Reducing Mains AC Voltage (220V50HZ) to 80~VAC at 1A without transformer - Page 1
Forum section: electronics / projects

author: ZeTeX
author_rank: Frequent Contributor
timestamp: February 25, 2016, 03:17:20 pm
content: Hi,I have a 90VDC 1A standard brushed dc motor, to run the motor ofcourse I need 90V DC, problem is I cant find 90V1A dc supply for reasonable price in my city, so is transformer.My question: Is there any topology that I can use to reduce the AC voltage to lower voltage while supplying about 1A max without alot of heat \ expensive, uncommon compoents?Ordering from ebay or something is not a option as I need it in a week or so.thanks.

author: Rickpercy87
author_rank: Contributor
timestamp: February 25, 2016, 03:26:50 pm
content: You will really need a transformer to output those kind of currents from such a large voltage supply.A power supply without a transformer could supply about 150mA at best I believe.They use an X rated capacitor to step the voltage down using the internal resistance of the capacitor I believe.Sent from my SM-N9005 using Tapatalk

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 25, 2016, 03:33:29 pm
content: You can connect a few 15V transformers in parallel.You can also get a 90V buck converter if you find one.

author: Rickpercy87
author_rank: Contributor
timestamp: February 25, 2016, 03:35:02 pm
content: Quote from: HAL-42b on February 25, 2016, 03:33:29 pmHe doesn't want to use a transformerSent from my SM-N9005 using Tapatalk

author: Cloud
author_rank: Regular Contributor
timestamp: February 25, 2016, 03:37:15 pm
content: Quote from: Rickpercy87 on February 25, 2016, 03:26:50 pmYou use reactance (I hope this is what it is called in english) Xc=1/(2pi*f*C)

author: ZeTeX
author_rank: Frequent Contributor
timestamp: February 25, 2016, 03:59:13 pm
content: Quote from: Rickpercy87 on February 25, 2016, 03:35:02 pmI would really like to avoid expensive transformer, cost is an issue.As for capacitior dropping supply, they work great for low current load, not 90V at 1A, the capacitors will be too expensive and large.I know a closed loop dc motor controller that uses SCR or something like that to control the voltage, is it possible and pracitial?

author: Siwastaja
author_rank: Super Contributor
timestamp: February 25, 2016, 04:03:13 pm
content: Getting two cheap off-the-shelf 48V 1A switchmode supplies and putting them in series - make sure the secondary GND is not connected to PE before you do that - might be easiest.If you want to go without any kind of transformer at all, then all I can see is that you rectify the mains (240V I suppose) and PWM the motor with 600V rated MOSFETs or IGBTs from your 330V DC link. Hopefully the motor insulation is ok for such "inverter duty" (so to speak) operation. Current feedback is a must, so this is not an easy solution after all.

author: Ian.M
author_rank: Super Contributor
timestamp: February 25, 2016, 04:10:39 pm
content: Not without a big hefty choke in a buck converter. If you simply rectified the mains, and chopped that without filtering, the motor would be seeing pulses of nearly 3.5 times its rated voltage. Inverter duty motor insulation is typically only rated up to double the nominal voltage. Also there are safety issues with a non-isolated supply as the insulation to the motor body certainly wont be mains rated.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 25, 2016, 04:45:46 pm
content: Quote from: Ian.M on February 25, 2016, 04:10:39 pmThis will be the problem with any solution that doesn't use some kind of transformer. Motor body must be properly tied to PE, which reduces the problem from the risk of shock to merely to a risk of a burning motor. But as you say, the motor insulation probably isn't up to the task.Transformer, either a mains frequency or inside a switch mode power supply is the only really acceptable way as far as I can see.Depends on the motor, though. Any specs?

author: ZeTeX
author_rank: Frequent Contributor
timestamp: February 25, 2016, 04:51:49 pm
content: Quote from: Siwastaja on February 25, 2016, 04:45:46 pmthanks for your replay and everybody elses replay.I'm not sure about the specs of the motor, its for my dad I will have to ask him later, but its a motor inside a pizza oven or something like that.hopefully I will get the datasheet for the motor.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: February 26, 2016, 01:27:01 am
content: For real quick and dirty, you could half-wave rectify the mains, and then feed the motor directly. Then you would be at 110 VDC RMS, with half-sine pulses at 50 Hz.To drop down to 90 VDC RMS, you could add one or two dozen series 1N540x diodes.The DC current draw on the mains won't be appreciated, so get something better when time permits. But your timing and cost restrictions are restrictive...What' the whole worry about isolation here? 90 VDC motors are often meant to be used with SCR motor drives, and these standard drives are not isolated. It's no more dangerous than standard mains-powered AC motors, which are not isolated from the mains.Edit: 90 VDC motors are meant for 120 VAC mains, and 180 VDC motors are meant for 240 VAC mains. So I suppose it's not "properly rated". I guess it depends on how badly you need this motor to run.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 26, 2016, 03:50:39 am
content: Quote from: ZeTeX on February 25, 2016, 04:51:49 pmYou really haven't given us enough information.If it is part of a pizza oven,the 90v DC will already be provided by the oven,so why do you need to make such a supply?Also,brushed motors are not always DC only,many are so-called "universal motors" which will run on both AC & DC.

author: Emi
author_rank: Contributor
timestamp: February 26, 2016, 09:15:58 am
content: Hi..I own some "proxxon" mini tools, they use a Very simple method good for dc 220v motors ..They drive a 4 diode rectifier with a triac dimmer. .this schemary allows also a lot of torque even at low rpm!

author: han
author_rank: Frequent Contributor
timestamp: February 26, 2016, 09:43:18 am
content: Use something like this (automatic voltage regulator) having 110V output,cheap and hackable

author: ZeTeX
author_rank: Frequent Contributor
timestamp: February 26, 2016, 04:04:17 pm
content: Quote from: vk6zgo on February 26, 2016, 03:50:39 amthe 90V DC for the motor was provided by some old closed loop dc motor controller, it failed after so many years , a new quality one cost over 300$ here brand new.I'm pretty sure those are only dc motor, and even then, why would it matter if its a universal motor?48V 1A Supply cost about 10$. might just buy 2 of them and be done with it.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 26, 2016, 04:16:19 pm
content: Couple of surplus 48V SMPSUs in series (trim down as necessary)

author: vk6zgo
author_rank: Super Contributor
timestamp: February 26, 2016, 05:09:03 pm
content: Quote from: ZeTeX on February 26, 2016, 04:04:17 pmIf it is a universal motor,& you only wanted to run it at a single speed,you would only need an AC supply--no rectifier.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: February 26, 2016, 10:09:01 pm
content: Quote from: vk6zgo on February 26, 2016, 05:09:03 pmits a DC motor.I just need 90V DC, I'm building a speed controller using 555 timer (PWM) for this motor.how about rewinding microwave oven transformer? would it work?

author: Ian.M
author_rank: Super Contributor
timestamp: February 26, 2016, 10:30:09 pm
content: It could be done that way but microwave oven transformers are Muntzed nearly to death and tend to only have just enough copper and iron to cope with intermittent operation. They tend to be designed to run right on the edge of core saturation and unless you can derate them by reducing the input voltage can run rather hot even on no-load. If you've got a good quality one that will run for an hour unloaded at your nominal mains +10% without an excessive temperature rise, it would be a possible candidate. However rewinding one for 100V RMS output will be a real P.I.T.A due to the number of turns required and the extreme difficulty of winding it without splitting the core.

author: nctnico
author_rank: Super Contributor
timestamp: February 27, 2016, 12:02:15 am
content: Quote from: mikeselectricstuff on February 26, 2016, 04:16:19 pmI second this suggestion if you don't want to use a transformer. The only thing to watch out for is that the PSUs must have a floating output (- not connected to AC ground) so you can put them in series.

author: Fraser
author_rank: Super Contributor
timestamp: February 27, 2016, 12:14:07 am
content: For information, hobby metal working lathes use a DC motor that is driven by a PWM control module. These modules are relatively cheap but well made. You set the voltage using an external potentiometer. They have soft start and everything needed for DC motor control. 90v is well within their capability. I will see of I can find some detail on the make.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: February 27, 2016, 12:29:46 am
content: Ok, here you go.......http://www.micro-machine-shop.com/Variable_Speed_DC_Spindle_Motor.htmThe KBIC-240 is the PWM controller used in UK hobby lathes. It includes soft start and current monitoring. Important when starting a powerful motor as the initial start current can be high.The controllers are an industry standard and are to be found in such items as conveyor belts and tread mills. anywhere that needs to control a high voltage DC motor within its power capabilities really.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: February 27, 2016, 12:33:37 am
content: Before going the 'bush engineering' route to drop mains voltage to 90V, do consider what will happen in the case of a failure. the failure could be the motor seizing, the 'dropper' going short or an over current situation. The controller copes with these situations without risk of fire or destruction of the motor. Worth considering before connecting a motor to the mains via a simple triac circuit. In all cases, make sure you fit a suitable fuse in the circuit.Fraser

author: Macbeth
author_rank: Super Contributor
timestamp: February 27, 2016, 12:46:41 am
content: Good thinking on the lathe idea. My own mini lathe uses a controller byhttp://www.kbelectronics.com/I was going to suggest an SCR/TRIAC circuit as I remember servicing some ancient Philips G8 TV's that had such PSUs, (big stud BT106 SCR IIRC) live mains chassis and all! Ah, the good old days...

author: Fraser
author_rank: Super Contributor
timestamp: February 27, 2016, 12:20:30 pm
content: @Macbeth,I have a couple of WARCO 7x12 mini lathes and rebuilt one of them. Mine also have the KB Elelectronics controllers. Neat little modules.Fraser

author: Macbeth
author_rank: Super Contributor
timestamp: February 27, 2016, 08:27:55 pm
content: @Fraser - ah yes, I have the similar 7x14 lathe from amadeal

author: Eutectic
author_rank: Newbie
timestamp: February 28, 2016, 11:43:01 pm
content: Quote from: Siwastaja on February 25, 2016, 04:03:13 pmCould you please clarify the term PE. I've never heard that term.

author: Macbeth
author_rank: Super Contributor
timestamp: February 28, 2016, 11:51:45 pm
content: Seems to me he means a (Physical Earth/Ground). Basically isolated secondaries.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: February 29, 2016, 12:18:55 am
content: Quote from: mikeselectricstuff on February 26, 2016, 04:16:19 pm+1 while it's the easiest way to do it. you may have to consider the motor start inrush current, it may trip the over current protection of the smps. an easy way to overcome this by adding a current limiting ntc with the value calculated according to the current limit of the power supply.

author: denelec
author_rank: Regular Contributor
timestamp: February 29, 2016, 12:22:36 am
content: Quote from: pmbrunelle on February 26, 2016, 01:27:01 amNo. If you half rectify the mains with a diode, the RMS voltage will be 0.707 x 220V = 156V, not 110V.The diode will half the power, not the voltage.