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Transformer Basic Question

Discussion in 'General Electronics Chat' started by Hurdy, Sep 11, 2008.

  1. Hurdy

    Thread Starter Senior Member

    Feb 27, 2006
    Hi everyone,

    I recently purchased a transformer that was rated as 240VAC-110VAC 3.3KVA.

    It finally arrived today but the terminals are labelled 240V, 0V and GND.
    Why would an AC transformer have 240V and 0V terminal? surely that's only DC. AC doesn't have a 0V.

    I'm assuming the terminals mean 240V = POSITIVE and 0V = NEGATIVE.

    Would anyone here agree with me on this?

    Thank you for your time to read this.

  2. vetterick

    Active Member

    Aug 11, 2008
    No, you can have 0V AC, its just relative from where you measure it.

    I'm a little confused though, are there more than 3 terminals on the transformer? If this is just one winding the ground will be the frame, check with a ohmmeter, if theres only the 3 terminals you have an autotransformer with only 1 winding, this may not work depending on what you want to use it for.

  3. Hurdy

    Thread Starter Senior Member

    Feb 27, 2006
    yes sorry the ground is wired to the chasis. Sorry I should have made that clear.

    Thank you for your post.

    I understand DC quite well. I play around alot with DC circuits such as microcorontrollers etc. I'm kind of new to AC concepts.

    Let me see if I've got this right. At any one point during the AC alternating cycles there will always be a 0v due to the difference in potential between the + and -?

    Is there a good website that anyone knows that can explain the basic concepts of AC.
  4. SgtWookie


    Jul 17, 2007
    Hello Robert,
    Have a look at our online E-books. Volume II - AC starts here:

    In the States, AC power outlets generally have three sockets, two that are flat and one circular, in a roughly triangular pattern. One of the flat sockets is longer than the other. That is the "Neutral" connection, or 0v. The shorter flat socket is the "hot" connection, which is the AC 120v 60Hz power. The round socket is earth ground.

    The purpose of the neutral line is the power return; where the current from the "hot" line gets sent back to the transformer. The earth/ground connection is there for the safety of the operator of the equipment. It is generally connected to the chassis. That way, if there is some breakdown of the insulation or other type of internal short, the operator is protected from exposure to lethal voltage levels by the chassis ground.

    I've never been to the UK, and I'm not an electrician - so I wouldn't do a very good job of describing your residential wiring I'm afraid - except that you get 220VAC at 50Hz.
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    Hello Robert,

    You have had some good advice here about getting clued up on AC.

    In particular if you wish to survive your experiments Vetterick has asked some very pertinent questions.

    What on earth do you want such a beast for?

    This sounds like an industrial transformer meant for supplying site tools. As such it may well be an autotransformer not an isolating transformer and you really need to know the difference.

    With an output capacity of 3.3KVA you also need to be able to calculate appropriate cable sizes and connection details.

    Is this thing cased and if so in plastic or metal?