Autotransformers

[Clint]

I understand the basic idea of an autotransformer, using a tapped
winding to raise or lower a voltage and no isolation (which isn't
necessary for my application anyway).


If I have a 60VA 240V primary mains transformer (the secondary
voltages are for this application, as I understand it, irrelevant as
long as I don't draw current from them) could I reasonably expect to
be able to draw, say, 40 watts from the centre tap to provide a 120V
output or does the 'misuse' of the centre tap derate the transformer
in some way (if it does, I'd be interested to know the reasons) It's
for a simple AC motor in a Christmas decoration.
Thanks..

 

[Tim Wescott]

On 10/05/2010 07:07 AM, Clint wrote:

I understand the basic idea of an autotransformer, using a tapped
winding to raise or lower a voltage and no isolation (which isn't
necessary for my application anyway).


If I have a 60VA 240V primary mains transformer (the secondary
voltages are for this application, as I understand it, irrelevant as
long as I don't draw current from them) could I reasonably expect to
be able to draw, say, 40 watts from the centre tap to provide a 120V
output or does the 'misuse' of the centre tap derate the transformer
in some way (if it does, I'd be interested to know the reasons) It's
for a simple AC motor in a Christmas decoration.
Thanks..

To be conservative you don't want to draw any more than the rated
current out of any one terminal, to avoid overheating that set of
windings. The 'real' allowable draw is somewhere between that amount
and an amount that causes the total winding dissipation to be equal to
the transformer used it it's maximum rating -- but without knowing the
details of how it's wound and how good it's thermal conductivity is, you
can't tell.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[John Larkin]

On Tue, 5 Oct 2010 07:07:56 -0700 (PDT), Clint <cjaysharp@gmail.com>
wrote:

I understand the basic idea of an autotransformer, using a tapped
winding to raise or lower a voltage and no isolation (which isn't
necessary for my application anyway).


If I have a 60VA 240V primary mains transformer (the secondary
voltages are for this application, as I understand it, irrelevant as
long as I don't draw current from them) could I reasonably expect to
be able to draw, say, 40 watts from the centre tap to provide a 120V
output or does the 'misuse' of the centre tap derate the transformer
in some way (if it does, I'd be interested to know the reasons) It's
for a simple AC motor in a Christmas decoration.
Thanks..

40 watts sounds fine to me. Try it and check after a while to see if
the transformer is getting hot. Too hot to touch, which is ballpark
55C, might be a reasonable limit.

John

 

[Rich Grise]

On Tue, 05 Oct 2010 07:07:56 -0700, Clint wrote:

I understand the basic idea of an autotransformer, using a tapped
winding to raise or lower a voltage and no isolation (which isn't
necessary for my application anyway).

If I have a 60VA 240V primary mains transformer (the secondary
voltages are for this application, as I understand it, irrelevant as
long as I don't draw current from them) could I reasonably expect to
be able to draw, say, 40 watts from the centre tap to provide a 120V
output or does the 'misuse' of the centre tap derate the transformer
in some way (if it does, I'd be interested to know the reasons) It's
for a simple AC motor in a Christmas decoration.
Thanks..

If it's rated for 60 VA at 240, I'd expect no more than 30VA at
120.

Good Luck!
Rich

 

[Paul E. Schoen]

"Clint" <cjaysharp@gmail.com> wrote in message
news:04423d62-c373-40c4-a1f8-99f16f61088c@30g2000yqm.googlegroups.com...

I understand the basic idea of an autotransformer, using a tapped
winding to raise or lower a voltage and no isolation (which isn't
necessary for my application anyway).


If I have a 60VA 240V primary mains transformer (the secondary
voltages are for this application, as I understand it, irrelevant as
long as I don't draw current from them) could I reasonably expect to
be able to draw, say, 40 watts from the centre tap to provide a 120V
output or does the 'misuse' of the centre tap derate the transformer
in some way (if it does, I'd be interested to know the reasons) It's
for a simple AC motor in a Christmas decoration.
Thanks..

You should be able to get 60 watts.

An autotransformer is generally rated at twice the VA as the same device
used as a transformer (with no common connection). If the center tap is on
the primary and it is designed to work at the same VA rating with 120 VAC
input, then that half of the winding should be able to handle the current
for 60 VA at 120 VAC, or 0.5 A.

If you have 240 VAC on the primary, and a 60 VA load on the center tap,
there will be 0.25 A flowing in the top half of the winding, and 0.25 A
flowing out of the common connection back to the source. The center tap will
provide a total of 0.5 amps into the load, with 0.25 amps in each winding.
The vector sum of all currents at each node will be zero.

The only thing that might limit this capability is the size of the wire to
the actual center tap of the transformer, but usually this is provided by
the two windings being brought out to a terminal or a flexible stranded wire
which is probably more than adequate to handle the current.

If you do an LTSpice simulation (as follows), you will see that with
V(IN)=240 VAC and R=240 ohms, and a transformer consisting of two coupled 10
H inductors:

I(IN) = 0.25 A @ 240 VAC = 60 VA
I(L1) = I(IN) = 0.25 A
I(L2) = 0.25 A (180 deg)
I(R1) = 0.50 A @ 120 VAC = 60 VA

Paul

------------------------------------------------------------------------

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SHEET 1 880 680
WIRE 112 80 16 80
WIRE 176 80 112 80
WIRE 16 128 16 80
WIRE 176 176 176 160
WIRE 272 176 176 176
WIRE 320 176 272 176
WIRE 176 192 176 176
WIRE 320 192 320 176
WIRE 16 288 16 208
WIRE 176 288 176 272
WIRE 176 288 16 288
WIRE 320 288 320 272
WIRE 320 288 176 288
WIRE 320 304 320 288
FLAG 320 304 0
FLAG 112 80 IN
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SYMBOL ind2 160 64 R0
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SYMBOL ind2 160 176 R0
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SYMBOL res 304 176 R0
SYMATTR InstName R1
SYMATTR Value 240
SYMBOL voltage 16 112 R0
WINDOW 3 -25 198 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(0 350 60 0 0 0 60)
TEXT 232 128 Left 0 !K1 L1 L2 1
TEXT -18 330 Left 0 !.tran .5 startup

 

[Paul E. Schoen]

"Paul E. Schoen" <paul@pstech-inc.com> wrote in message
news:AgKqo.8622$r15.8474@newsfe15.iad...

You should be able to get 60 watts.

An autotransformer is generally rated at twice the VA as the same device
used as a transformer (with no common connection).

I should clarify that you can get 120 VA if you have a 60 VA transformer
with two 120 VAC windings. For a center tap you only get the same VA as
rated.

Paul

 

[whit3rd]

On Oct 5, 7:07 am, Clint <cjaysh...@gmail.com> wrote:

I understand the basic idea of an autotransformer, using a tapped
winding to raise or lower a voltage and no isolation (which isn't
necessary for my application anyway).

If I have a 60VA  240V primary mains transformer ... could I reasonably expect to
be able to draw, say, 40 watts from the centre tap to provide a 120V
output

Yes, that'll probably work (assuming there IS a center tap).
The 60 VA rating, for primary and secondary current flowing
in the winding resistance, is lower than what you'd expect
from the autotransformer connection.
If the primary winding is 4 ohms, 60 VA at 240V creates 0.25W
of resistive heating in the primary, and another 0.25W in the
secondary.
60VA as a transformer yields half a watt of heat.
60VA as an autotransformer, there are two 2 ohm branches carrying
0.25A each (to make 0.5 A at the 120V tap), which yields a quarter
watt of heat.

It would seem that, on heat-generation alone, the autotransformer
rating would be higher than the transformer number.

 

[Clint]

Thanks all, I'd suspected it would work but confirmation is always
nice.

The pirmary is two 120V windings designed to be joined to make one
240V winding. I'll try it with a light bulb load this weekend.

 

[Jasen Betts]

On 2010-10-05, Clint <cjaysharp@gmail.com> wrote:

I understand the basic idea of an autotransformer, using a tapped
winding to raise or lower a voltage and no isolation (which isn't
necessary for my application anyway).


If I have a 60VA 240V primary mains transformer (the secondary
voltages are for this application, as I understand it, irrelevant as
long as I don't draw current from them) could I reasonably expect to
be able to draw, say, 40 watts from the centre tap to provide a 120V
output or does the 'misuse' of the centre tap derate the transformer
in some way (if it does, I'd be interested to know the reasons) It's
for a simple AC motor in a Christmas decoration.
Thanks..

Figure 30VA for each half of the primary, so 60 VA from the centre tap
(with no load on the secondary) should be fine.

You can probably do more as there's no heating in the secondary.

What's the VA rating of the motor?
and is it designed for the right mains frequency?

--
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