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Re: transformers in series



Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Tesla list wrote:
Original poster: Justin <rocketfuel@xxxxxxxxxxxxx>

Today I played with my 2 new potential transformers (60:1 ratio for 120V/7500V)
and I measured something which has me stumped.
With the two transformers not connected to eachother, I reduced the voltage
to 1V with my variac and measured 60V on each secondary, which I expected.
However, When I started measuring voltages between the two transformers,
I got values I wasn't expecting.  For example, if transformer A has HV
terminals labelled 1 and 2, and transformer B has terminals 3 and 4, this
is what I found:
1-2: 60VAC
3-4: 60VAC
2-4: 1VAC
1-3: 1VAC
2-3: 20VAC
1-4: 20VAC
Now the 1VAC I was seeing between 2-4 and 1-3 I understand, but why 20V
between 2-3 and 1-4?  I was expecting 120V, although I know from reading
about other people's configurations that 1-3 or 2-4 are typically
connected, and indeed connecting them together did produce 120V on the
non-connected terminals.
What I don't understand is why I got the 20VAC reading when they were
not connected together!

Any help in explaining the fundamentals here would be greatly
appreciated.

Thanks!
Justin


Hi Justin,

Although the HV windings were not physically connected together, they were in fact coupled (via leakage capacitance and resistance between the HV winding and the core (assuming the HV winding wound on another leg of the core) or between the HV and the LV winding (if the HV winding is layered on top of the LV winding). The end of the HV winding closest to the core or LV winding will a higher coupling capacitance than the other end. It's possible that your HV winding consist of two separate windings connected in series (similar to a center tapped NST) except that the center is tap is isolated from the core.

Try doing the following experiment. Ground your transformers (thereby grounding the cores) make sure that your variac is driving a 1 volt signal into the LV side versus neutral. Now measure the HV output from each transformer _versus ground_. Since each HV winding is "floating" relative to ground, the voltage that you'll measure versus ground will be a function of the coupling capacitance seen by that and, and the winding's output voltage.

The end of the HV winding that's closest to the ground will also have the lowest measured output voltage versus ground. If one of your transformers has two series-connected HV windings (common for a double-eared PT designed to measure voltage between phases in a delta system), then both output may float to a significant voltage (perhaps ~30 volts) versus ground. Slight variations in construction and materials can cause significant differences in leakage resistance and capacitance even in otherwise identical transformers. These differences are accounting for the differences in the "floating" voltages that you are seeing between transformers.

Bert
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