Volt Amps = ??

[Ross Irvine]

Hi All,

Ok, I know this is probably a silly question but what are "Volt Amps" as
used in America.

Eg. I have an American made device that wants 24 Volts AC, 50or60 Hz,
40VA (Volt Amps)

What does a Volt Amp translate to in what we'd normally use which is
just AMPS (Eg 1.5 AMPS or something) or is it something different?
Is there a formula to convert VA into AMPs?
Thanks.

 

[Phil Allison]

"Ross Irvine"

Hi All,

Ok, I know this is probably a silly question but what are "Volt Amps" as
used in America.

** Same as anywhere else.

Eg. I have an American made device that wants 24 Volts AC, 50or60 Hz,
40VA (Volt Amps)

What does a Volt Amp translate to in what we'd normally use which is
just AMPS (Eg 1.5 AMPS or something) or is it something different?
Is there a formula to convert VA into AMPs?

** Volt Amps ( VA) = Voltage x Current ( with both given in rms
values.)

Usually the voltage is known - like 240 or 120 volts.

So, VA divided by the supply voltage = current draw in amps rms.

The current may or may not be a sine wave - ie it can be distorted
as with most electronic loads.



.......... Phil

 

[Wild Wizard]

Ross Irvine wrote:

Hi All,

Ok, I know this is probably a silly question but what are "Volt Amps" as
used in America.

exactly what they sound like Volt Amps

Eg. I have an American made device that wants 24 Volts AC, 50or60 Hz,
40VA (Volt Amps)

VA = V * A
A = VA/V

V = 24
VA = 40

A = 40/24
A = 1.7

What does a Volt Amp translate to in what we'd normally use which is
just AMPS (Eg 1.5 AMPS or something) or is it something different?
Is there a formula to convert VA into AMPs?
Thanks.

Think i've just answered it

--
The Kennedy Constant:
Don't get mad -- get even.

 

[C3]

As far as I can tell, it just a way to express power in such a way that
allows you to see the voltage and current components that it comprises.

C3

 

[Bristan]

"Ross Irvine" <rirvineDOT@netspaceDOT..DOTnet.DOT.au> wrote in message
news:3FF24343.E798DB6C@netspaceDOT..DOTnet.DOT.au...

Hi All,

Ok, I know this is probably a silly question but what are "Volt Amps" as
used in America.

VoltAmps are watts (power = voltage times current)
VA are used instead of watts where there is a reactive component to the load
in ac circuits. eg inductance or capacitance and the watts used in the load
are not the total power used.
Transformers typically are rated in kVA not kw as a certain quantity of
power is taken up by inductance of the transformer itself. (power factor not
equal to one and the current lags the voltage).

Eg. I have an American made device that wants 24 Volts AC, 50or60 Hz,
40VA (Volt Amps)

What does a Volt Amp translate to in what we'd normally use which is
just AMPS (Eg 1.5 AMPS or something) or is it something different?
Is there a formula to convert VA into AMPs?
Thanks.

 

[Phil Allison]

"C3" <gned@telsMONOPOLYtra.com>

As far as I can tell, it just a way to express power .....

** VA is not power - a 55 uF capacitor across a 240 volt AC supply
dissipates no power but the effective load is nearly 1000 VA. VA is a
convenient way to describe the rms current draw of an item. This is
important to know so generator, cable and fuse ratings are not exceeded.

A sparkie just adds up all the VA numbers and compares that to the circuit
capacity in VA.




............ Phil

 

[Phil Allison]

"Bristan"

VoltAmps are watts (power = voltage times current)
VA are used instead of watts where there is a reactive component to the
load
in ac circuits. eg inductance or capacitance and the watts used in the
load
are not the total power used.

* The watts used in the load are always the total power - but not alway the
same as the VA.

There need be no reactive load either - electronic loads ( DC from AC
supplies) are rated in VA.

Transformers typically are rated in kVA not kw as a certain quantity of
power is taken up by inductance of the transformer itself. (power factor
not
equal to one and the current lags the voltage).

** The VA rating of a transformer ( along with the specified operating
voltages ) tells you the amount of **rms** current the secondary can supply
long term - without overheating.




......... Phil

 

[Ross Irvine]

Magic!

Thanks.



Wild Wizard wrote:

Ross Irvine wrote:

Hi All,

Ok, I know this is probably a silly question but what are "Volt Amps" as
used in America.

exactly what they sound like Volt Amps

Eg. I have an American made device that wants 24 Volts AC, 50or60 Hz,
40VA (Volt Amps)

VA = V * A
A = VA/V

V = 24
VA = 40

A = 40/24
A = 1.7

What does a Volt Amp translate to in what we'd normally use which is
just AMPS (Eg 1.5 AMPS or something) or is it something different?
Is there a formula to convert VA into AMPs?
Thanks.

Think i've just answered it

--
The Kennedy Constant:
Don't get mad -- get even.

 

[Ross Irvine]

Thanks Phil..

Ross.

Phil Allison wrote:

"Ross Irvine"

Hi All,

Ok, I know this is probably a silly question but what are "Volt Amps" as
used in America.

** Same as anywhere else.


Eg. I have an American made device that wants 24 Volts AC, 50or60 Hz,
40VA (Volt Amps)

What does a Volt Amp translate to in what we'd normally use which is
just AMPS (Eg 1.5 AMPS or something) or is it something different?
Is there a formula to convert VA into AMPs?

** Volt Amps ( VA) = Voltage x Current ( with both given in rms
values.)

Usually the voltage is known - like 240 or 120 volts.

So, VA divided by the supply voltage = current draw in amps rms.

The current may or may not be a sine wave - ie it can be distorted
as with most electronic loads.

......... Phil

 

[Ross Irvine]

Thanks.

Ross.

Bristan wrote:

"Ross Irvine" <rirvineDOT@netspaceDOT..DOTnet.DOT.au> wrote in message
news:3FF24343.E798DB6C@netspaceDOT..DOTnet.DOT.au...
Hi All,

Ok, I know this is probably a silly question but what are "Volt Amps" as
used in America.

VoltAmps are watts (power = voltage times current)
VA are used instead of watts where there is a reactive component to the load
in ac circuits. eg inductance or capacitance and the watts used in the load
are not the total power used.
Transformers typically are rated in kVA not kw as a certain quantity of
power is taken up by inductance of the transformer itself. (power factor not
equal to one and the current lags the voltage).

Eg. I have an American made device that wants 24 Volts AC, 50or60 Hz,
40VA (Volt Amps)

What does a Volt Amp translate to in what we'd normally use which is
just AMPS (Eg 1.5 AMPS or something) or is it something different?
Is there a formula to convert VA into AMPs?
Thanks.

 

[DJ]

On Wed, 31 Dec 2003 14:32:19 +1100, Ross Irvine
<rirvineDOT@netspaceDOT..DOTnet.DOT.au> wrote:

Hi All,

Ok, I know this is probably a silly question but what are "Volt Amps" as
used in America.

Eg. I have an American made device that wants 24 Volts AC, 50or60 Hz,
40VA (Volt Amps)

What does a Volt Amp translate to in what we'd normally use which is
just AMPS (Eg 1.5 AMPS or something) or is it something different?
Is there a formula to convert VA into AMPs?
Thanks.

There is a comprehensive discussion of this and related topics at
http://www.splatco.com/tips/pwrfact/pfarticl.htm

dj

 

[David]

On Wed, 31 Dec 2003 15:33:45 +1100, Phil Allison wrote:

"C3" <gned@telsMONOPOLYtra.com

As far as I can tell, it just a way to express power .....


** VA is not power - a 55 uF capacitor across a 240 volt AC supply
dissipates no power but the effective load is nearly 1000 VA. VA is a
convenient way to describe the rms current draw of an item. This is
important to know so generator, cable and fuse ratings are not exceeded.

A sparkie just adds up all the VA numbers and compares that to the circuit
capacity in VA.




........... Phil

A sparkie may add up all the VA numbers, but an engineer (or really clever
sparkie) would know that as VA's are vector quantities, you can't just add
them up. Power factor correction devices are often added to commercial
installations to reduce the total VA, and bring the power factor closer to
unity.

David

 

[Phil Allison]

"David" <no_way@hotmail.com> wrote in message
newsan.2004.01.01.02.16.29.235000@hotmail.com...

On Wed, 31 Dec 2003 15:33:45 +1100, Phil Allison wrote:


"C3" <gned@telsMONOPOLYtra.com

As far as I can tell, it just a way to express power .....


** VA is not power - a 55 uF capacitor across a 240 volt AC supply
dissipates no power but the effective load is nearly 1000 VA. VA is a
convenient way to describe the rms current draw of an item. This is
important to know so generator, cable and fuse ratings are not exceeded.

A sparkie just adds up all the VA numbers and compares that to the
circuit
capacity in VA.




........... Phil


A sparkie may add up all the VA numbers, but an engineer (or really clever
sparkie) would know that as VA's are vector quantities, you can't just add
them up.

** VAs may be quite resistive, have a known or unknown phase angle and
or/or have distorted (peaky) current waveforms. If you simply add them then
the answer is the same as the worst case value.

Power factor correction devices are often added to commercial
installations to reduce the total VA, and bring the power factor closer to
unity.

** That is only true where there is a phase angle to be corrected - what
if the load is 1000s of electronic devices with AC to DC power supplies in
them that only draw current at the crest of the AC voltage ??




.......... Phil

 

[David]

On Thu, 01 Jan 2004 13:41:48 +1100, Phil Allison wrote:

"David" <no_way@hotmail.com> wrote in message
newsan.2004.01.01.02.16.29.235000@hotmail.com...
On Wed, 31 Dec 2003 15:33:45 +1100, Phil Allison wrote:


"C3" <gned@telsMONOPOLYtra.com

As far as I can tell, it just a way to express power .....


** VA is not power - a 55 uF capacitor across a 240 volt AC supply
dissipates no power but the effective load is nearly 1000 VA. VA is a
convenient way to describe the rms current draw of an item. This is
important to know so generator, cable and fuse ratings are not exceeded.

A sparkie just adds up all the VA numbers and compares that to the
circuit
capacity in VA.




........... Phil


A sparkie may add up all the VA numbers, but an engineer (or really clever
sparkie) would know that as VA's are vector quantities, you can't just add
them up.


** VAs may be quite resistive, have a known or unknown phase angle and
or/or have distorted (peaky) current waveforms. If you simply add them then
the answer is the same as the worst case value.

If you simply add them for the worst case, then you will be over
specifying the power distribution system, requiring larger cables and
transformers etc, which will result in increased costs to the end user.
For a large industrial user this could well be significant.

Power factor correction devices are often added to commercial
installations to reduce the total VA, and bring the power factor closer to
unity.


** That is only true where there is a phase angle to be corrected - what
if the load is 1000s of electronic devices with AC to DC power supplies in
them that only draw current at the crest of the AC voltage ??


......... Phil

Non linear loads cause harmonic currents to appear in the electrical
distrubtion system. Harmonic filters can reduce the harmonic currents
generated by non linear loads in the phase and neutral conductors of the
three phase distrubution system. This will reduce the peak phase current,
and average phase current, which will reduce the system losses, and reduce
costs.

David

 

[Phil Allison]

"David" <no_way@hotmail.com>

Phil A wrote:

** VAs may be quite resistive, have a known or unknown phase angle and
or/or have distorted (peaky) current waveforms. If you simply add them
then
the answer is the same as the worst case value.


If you simply add them for the worst case, then you will be over
specifying the power distribution system,

** How so ?

You are assuming a MOST unlikely scenario where different loads onm the
SAME circuit have cancelling phase angles !!!!!

Power factor correction devices are often added to commercial
installations to reduce the total VA, and bring the power factor closer
to
unity.


** That is only true where there is a phase angle to be corrected -
what
if the load is 1000s of electronic devices with AC to DC power supplies
in
them that only draw current at the crest of the AC voltage ??


......... Phil

Non linear loads cause harmonic currents to appear in the electrical
distrubtion system. Harmonic filters can reduce the harmonic currents
generated by non linear loads in the phase and neutral conductors of the
three phase distrubution system. This will reduce the peak phase current,
and average phase current, which will reduce the system losses, and reduce
costs.

** Where does the "harmonic filter" go ??

Please describe such a filter .



BTW Are you quoting from some web site ??


It sure as hells looks to me like you are.




........ Phil

 

[Bristan]

To make it clear
I am not a power engineer and don't profess to be an expert however I am
prompted to recall an explanation from my university days, as you seem to
have a rather garbled idea of what is going on.

The average power in an ac circuit is given by
P =VIcos phi
where phi is the phase angle between the current and the voltage.
and the product VI is called the VOLT-AMPERES
cosphi is the power factor

When the current is in phase with the voltage then phi =0, power factor =1
and P =VI
In this case of a non reactive or balanced load the VA *ARE* equal to the
watts.

In most ac inductive or capacitive circuits this will not be the case and
the VA will be greater than the average power in watts. This leads to
larger currents and hence cables etc than are necessary for the power
provided by the device. This leads to power factor correction devices
(capacitors with inductive loads) As the current leads the voltage in a
capacitor this will bring the average current more in phase with the voltage
and make thew PF closer to one.

For an example if the current is lagging the voltage by say 60 degrees then
the PF will be 0.5 and the VA will be twice the average power in the
circuit.

Therefore as I said before, electrical machinery for ac is usually rated in
kva to take account of this occurrence.


"Phil Allison" <philallison@optusnet.com.au> wrote in message
news:3ff25348$0$18747$afc38c87@news.optusnet.com.au...

"Bristan"


VoltAmps are watts (power = voltage times current)
VA are used instead of watts where there is a reactive component to the
load
in ac circuits. eg inductance or capacitance and the watts used in the
load
are not the total power used.


* The watts used in the load are always the total power - but not alway
the
same as the VA.

There need be no reactive load either - electronic loads ( DC from
AC
supplies) are rated in VA.


Transformers typically are rated in kVA not kw as a certain quantity of
power is taken up by inductance of the transformer itself. (power factor
not
equal to one and the current lags the voltage).


** The VA rating of a transformer ( along with the specified operating
voltages ) tells you the amount of **rms** current the secondary can
supply
long term - without overheating.




........ Phil

 

[Phil Allison]

"Bristan" <daed>

To make it clear
I am not a power engineer and don't profess to be an expert however I am
prompted to recall an explanation from my university days, as you seem to
have a rather garbled idea of what is going on.

** Sorry - you have the garbled idea, and a very common form of garble
it is.

The average power in an ac circuit is given by
P =VIcos phi
where phi is the phase angle between the current and the voltage.

** Who says there is **any phase angle** or the current is a sine wave
?????????????????????????????????????

What you have forgotten from your Uni days is the crucial qualification: "
..... in an all sine wave system.... "

Therefore as I said before, electrical machinery for ac is usually rated
in
kva to take account of this occurrence.

** So what ?????????

Since when is this NG or was the OP speaking of "electrical machinery "
?????

Cripes - even my floor fan blowing on me as I type has a PF of 0.97.




.......... Phil

 

[David]

On Thu, 01 Jan 2004 17:25:50 +1100, Phil Allison wrote:

"David" <no_way@hotmail.com

Phil A wrote:


** VAs may be quite resistive, have a known or unknown phase angle and
or/or have distorted (peaky) current waveforms. If you simply add them
then
the answer is the same as the worst case value.


If you simply add them for the worst case, then you will be over
specifying the power distribution system,


** How so ?

You are assuming a MOST unlikely scenario where different loads onm the
SAME circuit have cancelling phase angles !!!!!

Example case. Assume pure resitive load of 1000VA, and a pure
inductive load of 1000VA. Simply adding the VA's would result in 2000VA,
where the real VA would only be 1414VA. This would result in approx 70%
less current requirement than your method, which represents a real
saving.

Power factor correction devices are often added to commercial
installations to reduce the total VA, and bring the power factor closer
to
unity.


** That is only true where there is a phase angle to be corrected -
what
if the load is 1000s of electronic devices with AC to DC power supplies
in
them that only draw current at the crest of the AC voltage ??


......... Phil



Non linear loads cause harmonic currents to appear in the electrical
distrubtion system. Harmonic filters can reduce the harmonic currents
generated by non linear loads in the phase and neutral conductors of the
three phase distrubution system. This will reduce the peak phase current,
and average phase current, which will reduce the system losses, and reduce
costs.



** Where does the "harmonic filter" go ??

Please describe such a filter .

The harmonic filter is on the three phase system. Depending on the

harmonics, the filter may be a delta transformer, which will eliminate
zero sequence harmonics (3rd, 6th, 9th etc). Other techniques can be used
to eliminate positive and negative sequence harmonics if needed. These can
range from simple LC shunt and series filters, to sophisticated active
power filters which are DSP based, and measure the harmonic currents and
inject currents into the supply to cancel out the harmonics.

A google search on Harmonic Mitigating Transformers and Harmonic
Filters will yeild plenty of examples.


David

BTW Are you quoting from some web site ??


It sure as hells looks to me like you are.




....... Phil

 

[Phil Allison]

"David" <no_way@hotmail.com

You are assuming a MOST unlikely scenario where different loads onm
the
SAME circuit have cancelling phase angles !!!!!


Example case. Assume pure resitive load of 1000VA, and a pure
inductive load of 1000VA. Simply adding the VA's would result in 2000VA,
where the real VA would only be 1414VA.

** Pure inductive loads are just a tad rare - so your example is not
typical.

If you use a 1000 VA load with a reactive PF of 0.7 then add a resistive
load of 1000 VA the sum is 1800VA.

If you have electronic loads with whatever PF then the VAs just add.

Non linear loads cause harmonic currents to appear in the electrical
distrubtion system. Harmonic filters can reduce the harmonic currents
generated by non linear loads in the phase and neutral conductors of
the
three phase distribution system. This will reduce the peak phase
current,
and average phase current, which will reduce the system losses, and
reduce
costs.



** Where does the "harmonic filter" go ??

Please describe such a filter .


The harmonic filter is on the three phase system.......

** Bad luck for single phase loads.

A google search on Harmonic Mitigating Transformers and Harmonic
Filters will yeild plenty of examples.

BTW Are you quoting from some web site ??

It still sure as hells looks to me like you are.




....... Phil

 

[David]

On Thu, 01 Jan 2004 23:12:29 +1100, Phil Allison wrote:

"David" <no_way@hotmail.com


You are assuming a MOST unlikely scenario where different loads onm
the
SAME circuit have cancelling phase angles !!!!!


Example case. Assume pure resitive load of 1000VA, and a pure
inductive load of 1000VA. Simply adding the VA's would result in 2000VA,
where the real VA would only be 1414VA.


** Pure inductive loads are just a tad rare - so your example is not
typical.

If you use a 1000 VA load with a reactive PF of 0.7 then add a resistive
load of 1000 VA the sum is 1800VA.

Still a saving of 10%. Well worthwhile saving.

If you have electronic loads with whatever PF then the VAs just add.

Only if the the power factors are the same. Transformers supplied loads
tend to be lagging, and switch mode supplies tend to be leading, so
they will cancel to some degree. Properly designed high power factor
electronic loads will have a power factor very close to unity.

Non linear loads cause harmonic currents to appear in the electrical
distrubtion system. Harmonic filters can reduce the harmonic currents
generated by non linear loads in the phase and neutral conductors of
the
three phase distribution system. This will reduce the peak phase
current,
and average phase current, which will reduce the system losses, and
reduce
costs.



** Where does the "harmonic filter" go ??

Please describe such a filter .


The harmonic filter is on the three phase system.......


** Bad luck for single phase loads.

Most single phase loads are supplied from three phase switchboards, where
the filter would be installed. Single phase active filters are available,
but cheaper to put in main swithboard, which for large loads are three
phase.


David