Brushless DC motor for machine tool power

Though I should of mentioned that in the first sentence instead of hiding it near the end. And sanity checks are always appreciated otherwise I would have burnt my house down trying to run a worm gear at 36000RPM.

To be more specific about it all I have had a mystery transformer which has been sitting around for a while. Have so far torn the copper out the middle (might be able to cast it into something later) and have it ready to wind my own transformer (after I do some maths to check it won't catch fire). Then the output goes into a rectifier. That will probably be in its own standalone box cause it might have uses other than motors.

Then I can stick an ESC to the outputs and power whatever motors I end up with (in theory).

Also damn that guy knows his stuff. A great link.

Also I only have a basic idea of choppers but wouldn't I only be able to get 36V 13A?

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Sorry ... My mistake

MichaelG.

No harm done

I see it is a bit too late now, but usually the best idea with a transformer is to just remove the secondary and make use of the existing primary. It saves a lot of winding.

A chopper supply will give the same effect as a transformer, eg the power out will approximately equal the power in. (less losses of course) So if you halve the voltage you will be able to get about twice the current.

I guess it depends on what your time is worth, but I suspect you might be better off just buying a suitable motor and inverter...you can get a 1hp package ready to go, eg motor, inverter and pendant control, in the UK for about 300 pounds. You can do it for less if you can manage to scrounge up a suitable three phase motor.

John

I'm not sure what machine you are trying to apply this to, but I think you should save yourself some time and money.

Earlier this year I had been looking for a mill to fit a space in my shed. A bloke at work had the machine below (I don't have a picture of it at the time, but it shows the type) for sale, and he only wanted £75 for it. The problem was that he had bought it from someone who had killed the original motor/drive, and had tried (with the help of someone else) to fit a new brushless DC motor to bring the machine back to life.

The machine came to me with a 400W 48V e-bay motor a 400W 48V Mean Well SMPSU, a home made coupling and a generic Chinese speed controller. Pictured below;

There were a lot of problems with the machine, and in particular the drive. The most basic problem was that the motor gear had a different pitch to the gearbox input shaft. They didn't mesh properly and consequently the result was a terrific howling noise since the motor shaft runs at 4000rpm. Some of the gears were plastic, and so the metal ones were gradually training the plastic ones to be the right size!

The head bearings were full of the thick white paint used to make the machine look pretty. As a consequence the bearings were shot. The machine didn't look like it had got a lot of use, and I think it was probably nearly impossible to use.

At 400W you might think that this motor and power supply were man for the task, but I can assure you they are not. I put the motor above in a vice, and at full tilt I could stop the piddly little shaft in my fingers. I put a meter on it, and at stall it was taking the full 400W!!!!

The thing with most DC motors is that they describe power in electrical terms. That is to say the electricity consumed, not the Horsepower developed.

I looked at this business of BLDC motors, and I actually found one on ebay with a drive that looked good for a proper 1/4 HP. It was not a direct fitment for the machine, and it was £250. Not only is it expensive, it is also rare. If I make the effort to fit it to the machine, then I want to be able to get a replacement easily, in case I burn it out.

Given that the gearbox inside the head was plastic, and that people say these machines are noisy anyway, I decided to go for a three phase AC motor with a Z-section vee belt drive. It turns out that you can fit a pulley directly to the spindle. Quiet, efficient, simple, cheap.

I got the motor from "inverter drive supermarket" and the motor and the drive were a fraction over £100 inc VAT.

I made the steel bracket from parts that I had laser cut, and I welded them together.

I decided to go a little to town with the control box. You wouldn't have to go that far.

If you really want a decent machine drive, it doesn't have to cost the earth. Once done, it's all standard parts, so it's done forever. If anything goes wrong, you just buy new bits and drop them in.

Andy,

Nice conversion on a mini mill there....

What size motor did you go for? I'm guessing you went 2 pole too?

Hi John,

Thanks.

I actually went for the four pole motor.

The spec sheet for the motor shows that it will happily run to 75Hz which I think is roughly around 2200rpm.

The actual motor is is characterised in the data to 4000rpm but I don't need more than 2000.

In fact I was more worried about the low speeds. I machine mostly steel and bronze, and prefer to use larger cutters where possible. I have allowed two pulley speeds. The high speed is 1:1 with the motor shaft. The low speed is 2:1 reduction.

I've been using the side of a 19mm cutter on steel this afternoon. The cut was about 3/4 of the full height of the cutter, and 20 thou per pass. It didn't even flinch. I couldn't be happier.

Speaking as someone who has developed alternator-based motors that act as a generator but can also restart an engine ("BSG" or belt starter alternator) for production, I'd suggest it's not a very good candidate for driving a machine spindle. You could do it for a bet or if you were stuck on a desert island but otherwise it's far from ideal.

To do it properly you really need 3 Hall effect switches to report the position of the rotor, so you can switch the windings at the correct time (angle). You also need a ring magnet to drive said switches and some circuit or software to commutate the phases. This is normally done using a "6 step" or "trapezoidal" scheme. Of course, you can run them open loop (drive the field windings in sequence and hope the rotor manages to follow) but you have to be careful to accelerate the rotating field slowly enough for the loaded rotor to follow.

You will need a 6 switch (3-phase) FET bridge complete with high side and low side drivers. These drivers take the commutation signals and drive the FETs.

The Kv for an alternator varies with the current in the rotor, so you really need to be able to control it too. You'll also struggle get it to run at a decent speed.

One key issue with alternators is their lousy efficiency. And if you are trying to use a standard alternator as a motor, the efficiency will be abysmal. The "real" BSGs also have magnets in the rotor, although these are acting more like flux concentrators than drive magnets.

As pointed out, there are much better solutions available unless you are short of something to brag about down the pub.

Murray

The transformer had been sitting around for a while. Makes a useful doorstop, anvil, glueing weight... Winding were a bit frayed when I thought of using it for actual transformer stuff. Windings had to go.

I was about to say that to get 1.5kw 3 phase inverter + motor is £240 or more.

I may or may not be able to get a bargain, by this time tomorrow I might be very strongle on the path to three phase. Shame about no being able to hook stuff up to a computer but oh well.

Andy,

What about horse power size? Did you go for same as existing, 400 watts?

It's a pity you have been put off Rainbows because no one will know what can be done with them unless they try it.

As I mentioned I think 1.5kw is rather ambitious unless you make one yourself. That would need a source of lamination stacks of some what larger diameter than the model motors that are about. I posted a link that shows the shape of these which is needed. Power can be increases by using more than one next to each other. A site linked to off the one Michael posted has some practical aspects on it. There are others.

**LINK**

That site shows a motor made from DC motor lamination stacks and no details. There may be a catch with that method which will spoil the efficiency. The laminations are likely to be thicker than what is ideally needed. That's the laminations that make up the stack of them. This may or may not matter. There is a lot of that in self building. Ideally the stack needs to be epoxy coated to save messing about with kapton tape and having sharp corners etc.

The model motors may be a bit tat but they do have what all brushless motors have in them. They could be modified in all sorts of ways even down to completely dismantling to get the parts or even just rewinding. If some one takes one apart they will see why the shaft and bearings are rather small. I have a couple of 22mm laminations. Sort of size used in ones built from CD motors etc. The hole in the middle of them is 10mm dia.

The ESC's for models are made to be extremely small and light. One way of improving them might be to change the surface mount power devices to leaded types with a heatsink but one of the self builds with some modifications might be a better idea. When these motors stall the current levels are huge so some sort of current monitoring would be a good idea. It could be added to the software used to drive the things.

As you can see it's not a project for a typical defeatist who wants everything laid out in black and white complete with detailed instructions. It's also likely to take some time and be a bit hit and miss.

There seems to be a lot less defeatism about in people who speak English as a 2nd language or not at all. A sign of the times I suspect.

John

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Rainbows,

Over on RC Groups.com there is a whole section devoted to building brushless motors, various details on lamination sizes and thicknesses, winding patterns LRK method etc. Magnets, clearances etc....All very informative stuff, there may evn be supplier details iirc. Then there's some info in building your own brushless controller in the electronics section....

I dont want to start cluttering up this thread with too much info as its not really model engineering stuff, so best if you have a look there yourself

Edited By JasonB on 18/07/2016 11:52:06

The idea does still appeal to me but building a motor is something I would look into after having wired up my inverter and motor. Mixture of low cost, proven equipment and low effort all point to three phase for the short term. (Well usually not low cost but the second hand market is a beautiful thing)

Hi John,

The motor is 1/4 HP which is 180 Watts, considerably less than the rating of the DC motor and PSU I showed.

I just looked up the stall current on the 1/4 HP AC motor. I wanted to compare like with like.

The motor I chose looks like it comes out at 1500 Watts (2HP) in those terms, but there is no way you could use it like that continuously.

A real 1/4 HP (continuous/shaft) is more than enough to drive a small lathe or mill.

Andy,

Thanks for the info on the motor, it gives me an idea for my own mini mill...

@ Jason,thnx for the hyperlink addition....guess I'm just too damn lazy....

This is an interesting paper on brushless motor design

**LINK**

Some pretty thick books mention that even complex modelling has it's limitations and wont be entirely accurate.

John

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