Automatic Tap Changer (or AC Voltage regulator) - Ideas please!! - Page 1
Forum section: electronics / projects

author: xibalban
author_rank: Regular Contributor
timestamp: December 24, 2014, 11:10:46 am
content: Hello goodfellas,So, being recently turned into a salvage guy, I've learnt that power transformer is the only "reliable" component that I could safely retrieve from the dumpster. I've recently come across a transformer from a manual voltage stablizer. You see, in our neighbourhood the voltage fluctuates quite considerably and could go below 90 Volts in the evening (the rated voltage being 220 V ac). Stepping up the mains voltage, therefore, is required for operation of most appliances. The typical voltage booster available is as follows:The manual selector switch is used to change the taps of the secondary windings of the power transformer to attain the rated voltage, and a simple relay acts as a protection device. If the output voltage exceeds the rated voltage (220V), the relay isolates the load and provides visible annunciation using an LED. The rotary switch then needs to be adjusted to resume load operation.As is evident, this is a very annoying design. I'd want to automate the rotary switch (using a micro-controller or what?) and replace the relay with a solid state device. Ideas please.

author: rob77
author_rank: Super Contributor
timestamp: December 24, 2014, 11:34:04 am
content: variac with a servo drive would be kind of ideal

author: DanielS
author_rank: Frequent Contributor
timestamp: December 24, 2014, 11:42:12 am
content: I cannot see your image but if I wanted to do something like that, I would use TRIACs to pick the tap and zero-crossing opto-isolators to synchronize tap changes. Use an RC filter with ~100µs time constant to peak-detect the input line voltage, sample that voltage at 10kHz, apply a rolling average or RMS calculation to further filter out noise, and pick the tap for the next half-wave based on the result.That would give you a solid-state tap-changer with ~6ms response time.

author: xibalban
author_rank: Regular Contributor
timestamp: December 24, 2014, 11:47:59 am
content: Quote from: rob77 on December 24, 2014, 11:34:04 amSince I'm a beginner, my little understanding tells me that you're suggesting a switch driven by a motor. Thanks, but I'll need to do some research to begin with.I'd prefer a solid state switch though, kind of constrains the circuit to a purely electronic type!Quote from: DanielS on December 24, 2014, 11:42:12 amHere'sthe link to the image.

author: rob77
author_rank: Super Contributor
timestamp: December 24, 2014, 11:57:13 am
content: i was suggesting a variac driven by a servo drive (a geared down motor is one component of a servo drive). variac is a transformer with continuous rate change (in fact smooth change between "taps" which are every single turn) you turn a knob and the output voltage changes.the advantage would be continuous operation - no switching delays and it would be very simple... a bridge rectifier with a small smoothing capacitor and voltage divider (divider with resistance low enough to act as a bleed resistor) , a voltage reference + a servo drive tracking the changes and bringing the voltage coming from the divider to the same level as the reference is.

author: DanielS
author_rank: Frequent Contributor
timestamp: December 24, 2014, 12:25:44 pm
content: Quote from: xibalban on December 24, 2014, 11:47:59 amYup, that was pretty much what I thought you meant.You could do the auto-switching using relays too. With relays, you would need to use SPDTs so each relay switches between tap and pass-through configuration so taps cannot short each other.If you use relays, you might want to put a 100nF X-cap across your transformer's secondary to catch the transformer's leakage inductance when relays switch and an RCD snubber on the output (500V fast diode bridge, 450V 100µF cap, 100k 1W bleeder resistor - though you can go fancy with zeners and transistors to avoid wasting 1W when capacitor voltage is below 320V) to catch inductive load currents during relay switching to minimize arcing.

author: DanielS
author_rank: Frequent Contributor
timestamp: December 24, 2014, 12:28:36 pm
content: Quote from: rob77 on December 24, 2014, 11:57:13 amWith a 90-220V input range, getting the variac's wiper from end to end during a large line input transient may take several seconds of response time. With the TRIAC or relay implementation, the output can be corrected within milliseconds regardless of swing.

author: rob77
author_rank: Super Contributor
timestamp: December 24, 2014, 12:55:00 pm
content: Quote from: DanielS on December 24, 2014, 12:28:36 pmfully agree, no question about it.but the servo driven triac is good enough because the voltage never swings from 220 to 90 and vice versa in a instant - the voltage drop is coming from a overload in the distribution network (the "branch" of it behind the local distribution transformer) - it goes slowly down and then up as the load is increasing and decreasing on the network.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 24, 2014, 01:17:47 pm
content: There are commercial rack-mount power strips with that kind of voltage-adjust circuit. IIRC they use a bunch of Triacs to select which tap to use depending on the mains voltage from moment to moment.

author: DanielS
author_rank: Frequent Contributor
timestamp: December 24, 2014, 01:33:22 pm
content: Quote from: rob77 on December 24, 2014, 12:55:00 pmMaybe not across the whole range but on such a grossly overloaded and presumably neglected power distribution network, I imagine some pretty nasty stuff must happen when distribution fuses blow - assuming there are any.

author: xibalban
author_rank: Regular Contributor
timestamp: December 24, 2014, 01:49:55 pm
content: Quote from: DanielS on December 24, 2014, 01:33:22 pmSpoken like my neighbour, sounds kinda funny but is a plight in reality.The frequency of fluctuation is indeed quite high. There's a rice husking mill loaded on the same distribution transformer and when the motor starts/stops, we get unpredictable transients in our mains supply. Anyway, so are we ruling out the servo driven triac idea?

author: compet17
author_rank: Regular Contributor
timestamp: December 24, 2014, 02:54:11 pm
content: Quick changes will be quite difficult to handle mechanically. In your situation, I would look for an user UPS - uninterruptable power supply. These are made to handle under and overvoltages and because of the batteries you will get power even if the mains fails completely.On the other hand... where do you live? I mean a voltage drop from nominal 220V down to 90V would be a case for your local power company. They are in charge to provide you with clean and stable electricity. Call them to fix that....

author: rob77
author_rank: Super Contributor
timestamp: December 24, 2014, 03:18:07 pm
content: i think we should distinguish between transients and the mains voltage. you can't handle the transients even with solid state switching of taps... i think the goal is to "regulate" the mains voltage, not get rid of transients.considering the fact that we're talking about Nepal - any fancy out of the box solution is simply out of the budget. i think Xibalban's idea is to build anaffordabledevice which will make the life a bit more comfortable.

author: xibalban
author_rank: Regular Contributor
timestamp: December 24, 2014, 03:22:59 pm
content: Quote from: compet17 on December 24, 2014, 02:54:11 pmI'm glad you asked. I live in Nepal and ours is a community managed 400 kW small hydro plant, donated by the Swiss in 1970. The load is ever increasing with not enough capital for plant upgradation.

author: calzap
author_rank: Frequent Contributor
timestamp: December 24, 2014, 03:26:51 pm
content: Xibalban,Sounds like a zero-sum game you and fellow utility customers are playing, at least those of you on the same substation. What happens when a lot of customers begin using voltage boosters? Voltage for those that don't have them drops even lower, utility company equipment may have problems, and customer complaints increase. Then the utility company starts rotating block outages because it's a lot quicker and cheaper to install some switches than to increase generating capacity. Then lots of small petro-powered generators start appearing. Now the zero-sum game has become a declining-sum game with regard to economics and energy efficiency.Mike in California

author: Zero999
author_rank: Super Contributor
timestamp: December 24, 2014, 03:50:46 pm
content: Quote from: calzap on December 24, 2014, 03:26:51 pmI agree. if the popularity of voltage boosters increases it will make things worse in the long term.When the voltage is boosted, the current consumption is more than doubled, to give the same power. Eventually a circuit breaker is going to trip or the power company starts shut parts of the grid down.

author: DanielS
author_rank: Frequent Contributor
timestamp: December 25, 2014, 02:24:29 am
content: Quote from: Hero999 on December 24, 2014, 03:50:46 pmI'm just surprised the generator is allowing its output to go this far out of spec without burning out the stator wiring - unless it uses rotor magnetizing current fold-back to limit stator output current. In that case, the gross overload would simply cause the output voltage to collapse until current drops below limits. This would cause the generator to operate well below its designed power during overloads and thoroughly screw everyone on that power plant.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 25, 2014, 03:01:17 am
content: These exist:http://www.ebay.com.au/itm/MATSUNAGA-STAVOL-AC-AUTOMATIC-VOLTAGE-REGULATOR-MODEL-SVC-350N-MADE-IN-JAPAN-/191426783293I posted some teardown pics of one somewhere in eevblog maybe a year ago. Didn't make a note of the thread, sorry.Anyway, yes it's a variac with a servo driven wiper, producing a regulated output AC voltage. Fairly slow response, of course.

author: SeanB
author_rank: Super Contributor
timestamp: December 25, 2014, 04:17:48 am
content: Generator probably is operating at design voltage, just the cabling is underrated for load and length, so the large voltage drops. It probably has a maximum distribution voltage of 11kV direct from the generator.

author: Circlotron
author_rank: Super Contributor
timestamp: December 26, 2014, 07:56:48 am
content: If you do use triacs to switch tappings, put a voltage sensing cct across each one. When the triac gate drive removed it will take somewhere between 0 and 90 degrees of the AC voltage waveform to switch off because the current is lagging the voltage. When it does turn off the voltage across the triac will jump from ~1V to a much higher figure. That is the time to turn on the next triac. This will allow you to "change gears" smoothly with no grinding.

author: compet17
author_rank: Regular Contributor
timestamp: January 02, 2015, 12:24:22 pm
content: Ok... Nepal... that's a different story. Donated by Swiss in the 1970s... Maybe I should write to some companies to "update" the old power plant in your area. Do you know the manufacturer of the plant?

author: xibalban
author_rank: Regular Contributor
timestamp: January 07, 2015, 09:54:09 am
content: Quote from: TerraHertz on December 25, 2014, 03:01:17 amThanks for the link. The idea of a mechanical switch (although an automatic one), still doesn't seem to be good enough (to me).The reason, partly, is due to my fresh exposure to microcontrollers and solid-state switches like MOSFETS.I was thinking of using a MCU to read the input voltage level, and let it decide precisely which MOSFET to activate; each being connected to each tap of the transformer. For safety's sake, I'd leave the original transistor driven relay to cut the output supply off if the MCU happens to fail someday.Besides, I could also add some seven segment LEDs to display the current (actual) input voltage level from the mains.Has anyone attempted this, ever?

author: oldway
author_rank: Super Contributor
timestamp: January 07, 2015, 10:40:58 am
content: That's not that easy: with inductive load, if you simply switch triacs at 0V, two triacs will conduct together and short circuit a transformer tap.Triacs will be destroyed by over-current.

author: KJDS
author_rank: Super Contributor
timestamp: January 07, 2015, 10:46:30 am
content: Quote from: calzap on December 24, 2014, 03:26:51 pmI didn't realize the situation had become that bad!

author: xibalban
author_rank: Regular Contributor
timestamp: January 07, 2015, 01:58:04 pm
content: Quote from: oldway on January 07, 2015, 10:40:58 amWhat if we introduced some delay, between successive switches? Or, perhaps design some sort of load snubber?

author: AF6LJ
author_rank: Unknown
timestamp: January 07, 2015, 02:00:27 pm
content: Line voltage regulators usually do not use relays or switches, not even thrysistors means to regulate the output voltage. Ferroresonant transformers are used.http://en.wikipedia.org/wiki/Voltage_regulatorSwitching taps under heavy load will result is voltage spikes that may be hard to manage.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 07, 2015, 02:28:36 pm
content: Instead of boosting the voltage, you should disconnect any device that isn't supposed to have 90Vac and use devices that will operate on 90-240 Vac 50/60Hz. Such as almost any modern switchmode supply.But that is what ideal consumers do. Normal consumers only do the selfish thing of course.Measure the frequency drops as well, since a drop from 220 to 90 is probable accompanied by a large drop in frequency. That is far more damaging than the voltage drop if you try to maintain the current.Generators typically reduce the voltage to <100 Vac (on nominal 400) if the frequency gets below 45Hz to prevent it from burning out.

author: bktemp
author_rank: Super Contributor
timestamp: January 07, 2015, 02:31:18 pm
content: It is possible to smoothly switch taps with relais:Before switching the main relais, activate an auxiliary relais that connects 2 small resistors between the current tap and the output and the next desired tap and the output. Then disconnect the tap with the main relais. There will be a voltage drop due to the resistors, but since these are small it will be tolerable for a short time. After another small time, activate the main relais for next tap and disable the auxiliary relais.This is exactly the same way a variac works:Because the wiper can touch 2 wires at the same time, it uses a carbon brush with some resistance to limit the current. Since the voltage between the wires is small compared to the output voltage, it works well.The triac solution is much better because they can switch in microseconds instead of milliseconds, but it is much harder to make them work reliably.

author: KJDS
author_rank: Super Contributor
timestamp: January 07, 2015, 02:46:30 pm
content: Many years ago I worked on some equipment that had been designed before switch mode PSUs were common and the designer had implemented an autoswitching PSU.Unfortunately, anyone farting near the mains lead when the wind was blowing in the wrong direction would cause half the devices in the supply to kill themselves. Whilst it was eventually made stable, it took a lot of skill to do so. I'd be very wary of plugging anything into such a device if I wanted it to continue working reliably.

author: Kalvin
author_rank: Super Contributor
timestamp: January 07, 2015, 02:54:00 pm
content: Use the transformer as a car battery charger and hook a  inverter to the car battery and you will get UPS at the same time.

author: oldway
author_rank: Super Contributor
timestamp: January 07, 2015, 05:02:51 pm
content: Will not work because transformer ouput voltage will be too low to charge the battery.QuoteThere are differents possible solutions:- use SCR in anti-parallel instead of triacs. Each SCR must have his own trigger pulses only during half of the cycle.- use a low value power resistor in serie with triacs to limit the short circuit current when switching taps with inductive load. Tap switching must be sequential (1 tap per cycle) so voltage difference stay low.- delay could also work but it has to be of 1 cycle, not of 1/2 cycle for not applying 2 times the same polarity cycle to the load. This could saturate transformers, if any.Load snubber has nothing to do with this problem, but with too high dV/dt. You loose control of the triac who stay conducting and shorten the taps.Use snubberless 3 quadrants triacs, they are far better.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 07, 2015, 06:46:17 pm
content: Quote from: Kalvin on January 07, 2015, 02:54:00 pmYou can also find a used UPS, a line-interactive (or double conversion) type. These should be able to handle dip and swell conditions.You don't even need much battery capacity, since that is not its main purpose anymore.That is compared to the suggested transformer and diy control circuitry a possibly affordable solution.

author: oldway
author_rank: Super Contributor
timestamp: January 07, 2015, 08:14:17 pm
content: A 220V UPS can't work on main supply with only 90V ac input voltage.Load is then feeded with energy coming from the battery.Your solution will not work.What could be possible:- one SMPS with PFC for telecomunication: input: 90V to 240Vac, output 48V- one inverter 48Vcc/220VacNo battery needed.

author: xibalban
author_rank: Regular Contributor
timestamp: January 08, 2015, 05:51:13 am
content: Quote from: AF6LJ on January 07, 2015, 02:00:27 pmTrue! But hey, wait a minute. Is there any ideal switch which does not cause this effect? Weren't solid state devices devised to mitigate the effect?

author: oldway
author_rank: Super Contributor
timestamp: January 08, 2015, 06:46:27 am
content: Ac solid state relays are only triacs with their trigger circuits.With inductive load, they have the same problem as triacs: risk to shorten taps of the transformer when switching.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 08, 2015, 07:39:20 am
content: Quote from: AF6LJ on January 07, 2015, 02:00:27 pmOTOH, you could argue that, since it is just another (nearby) tap on the SAME inductor, there is no significant difference in phase, and only a small difference in voltage. So the effect isn't that different than (or even perhaps not as demanding as) phase-angle dimming, etc.

author: Kalvin
author_rank: Super Contributor
timestamp: January 08, 2015, 09:13:18 am
content: Quote from: oldway on January 07, 2015, 05:02:51 pmYes, you are correct. In the spirit of making use of the salvaged transformer and using the tap switching concept, the tap switching may be used to keep the car battery charger running. And then using an (sinewave) inverter to convert the 12VDC car battery to 240VAC. In this concept the tap switching doesn't act in realtime which might make the design more relaxed and easy to implement, and the use of the car battery eliminates possible brown-outs or short blackouts.

author: xibalban
author_rank: Regular Contributor
timestamp: January 08, 2015, 09:28:11 am
content: Let us forget about multi-taps for a brief moment, but only for a moment. Let's try to use just one tap of the secondary winding - the one with the highest voltage output. For argument's sake, say this tap outputs 400 Volts at 220 Volts primary (i.e. under normal supply conditions).Now, is it possible to electronically chop this voltage down to 220 Volts and feed it to the load terminals? In other words, does any high voltage AC equivalent of a low voltage DC regulator (like LM317) exist?

author: Kalvin
author_rank: Super Contributor
timestamp: January 08, 2015, 09:34:52 am
content: xibalban, what is you budget ie. how much money you have for this project and what kind of a load you have (a laptop? a lamp? a TV? a washing machine? an electrical oven?)

author: Jeroen3
author_rank: Super Contributor
timestamp: January 08, 2015, 09:48:04 am
content: Quote from: xibalban on January 08, 2015, 09:28:11 amYes, AC/AC converters such as motor drivers and frequency converters. These are pricey when new. They use high voltage DC and some fancy high voltage transistors mounted on a few kilos of aluminium.You can probably get a used one on ebay for less than 1000$. Few kW.Not sure if your devices will like the output.... And most of them generate an awful lot of noise and heat.

author: xibalban
author_rank: Regular Contributor
timestamp: January 08, 2015, 09:59:33 am
content: Quote from: Kalvin on January 08, 2015, 09:34:52 amGood question!!The unit is proposed to be used as a mains outlet for my electronics workbench, the loads being:A Hakko 888 Analog Soldering StationAn ATTEN (or similar brand) hot air station - plannedA bench power supply with two pairs of variable outputs - constant voltage & constant currentA desktop computer/laptopLighting systems and other peripheralsThe budget?I wish I could get away with salvaged components!! I already have the transformer in question, some power transistors, MOSFETs, and the like.

author: DanielS
author_rank: Frequent Contributor
timestamp: January 08, 2015, 03:32:57 pm
content: Quote from: AF6LJ on January 07, 2015, 02:00:27 pmYou will not have large voltage spikes if you use zero-crossing triac drivers: the currently conducting triac only turns off when current reaches 0A while the next triac only turns on when line voltage crosses 0V. If the load is heavily inductive, there may be some overlap between the two taps but it will be fairly short-lived. Worst case, you can add a small inductor in series with the multi-tap transformer on the primary side to limit the momentary cross-conduction current between output taps during switching.

author: AF6LJ
author_rank: Unknown
timestamp: January 08, 2015, 05:53:45 pm
content: Quote from: Richard Crowley on January 08, 2015, 07:39:20 amI am not so sure the voltage spike would be that small, there is a useful experiment in there to settle the issue.

author: AF6LJ
author_rank: Unknown
timestamp: January 08, 2015, 05:59:17 pm
content: Quote from: DanielS on January 08, 2015, 03:32:57 pmThis is true and a reasonable implementation albeit a bit complex.Thinking about this further this means could be less complicated than some of the other schemes other posters have proposed.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 08, 2015, 07:05:42 pm
content: Quote from: AF6LJ on January 08, 2015, 05:59:17 pmActually it is very difficult to even find an SSR that does NOT have zero-cross switching built-in.I discovered that when trying to design some phase-angle lamp dimmer modules.99.98% of all the SSRs on Ebay have built-in zero-cross switching.And the ones that don't claim they do just because the sellers don't know any better

author: oldway
author_rank: Super Contributor
timestamp: January 08, 2015, 08:17:26 pm
content: To switch taps with triacs without any precaution will not work correctly with inductive loads.You will have to replace the triacs every day !I have already projected an AC voltage regulator working on this principle and I know very well the problem.Zero crossing switching don't solve this.

author: Circlotron
author_rank: Super Contributor
timestamp: January 09, 2015, 04:03:36 am
content: Back in 1988-89 I used to work at Sola Basic Australia. We used to make UPSs, ferro resonant transformers and tap switchers. I used to work on them all day every day.Did anyone actually read my post #19?

author: DanielS
author_rank: Frequent Contributor
timestamp: January 09, 2015, 04:38:03 am
content: Quote from: Circlotron on January 09, 2015, 04:03:36 amLooks like a neat little box. Only problem for OP is his line input voltage varies from 90V to 250V due to massively overloaded power distribution and generator, so two separate ranges for 50-130V and 160-240V might be no good to him: if he is not there to flip the switch when line voltage changes, something might blow up.

author: edavid
author_rank: Super Contributor
timestamp: January 09, 2015, 04:45:45 am
content: The cheap commercial units on eBay use simple relay tap switching, so that's what I would try. Here are some photos of the guts of one.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 09, 2015, 08:24:17 am
content: Quote from: Richard Crowley on January 08, 2015, 07:05:42 pmThat is because thyristors can only turn off when there is no current. And cannot stay on without current.They have a special control circuit generating gate current pulse trains.They ebay thing doesn't look like something I would run 1 kW through....

author: oldway
author_rank: Super Contributor
timestamp: January 09, 2015, 08:29:17 am
content: Quote from: Circlotron on January 09, 2015, 04:03:36 amYes, your post #19 is all right.With inductive load, we must ensure 2 triacs will never conduct together or use a low value power resistor to limit short circuit current between taps.On low end triac voltage regulators used for computers, I often had to repair them (all triacs faulty) only because user had connected his fan at the output (inductive load).Slow acting voltage regulator (tap's switching by relays) is dangerous for the load: let's think mains voltage rapidly vary from 90 to 220V, the load voltage will then go to 537V for a short time before switching from 90V tap to 220V tap.

author: xibalban
author_rank: Regular Contributor
timestamp: January 09, 2015, 08:31:15 am
content: Quote from: edavid on January 09, 2015, 04:45:45 amThanks for the pictures! I see that the unit uses relays, but apart from a bunch of transistors where is the intelligence of the electronics to change between taps?Coming back to the pertinent question of inductive loads and the challenges associated with switching those, if I could live with the manual knob system - I ought to be fairly content with the use of triacs. The caveat would be to feed the MCU with information about "completely off" state of the preceding triac, thus compromising on speed of operation.Quote from: oldway on January 09, 2015, 08:29:17 amDo you remember my stating that I would retain the analog over-voltage relay (that senses the output voltage), which originally came with the manual regulator which simply disconnected the load if the output voltage exceed some pre-set value?

author: oldway
author_rank: Super Contributor
timestamp: January 09, 2015, 08:49:11 am
content: QuoteAbsolutely not. It does not compromise speed of operation. If you are sure previous triac does not conduct any more, you can trigger any other triac without risk, it has not to be sequential.In my project, I was integrating every positive half cycle of input voltage and choosing by comparators what was the right tap. This is possible because such a voltage regulator has a low internal impedance and voltage drop is also low.

author: xibalban
author_rank: Regular Contributor
timestamp: January 09, 2015, 08:55:10 am
content: Quote from: oldway on January 09, 2015, 08:49:11 amDo you have a sample schematic to share, if that's not much of a problem?

author: oldway
author_rank: Super Contributor
timestamp: January 09, 2015, 09:59:40 am
content: QuoteI make this project in 1983, I don't have this schematic any more, and I can't have it because I am now living 10.000 Km's far from where I lived in this time and the company where I worked does not exist any more.I can only give you informations that I can remenber.That was not a simple tape changer because in such case, triacs had to be high current triacs and this was too expensive.In my project, it added or substracted voltages with 9 levels, using 6 triacs TIC226.Nominal input voltage was 80 to 140V, output voltage was 110V or 127Vac +/-5% (two models) 1KwTwo auxiliary transformers where used, adding or substrating voltages in proportions of 1 and 3.The steps where:-4 : triacs of -3 and -1 on-3: triacs of -3 and 0 on-2: triacs of -3 and +1 on-1 ; triacs of 0 and -1 on0 : both triacs 0 on+1 : triacs of 0 and +1 on+2 : triacs of +3 and -1 on+3: triacs of +3 and 0 on+4: triacs of +3 and +1 onTo avoid changing of taps continually, comparators had an hysterese.There also was a delay avoiding changing taps before to stay a complete cycle on the same tap so DC voltage can't be applied to the load.As the auxiliary transformers had their secundaries in serie with the load, they could not stay with open primary, as they would act as current transformers and develop high primary voltages, I had to accept the occurrence of short circuits during steps changes with inductive loads and protect the triacs with low value power resistors.Here is an example of a voltage regulator with 115Vac nominal output and 8 taps.

author: oldway
author_rank: Super Contributor
timestamp: January 09, 2015, 12:53:50 pm
content: QuoteYou need 30 or 40ms for actuating the relay, more some ms or tenth's of ms for the over-voltage sensor. This could be too slow to protect electronic instruments.What seems to be the best and more secure solution would be:- a SMPS with PFC (input 90 to 230Vac), output 48V 20A (for telecomunications)- an inverter 48V / 220V 1KVA (seems to be enough if you don't use all your equipment at the same time)

author: xibalban
author_rank: Regular Contributor
timestamp: January 10, 2015, 05:12:19 am
content: Quote from: oldway on January 09, 2015, 12:53:50 pmI get what you imply, and I am seriously going to consider it. Here, my multitap-transformer wouldn't be much useful though.Your advice and feedback are highly appreciated. Thank you!

author: edavid
author_rank: Super Contributor
timestamp: January 10, 2015, 05:18:30 pm
content: Quote from: oldway on January 09, 2015, 12:53:50 pmA small relay will actuate in under 10ms. How long does the generator take to ramp up? Isn't it at least a few cycles?