Loading ...
Sorry, an error occurred while loading the content.

Re: Back-to-back two transformers

Expand Messages
  • Kerim F
    I used my old digital scope (FLUKE 199C) and its 10M probes. Also I set its readings for Vac (RMS). I forget measuring the low voltage, as you suggested,
    Message 1 of 9 , Jun 22, 2013
    • 0 Attachment
      I used my old digital scope (FLUKE 199C) and its 10M probes. Also I set its 'readings' for Vac (RMS).

      I forget measuring the low voltage, as you suggested, before and after connecting the two transformers. Doing this is not easy now since I already enclosed the two transformers after covering all internal connections :(.

      When I noticed this relatively large drop of voltage between input and output (with no load) I thought that, unlike I, many of you may expect it from experience hence already explained it. But there is always the possibility that my two ready-made small transformers are rather a special case, I mean they could be of a bad quality. If this is the case I think this simple experiment can quickly compare the quality of similar transformers made by different manufacturers.

      Kerim


      --- In Electronics_101@yahoogroups.com, wharpt@... wrote:
      >
      > Do you have a load on the output of the second transformer besides the volt
      > meter? When not tied together, what is the measured output voltage of
      > your first transformer at 220V in? Same question for your second transformer.
      > When tied together, what do you measure at the tie points? What do you
      > measure at the tie points and the output if you cross the tie points ( 220v = !!
      > X !! = ?)
      >
      > Wharpt
      >
      >
      > In a message dated 6/22/2013 5:05:03 P.M. Central Daylight Time,
      > ahumanbeing2000@... writes:
      >
      >
      >
      > To get a floating 220V AC supply (for very low power), I connected two
      > small transformers (220/12V, 3W) back to back; the two terminals of the low
      > voltage side of T1 (12V) to their counterparts of T2 (12V).
      >
      > I supplied the high voltage side of T1 with 220V and I expected getting
      > 220V (rms) on the high side terminals. I got 146V (rms) only.
      >
      > I decreased the input voltage to 25V, the output became 10V!!! (more tests
      > below).
      >
      > I wonder which characteristic of the transformer affects the most on this
      > voltage drop.
      >
      > On the other hand, I assume the higher the voltage we get from this simple
      > configuration, the better the type of the transformer under test is (if
      > the comparison is made among transformers that are rated for the same power).
      >
      > Kerim
      >
    • Jan Kok
      I think Andy is correct (although he got the turns ratio wrong - should be about 18:1 not 4:1). Playing around with circuit simulators can be enlightening. The
      Message 2 of 9 , Jun 22, 2013
      • 0 Attachment
        I think Andy is correct (although he got the turns ratio wrong - should be
        about 18:1 not 4:1).

        Playing around with circuit simulators can be enlightening. The following
        can be saved as a .asc file and loaded into LTSpice.

        This simulation shows two transformers connected as described, with
        coupling factors of .9 which gives approximately the results that Kerim
        reported.


        Version 4
        SHEET 1 880 680
        WIRE 240 64 192 64
        WIRE 336 64 240 64
        WIRE 32 80 -32 80
        WIRE 80 80 32 80
        WIRE 496 80 448 80
        WIRE 512 80 496 80
        WIRE -32 112 -32 80
        WIRE 80 112 80 80
        WIRE 192 112 192 64
        WIRE 336 112 336 64
        WIRE 448 112 448 80
        WIRE -32 208 -32 192
        WIRE 80 208 80 192
        WIRE 80 208 -32 208
        WIRE 192 208 192 192
        WIRE 192 208 80 208
        WIRE 336 208 336 192
        WIRE 336 208 192 208
        WIRE 448 208 448 192
        WIRE 448 208 336 208
        FLAG -32 208 0
        FLAG 32 80 VP
        FLAG 240 64 VS
        FLAG 496 80 VT
        SYMBOL ind2 64 96 R0
        SYMATTR InstName L1
        SYMATTR Value 600
        SYMATTR Type ind
        SYMATTR SpiceLine Rser=100
        SYMBOL ind2 176 96 R0
        SYMATTR InstName L2
        SYMATTR Value 1.5
        SYMATTR Type ind
        SYMATTR SpiceLine Rser=1 Rpar=1meg
        SYMBOL voltage -32 96 R0
        WINDOW 3 -85 145 Left 2
        WINDOW 123 0 0 Left 2
        SYMATTR InstName V1
        SYMATTR Value SINE(0 240 50 0 0 0)
        SYMATTR SpiceLine Rser=1 Cpar=0
        SYMBOL ind2 320 96 R0
        SYMATTR InstName L3
        SYMATTR Value 1.5
        SYMATTR Type ind
        SYMBOL ind2 432 96 R0
        SYMATTR InstName L4
        SYMATTR Value 600
        SYMATTR Type ind
        TEXT -118 262 Left 2 !.tran 5
        TEXT 136 -8 Left 2 !K1 L1 L2 .9
        TEXT 352 0 Left 2 !K2 L3 L4 .9





        On Sat, Jun 22, 2013 at 6:45 PM, Andy <ai.egrps@...> wrote:

        > Kerim F <ahumanbeing2000@...> wrote:
        >
        > > I decreased the input voltage to 25V, the output became 10V!!! (more
        > tests below).
        >
        > There is a property (nonlinear magnetizing inductance??) of iron core
        > power transformers that causes their efficiency to drop when supplied
        > with lower voltages. At least that explains the drop in Vout/Vin at
        > lower voltages.
        >
        > Now, just guessing about the lower voltage in general. You'd think
        > the turns ratio should be 4.28:1 to get the 12 V. But let's say it is
        > less, maybe 4.0:1 ... so one might expect 13.75 V unloaded. But you
        > get only 12 V because of leakage inductance (not all the flux is
        > coupled between windings). In effect there is a built-in voltage loss
        > due to less than perfect mutual inductance. That loss gets you both
        > ways, stepping down and again stepping up.
        >
        > The same explanation works for I*R voltage drops when the transformers
        > are under load.
        >
        > Regards,
        > Andy
        >


        [Non-text portions of this message have been removed]
      • Andy
        ... Oops! Yes indeed. I took the square root of the voltage ratio. My mistake. I was in the habit of doing that when going from impedance ratio to turns
        Message 3 of 9 , Jun 22, 2013
        • 0 Attachment
          > I think Andy is correct (although he got the turns ratio wrong - should be
          > about 18:1 not 4:1).

          Oops! Yes indeed. I took the square root of the voltage ratio. My
          mistake. I was in the habit of doing that when going from impedance
          ratio to turns ratio.

          Andy
        • John Popelish
          I think you are seeing the effect of magnetizing current for the second core, passed through the resistance of three windings, instead of just one (in addition
          Message 4 of 9 , Jun 22, 2013
          • 0 Attachment
            I think you are seeing the effect of magnetizing current for the second
            core, passed through the resistance of three windings, instead of just one
            (in addition to the resistive losses, supplying the magnetization current
            for the first core).

            You might try connecting a 400 volt mylar capacitor across the 220 volt
            output to supply some of that current, via resonance, to see if it pumps
            the voltage up a little. There is some optimum value that puts the system
            into resonance, so, if you have a few sizes, you might get some idea what
            the optimum value is.

            It should work best if half that resonance current was supplied across the
            12 volt windings, also, but that would require a lot higher value of
            capacitance (I think ((220/12)^2)/2 times as much to reduce the 220 volt
            resonating capacitor by half.

            --
            Regards,

            John Popelish

            On Sat, Jun 22, 2013 at 6:04 PM, Kerim F <ahumanbeing2000@...> wrote:

            >
            > To get a floating 220V AC supply (for very low power), I connected two
            > small transformers (220/12V, 3W) back to back; the two terminals of the low
            > voltage side of T1 (12V) to their counterparts of T2 (12V).
            >
            > I supplied the high voltage side of T1 with 220V and I expected getting
            > 220V (rms) on the high side terminals. I got 146V (rms) only.
            >
            > I decreased the input voltage to 25V, the output became 10V!!! (more tests
            > below).
            >
            > I wonder which characteristic of the transformer affects the most on this
            > voltage drop.
            >
            > On the other hand, I assume the higher the voltage we get from this simple
            > configuration, the better the type of the transformer under test is (if the
            > comparison is made among transformers that are rated for the same power).
            >
            (snip)


            [Non-text portions of this message have been removed]
          • epa_iii
            We were not given a lot of information on the transformers. Some transformers that are intended for installation under building codes, have current limiting
            Message 5 of 9 , Jun 24, 2013
            • 0 Attachment
              We were not given a lot of information on the transformers. Some transformers that are intended for installation under building codes, have current limiting resistors installed in series with the windings. Low Voltage wiring is often installed in buildings without conduit or other protection. These resistors are to limit the current and therefore the danger of creating a fire in the case of a short circuit. These resistors are generally not visible or accessible from the outside, probably to prevent the users from removing them. If these transformers are of that, current limiting, type then there would be two such resistors in series with the "back-to-back" secondary windings and that would drop a significant fraction of the output Voltage generated in the first transformer.

              You will always have some losses, but with the proper, identical transformers (not current limited ones), I would expect that under no load conditions the output Voltage of such a connection would be within 5 or 10 percent of the input Voltage. Adding your load will change that number.

              Paul A.


              --- In Electronics_101@yahoogroups.com, John Popelish <jpopelish@...> wrote:
              >
              > I think you are seeing the effect of magnetizing current for the second
              > core, passed through the resistance of three windings, instead of just one
              > (in addition to the resistive losses, supplying the magnetization current
              > for the first core).
              >
              > You might try connecting a 400 volt mylar capacitor across the 220 volt
              > output to supply some of that current, via resonance, to see if it pumps
              > the voltage up a little. There is some optimum value that puts the system
              > into resonance, so, if you have a few sizes, you might get some idea what
              > the optimum value is.
              >
              > It should work best if half that resonance current was supplied across the
              > 12 volt windings, also, but that would require a lot higher value of
              > capacitance (I think ((220/12)^2)/2 times as much to reduce the 220 volt
              > resonating capacitor by half.
              >
              > --
              > Regards,
              >
              > John Popelish
              >
              > On Sat, Jun 22, 2013 at 6:04 PM, Kerim F <ahumanbeing2000@...> wrote:
              >
              > >
              > > To get a floating 220V AC supply (for very low power), I connected two
              > > small transformers (220/12V, 3W) back to back; the two terminals of the low
              > > voltage side of T1 (12V) to their counterparts of T2 (12V).
              > >
              > > I supplied the high voltage side of T1 with 220V and I expected getting
              > > 220V (rms) on the high side terminals. I got 146V (rms) only.
              > >
              > > I decreased the input voltage to 25V, the output became 10V!!! (more tests
              > > below).
              > >
              > > I wonder which characteristic of the transformer affects the most on this
              > > voltage drop.
              > >
              > > On the other hand, I assume the higher the voltage we get from this simple
              > > configuration, the better the type of the transformer under test is (if the
              > > comparison is made among transformers that are rated for the same power).
              > >
              > (snip)
              >
              >
              > [Non-text portions of this message have been removed]
              >
            Your message has been successfully submitted and would be delivered to recipients shortly.