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| Transformer phase angle error |
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| Benta:
--- Quote from: fourtytwo42 on December 19, 2018, 06:52:22 pm --- --- 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. --- End quote --- No reason to be testy, I apologise for my 'advanced opinion'. I have no doubts about your ability using a 'scope. My comment refers to the "load it correctly" part. Your measurement is obviously on a very small transformer operating at saturation limit with no load. In that case, anything is possible. |
| Jester:
--- Quote from: wraper on December 19, 2018, 08:54:04 pm --- --- Quote from: rstofer on December 19, 2018, 08:35:20 pm ---If I could prove that the phase shift of a given transformer was consistent over a range, I would calibrate it out in code.. --- End quote --- There are more problems than phase shift. Output signal(voltage) just won't be accurate. --- End quote --- Specification for this device states PT's must have galvanic isolation. Hopefully the transformer errors are consistent enough that we can calibrate them out in software, our calibration code accommodates up to 20 points across the range, with our old Rogowski coil / resistor divider metering system we can calibrate to better than 0.1%. I'm not sure how "cheap" we can go on the transformers for this project, the software should go a long way. |
| wraper:
Why cannot you make measurment on the primary side and then transfer data via optoisolator? This is how bench multimeters work. IMHO you are trying to invent the wheel. |
| Jester:
--- Quote from: schmitt trigger on December 19, 2018, 02:58:16 pm ---A trick that may work is to purchase a transformer with a 240 volt rated primary and operate it at 120 volts. Of course, to obtain the same required output voltage, the secondary rating must also double. Another trick I read -if I recall correctly- on an Analog Device's app note is to use an RC network to introduce a phase shift in the opposite direction, which will cancel out the transformer's own shift. --- End quote --- Thanks schmitt trigger Well I dug through my transformer box and found an E-Core style 240 to 24V, and did a few unloaded measurements (just visual zero cross on the scope). With 120V into the 240V transformer, I'm only seeing about 18us lag, so about 0.4 degrees, not so bad. Considering that the load will be minimal and fixed, and the temperature stable, I expect the ratio will also be fairly stable, need to do more testing. --- Quote from: Kleinstein on December 19, 2018, 07:17:43 pm ---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. --- End quote --- Thanks Kleinstein Good point, I measured the magnetizing current at about 2.3ma |
| Jester:
--- Quote from: wraper on December 19, 2018, 10:01:54 pm ---Why cannot you make measurment on the primary side and then transfer data via optoisolator? This is how bench multimeters work. IMHO you are trying to invent the wheel. --- End quote --- Well we could but that would be like reinventing the wheel, when you can purchase a 0.1% accurate Energy Metering IC for $1 https://www.digikey.com/products/en/integrated-circuits-ics/pmic-energy-metering/765?FV=ffe002fd%2Cbfc0002&quantity=0&ColumnSort=1000011&page=1&pageSize=25 |
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