View Full Version : Do a transformer polarity test

February 16th, 2007, 12:42 PM
There has a single phase transformer, and I want to do a test on polarity. First, I make sure there is power off. Then I use ohm meter to identify H1, H2, X1, X2. Secondly, put a jumper between H and X. And put volt meter on H and X which not with jumper. Third, connect H1 and H2 toi AC supply. If the voltmeter reads less than the value of the applied voltage, the polarity is subtractive. Or if more, there is additive. Please to give me a comment if I don't do it properly.

February 16th, 2007, 04:42 PM
The correct term is "step up" for a transformer whose output (secondary) voltage is greater than the input, and "step down" for a transformer whos output voltage is less. A step up transformer has more turns in the secondary coil (side; winding) than in the primary coil and a step down transformer has more turns in the primary coil. But practically speaking, read the next paragraph.

You have to know in advance which side is intended to be the input (primary) and what voltage and power line frequency the transformer is intended for. You may not arbitrarily make a step down transformer step up simply by connecting it backwards and you may not arbitrarily feed in power of a lesser frequency than the transformer was intended for. Normally the power drawn by the transformer primary varies with the load on the secondary. However connecting the power to the wrong side may result in burnout within seconds even with nothing connected to the output (other side) if that side used as the input was not intended to accept the voltage applied.

February 16th, 2007, 11:16 PM
Thanks for notice that for me. So to speak that method is only to find the polarity H1, H2, X1, X2 only. And at least I need to know all the information about that transformer before do the test. If there has no nameplate or markings on the transformer. It will becomes a garbage.

February 17th, 2007, 08:23 AM
Come to think of it, if you can see the wires coming out from under the terminal screws or lugs and going into the rest of the transformer, it is generally safe to apply 120 volt power to the pair of terminals whose wires are thinner. This will generally cause the transformer to work as a step down transformer and you can then measure the voltage across the other pair of terminals. A step down transformer alwasy gives you more amperes of output than the amperes drawn from the line. (And vice versa.) The side that handles more amperes will have thicker wire.

Let's say you connected 120 volts and got 12 volts out the other side (a tenfold decrease). If the transformer connected backwards could take 120 volts in the other side, you would get a tenfold increase.

But the transformer is probably still useless if you don't know what it was meant for. You would not know how much load you could apply (how many amperes or watts you could get from) the secondary before the transformer overheated, unless you were told in advance.

Also you need to be careful if you try to connect a "strange" transformer as a step up transformer even if you know the transformer will take the 120 volt feed. The step up could be to thousands of volts.

February 17th, 2007, 12:13 PM
I am not going to connect any "unknown" transformer. The reason why I ask about this question because I just came out of school. Since I am still looking for a job as apprentice. And I have some times at home, so I do some review on chapter transformer.

February 17th, 2007, 03:39 PM
The word polarity is negative or positive polarity. This is found in DC wiring a transformer has to have alternating polarity in order for it to operate. A transformer set with a fixed polarity would only produce voltage when the power is turned on or off and no power output on a fixed polarity wiring style can produce power through that transformer at any other time.

AC alternating polarity is required for a transformer to work. That transformer can not have a permenant polarity for it to be able to operate.

Either you need to relearn about transformers or you need to relearn about word terminology.

Good Luck