High accuracy voltage transformer with minimal phase angle error - Page 1
Forum section: electronics / projects

author: Karel
author_rank: Super Contributor
timestamp: June 22, 2015, 08:10:10 am
content: Hello all,For an energy measuring project, I'm looking for a high accuracy voltage transformer with minimal phase angle error.Frequency range should be from 50Hz to 400 Hz. Mainvoltage is 230V.The transformer is not used for a powersupply. It will be used as an isolation transformer and to measure the mainvoltage and phase.The only thing I found so far is this chinese website:http://yuanxing.net/1-1.aspAny advice from you about where to find these kind of transformers would be appreciated.

author: rs20
author_rank: Super Contributor
timestamp: June 22, 2015, 08:13:29 am
content: Not what you're asking, but:Another technique which doesn't require special transformers is to have your ADC on the hot side of the isolation, and transfer the data through the isolation digitally. That way, the galvanic isolation isn't able to detrimentally affect the accuracy (phase or otherwise). Same for current measurement; quite straightforward if you do it on the hot side.

author: coppice
author_rank: Super Contributor
timestamp: June 22, 2015, 08:31:01 am
content: Current transformers are still the most popular way to sense mains current in an isolated manner, but very few people use potential transformers for sensing mains voltage, unless the voltage is rather high. As rs20 said, for 110V and 220V/230V mains people sense on the hot side, generally with just a resistive divider, and then isolate in one of several ways.

author: Karel
author_rank: Super Contributor
timestamp: June 22, 2015, 09:09:00 am
content: Thank you for your input.The reason that I want to avoid to use a resistor divider and digital isolators,is because the current clamp we would like to use is an external part that comes with a cable and mini-jack plugwhich are not safe to connect to the hot side. It would make the design cheaper if there should be a small transformeravailable with an acceptable phase angle error.I did some measurements with a cheap, small, 0.35VA powertransformer (230V -> 12V) but the phase delay is 12 degr. at 50Hzand decreases to 3 degr. at 400Hz. The biggest problem is the non-linearity of the phase angle with regard to the frequency.Otherwise it could be compensated for in software.Any other ideas?

author: bktemp
author_rank: Super Contributor
timestamp: June 22, 2015, 09:24:10 am
content: Avoid any small transformers less than 5VA. Those are very inefficent and have significant losses even without any load and have therefore a high phase shift.You could use some toroidal transformers. Toroidal transformers have a very low idle current and therefore exhibit almost no phase shift.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 22, 2015, 01:29:26 pm
content: Keyword: "potential transformer"Usually pretty special-purpose, because you can measure the voltage with high-value resistors and an op-amp, most of the time.Agree with toroids, the leakage inductance is low (I would say this is the more important attribute). Definitely avoid bank-wound style transformers; they have so much leakage inductance, they barely work at line frequency already!Tim

author: The Electrician
author_rank: Frequent Contributor
timestamp: June 22, 2015, 10:18:17 pm
content: High accuracy transformers for measuring current and voltage are known collectively as "instrument transformers". They are not cheap.Here are some new ones:http://www.powermeterstore.com/category/potential-transformers-for-sub-metersand I see used ones on eBay:http://www.ebay.com/itm/Instrument-Voltage-Transformer-Type-DB-Ratio-5-1-600-120V-/121685485791?pt=LH_DefaultDomain_0&hash=item1c550528df

author: Circlotron
author_rank: Super Contributor
timestamp: June 23, 2015, 03:25:26 am
content: Quote from: T3sl4co1l on June 22, 2015, 01:29:26 pmYeah, those transformers with primary and secondary wound side by side at opposite ends of the bobbin would be hopeless for measurement purposes. Seeing you are looking at 230VAC mains, maybe if you could get atoroidaltransformer with a primary rated somewhat higher than this so that it will be well away from saturation and therefore more linear. Perhaps two 230VAC trannies in series with the secondaries paralleled?

author: johansen
author_rank: Super Contributor
timestamp: June 23, 2015, 06:28:01 pm
content: Quote from: Circlotron on June 23, 2015, 03:25:26 amthe real problem is that small transformers are intrinsically inefficient due to their small size.most sub 24volt amp transformers "regulate away" the primary copper losses by.. running the transformer saturated.if you want to use a pcb mount 10 watt or smaller transformer as an instrument transformer, configure it for 240vac and run it at 120, it should work fine.

author: Karel
author_rank: Super Contributor
timestamp: June 23, 2015, 07:10:03 pm
content: Quote from: johansen on June 23, 2015, 06:28:01 pmThanks everybody. I'm going to do a test with two 230V->12V (5VA each) transformers in series.I'll keep you updated.

author: woodchips
author_rank: Frequent Contributor
timestamp: June 23, 2015, 07:56:11 pm
content: A possible solution is to use a 400V primary transformer to reduce the non-linear magnetising current.If the secondary is resistive then there won't be any phase shift, other than losses.As has been said, use a good quality largish toroidal transformer.

author: Alphatronique
author_rank: Regular Contributor
timestamp: June 23, 2015, 08:36:53 pm
content: Hi try capacitive coupling whit a calibration routine we may reach very good result whit this