Rewinding reluctance motor?

[The Blight]

At first I thought the motor I have in front of me was a standard bldc motor, but after opening it, and finding no magnets, I started searching. Figured out it had to be a synchronous reluctance motor. Don't have the equipment to get it up and running, so thats not something I will be spending time on.

After doing some reading, I started thinking about rewinding the motor, and making a new rotor for it with magnets, so that I could use an rc esc to drive it. Essentially making it a bldc motor. Is this possible? Any comments? Suggestions? Right now it's rated at about 400W. I want to rewind it to about 1000W and use it as a spindle motor. Am I nuts? It's 75mm long (housing), and 71mm in diameter.

[The Blight]

Change of plan!

I will still re-wind the motor, but I'm thinking about keeping the rotor the way it is. If the rotor is magnetized by the first 4 windings, would this mean that the rotor has 4 "magnets"? I'm new to this, so it might be a stupid question.

[vger]

You might rewind it for a bit more power, say maybe 500 W, but.... the field laminations were probably designed at or near magnetic saturation. Most manufacturers will put only as much material into a product as needed. Just as a given wire size can carry only a certain amount of current, magnetic materials can carry only a certain amount of magnetic field. There are even control transformers that use that property called "Saturable core reactors" generally used for very high power controls where solid state devices would be too costly. Synchronous reluctance motors are generally used where speed regulation is critical such as merering pumps, clocks, and equipment that must remain syncronized to something else. They are not as efficient as standard induction motors.

Steve

[The Blight]

I was afraid of that answer. I will have to do some more reading, and see what I can find out. It's weird though. Most of the brushless motors, for RC, are smaller then the one I have. A lot smaller in fact. A lot of those are also rated at high power. I know you can probably not compare it that way, but because I don't know better, it seems logical.

This motor is very old, and it was made for a cassette reader of some sort. Good German quality though.

Thanks for your help Steve! It's nice to have some information from someone who knows a bit about motors.

[wildwestpat]

Treat rewinding as a educational exercise but it would be simpler to learn on a bigger motor. The mechanics of winding the coils and inserting them into the stator iron work is not a simple process and requires adequate insulation as well as mechanical stability but can be done on a DIY basis.

From your description what you have is synchronous motor designed for a reasonably constant mechanical load – the volumetric efficiency of these motors also tends to be very poor compared to the modern high speed brush less DC motors used by the model aeroplane fraternity.

Sorry to have to be so pessimistic - but have fun learning the art rewinding field coils.

Regards - Pat

[The Blight]

It's good to know this stuff before I start out with this project. I'm doing this for fun, so it is for learning. A high power bldc motor costs around $35, so I'm not saving any money by doing this. In the end I might end up having to buy the motor as well as the esc, but right now I have more time then money, so that will have to wait.

Just got 2kg of 0.18mm wire for some electromagnets I'm making. I see that a lot of the bldc rc plane motors are wound with multi strand wire. Maybe I will try this too. What do you guys think?

[The Blight]

After doing some calculations, it seems like the core is close to saturation as it is.

I have also tried calculating the power of this thing as a bldc motor, but I'm having no luck at all. I'm just getting more and more confused, and I'm close to giving up. I don't want to burn out an esc just because I did something stupid. It seems like these will go back into a box, and a new motor will be bought for the spindle.

[Zig]

from a given volume of iron You may squeeze more power only if You go to a higher frequency.

But then again the losses ( both winding and iron losses) go up. Non linearly as a greater than unity power of frequency ( power loss ~ frequency^(1 +n))

[The Blight]

I have been doing some more thinking (I don't know if it's good or bad), and I will be making a topic on this over at the RC forum (found one where they have a lot of topics on re-winding bldc motors). If they can help me figure out how many windings I have to make, with what wire, and can give an estimate on what I can expect, then I will proceed with the project.

[vger]

You will find that the area in the stator core is limited as far as how many strands (loops) you can fit in each slot. Yes, a larger wire will carry more current, but you will not be able to get as many turns in the core. The magnetic field produced by a coil is related directly to "Ampere-turns". So with a larger wire size, you will lower the voltage and increase the current. The power will stay about the same unless you can squeeze a few more turns in there. Best of luck.

Steve

[The Blight]

With a 1mm wire, I will be able to fit about 40 turns per pole. This is more then enough.

This might be a stupid question, but what is it that limits the current in these motors?

Thanks!

[The Blight]

Took some pictures of it.