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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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16th Aug 2018, 1:06 pm | #1 |
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
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RS Autotransformer question
Here is an unusual mains transformer. Can anyone please advise how to connect it and what can it be used for. It does not seem to have a secondary winding. All I can see is 6 wires in two groups of three each. It seems to be a discontinued model.
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16th Aug 2018, 1:33 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
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Re: RS Autotransformer question
It can be used to provide 110V but without isolation.
It is the same as the internal bit of an ordinary 110V adapter. |
16th Aug 2018, 1:39 pm | #3 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
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Re: RS Autotransformer question
I've used one very similar in the past to supply old US communications-gear that's only rated for 220V from my domestic supply which frequently heads towards 250V.
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16th Aug 2018, 1:54 pm | #4 |
Dekatron
Join Date: Jan 2012
Location: Brentwood, Essex, UK.
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Re: RS Autotransformer question
It is one of a range of auto-transfomers listed in a 1975 RS catalogue, which says that they can be used for step- up or step-down of mains voltages, and that "These transformers all employ vacuum-varnish impregnated windings.". It cost £6.40 but VAT was extra.
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16th Aug 2018, 2:00 pm | #5 |
Octode
Join Date: Dec 2017
Location: Morden, Surrey, UK.
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Re: RS Autotransformer question
£6.40, stroll on.....
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16th Aug 2018, 2:06 pm | #6 |
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 1,051
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Re: RS Autotransformer question
OK. So I do need to connect the earth, neutral and live of my mains supply to the transformer terminals marked E, N and 240 V respectively ? And ,for example, can I get 220V output by connecting again a three core wire to the earth, neutral and 220v terminals ?
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16th Aug 2018, 2:09 pm | #7 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: RS Autotransformer question
Whatever tap you use please remember that the output will NOT be isolated from the mains, the transformers earth wire must always be connected via the power cables earth wire to the mains earth pin in the mains plug.
Lawrence. Last edited by ms660; 16th Aug 2018 at 2:24 pm. Reason: Clarification |
16th Aug 2018, 2:17 pm | #8 | ||
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 1,051
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Re: RS Autotransformer question
Quote:
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16th Aug 2018, 2:30 pm | #9 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
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Re: RS Autotransformer question
You should be able to sell it.
The copper will be bonded with varnish. It will be hard to recover for reuse. |
16th Aug 2018, 2:37 pm | #10 |
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
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Re: RS Autotransformer question
Cheers. I might put it up for sale and see if anyone's interested.
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16th Aug 2018, 3:03 pm | #11 |
Dekatron
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Location: Surrey, UK.
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Re: RS Autotransformer question
250VA is a useful rating, someone will bite. As mentioned, some older/foreign kit is happier with the mains knocked down a bit, also some 115V kit is cheap over here!
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16th Aug 2018, 10:00 pm | #12 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
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Re: RS Autotransformer question
£51 in today's money- much the same as RS's current offerings.
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16th Aug 2018, 10:40 pm | #13 | |
Dekatron
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Re: RS Autotransformer question
Quote:
Cheers, GJ
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16th Aug 2018, 11:14 pm | #14 |
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
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Re: RS Autotransformer question
In this connection, I remembered that I do have a new 220V AC motor that I purchased in Japan. Japan doesn't use 220V so the motor was heavily discounted. Would I be ok to connect it directly to 240V UK mains as it is not sensitive equipment. I am not sure what project I'll end up building, but am curious. It is only 10% above its voltage rating, but surely mains supplies are not constant either ?
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17th Aug 2018, 9:10 am | #15 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
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Re: RS Autotransformer question
Mains supplies are indeed not constant. So your 240V supply might be 253V (that's the limit on the spec) in which case your 220V motor would be running at 15% over its rating. How long it would last at that voltage depends entirely on the quality of its construction which, of course, we can't help you with without knowing a great deal more detail. I'm afraid there's only one way to find out really. If, after it's been running for a while, you see the paint on the motor starting to go brown and bubble off then I'd unplug it. Do make sure any exposed metal is earthed too - if the insulation burns through then there would be a shock risk.
Cheers, GJ
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17th Aug 2018, 11:18 pm | #16 |
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
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Re: RS Autotransformer question
Thank you. Here's a picture. The transformer is to the left. The motor has no brushes, which sits inside the iron core. I am guessing the AC supply generates electro magnetic pulses to rotate the motor without contact ?
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18th Aug 2018, 4:04 am | #17 |
Nonode
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
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Re: RS Autotransformer question
It looks to me like a shaded pole induction motor.
Very popular for small AC motors, a single phase supply produces a rotating magnetic field due to the "shading ring" which is a strip of copper around part of the magnetic circuit. The main merit of this type is extreme cheapness and simplicity. They contain only a single winding, no brushes, and need no capacitors. Most types will work as a very crude alternator. Try connecting a low current 6 volt or 12 volt pilot lamp to the input connections and spinning the shaft by hand. The lamp will probably light. Be wary of re-purposing such motors, some are intended for only short term use at the rated voltage, not for continuous duty. A very common application is the water pump in a washing machine which is energised for only a few minutes at a time. |
18th Aug 2018, 4:37 am | #18 |
Moderator
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Location: Fife, Scotland, UK.
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Re: RS Autotransformer question
Magnetic cores with mains powered windings can be running quite close to saturation. A small increase in voltage causes an increase in current. The increase in current increases the magnetic flux density closer to saturation and this causes the inductance to drop, so the current gets greater still, and this moves it nearer to saturation, so the inductance drops more still and the current goes up more still.
So the current taken doesn't go up in simple proportion to the voltage. If you do measurements and plot a graph, it rises gently and then it starts accelerating upwards getting very steep. For cost saving, profit and size, things tend to get designed to be part way up this ski-jump and close to things going dramatic. For this reason, small increases in mains voltage can be a big deal for motors and transformers. So can running on a lower frequency, reducing the impedance the inductance provides, increasing currents. Japan has mixed 50 and 60 Hz supplies, I believe. Americans use 220V/60Hz for cookers and large appliances. This could be something made for export. If it is for 60Hz, then the issue of increased voltage is made worse. Autotransformers can change the voltage for you, but not the frequency. David
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18th Aug 2018, 7:40 am | #19 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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Re: RS Autotransformer question
An auto transformer works like a voltage divider, more info here - https://www.youtube.com/watch?v=RblT05CLPw8 If nothing else save it for the core, you can wind your own tfmr if it comes apart easily.
Andy.
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18th Aug 2018, 10:37 am | #20 |
Nonode
Join Date: Jan 2010
Location: North Somerset, UK.
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Re: RS Autotransformer question
220 volts is not a standard supply voltage in the USA.
Standard voltages are 120 for lighting and small appliances. 208 volts for heavy loading appliances. 240 volts for heavy loading appliances. And in non-domestic premises 277 volts for lighting. Sometimes appliances are referred to as "220 volt" but what is usually meant is that the appliance is designed for operation on 208 volts or from 240 volts. The 220 volt motor is therefore unlikely to be for the American market. |