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Transformer phase angle error
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Kleinstein:
There is some current needed to magnetize the core. This current together with the ohmic drop at the windings causes a little phase shift.
Just as a crude estimate:  The magnetizing current for a small transformer could be in the 5 mA (for a ring core type) range on the primary. Assuming some 70% efficiency this would be about 15 % loss due to primary resistance.  For a 5 W 110 V load the impedance is some 2420 Ohms and 15 % of this would be something like 360 Ohms for the primary. At 5 mA this would be something like 1.8 V.  So the phase shift would be at atan(1.8/110) = 0.93 degree. This would be something like 0.04 ms for delay.

With a more normal EI type core transformer the error could be larger, and with a poor little 1 VA type the 1 ms are very plausible.

To get a low phase shift it helps to have a high voltage rating (like a 230 V transformer), a ring core type and also a little larger power rating (e.g. 5-10 VA), as the larger transformers have higher efficiency (lower wire resistance).
Benta:
Kleinstein, we're 100% on the same page. Primary resistance certainly plays a role, and very small mains transformers are notoriously inefficient.
But with the 12.6 VA type the OP has chosen, it should be OK.
Jester:

--- Quote from: Benta on December 19, 2018, 06:30:30 pm ---Kleinstein, we're 100% on the same page. Primary resistance certainly plays a role, and very small mains transformers are notoriously inefficient.
But with the 12.6 VA type the OP has chosen, it should be OK.

--- End quote ---

I appreciate the additional information. :-+

Fortunately load current will be very low, so this will help.
fourtytwo42:

--- Quote from: Benta on December 19, 2018, 05:18:34 pm ---That being said, there's no reason at all that you would have phase shift from primary to secondary if you load it correctly. Fourtytwo42's trace is highly questionable and must reflect something else. Leading phase would normally never occur.
--- End quote ---

Really, highly questionable in exactly what way ? Are you perhaps implying I am unable to use a scope ? Please tell me what you would have done as an improvement upon my obviously mistaken (in your advanced opinion) method.
Kleinstein:
The phase shift depends on the transformer used. Especially small transformers like < 1.5 VA are far from ideal, especially if a classical EI core is used. The measured  0.8 ms delay and also the rather distorted waveform suggests a relatively poor transformer, likely a small one.

The problem is not so much with the load current - load current is usually in phase with the voltage and would thus only reduce the voltage a little. It is the magnetizing current that is the problem, because this current is phase shifted by nearly 90 degree. A inductive load to the transformer would also also cause a similar problem - an capacitive load could kind of compensator. One could see the capacitor as an additional source of the magnetizing current.  So an added capacitor to the secondary could somewhat compensate for the magnetizing current. Compensation should be rather good if enough capacitance is added to reduce the primary no load current to a little more than 50%.

In addition the magnetizing current can be nonlinear, if the core is drive relatively close to saturation. This would cause a distortion in the waveform. Using a 230 V primary would to a large part solve that 2nd problem. A nonlinear load (e.g. rectifier and Filter cap) could also add distortion.
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