and "Apparent power is conventionally
expressed in volt-amperes (VA) since it is the product of rms voltage
and rms current." and "While real power and reactive power are well
defined in any system, the definition of apparent power for
unbalanced polyphase systems is considered to be one of the most
controversial topics in power engineering"

Heh? I'm not a specialist, you know! I want to know in ordinary
English. :)
Click to expand...
 
jj wrote:
Let me try to give a simple explanation :

when you measure voltage and current in a device, then you multiply
Volts by Amps, you get apparent power.

because the Current and voltage may not be exactly in phase.

for a heater, a resistor, they are in phase, no problem

for a transformer, the current is always "late" regarding to voltage,
this is due to the coil. The best example is a microwave (big
transformer inside) or a motor.

As the current is not at the maximum at the same time than voltage, if
you mesure maybe every millisecond the current and voltage and then
multiply, you measure real power.

Your device is able to mde both measurements, apparent power and real
power.

The issue is that if your apparent power is too big, you will get higher
current in your wires, even if the real power is not so high !

at home, this is a minor problem, but in plants, when you play with
hundreds of kilowatts, it makes a sense to check both.

JJ

Allie a écrit :
On Sat 27 Dec08 15:21, nospam <nospam@please.invalid> wrote
Allie <billgates@microsoft.com> wrote:

It mentions "apparent power" but what is it?

How hard is it to type "apparent power" into google?

I asked several connected questions ... (1) How to read the display
of the meter. (2) "There seems to be some difference between V
multiplied by A and Watts?" (3) "It mentions "apparent power" but
what is it?"

When I had keyed "apparent power" into Google I got gobbledegoogle
back. For example, the Wikipedia had said
"Apparent Power (|S|) ... is, the absolute value of complex power S -
unit: volt-ampere (VA)" and "Apparent power is conventionally
expressed in volt-amperes (VA) since it is the product of rms voltage
and rms current." and "While real power and reactive power are well
defined in any system, the definition of apparent power for unbalanced
polyphase systems is considered to be one of the most controversial
topics in power engineering"

Heh? I'm not a specialist, you know! I want to know in ordinary
English. :)
Click to expand...
This is almost correct. However, if you measure the product of voltage
and current at microsecond intervals, AND take the average over a cycle,
then you get the "real" power. Consider that use of rms values of
voltage and current implies averaging over a cycle.

I may be nit picking but .....


As an aside for the original questioner- the WIKI article does assume
some knowledge of complex number theory as applied to AC circuits- there
are simpler references.
look up power factor.
one such is

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powfac.html
 
Allie submitted this idea :
I am in the UK. A home power meter I bought says this in its
Chinese-English instructions. Can anyone make sense of it?

Press 'WATT' display Watt meter, then press
key to display VA meter, and active power
for 1 second each automatically.

'WATT/VA' Key is a toggle function Key, the
LCD will show Watt is the active power,
and VA is the apparent Power. (VA = Vrms Arms)


There seems to be some difference between V multiplied by A and
Watts?

It mentions "apparent power" but what is it?

-------

This is the meter:
http://www.prodigit.com/img/product/2000mu-01.jpg

These are the instructions (see #5):
http://www.prodigit.com/pdf/2000mu-01.pdf

It looks quite like the US's "Kill-A-Watt" Electric Usage Monitor
http://store.servomagazine.com/image.php?type=T&id=16502
Click to expand...
Apparent Power (VA) is the voltage measured * the current measured.
True Power (Watts) is the actual amount of power being used.
If the load is purely resistive, then the Apparent power and true power
will be the same. However if you introduce a capacitive or inductive
load you get reluctance.
So know you have 3 factors:
VA, Watts, and Q (reactive power).
Thier relationship is called the power factor.
Power factor is true power divided by apparent power
Apparent power = Sq.Rt. of (VA sq. + Watts sq.)

Think of it as a right triangle, the vertical leg is reactive power.
The Horizontal leg is true power and the hypotenuse is VA or apparent
power.
 
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