Reducing mains voltage for inductive input application - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 14, 2020, 03:19:22 pm
content: I'm trying to look for a solution to drop my mains voltage input which is 240 Volts AC. What I am looking for is around 150 Volts to 180 Volts, but with out using a Variac. I can't say precisely at which voltage would be best for my application, until I test both input voltages to determine which gives the better efficiency. It's input to an inductive load, that of a transformer. Can anyone suggest a solution where by I can safely drop 240 Volts AC to the level of the voltages above. I don't really want to use another transformer to achieve this as it adds up to to much weight in the final project. And the input current will be around 1.5 Amp to 2.0 Amp AC, but could reach 3.0 Amp AC for very brief periods. So what I'm looking for is 150 Volts to 180 Volts AC, and up to 3 Amps of power from that lowered AC mains voltage. Any help would be greatly appreciated.Thanks for reading.

author: Gyro
author_rank: Super Contributor
timestamp: September 14, 2020, 04:57:56 pm
content: Yes, you can use a standard transformer wired as an autotransformer. Work out the ratio of input to desired voltage and choose the appropriate secondary voltage to drop the approximately desired volts. Then wire the primary and secondary in series (making sure the phasing is correct). Connect the 240V mains across the series connected windings and take the lower voltage output from the junction of the two windings (ie. across the Primary). The output current capability will the the same as the secondary current rating.Some people will suggest a 'bucking transformer' where the input is applied to the 240V Primary and the Secondary is used 'backwards' in series with the output to drop the voltage. The autotransformer configuration is more efficient though, and has lower transformer core losses, leading to cooler running.This method is normally used to drop (or boost) a few volts, but there's no reason that it can't be used for higher ratios - apart from bigger transformer (defined by the secondary current rating, eg. a transformer with a 100V 2A secondary is going to be a lot bigger than one with a 10V 2A secondary!).It's going to be difficult to do without a transformer - you don't really want something like a triac light dimmer driving your transformer load.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 14, 2020, 07:58:01 pm
content: Thanks for replying, I'm not to familiar with autotransformers. But I'm always up for trying new ideas. I will mention why I wanted 150 Volts to 180 Volts AC, I will keep it brief, most people run for the hills when you mention microwave oven transformer. I need that voltage for a pair of MOT transformers wired in series, that is both primary and secondary windings. So I'm dropping to much voltage on the secondary windings with 120 Volts per transformer, in series. If I can raise that 120 Volts per transformer to 150 Volts AC to 180 Volts AC, I think I will get the output secondary voltage under load conditions. And a good trade off between input voltage and heat generated in the primary windings. I didn't want to add additional weight, the pair of transformer weight is substantial already. So I'm trying to find a way to drop between 60 Volts AC and 90 Volts AC. I don't think there is a solution to do that. Anything I can think of is going to generate heat as a by product of dropping that much voltage. I'm still thinking of ideas, I know most think why bother with those lumps of old iron. But it's a challenge to try and achieve a power supply out of a pair of transformers that where repurposed from a pair of old ovens where the magnetrons have died.

author: Gyro
author_rank: Super Contributor
timestamp: September 14, 2020, 08:11:46 pm
content: Ah, I remember your thread now. Leave your primaries in series and adjust the secondary windings. A far cleaner solution.

author: Ian.M
author_rank: Super Contributor
timestamp: September 14, 2020, 08:24:42 pm
content: In theory it could be done electronically by full wave rectifying (but not smoothing) the incoming mains supply, and reducing the amplitude by fast PWMing a H-bridge, reconstituting the AC by switching H-bridge direction at every zero crossing and heavily low pass filtering the result, but if you've got the skills to make that work without first filling a quart canning jar with silicon shrapnel from all the semiconductors you've killed developing it, you've got the skills to design and build your own high power high efficiency SMPSU from scratch without any need for salvaged microwave oven transformers!The transformer used to 'buck' the incoming mains only needs a fraction of the VA rating of the MOTs. If you don't have a Variac to test for the optimum voltage to run your MOTs at, I'd target 192V, which is 20% less than the normal UK mains voltage, and can be done with a transformer with 2x 24V secondaries in series. It only needs 1/4 of the VA rating of the MOTs its feeding, as its secondary current is their primary current.As Gyro has pointed out, its somewhat less efficient to wire it as a buck transformer rather than an autotransformer, but it will buck by its nominal secondary voltage, whereas wired as an autotransformer., it will buck by less, in this case only 40V with the same 2x 24V secondaries in series wired as the upper limb of the autotransformer. However if you are using a MOT for the buck transformer, you must use the autotransformer configuration so it can buck its own primary supply, and will simply have to increase the number of turns you wind on to get the desired buck ratio.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 14, 2020, 10:29:27 pm
content: If the load is not bothered much by harmonics, you can use thyristors to phase angle modulate the voltage. Some Carver amplifiers do that to get more power out of a smaller transformer.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 15, 2020, 12:10:43 am
content: That's quite some theory to get equated with, its interesting though. Extra transformers will just add volume and area, in i case i'm unlikely to find. I had thought about some kind of SCR type dimmer or something like that, but my understanding of those is not a good idea on an inductive load. Only because it doesn't lower the voltage, but chops the AC wave form, tricking the usually restrictive load in to behaving as if the voltage was lower than it actually is. I'd try such a device if it didn't overheat the transformers primary series winding's. Its highly unlikely i could design an electronic switching circuit tailored to drive the pair of series primaries. Looking under the transformers fast on tags there is a third tap of some description, i haven't got round to buzzing it out yet, i'd hazard a guess that it may be redundancy for 110 Volts maybe. 192 Volts AC still seems a bit high, but i'm not an expert, would a 40 Volt drop temper the excess heat, and be stiff enough to mutually energize the secondary winding's. At the moment i'm stuck with two choices, run said transformers in series, avoid the heat, but have a dismal secondary voltage and current. Or run the transformers in parallel at full voltage, and suffer the heat and associated turbo fan cooling that would be needed, over volting 12 Volt axial fans. Its frustrating, because in series the primary's run very cool.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 16, 2020, 12:35:30 am
content: Today i rewired the transformers in series again, but this time i really went heavy on the wire i used. Trying to minimize any losses in line load drops. I was quite surprised, with a 50 Amp DC rectifier package and the same 90000uf capacitor bank (three rows of three 10000uf capacitors in parallel) I got a no load DC voltage of 19.10 Volts. I loaded this up with 4 x 50 watt halogen lamps, and got 13.87 Volts steady state for more than 2 hours continuous. Taking a few measurements, including temperature of the transformers, the hottest was the transformer core at 51 degrees C. I measured the primary current drain with this secondary 200 watt load still running, primary current was 1.25 Amps AC. So i don't know 100% but the LM2940CT 12 data sheet says minimum input of 13.60 Volts DC. D you think it possible that this regulator IC might just have enough headroom for up to 200 watts, with series pass transistors that is ? the testing above was rectified and filtered, but not regulated. thoughts would really be appreciated.