Rewinding ac motor to dc?

[raitl]

Can anyone point me a decent information source on how to rewind an AC motor to run on DC?

 

[RODALCO]

The only type of motor which will run on AC or DC is the series motor with commutator.

I don't know what type of motor you want to convert, but you need a commutator set up which will be a complicated set up.

Unless you have a 400 Volts dc accu bank and via a 6 scr or power fet type drive you create via a driver circuit a 3Ø field from which you can drive your 3Ø AC motor like in a variable speed drive.

 

[raitl]

actually my goal is to run a small 220V AC motor off a 12V battery. 220V to 12V calculations I can handle, it's the AC to DC part I'm puzzled with.

 

[Nigel Goodwin]

actually my goal is to run a small 220V AC motor off a 12V battery. 220V to 12V calculations I can handle, it's the AC to DC part I'm puzzled with.

You can't 'rewind' an AC motor to make it DC, it's a completely different type of motor.

But as you're starting from 12V anyway, you have to convert to AC to get to 220V - so leave the 220V as AC instead of rectifying it!.

 

[raitl]

Risking to make myself look even more stupid....what exactly is the major difference? I mean, there are DC motors that aren't permanent magnet ones, right? So basically it should be just the manner the coils are connected... or am I way off here?

 

[raitl]

But as you're starting from 12V anyway, you have to convert to AC to get to 220V - so leave the 220V as AC instead of rectifying it!.

The motor in question is an electrical grass trimmer motor which I'm trying to convert into a small boat motor. (Talking about a light ply boat, for 2-3 ppl)

 

[Nigel Goodwin]

Risking to make myself look even more stupid....what exactly is the major difference? I mean, there are DC motors that aren't permanent magnet ones, right? So basically it should be just the manner the coils are connected... or am I way off here?

Essentially DC motors have brushes and a commutator, which switch the different coils in turn to create the rotary movement. A DC brushless doesn't have mechanical brushes or a commutator, but does the exact same thing electronically.

AC motors don't need this switching action, as the waveform is already 'switching'.

 

[Klaus]

The motor in question is an electrical grass trimmer motor which I'm trying to convert into a small boat motor. (Talking about a light ply boat, for 2-3 ppl)

First: check if your motor has brushes. If it has it belongs to the 'universal' type of motors which 'could' run on DC. But be careful there, the voltages and power figures might be quite different.

Second: what you are trying to do does not promise a lot of success.
The motor RPM must match the Propellor specs to get any worthwhile efficiency. A gearbox would really complicate the job and suck up more power in the bargain.
Its NOT just a question of spinning a propellor underwater!
Then, driving a submerged propellor demands power, very likely considerably more than your grass trimmer motor could provide.

Some people have tried to use a petrol (benzin) powered weed trimmer motor to propel a boat. They have a bit more power but at high RPM so there is still the problem of finding a suitable propellor. A plastic model aircraft type prop might stir up some useful water- experiment.

For small electric boat motors look up the 'Minnkota' series of motors on the net and see how they do it. You might get some ideas. They use 12V high current motors, something you will NOT be able to rewind your electric motor for. Its simply physically not practical to wind heavy gauge wire where previously you had many turns of very thin wire. Nor will the communtator or the brushes be able to handle the large current.

And, if you are thinking to use high voltage DC, remember that water conducts and you could end up with a lethal trap.

Good luck with more research. BTW, 2-3PPL do increase the weight of a light boat substantially. I would think a 2Hp motor might be required if you want to move faster than rowing speed.
Klaus

 

[raitl]

gasoline and any kind of internal combustion motors are out of the question, as they are not allowed on that specific lake, just electrical motors. And I would be quite happy with achieving rowing speed.

 

[Hero999]

Build/buy a small inverter to convert your 12V to 220V AC.

Even better, just get a 12V motor.

 

[raitl]

ok, turns out it's a motor that will run both on AC or DC. But also turns out that the Ohms law doesn't work when trying to calculate the resistance of motor stator and rotor windings... how DO you actually calculate the resistance, when you know power(P) and voltage(U)?

220*220/200=242 whereas the actual resistance of the entire motor is 80 ohms.

I want to know what should the resistance be, to run the motor on 12V

 

[Oznog]

Here's the thing. OK, it's Universal. At that, if you found a way to strip off 90% of the wire off of EACH winding it would create the same magnetic field on 12v. But the wiring (and brushes) were sized for 1/10th the current so in reality all wire would need to be replaced, and preferrably enameled. There's a lot of technique to doing this and the wire alone may cost you a bundle.

This is one of those times where we say you're better off buying something. I mean aside from the rewinding- which is not really practical- there's an angled gearbox, prop, thrust bearings, grease seals, and hull mounting issues. These electric trolling motors aren't expensive, heck a yard sale, craigslist, flea market etc will get you one for next to nothing.

That motor will run fine on a 12v inverter. But... the inverter is going to cost more than the trolling motor, and the project is only 10% closer to being completed to the point where it'll push a boat. And frankly that motor and inverter won't work nearly as well for several more reasons I'm not even going to get into.

 

[Nigel Goodwin]

ok, turns out it's a motor that will run both on AC or DC. But also turns out that the Ohms law doesn't work when trying to calculate the resistance of motor stator and rotor windings... how DO you actually calculate the resistance, when you know power(P) and voltage(U)?

220*220/200=242 whereas the actual resistance of the entire motor is 80 ohms.

I want to know what should the resistance be, to run the motor on 12V

Motors don't have 'resistance', it varies massively with the load applied, so you can't really measure it.

Bear in mind, you would have to rewind both rotor and stator, with many less turns of MUCH thicker wire - probably thick enough to make it very difficult?.

 

[G8RPI]

Motor conversion

Hi,
You have a brushed "universal" motor it will run of AC or DC. There are two sets of windings the Stator (on the body of the motor) and the Rotor (the bit that oges around) the rotor is connected by the commutator and brushes, these reverse the current through the rotor at the correct time to cause the motor to rotate. Reversing ONE set of coils (stator or rotor) will reverse the direction of the motor. Reversing both makes no difference, this is what happens with AC.
If you connect your motor to a 12v car battery it will almost certainly run, but at much reduced power.
Rewinding the coils is not really practical, there is however one easy option.
On a small mains motor like this the windings are almost certainly in series. This is called "series connected" and provides a controlled speed/torque response. There is an alternative were they are the stator and rotor are in parallel, called "Shunt" wound. Car starter motors are wound like this. It provides high torque but with no load the motor can destroy itself.
As your motor has windings designed for 220V series operation, you can connect them in parallel (Shunt) to improve the performance at 12V. You will not get full power (not enough current through the windings) but it will be much better than the series connection. It would be a start.
Using two (or more) batteries in series will give more power. If you think about it you can arrange to switch the batteries in parallel for longer duration at low speed or series for high speed.
You still need to find a suitable propeller. Some fans used on industrial Air Con units have adjustable pitch blades. You loosen the hub to twist the blades. One of these with only 2 or 3 (half the original number) fitted migh be a starting point.

Robert G8RPI.

 

[Hero999]

As it's a universal motor you could build a 12V-220V DC-DC converter, it wouldn't cost much and it wouldn't be very big either.