Transformer phase angle error - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: December 19, 2018, 01:00:36 pm
content: Can anyone shed any light on typical phase angle error of small (non metering accuracy) transformers for example something like this:https://www.digikey.com/product-detail/en/hammond-manufacturing/166J12/HM519-ND/270120

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 19, 2018, 01:07:47 pm
content: Yup I recently had occasion to measure a similar one and found it to have about 0.8mS lead on 50hZ see pic

author: Jester
author_rank: Frequent Contributor
timestamp: December 19, 2018, 02:48:33 pm
content: 0.8 ms, that’s a considerable error. I need to find a low cost 120V to 12V transformer with a lot less phase error. VA is really low 1-2 tops

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 19, 2018, 02:58:16 pm
content: 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.

author: wraper
author_rank: Unknown
timestamp: December 19, 2018, 03:18:50 pm
content: Why do you need it in the first place? There might be solutions to your problem without getting such transformer.

author: Jester
author_rank: Frequent Contributor
timestamp: December 19, 2018, 03:21:08 pm
content: Quote from: wraper on December 19, 2018, 03:18:50 pmIsolation, transformation from 120V to 12V

author: wraper
author_rank: Unknown
timestamp: December 19, 2018, 03:25:41 pm
content: Quote from: Jester on December 19, 2018, 03:21:08 pmThat is not an answer and is irrelevant to why you need low phase shift.

author: Jester
author_rank: Frequent Contributor
timestamp: December 19, 2018, 04:20:20 pm
content: Quote from: wraper on December 19, 2018, 03:25:41 pmIf your asking what the application is, the answer is a metering instrument.

author: Benta
author_rank: Super Contributor
timestamp: December 19, 2018, 05:18:34 pm
content: Schmitt trigger's advice about using a 230 V transformer on 120 V is good, simply because you get much more linear operation from it. Modern transformers are close to the edge on core saturation and thus non-linear.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.Go for it, but use a 230/24 V transformer instead.

author: Jester
author_rank: Frequent Contributor
timestamp: December 19, 2018, 05:50:51 pm
content: I will order a 240:24 and test, thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 19, 2018, 05:57:45 pm
content: 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).

author: Benta
author_rank: Super Contributor
timestamp: December 19, 2018, 06:30:30 pm
content: 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.

author: Jester
author_rank: Frequent Contributor
timestamp: December 19, 2018, 06:36:09 pm
content: Quote from: Benta on December 19, 2018, 06:30:30 pmI appreciate the additional information.Fortunately load current will be very low, so this will help.

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 19, 2018, 06:52:22 pm
content: Quote from: Benta on December 19, 2018, 05:18:34 pmReally, 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 19, 2018, 07:17:43 pm
content: 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.

author: wraper
author_rank: Unknown
timestamp: December 19, 2018, 08:11:10 pm
content: Quote from: Jester on December 19, 2018, 04:20:20 pmThis is not a good answer as well. What do you want particularly? Zero cross detect, or what?

author: Jester
author_rank: Frequent Contributor
timestamp: December 19, 2018, 08:20:56 pm
content: Quote from: wraper on December 19, 2018, 08:11:10 pmWe're measuring energy, aiming for better than 1%

author: wraper
author_rank: Unknown
timestamp: December 19, 2018, 08:26:34 pm
content: Quote from: Jester on December 19, 2018, 08:20:56 pmThen you should not use transformer to begin with.

author: rstofer
author_rank: Super Contributor
timestamp: December 19, 2018, 08:35:20 pm
content: You should be using instrument grade transformers.https://www.gegridsolutions.com/products/manuals/ITITechInfo.pdfSee page 9 (top right) for a test card of a typical transformer. Phase shift is on the order of 5 minutes of angle - less than 0.1 degree.The problem will be to find an instrument transformer with the ratio you need. And they probably aren't cheap!If I could prove that the phase shift of a given transformer was consistent over a range, I would calibrate it out in code..

author: wraper
author_rank: Unknown
timestamp: December 19, 2018, 08:54:04 pm
content: Quote from: rstofer on December 19, 2018, 08:35:20 pmThere are more problems than phase shift. Output signal(voltage) just won't be accurate.

author: Benta
author_rank: Super Contributor
timestamp: December 19, 2018, 09:00:07 pm
content: Quote from: fourtytwo42 on December 19, 2018, 06:52:22 pmNo 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.

author: Jester
author_rank: Frequent Contributor
timestamp: December 19, 2018, 09:53:47 pm
content: Quote from: wraper on December 19, 2018, 08:54:04 pmSpecification 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.

author: wraper
author_rank: Unknown
timestamp: December 19, 2018, 10:01:54 pm
content: 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.

author: Jester
author_rank: Frequent Contributor
timestamp: December 19, 2018, 11:04:32 pm
content: Quote from: schmitt trigger on December 19, 2018, 02:58:16 pmThanks schmitt triggerWell 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 pmThanks KleinsteinGood point, I measured the magnetizing current at about 2.3ma

author: Jester
author_rank: Frequent Contributor
timestamp: December 19, 2018, 11:20:15 pm
content: Quote from: wraper on December 19, 2018, 10:01:54 pmWell we could but that would be like reinventing the wheel, when you can purchase a 0.1% accurate Energy Metering IC for $1https://www.digikey.com/products/en/integrated-circuits-ics/pmic-energy-metering/765?FV=ffe002fd%2Cbfc0002&quantity=0&ColumnSort=1000011&page=1&pageSize=25

author: wraper
author_rank: Unknown
timestamp: December 19, 2018, 11:37:02 pm
content: Quote from: Jester on December 19, 2018, 11:20:15 pmAnd what's the problem placing it on high voltage side?

author: wraper
author_rank: Unknown
timestamp: December 19, 2018, 11:45:18 pm
content: LOL, did you bother to read the datasheet? It's freakingintended to sit on mains voltage side. And you telling me you are not reinventing the wheel.

author: djacobow
author_rank: Super Contributor
timestamp: December 19, 2018, 11:58:28 pm
content: if it's power measurement, consider using a chip designed for the job. My adventures with the atm9e26 are well-documented on this forum. That particular chip lives on the mains side, uses a voltage divider for voltage and a shunt or a CT for current. I only used a shunt, though I suspect that the phase error with a CT is lower than for a power transformer, since they are intended specifically for this type of instrumentation.One problem I did have with my project (still do) is lack of proper equipment to calibrate it. If you want a 1% instrument and it needs to perform that accurately even at very low power factors, you're probably going to want a lab quality programmable AC load, and maybe AC source, too.

author: Jester
author_rank: Frequent Contributor
timestamp: December 20, 2018, 01:35:17 am
content: Wraper,I was not asking for advice on how to design a circuit, it was a simple question about phase delay across a transformer. We were designing optically isolated circuits running on the high side decades ago. I'm not at liberty to share the details of the specification that we're working to, I can tell you that it must have a transformer, even if there may be better approaches. BTW I did read the data sheet and have 10's of thousands of a design that is running resistor coupled inputs that have been in the field for years.Have a nice day, no need to respond as none of what you have posted as been the least bit useful.

author: wraper
author_rank: Unknown
timestamp: December 20, 2018, 01:48:43 am
content: Quote from: Jester on December 20, 2018, 01:35:17 amYour post about ICs and link suggested exactly opposite. Providing link to ICs which are supposed to sit on the mains side and use isolators, just as I suggested. From the beginning I felt you want to do something fishy. And none of what I asked was anything too specific. Yet you gave useless answers instead of just saying it's confidential. And the fact you are going to use usual small transformer and expecting achieving better than 1% accuracy shows you have no clue what you are doing. You might somewhat get it with particular transformer but when that gets into mass production, don't expect anything good. Also don't expect it working well over wide temperature range.

author: duak
author_rank: Super Contributor
timestamp: December 20, 2018, 05:34:40 am
content: I wonder if the steel used in audio transformers is significantly different as they are designed to minimize distortion. I'll bet power transformers try to minimize losses and cost but let the other chc's fall as they may.PA systems often distribute the speaker drive over 70 V rated signal pairs so transformers for that might be one source to try out.Cheers,

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 21, 2018, 03:16:54 am
content: Quote from: Jester on December 20, 2018, 01:35:17 amMy post did provide useful information.

author: djacobow
author_rank: Super Contributor
timestamp: December 21, 2018, 04:44:41 am
content: short:"Can someone here characterize parameters of this part that the manufacturers do not characterize or control, because those parameters are not remotely relevant to the part's intended usage, so that I can design the part into a high volume precision measurement device that utterly depends on that uncontrolled, characterized parameter?No? Geez, you must all be f*king idiots."

author: Zero999
author_rank: Super Contributor
timestamp: December 21, 2018, 11:17:58 am
content: Quote from: Jester on December 19, 2018, 11:04:32 pmThat's good, now what voltage do you get on the secondary? I bet it's not 12V, with 120V in.The turns ratio is not accurately controlled and is usually slightly lower, than the primary/secondary because the secondary voltage drops under load and the transformer will be specified to give the rated voltage, under full load.If you just want to do zero crossing then fine, but if you want to perform voltage measurement, then each transformer will need to be individually calibrated. Get a voltage instrument transformer. It will be more expensive but probably cheaper than having to calibrate an ordinary transformer in house.

author: soldar
author_rank: Super Contributor
timestamp: December 22, 2018, 12:52:21 am
content: Quote from: Jester on December 19, 2018, 01:00:36 pmAn ideal transformer has no losses and provides ideal V*I transformation between both windings. An ideal transformer does not exist in reality. That is why it is called "ideal".A real transformer has parasitic resistances and inductances. It is far from ideal. Have a look at the equivalent circuit of a real transformer.http://electricalacademia.com/transformer/equivalent-circuit-transformer-referred-primary-secondary-side/You can see in parallel with the input an inductance and resistance that represent the magnetizing current. No matter what you do the input and output current can never be totally in sync. The only thing you can do is design a transformer so as to minimize the unwanted magnetizing current (which basically means raising the inductance).Also, the magnetising current is a small part of the input current when the transformer is under full load but the percentage increases as the load decreases. This also means that the phase difference changes with load. It is not constant.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 23, 2018, 03:23:48 pm
content: There can be a valid point in using a transformer to couple the line signal. One is if one wants to keep it isolated from the grid and does current measurement with a current transformer or Rogowski coil anyway.The 0.4 degree measurend phase angle sounds perfectly plausible for a EI core transformer. Using the 230 V windings helps to keep the magnetizing current low. With a ring core type transformer one can expect even better values, though usually at a higher price. It might still be worth looking at, as the ring core type could be lower power rating (e.g. 3 VA instead of 5 VA).How stable the phase shift is, likely depends on the magnetizing current, as the resistance would not change much. The magnetizing current could change with mechanical stress on the transformer core and if the small residual air gap in an EI core changes. Another possible factor could be DC on the grid, that shifts the magnetization out of center.Even if the winding number it not accurate between transformers, it is stable over time.