DC Across distribution transformer primary

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wirenut1980

wirenut1980

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Plainfield, IN
Hello, If DC is applied to a single phase 7200V-120/240 V transformer, will it provide power on the secondary side? I think that it will output dc on the secondary side with a magnitude of the primary DC voltage divided by the turns ratio of the transformer, in this case (7200/240=30). And could this cause damage to electronics in a house.

The story is the line crew for my utility company was thumping a bad underground single phase line and accidentally hooked it up backwards and they thumped the transformer with DC. I am not sure what magintude of DC they used as of now. Thanks!
 
A transformer will not pass DC, though there will be an initial pulse on the secondary as the DC is hooked up and removed. A transformer works when the magnetic field from one winding magneticly couples with the other winding, but the magnetic field has to be constantly changing, as in AC or a fluctuating DC.
 
my guess
the secondary would only pulse as the DC is turned on. As I understand the operation of a transformer , it would require two coils in close proximity one with a magnetic field about it and relative motion between the two coils.

Typically the relative motion in a transformer is provided by the fluctuating magnetic field (AC) or possibly pulsating DC.
For DC the magnetic field only varies in strength during the on and off cycle of the supply. or during changes in intensity of current flow varying resistance or loads.

If the DC is pulsating then you should get an output. If the Direct Current flow does not change in ampltude with a frequency then there may be damage to the windings. No inductive reactance
 
Just so you know, the collapsing magnetic field from a DC-charged primary creates a much larger induced voltage in the secondary than does the initiation of the field.

In an automotive ignition, for example, the spark plugs fire upon the points opening, not closing. But to answer the original question, no, a transformer cannot pass DC.
 
Another highly controlled experiment from my testing lab:) . Shows a 7 volt battery (used to be 9) capable of delivering a 8.8 vdc pulse max open circuit, when measured across the primary side of a 16v to 120v bell transformer. Adding a 20 ohm load resistor dropped the voltage to 2.5
 
the transformer will not pass the dc signal. the tranformer will pass the initial pulse signal when the switch is closed. you would need a time varying signal to have a time varying magnetic field. L di/dt. in the case dc there is no flux. no secondary voltage would be measure beyond the intinial pulse after the switch is closed.
 
72.5kv said:
. . . no secondary voltage would be measure beyond the intinial pulse after the switch is closed.
Click to expand...
Except the much larger pulse when the switch is re-opened.
 
LarryFine said:
Except the much larger pulse when the switch is re-opened.
Click to expand...

And that pulse will not be governed by the "turns ratio"of the primary/secondary windings; resulting in a potentially catastrophic (for sensitve electronic equipment) over-voltage surge. Which may have been the point of the thread question in the first place.
 
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