Why in a practical transformer short circuit test primary voltage is not zero?

I would like to know, why is there much lesser noise in the voltage waveform of open-circuit test compared to short circuit test?

also

Why in a practical transformer short circuit test primary voltage is not zero?

Hope someone could let me know.

Thanks

Re: Transformer Analysis

When you put a short on a transformer, current flows in the secondary through the resistance of the secondaries winding. The resistance of this winding is reflected back into the primary by a factor k^2 (k = Np/Ns, turns ratio). Add to this the resistance of the primary winding and you get a voltage drop according to the current in the primary.
Frank

ah thanks, really helped me understand.

but, does anyone know why is there much lesser noise in the voltage waveform of open-circuit test compared to short circuit test?

The general definition of short circuit test doesn't say, how the input voltage is set. If it's a low impedance variable transformer (the usual setup presented in text books, I think) there won't be necessarily more "noise". If it's effectively a current source, the inductive transformer impedance will differentiate the input current, so harmonics will "amplified" proportional to their frequency.

the voltage sensor used is capable of measuring up to 500Vrms. Is it possible that as the percentage of the measured short-circuit voltage to the voltage sensor rating is below 1%, & the percentage is 40% for open-circuit voltage, we can safely say that there is more noise in the voltage waveform of short-circuit than open-circuit?

Please let me know if I'm wrong.

Thanks

@intesified: Could you quantify the noise you are talking about?

Depending on the transformer design, the input of the short-circuited transformer may have a large inductive component due to the leakage induction (because of k<1 in real transformers).

WimRFP said:

@intesified: Could you quantify the noise you are talking about?

Depending on the transformer design, the input of the short-circuited transformer may have a large inductive component due to the leakage induction (because of k<1 in real transformers).

Click to expand...

Just asking, what do you mean by quantify the noise?

I was talking about the input voltage waveform with distortion present in the short circuited transformer, unlike when it is open circuited, distortions present are much less.

@intensified: You mentioned noise in your postings only, and that can be anything. With the additional info it is clear.

In the open circuit case, the transformer has a non linear input impedance that will be high w.r.t. the source impedance. So the voltage across the primary winding is dictated by the driving voltage source. When that source has good sinusoidal output, the voltage you measure will be a sine wave. The primary input current can be non-sinusoidal.

When short circuited, the source may behave like a current source, as impedance of short-circuited transformer may be less then the output impedance of the source . Short-circuiting a mains transormer acutally makes it more linear (as the core flux is reduced because of the low primary voltage). As FVM says, harmonics in the source output current will be enhanced (provided that Ztransformer < Zsource).

Other thing can be that with the much lower fundamental frequency component, you recognize the harmonics better. Are they really stronger (absolute value)?