another transformer question

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mikezx
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another transformer question

Post by mikezx »

Why are transformers rated in KVA instead of just watts? I know that it has something to do with the power factor but I'm not sure what. Thanks!
bodgy
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Re: another transformer question

Post by bodgy »

Transformers are rated in VA rather than Watts because technically Watts are the result into a resistance of Volts and Current mainly Direct Current.<p>Alternating Current on the other hand (Left or Right theory :D ), has the added fact that the current and voltage are not in phase with each other and as a transformer is a type of coil there is going to be resistance of Inductance/Capacitance to take into consideration which is Reactance. <p>For rough calculations you can just multiply V * A and call it watts, but from a design and strictly correct point of view as there is not pure resistance there is a difference.<p>Look at http://members.tripod.com/~schematics/x ... ormer1.htm for some transformer theory<p>bodgy
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MrAl
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Re: another transformer question

Post by MrAl »

H there,<p>To add to the other posts:<p>Some loads have pure reactance and so
when you calculate the power in watts
you get a total of zero watts, but you
still have much current flowing that the
transformer still has to be able to handle.
This is why we need units of VA or something else
suitable that will allow us to match a given
transformer to a particular application.
The best way to understand all of this is to
look at both the watts and the VA for a given
load, not just one or the other.<p>Example:
You have a 1:1 transformer plugged into 120
volts ac, 60Hz, and your only load is a
large capacitor. Let's say the capacitor draws
10 amps from the transformer. <p>Let's look at both the watts and the VA:<p>We calculate the watts:
120*10*cos(90 degrees)=0 watts
We also calculate the VA:
120*10=1200VA.<p>Now the question is, can we use a transformer
rated at 1KVA (1000VA) for this capacitor load?<p>If we look at the watts, it looks like we can
use this transformer, which would be an incorrect
conclusion.
When we look at the VA we see that 1200 is higher
then the rating of the transformer in question
(1000) so we find that we cannot use this
transformer for this cap load. Most likely it
would overheat.<p>From this we can see that some loads consume
zero power but the transformer windings still
have to pass high currents. Since the copper
used in the wire for the windings has a certain
max current rating (before it overheats) we end
up rating the transformer mostly because of the
wire size, although we dont always rate the
transformer according to current flow either
because one side of the transformer can
have a different current flow then the other for
ratios other then 1:1. This is why VA ratings
work very well.<p>Sometimes we do rate the transformer secondary
in units of current but only when it's assumed
that the input is a set known input (such as
120vac).<p>Good luck with your circuits,
Al
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mikezx
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Re: another transformer question

Post by mikezx »

Hey thanks alot, it's clear as a bell to me now. So if you had say a motor that was rated in hp or watts, then you would just simply divide by the power factor (cos theta) to get the va rating. Cool, thanks again.<p>One other question, for a 3 phase transformer, if we know the full load amps, then do we still have to multiply (V*A) by 1.73?<p>[ July 03, 2002: Message edited by: mj ]</p>
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MrAl
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Re: another transformer question

Post by MrAl »

Hello again,<p>If i understand your second question correctly, you are looking at the
factor sqrt(3)=1.73 and trying to relate this to the full load power.
The factor 1.73 is used to convert to/from line to line voltage and
line to neutral voltage in a three phase Y system.
The line to line (L-L) voltage is equal to the line to neutral (L-N) voltage
times the square root of 3, or approximately 1.73.
To compute the VA, if you know the L-N voltage all you have to do is
multiply by the current in one phase (assuming a balanced load) and
multiply that by 3. If you know the L-L voltage then you would first
divide by 1.73 to get the L-N voltage.<p>Since<p>P=VA*cos(angle)=VA*power_factor<p>the power or the VA for any device that also has a power factor rating
can be calculated using this last equation as you suggested.<p>If we know the power and the pf and the L-L voltage V then
P=3*(V/1.73)*A*pf<p>If we know the power and the pf and the L-N voltage V then
P=3*(V)*A*pf <p>In other words, we dont divide by 1.73 for L-N voltages.<p>Example1:
We have L-N voltage of 120vac at 10 amps per line and pf=0.9.
VA=3*120*10
P=3*120*10*0.9<p>Example2:
We have L-L voltage of 208vac at 10 amps per line and pf=0.9.
VA=3*(208/1.73)*10=3*(120)*10
P=3*(208/1.73)*10*0.9=3*(120)*10*0.9<p>In this last example, we rounded 120*1.73 to 208 volts,
and rounded 208/1.73 back to 120 volts to be able to work
in whole numbers.<p>We get the same answers in both examples because it is the same system
in which we are just measuring the three phase voltages differently.<p>Good luck with your circuits,
Al
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