Meaning of V.C.T.?
Will White
the secondary/output of power transformers. Can someone please spell this
abbreviation out for me and explain what it refers to?
Also, why are power ratings described in terms of volt-amps (V.A.) instead of
watts in some components?
--
Will White, KD7BFX
to reply, please click this link mailto:w...@asu.uswest.net
Larry Gauthier
V.C.T. stand for Volts Center Tapped, which means that the secondary of the
transformer has a connection that is mid-way (electrically) between the two
outputs. A transfmormer labeled 1000 vct has a secondary with three
connections: 1000 volts end-to-end, or 500 volts from either side of the
center tap.
Back in the days of vacuum tube rectifiers, the center tap was generally
tied to ground, and two tubes provided full-wave rectification - one on each
leg. That can still be done today with two diodes, or you can use a
"full-wave bridge rectifier" four-diode combination which would ignore the
center tap. Generally, a VCT transformer will provide you with more
flexibility in designing the output voltage from a power supply (this may or
may not be on any consequence to you...).
Regarding the terms V.A. - that term is far more prevalent in the electric
and power industry circles, where industrial equipment is rated in VA of KVA
(Kilo volt amps) terms. Why? Cause that's the way they've always done it...
;-)
-larry
wb8bgo
Will White wrote in message <372ECA73...@asu.uswest.net>...
Bob Lewis
Amps) is the apparent power delivered to the load while Watts is the
true power. Because the nature of the load (and therefore the phase
difference) is often unknown in power systems, the term VA is used.
The formula Watts = voltage times current only applies when the voltage
and current are in phase but VA = voltage time current always applies.
larry d. clark
larry
kd5foy
Gerard Butler
--
Dr. Gerard Butler CEng MIEE, EI0CH, WEMT
TELTEC Propagation Group, Trinity College, Dublin Ireland
Dublin+Wicklow Mountain Rescue Team
Will White <w...@asu.uswest.net> wrote in message
news:372ECA73...@asu.uswest.net...
>
>Also, why are power ratings described in terms of volt-amps (V.A.) instead
of
>watts in some components?
>--
>Will White, KD7BFX
Volt-amps = max volts x max-amps without regard to the phase shift which
occurs for inductive or capacitive circuits.
Watts = volts x in-phase amps, resistive circuits only.
Jim
Larry Gauthier wrote in message <372e...@news.ismi.net>...
>Regarding the terms V.A. - that term is far more prevalent in the electric
>and power industry circles, where industrial equipment is rated in VA of
KVA
>(Kilo volt amps) terms. Why? Cause that's the way they've always done it...
>;-)
There is a very sound reason for using VA for rating a transformer.
If you are using an entirely resistive load the current and voltage supplied
by the transformer are in phase, so you can use ohms law: watts = volts
times amps.
Now what happens in an AC power grid when you have lotsa motors and
transformers? The load that the transformer is supplying is far from
resistive and is usually highly inductive. Therefore the current and
voltage are not in phase. Transformers are then rated at the rms voltage
times the rms current.
To determine the actual power you need to know the phase angle between the
current and voltage. Power companies sometimes put capacitors across the
lines to keep the phase angle under control.
It has been almost 25 years since I had a Power Distribution class, so
perhaps there are some power engineers on here that can explain it better.
Jim
N8EE
Robert Sheahan - remove the SPAM to reply
>Regarding the terms V.A. - that term is far more prevalent in the electric
>and power industry circles, where industrial equipment is rated in VA of
KVA
>(Kilo volt amps) terms. Why? Cause that's the way they've always done it...
There is a usefull side too - V.A. is a better indicator of current
requirements of house wiring, fuses, extension cords, etc. than Watts if the
equipment has a reactive component instead of just a resistive component.
Watts tells you how much work it can do (heat + light + sound + radio
waves), V.A. tells you how much current it will draw.
Robert Sheahan
KA1VYZ
N2EY
writes:
> keep coming across the abbreviated voltage rating "V.C.T." with respect to
>the secondary/output of power transformers. Can someone please spell this
>abbreviation out for me and explain what it refers to?
>
>Also, why are power ratings described in terms of volt-amps (V.A.) instead of
>watts in some components?
Hello Will,
As others have explained, "VCT" means "volts center tapped" and refers to the
entire secondary voltage. It's a shorthand way of telling you the voltage of a
secondary, plus the fact that it is center tapped.
VA means Volt Amperes. As others have explained, this term has to do with
reactive loads. It is the product of the voltage and curent delivered,
regardless of the phase angle involved.
If you are still puzzled by explanations involving phase angles, here's another
way to look at it:
Suppose I have a transformer with a 10 volt secondary. If I connect a 10 ohm
resistor across the secondary and turn on the power, the resistor will draw 1
ampere and dissipate 10 watts. The transformer must deliver 10 volts at 1 amp
to the secondary, which means 10 watts.
Now suppose I replace the 10 ohm resistor with an (ideal) capacitor having 10
ohms reactance. (At 60 Hz that works out to 265 uF). The capacitor will draw 1
ampere - but that 1 ampere current will not be in phase with the voltage, and
no power will be dissipated in the capacitor. (It's ideal). But the transformer
still has to deliver 1 ampere at 10 volts. The fact that the current and
voltage are not in phase means nothing to the transformer - it still has to
deliver 10 volts and 1 ampere. 10 volt-amperes, in other words.
The same is true if a 10 ohm reactance ideal inductor is used, or various
combinations of resistors and capacitors or inductors.
So the only realistic way to rate transformers is by their VA, because if you
rate them by watts, it could be assumed that they will be unaffected by
reactive loads - which is not the case.
73 de Jim, N2EY