# how do simple mains inverters work? modifiable?

#### daveyjones97

Joined Oct 21, 2011
46
i have a 12/24vdc to 240vac 300w (im in the uk) mains adapter i want to drive a universal motor. the motor is rated 240v 50hz 2000w. i do have basic 12v auto experience and am reasonably competent knocking stuff up. so the plan as it is this minute is to adapt the mains inverter for variable gain on the low voltage side and adapt/add/replace parts as required to get the current needed for 1000+ watts at the motor.

the problem i have is negligible circuitboard experience and have better than average, but patchy understanding of electrics. not being proud i even played with a kids spring terminal electronic kit, 100 fun projects type and got on fine with that. i find i can follow and understand most of the tech here and like to think i have enough sense to turn on power last and whilst keeping a safe distance on anything i do mess with, i understand capacitance and the need to check for residual power and heat before diving in. its just lack of experience with consumer circuits where there are no schematics and little information available i struggle.

anyways what im after is advice on what parts of the inverter to change and for what components to allow 0/10w 1000w + output. im not too worried about finnesse, i just want to scratch the itch to make something that will work and the universal motor would probably run on gas if i asked it nicely

ive got the mains adapter apart tonight to answer the detail questions you guys will probably ask.
ALL HELP GRATEFULLY APPRECIATED, THANK YOU

http://i1096.photobucket.com/albums/g325/daveyjones97/DSCN5944.jpg
http://i1096.photobucket.com/albums/g325/daveyjones97/DSCN5943.jpg
http://i1096.photobucket.com/albums/g325/daveyjones97/DSCN5946.jpg
http://i1096.photobucket.com/albums/g325/daveyjones97/DSCN5949.jpg

just realised i missed the 2 components fets/diodes on the opposite side of the board with the camera, they look the same as the pair on the other side. im assuming theyre part of a H bridge? its 23:25 here now so googling for data sheets will have to wait for tommorrow.

#### daveyjones97

Joined Oct 21, 2011
46
oh, i know that a no loaded universal motor is a pinless grenade. i could go upto 72v/80v max dc. as that would mean a different transformer coil would i be better adapting this or salvaging parts from it?

Last edited:

Joined Dec 26, 2010
2,148
You can't really expect to "adapt" an inverter designed for 300W fixed voltage output to operate at over three times the power level with variable output.

It's a bit like starting with a basic lawn mower engine and expecting to make a high power motorcycle engine and transmission out of it.

Last edited:

#### daveyjones97

Joined Oct 21, 2011
46
i see that, but what about copying the layout and reusing the ic? Are there any other components likely to be reusable for a more high power ap? I didnt think it likely the high power rated inverters are totally different inside, but are they? I planned on using this as a basis as its been cluttering up my dry shed for years

#### kubeek

Joined Sep 20, 2005
5,753
If you just want a variable speed, then since it is an universal motor rated for 240V and UK mains has 240V AFAIK, then just go to some hobby shop and buy a dimmer rated for that power.
It will get you there much faster, cheaper and safer than modifying and inverter.
Or do you want to run it off 12/24V? Then buying the right inverter will most likely still be cheaper than making your own. And definitely less dangerous, 2000W at 12V equals to 160A of current which definitely is not a safe nor managable amount for a beginner. Also you still would have to find a 24/240 transformer rated for 2kW, which will cost about the same as a new inverter.

#### daveyjones97

Joined Oct 21, 2011
46
i need it for a remote application, i forgot to state in the original post i planned on using 72vdc on the low side to try and keep current and heat down. Would adapting an inverter for 72v bring much advantage over a 24vdc in off the shelf inverter?
If i do buy one with the right spec would a simple pwm circuit between the battery and the inverter be the best speed control method?

#### praondevou

Joined Jul 9, 2011
2,942
i need it for a remote application, i forgot to state in the original post i planned on using 72vdc on the low side to try and keep current and heat down. Would adapting an inverter for 72v bring much advantage over a 24vdc in off the shelf inverter?
If i do buy one with the right spec would a simple pwm circuit between the battery and the inverter be the best speed control method?
In this context the term "inverter" is commonly used for circuits that transform DC to AC, like in a UPS.

I guess one of the best methods would be the dimmer method Kubeek mentioned.

In case you want to modify that mains adaptor you will first need to draw it's schematic. This will be quite easy (apart from the transformer maybe) since it's a single layer board . If you want to increase it's output voltage a great part of the secondary sides components will need to be changed (including the turns of the transformer. As Adjuster said, the power the adapter will need to provide doesn't match the motor power rating at all.

Since this is an universal motor you can drive it with AC or DC, right?
If you wanted AC you would need to modulate the mains adapter PWM with 50Hz which is not possible because of the transformer.
It could work with DC (up to a certain voltage/power level, not the 2000W)

Last edited:

#### thatoneguy

Joined Feb 19, 2009
6,359
1500W Inverters I have are about 18" long, 12" wide, and 3" tall, nearly all of which is heatsink fins.

One pulls 300A@13.8V when running an air compressor, so efficiency isn't perfect, and goes down a bit as the load goes higher.

There's literally nothing inside of it that is also inside my 100W "lighter jack" inverter. Transformer is tiny in small inverter, while large inverter has over a dozen high current transformers to drive the load.

The higher power inverters also have a great deal more protection circuitry due to the lethal amount of power moving around.

A universal motor will run on AC or DC just fine, for more RPM under load, increase voltage, for lower RPM, use a TRIAC chopper circuit such as a light dimmer.

#### shortbus

Joined Sep 30, 2009
8,593
Not what you asked but, you would be farther ahead to find a different motor that operates at the battery voltage you have. Every time you convert a voltage from one form to another you will have a certain amount of loss in the conversion. There are motors out there that operate at your battery voltage that can be adapted to fit, unless the motor is buried some where in the design of your item.

Tell us/show us, what the motor is in, and it will open up ideas for you.

#### daveyjones97

Joined Oct 21, 2011
46
oh dear, its an ebike on a shoestring project. yes i know about the T O S no vehicles, but am stumped. the low power mains inverter was effectively free and the universal motor was $16. the budget is for 72v worth of nimh or better if i can get a bargain. a proper ebike kit is$1600 which is a good $1000 more than i can find behind the sofa. i need someone to tell me how to achieve in the region of 30A (torque) 1500 - 2000w from upto 72vdc less than 10,000 rpm motor speed for less than$600. and the holy grail would be nice too....
as i mentioned above, my patchy knowledge is leading me down blind alleys as im finding the ideas i have arent feasable and struggle for detailed how to's.
a mains inverter seemed quite close except for the ability to control output from 0-100%. i dont mind wether the motor is fed dc or ac.
detailed advice/how to's before the post is canned would be wonderful.

so far this motor controller project has only made me learn that patient prolonged saving up to buy off the shelf is the only way to achieve what i want (a diy ebike) and if/when noone wants/can help im going to have to give up, which frankly sucks.
sorry for the negativity, just fustrated and appologies for contravening the TOS. thanks

#### kubeek

Joined Sep 20, 2005
5,753
First, ebike is not agains TOS, it´s modification of road legal vehicles that is a problem.

Now that you said what you actually want to achieve you will get much better responses.
The circuit that will do what you need, maybe not for the power you ask for but something smaller is doable, and 600\$ if you don't count the batteries should be more than enough.
Stepping the voltage up from 72V to the voltage the motor needs isn't trivial, but not impossible. I personally would use current controlled boost converter topology, but maybe others will have different opinions. This will allow you to control the motor 0-100% without resorting to stuff like DC-AC inverters and dimmers.

Anyway, how much does the motor weigh? AFAIK 500W should be enough even for a larger person.

#### kubeek

Joined Sep 20, 2005
5,753
Also you should know that in many countries your ebike must be only able to use the motor up to 25km/h, above that it should not be helping, and I think UK will have similar limits. It is of course up to you, but you better know it before you get fined for going 50 on a modified bicycle

#### kubeek

Joined Sep 20, 2005
5,753
Also I would stay with nimh, nicd or pb batteries, because li-ion and li-pol are too tricky to charge properly and are dangerous if charged wrong.

#### daveyjones97

Joined Oct 21, 2011
46
thanks kubeek, im close to giving up tbh, finally some hope! shall i say my ideal spec of 30ish top speed and nearly 10x the uk legal max power is with a view to "using my ebike on private land, officer". i fully understand peoples fear of litigation and why people fear for my and other peoples welfare with a project like this. all i can say is i have worked with dangerous things in the past and am not stupid, although not neccessarily educated fully (yet) in electronics and inevitably algebra and physics.
i have a motorbike licence and have had weak and powerful gasbikes in the past and as i need my driving licence for my job i will be careful.
ironically trail off road riding is why id like 2000w.
before i wander totally off topic, i dont get a lot of free time hence the interest in adapting something or a painting by numbers/motor controller for dummies type guide.
anxiously awaiting any suggestions, you have my hopes up now!

#### kubeek

Joined Sep 20, 2005
5,753
That won´t do, because it´s for a motor that is rated for 72v. You got 72v but your motor needs 3-4x more, so you need to step up the voltage somehow. I will try to post my circtuit idea and will see what others will have to say about it.

#### kubeek

Joined Sep 20, 2005
5,753
So this is my idea, controller keeps M1 on until current on sense resistor reaches set current, then M1 switches off very fast and dumps the current through D1 into the load. After some dead time the cycle repeats.

I think it will be doable with high enough frequency to keep L1 small, but M1 and D1 will need to withstand the max current and peak voltage. Values in schematic are wild guesses.

So guys, is this doable at 30A and ~250V output, in terms of physical size of L1 for example?

#### Attachments

• 5.2 KB Views: 17

#### Audioguru

Joined Dec 20, 2007
11,249
Hee, hee and boo, hoo.
All the E-bikes I have seen are creeping dangerously slowly along the road in the dark with very dim lights. They should be banned.

#### daveyjones97

Joined Oct 21, 2011
46
Hee, hee and boo, hoo.
All the E-bikes I have seen are creeping dangerously slowly along the road in the dark with very dim lights. They should be banned.
this exactly why id like 1500-2000w, but the uk has a lot of cycle paths for pottering along at 10-15mph. as i stated above, lack of tech knowledege does not = lack of common sense. i respectfully point out im here for help and education not for demotivation and savings advice.
i guess a controller that would use the universal motor would have elements of a regular brushed motor controller and a high power mains inverter. is it likely to be expensive to get a 72v-240 transformer wound? im pretty sure theres a rewind place not far from me. have to go out now but will be racking my brains later. not given up yet!

#### kubeek

Joined Sep 20, 2005
5,753
I don´t think that going the way of inverter + regulator will get you anywhere, because either the transfromer will weigh 20kg or you get into the realm of designing high frequency transformer, which will be lighter but much more hard to do right.

What instruments do you have avilable to build this? Because wihtout at least some multimeter this is impossible, and without oscilloscope it will be still very hard. Also some stuff like soldering iron etc. will come handy.

Anyway, can anybody guesses on the ballpark inductance and resistance of a 2kw 240V universal motor, or at least a way to model it properly? Could anyone who has some similar sized motor measure his?