Variac input/output current

[Daniel Watman]

Hi all,

I need to power some US equipment at 110V and am thinking of getting a
variac rather than a fixed transformer since it will be useful for other
things as well.

I have a question about the ratings, if a variac has a 10A rating (no power
rating given) does that mean:
a) it will be able to draw 10A at 240V (higher current at lower voltage)
b) 10A at all voltages
c) 10A at 240v and decreasing at lower voltages
d) none of the above

If anyone knows of a source of cheap 1kW or higher 240-110V transformers or
large variacs I would like to know that too! Especially if in Sydney.

Thanks for any help...
-Daniel

 

[Trevor Wilson]

"Daniel Watman" <pywo@optushome.com.au> wrote in message
news:40ad9fa8$0$8987$afc38c87@news.optusnet.com.au...

Hi all,

I need to power some US equipment at 110V and am thinking of getting a
variac rather than a fixed transformer since it will be useful for other
things as well.

I have a question about the ratings, if a variac has a 10A rating (no
power
rating given) does that mean:
a) it will be able to draw 10A at 240V (higher current at lower voltage)

**No.

b) 10A at all voltages

**Yes.

c) 10A at 240v and decreasing at lower voltages

**No.

d) none of the above

**No.

A caution, when using Variacs at 117VAC, is that you may end up with a dodgy
spot (mine has one) when left of that Voltage, at high currents, for
extended periods. Or not. YMMV. Mine is a 4 Amp variable autotransformer
(Variac is a trade name) and has served me well for several decades. I would
prefer a 10 Amp one, but they do take up rather a lot of bench space.

If anyone knows of a source of cheap 1kW or higher 240-110V transformers
or
large variacs I would like to know that too! Especially if in Sydney.

**I have some large transformers (1kVA - 3kVA) which would probably suit
your needs. They ain't purty, but they're usable.


--
Trevor Wilson
www.rageaudio.com.au

 

[Mark Hathaway]

beware of the Jaycar/electus ones, I went through three of them before
giving up on them and was refused a refund, "you obviously drew more then 4
amps" , they do not come close to their amperage rating. It was suggested
here on this forum that the contact brush is too wide and engages too many
turns, causing the windings to overheat.


I now own a very old but super sturdy 10 amp Westinghouse autotransformer.


Mark Hathaway


"Trevor Wilson" <trevor@rageaudio.com.au> wrote in message
news:40adae82$1@news.comindico.com.au...

"Daniel Watman" <pywo@optushome.com.au> wrote in message
news:40ad9fa8$0$8987$afc38c87@news.optusnet.com.au...
Hi all,

I need to power some US equipment at 110V and am thinking of getting a
variac rather than a fixed transformer since it will be useful for other
things as well.

I have a question about the ratings, if a variac has a 10A rating (no
power
rating given) does that mean:
a) it will be able to draw 10A at 240V (higher current at lower voltage)

**No.

b) 10A at all voltages

**Yes.

c) 10A at 240v and decreasing at lower voltages

**No.

d) none of the above

**No.

A caution, when using Variacs at 117VAC, is that you may end up with a
dodgy
spot (mine has one) when left of that Voltage, at high currents, for
extended periods. Or not. YMMV. Mine is a 4 Amp variable autotransformer
(Variac is a trade name) and has served me well for several decades. I
would
prefer a 10 Amp one, but they do take up rather a lot of bench space.


If anyone knows of a source of cheap 1kW or higher 240-110V transformers
or
large variacs I would like to know that too! Especially if in Sydney.

**I have some large transformers (1kVA - 3kVA) which would probably suit
your needs. They ain't purty, but they're usable.


--
Trevor Wilson
www.rageaudio.com.au

 

[Brian Goldsmith]

"Mark Hathaway" wrote


beware of the Jaycar/electus ones, I went through three of them before
giving up on them and was refused a refund, "you obviously drew more
then 4
amps" , they do not come close to their amperage rating. It was
suggested
here on this forum that the contact brush is too wide and engages too
many
turns, causing the windings to overheat.

**** Mark,did you happen to notice the width of the brush in your
"variac"?

Brian Goldsmith.

 

[Patrick Dunford]

In article <40ad9fa8$0$8987$afc38c87@news.optusnet.com.au>,
pywo@optushome.com.au says...

Hi all,

I need to power some US equipment at 110V and am thinking of getting a
variac rather than a fixed transformer since it will be useful for other
things as well.

I have a question about the ratings, if a variac has a 10A rating (no power
rating given) does that mean:
a) it will be able to draw 10A at 240V (higher current at lower voltage)
b) 10A at all voltages
c) 10A at 240v and decreasing at lower voltages
d) none of the above

there is no power rating because power is measured in VA not in watts, it
varies depending on the phase angle.

 

[Patrick Dunford]

In article <40ad9fa8$0$8987$afc38c87@news.optusnet.com.au>,
pywo@optushome.com.au says...

Hi all,

I need to power some US equipment at 110V and am thinking of getting a
variac rather than a fixed transformer since it will be useful for other
things as well.

A variac is a variable autotransformer. The electrical regulations in NZ
say that autotransformers must not be used for stepdown, because if a
wire breaks the full voltage will appear on the output.

This is probably just a guideline to your situation, but a variac will
typically be used to restrict the power of a device that is capable of
working on the full mains voltage.

 

[Phil Allison]

"Patrick Dunford" <patrickdunford@nomail.invalid>

there is no power rating because power is measured in VA not in watts,

** Sorry Patrick - power is always measured in watts.

VA is measured in " VAs "

it varies depending on the phase angle.

** Since we are on an electronics NG it ought to be cool to mention that
electronic devices have little or no phase angle between the current and
voltage drawn from the AC supply. They do however draw current only at the
peaks of the AC voltage resulting in a very distorted current waveform -
this increase the RMS value of the current ( compared to the average
rectified one) which produces a less than unity power factor so the VA
figure is greater than the watts figure.



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

 

[Phil Allison]

"Patrick Dunford" <patrickdunford@nomail.invalid>

A variac is a variable autotransformer.

** With the important difference that unlike a real autotransformer the
winding wire is **all** made from the same gauge - this *prevents* an
increase in the maximum current available as the voltage is reduced.

The electrical regulations in NZ
say that autotransformers must not be used for stepdown,

** For general purpose stepdown to a low voltage ( ie 50 volts AC or less )
that is no doubt true.

because if a
wire breaks the full voltage will appear on the output.

** The full AC voltage may appear **on** the output wires (referenced to
ground ) - but not *across* the output wires.

This is probably just a guideline to your situation, but a variac will
typically be used to restrict the power of a device that is capable of
working on the full mains voltage.

** A variac should NEVER be used with any other kind of device - since
that is an accident waiting to happen.



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

 

[KLR]

On Fri, 21 May 2004 16:20:23 +1000, "Daniel Watman"
<pywo@optushome.com.au> wrote:

I would assume that it would be able to SUPPLY 10A maximum output at
any voltage (within the range of its variable scale)

The current actually drawn from the 240v source would depend on how
big the output load is and would vary according to the voltage being
supplied out as well..

Another point that you must be aware of is that a Variac DOES NOT
isolate the output - so you could have it wound down to say 20v or so
and one output terminal is still at mains potential and therefore
potentially lethal depsite the low output voltage.

Finally there is the risk that someone will accidentally change the
setting and do a lot of damage. I doubt it would be legal - or safe
for continuous use..

If anyone knows of a source of cheap 1kW or higher 240-110V transformers or
large variacs I would like to know that too! Especially if in Sydney.

Oatley Electronics often has good sized 240/120v transformers - (often

housed) used or surplus buys for extremely good prices ZB0396 (1000W)
With this sort of grunt I think would be more than adequate for
running most consumer items . It doesnt say if its an isolation
transformer though so you should confirm this if needed before buying.

ZC0329 is smaller again.

Thanks for any help...
-Daniel

 

[Daniel Watman]

<KLR> wrote in message news:nh40b0h2tirq723smf9ki8ek3789hpptke@4ax.com...

On Fri, 21 May 2004 16:20:23 +1000, "Daniel Watman"
pywo@optushome.com.au> wrote:



I would assume that it would be able to SUPPLY 10A maximum output at
any voltage (within the range of its variable scale)

The current actually drawn from the 240v source would depend on how
big the output load is and would vary according to the voltage being
supplied out as well..

Another point that you must be aware of is that a Variac DOES NOT
isolate the output - so you could have it wound down to say 20v or so
and one output terminal is still at mains potential and therefore
potentially lethal depsite the low output voltage.

Finally there is the risk that someone will accidentally change the
setting and do a lot of damage. I doubt it would be legal - or safe
for continuous use..

If anyone knows of a source of cheap 1kW or higher 240-110V transformers
or
large variacs I would like to know that too! Especially if in Sydney.

Oatley Electronics often has good sized 240/120v transformers - (often
housed) used or surplus buys for extremely good prices ZB0396 (1000W)
With this sort of grunt I think would be more than adequate for
running most consumer items . It doesnt say if its an isolation
transformer though so you should confirm this if needed before buying.

ZC0329 is smaller again.

Thanks for reminding me of oatley, I looked there for variable transformers
but now I think a fixed one would be better.

I need 10A output, so 1000W is not enough. One option would be to parallel 5
of their 250W transformers together: is this likely to cause a lot of
problems? Besides making sure the transformers arent out of phase with each
other are there any other issues with doing this? It's probably not the
safest thing to do but I can't think of why, as long as the whole bundle is
fused and in a nice safe box. They might need something in between them so
the magnetic fields dont interact too.

Thoughts anyone?
-Daniel

 

[Phil Allison]

"Daniel Watman" <pywo@optushome.com.au>

I need 10A output, so 1000W is not enough

** Arrange a 120mm fan to blow onto it - that will increase the rating
to at least 1500 watts.


BTW What is the 120 volt device you wan to operate and how do you know it
needs 10 amps ?




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

 

[Daniel Watman]

"Phil Allison" <philallison@tpg.com.au> wrote in message news:<2hbh5qFaq7kgU1@uni-berlin.de>...

"Daniel Watman" <pywo@optushome.com.au

I need 10A output, so 1000W is not enough



** Arrange a 120mm fan to blow onto it - that will increase the rating
to at least 1500 watts.


BTW What is the 120 volt device you wan to operate and how do you know it
needs 10 amps ?




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

It is an argon laser that has a max tube current of 10A. I think the
supply uses PWM on the rectified and filtered line voltage to control
the current (voltage across the tube is ~100V), so line current must
be >= 10A. It will not always be run at this current so your idea is
probably the best solution. Thanks!

So it is heat, not resistance/impedance that limits the max current?
Out of interest, what are the major reasons not to parallel
transformers?
Differences in output voltage could cause some problems I guess.

Thanks,
-Daniel

 

[Phil Allison]

"Daniel Watman" <pywo@optushome.com.au> wrote in message

It is an argon laser that has a max tube current of 10A. I think the
supply uses PWM on the rectified and filtered line voltage to control
the current (voltage across the tube is ~100V), so line current must
be >= 10A.

** So you only have a very vague idea of the actual **** RMS*** current
draw ???

Is there no AC rating plate on the laser's PSU ???

A DC supply draws much more AC current than simple math suggests.



It will not always be run at this current so your idea is

probably the best solution. Thanks!

** Fan cooling works 100% of the time.

So it is heat, not resistance/impedance that limits the max current?

** Absolutely - transformer VA ratings are determined by temperature rise
alone.

A rise of about 60 degrees C is considered normal, which may take an hour
or more to happen - in the short term 100% or even 200 % overloads are
easily tolerated.

Out of interest, what are the major reasons not to parallel
transformers?

** 1. It is not cost or weight efficient.

2. The transformers must be identical with the **exact** same number
of turns or power will not be shared.







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

 

[Phil Allison]

"Daniel Watman" <pywo@optushome.com.au> wrote in message

It is an argon laser that has a max tube current of 10A. I think the
supply uses PWM on the rectified and filtered line voltage to control
the current (voltage across the tube is ~100V),

** Hmmm, 10 amps at 100 volts = 1000 watts.

So Daniel has an 1 KW argon laser ??????

That is nothing short of a a military weapon.





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

 

[Richard Freeman]

"Daniel Watman" <pywo@optushome.com.au> wrote in message
news:40ad9fa8$0$8987$afc38c87@news.optusnet.com.au...

Hi all,

I need to power some US equipment at 110V and am thinking of getting a
variac rather than a fixed transformer since it will be useful for other
things as well.

I have a question about the ratings, if a variac has a 10A rating (no
power
rating given) does that mean:
a) it will be able to draw 10A at 240V (higher current at lower voltage)
b) 10A at all voltages
c) 10A at 240v and decreasing at lower voltages
d) none of the above

If anyone knows of a source of cheap 1kW or higher 240-110V transformers
or
large variacs I would like to know that too! Especially if in Sydney.

I got my Variac (10A) from Rockby in Melbourne but the biggest they have at
the moment is a 9A for around $577

My Isolation Transformer is a custom wound job (with 120 or 240V out) from
Harbuch in Hornsby but even that cost (I foret exactly) around $300 for just
the Transformer ...

 

[Patrick Dunford]

In article <a2198a3e.0405231739.dc440a9@posting.google.com>,
pywo@optushome.com.au says...

"Phil Allison" <philallison@tpg.com.au> wrote in message news:<2hbh5qFaq7kgU1@uni-berlin.de>...
"Daniel Watman" <pywo@optushome.com.au

I need 10A output, so 1000W is not enough



** Arrange a 120mm fan to blow onto it - that will increase the rating
to at least 1500 watts.


BTW What is the 120 volt device you wan to operate and how do you know it
needs 10 amps ?




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

It is an argon laser that has a max tube current of 10A. I think the
supply uses PWM on the rectified and filtered line voltage to control
the current (voltage across the tube is ~100V), so line current must
be >= 10A. It will not always be run at this current so your idea is
probably the best solution. Thanks!

So it is heat, not resistance/impedance that limits the max current?

The resistance determines how much heat is going to give off. Too much
heat and it catches fire or burns out. Compare it to a power cord wound
up on a reel that will melt at 500 watts or so, that is fine if unwound.

 

[Stan Blazejewski]

On Sun, 23 May 2004 11:25:03 +1000, "Phil Allison"
<philallison@tpg.com.au> wrote:

"Patrick Dunford" <patrickdunford@nomail.invalid


A variac is a variable autotransformer.


** With the important difference that unlike a real autotransformer the
winding wire is **all** made from the same gauge - this *prevents* an
increase in the maximum current available as the voltage is reduced.


The electrical regulations in NZ
say that autotransformers must not be used for stepdown,


** For general purpose stepdown to a low voltage ( ie 50 volts AC or less )
that is no doubt true.


because if a
wire breaks the full voltage will appear on the output.


** The full AC voltage may appear **on** the output wires (referenced to
ground ) - but not *across* the output wires.

'UNLESS' the wire that broke was somewhere between the output wires
making the resultant circuit a inductor in series with the load
(effectively or close to the full AC voltage).

This is probably just a guideline to your situation, but a variac will
typically be used to restrict the power of a device that is capable of
working on the full mains voltage.


** A variac should NEVER be used with any other kind of device - since
that is an accident waiting to happen.



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

--

Australia isn't "down under", it's "off to one side"!

stanblaz@netspace.net.au
www.stanblaz.customer.netspace.net.au
www.cobracat.com (home of the Australian Cobra Catamaran)

 

[Daniel Watman]

"Phil Allison" <philallison@tpg.com.au> wrote in message
news:2hdcmqFbbbl6U1@uni-berlin.de...

"Daniel Watman" <pywo@optushome.com.au> wrote in message

It is an argon laser that has a max tube current of 10A. I think the
supply uses PWM on the rectified and filtered line voltage to control
the current (voltage across the tube is ~100V),



** Hmmm, 10 amps at 100 volts = 1000 watts.

So Daniel has an 1 KW argon laser ??????

That is nothing short of a a military weapon.

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

I wish
You arent factoring the super high efficiency of argon lasers... 1000W in,
~50mW out.
Its a heater that happent to output a bit of light as well!

-Daniel

 

[Daniel Watman]

"Phil Allison" <philallison@tpg.com.au> wrote in message
news:2hd3vaFbcgmeU1@uni-berlin.de...

"Daniel Watman" <pywo@optushome.com.au> wrote in message

It is an argon laser that has a max tube current of 10A. I think the
supply uses PWM on the rectified and filtered line voltage to control
the current (voltage across the tube is ~100V), so line current must
be >= 10A.


** So you only have a very vague idea of the actual **** RMS*** current
draw ???

Is there no AC rating plate on the laser's PSU ???

A DC supply draws much more AC current than simple math suggests.

Yep I barely have any idea how much it draws, I bought it off ebay and don't

have it here yet. I will start a new thread when it arrives and I have more
details...

It will not always be run at this current so your idea is
probably the best solution. Thanks!



** Fan cooling works 100% of the time.



So it is heat, not resistance/impedance that limits the max current?


** Absolutely - transformer VA ratings are determined by temperature
rise
alone.

A rise of about 60 degrees C is considered normal, which may take an
hour
or more to happen - in the short term 100% or even 200 % overloads are
easily tolerated.



Out of interest, what are the major reasons not to parallel
transformers?


** 1. It is not cost or weight efficient.

2. The transformers must be identical with the **exact** same
number
of turns or power will not be shared.

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

I was thinking of some of oatleys transformers: they have some 250W

240v-110v toroidals for $25 (TOR1 in bargain corner 3). They should be
identical, and the price for 5 of them is still cheaper than their 1kW
transformer. Weight is not too bad at 4kg each.
I will wait until I get the actual laser before I buy anything, but what do
you think about using these? (if I can't find a suitable single transformer)

Thanks!
-Daniel

 

[Phil Allison]

"Stan Blazejewski" <stanblaz@netspace.net.au> wrote in message

** The full AC voltage may appear **on** the output wires (referenced to
ground ) - but not *across* the output wires.

'UNLESS' the wire that broke was somewhere between the output wires
making the resultant circuit a inductor in series with the load
(effectively or close to the full AC voltage).

** In the case of a 240 volt to 120 volt auto-transformer this is not a
hazard to life as just like with 240 volt AC wires, 120 volt AC wires are
not exposed nor touchable in normal use.

If the internal connection to neutral is lost on an auto-transformer
240/120 stepdown then the voltage at the output terminals will be the same
as the supply albeit with a half a 240 volt winding appearing in series.
This is still not likely to overvoltage a typical 120 volt load as only the
normal magnetising current can flow.

In case of a ( hypothetical ) 240 volt to low voltage auto-transformer the
output wires and metal parts of items connected directly to them are *very
likely* to be exposed and touchable in normal use. In this case even the
reversal of the A and N connections at the AC outlet or simply the loss of
the N connection would result in a lethal situation. Such transformers are
understandably completely banned for that reason.




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