Correcting AC line sag - Is this do-able? - Page 1
Forum section: electronics / beginners

author: Cliff Matthews
author_rank: Unknown
timestamp: March 31, 2019, 12:33:24 am
content: I have 2 beefy 500VA transformers I'd like to pair-up with 2 100VA units as shown (see pic*).Q: Can over-all current draw boost the smaller transformer and cancel-out good portion of output line sag?*I have a large variety of the toroid chokes (some up to 64oz.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 31, 2019, 01:06:04 am
content: Nope! Draw out the node equations if you can. It looks right if you consider just current going forward, but transformers are reciprocal devices, i.e., the same thing happens in both directions. So you need to consider the current going backwards, and the voltage as well.Tim

author: Cliff Matthews
author_rank: Unknown
timestamp: March 31, 2019, 01:16:49 am
content: Ha! I guess you knew I'd finally get around to either using or chuckingthat stash. I'll digest that more before they get trashed (god I hate throwing stuff out..)

author: IanB
author_rank: Super Contributor
timestamp: March 31, 2019, 01:30:36 am
content: If you have a transformer with a low voltage secondary you can always use it as an auto-transformer to boost the line voltage...

author: Cliff Matthews
author_rank: Unknown
timestamp: March 31, 2019, 02:01:43 am
content: Quote from: IanB on March 31, 2019, 01:30:36 amWhat I was looking for was some linear correction..As my semi retired brain understands it, transformer output voltages are rated at full load and typical output regulation from no load to full is -10%. In this configuration, I have little problem with raising the input voltage on the 100VA unit to reduce total output sag to -6% before the fuse blows. It's really just a cheap novice attempt to get a 600VA isolation rig to act a bit more like 1000VA rig with the mountain of toroids I have hanging around.In mains distribution, I've heard they regulate sag sometimes with commutator taps (now that's beyond me..)

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2019, 01:59:00 pm
content: One design for a line voltage stabilizer uses a linear amplifier to drive a step down transformer with the secondary in series with the line input to produce a corrected output. With a limited correction range, most of the output power is supplied directly from the line so the linear amplifier efficiency is less significant. Of course now the linear amplifier could be replaced with an inexpensive class-d audio amplifier.A simple design could compare the output to the line input and remove the sag due to the transformer without adjusting for input line variation.

author: innkeeper
author_rank: Unknown
timestamp: May 14, 2019, 06:01:39 am
content: I realize this is an old thread, but, you might want to look at something called Ferroresonant transformers or also called CVT Constant Voltage Transformer.What these are is a transformer with an additional winding and a capacitor that forms a resonant tank circuit at your line frequency.they have many advantages,good regulation handling +/- 40%+ input voltage change.the output is a clean sinewave due to the resonant circuit.good noise and spike isolation and electrical isolation.Simple designSelf-regulatinghandles non-linear and high crest factor loads well.disadvantages arethey are bulky/heavy compared to other line regulation or just standard 1:1 transformerthere noisy due to operating at resonance.they are inefficient but efficiency goes up with the load.very sensitive to line frequency changes (not good to put on a generator for example)

author: MrAl
author_rank: Super Contributor
timestamp: May 15, 2019, 01:33:28 pm
content: Hi,The simplest method if you already have a low voltage transformer is to wire the primary to the line as usual so you get the secondary voltage of say 6.3 volts AC, then wire the secondary in series with the line itself, then use that as the output. The output will be approximately the line voltage plus the secondary voltage.So if you have 120vac and secondary 6.3vac with no load the output will be 126.3vac. With load however, the output depends partly on the resistance of the primary and secondary winding of the transformer. The lower these resistances the less voltage sag you will see once a load is applied.Obviously if the line itself sags to say 114vac, then the no load output will be 120.3vac.The phasing of the secondary must be wired in configuration known as "series aiding" which simply means the output gets boosted. If you wire the secondary backwards you will actually subtract from the line not add to it, so measure the output and make sure it went up not down.Also be aware that the secondary current rating of the transformer must be greater than or equal to the total current draw of the load.If the primary and/or secondary resistance is too high, you could actually see a drop in voltage with load so check that too.You can end up with some nasty problems though if something shorts out, so you absolutely must use a fuse in series with the line coming in.In installations like this it is also customary to use a thermal fuse to detect a temperature rise in the transformer. If it gets too hot it must be disconnected and the thermal fuse placed in the windings will sense that and disconnect the power and thus prevent fire. I've actually seen buildings burnt up because of faulty doorbell transformers.

author: Cliff Matthews
author_rank: Unknown
timestamp: May 16, 2019, 01:36:05 pm
content: Quote from: MrAl on May 15, 2019, 01:33:28 pmThat's effectively what is driving for in my rough schematic, not a DIY auto transformer but 2 in full isolation. I had thought an overall current increase could be inductively used to boost the smaller 12v unit (canceling-out a portion of output sag). Tim's sage-like reply "Draw out the node equations if you can.." was all I needed.. he know's I'm an old dolt and can't, so that's all I needed to trash the idea.