Brushless DC motor for machine tool power

[AjohnwParticipant @ajohnw51620]

i've seen helical gears used in model helicopters Rainbows and I think some more like a multi start worm.

I thought Jason may have found a test that actually measured the output power. I never have but not looked for a long time. Another maker, Scorpion do I think give continuous current ratings.

Price seems to rise at an astronomical rate with shaft diameter. The go cart one may be £200 plus or more. 8 or maybe 10mm shaft. 6mm used to be common and cheap.

Here's a home made one

**LINK**

There is also a 240v one around and details on rewiring a washing machine type for lower voltages. All on YouTube. There were several design sites about and some laminations were available.

There is still some design info around eg

**LINK**

It may help people understand what they are buying.

John

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[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ajohnw on 16/07/2016 19:30:08:

Price seems to rise at an astronomical rate with shaft diameter. The go cart one may be £200 plus or more.

.

**LINK**

[John RuddParticipant @johnrudd16576]

A car alternator may provide an alternative……

Remove the rotor winding replace with perm magnets,( or feed with a fixed excitation voltage) use the stator energised by the speed controller…..

[Neil WyattModerator @neilwyatt]

Posted by JasonB on 16/07/2016 16:41:54:

Here is a vid of one putting out more watts than it is rated at so sounds like they are rated about right

**LINK**

About 10% more input power than rated?

That is about the right sort of max power for a 5mm shaft. 380-odd watts rather than 1.5kW

Neil

[AjohnwParticipant @ajohnw51620]

Geee Neil I mentioned 1/2 hp I think earlier – output too not input and at sub 10,000 rpm which I didn't mention. I think you need to consider the various numbers on them. It's also a bit weird that people fly things around using motors like that.

I doubt if many people who bought the motor initially linked to would run it with 6 lipo cells. I wouldn't anyway. If Rainbows wants make use of this style of motor I feel he is looking at the wrong type and wont be running it at max voltage either. There seems to have been a shift from 6mm shafts to 5 but 6 and 8 are still available – generally at a cost though.

Here is one with a 4mm shaft with figures that I might use on it. It can be run at 42 amps.

**LINK**

Thought I had posted a link for Michael that showed a motor that might well be used on the go cart – price over £300 and some makes can be more expensive than that.

John

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[John OlsenParticipant @johnolsen79199]

The outrunner motors actually do have two bearings, one at each end. You can see in Ajohnw's picture above that the shaft extends out the back of the stator.

There are also brushless inrunner motors used sometimes, although not nearly as common these days.

Mostly in a model aircraft they would be run at 10K or so Rpm, maybe 20K or so at the outside. The propellors are less efficient at higher rpm. I'm not sure how good the bearings would be at really high speeds. It is not unusual for them to need changing.

They do get quite hot, I have had magnets come loose which is because the glue that holds them on is getting too hot. That particular motor was being run at up to about 1200W. (input) The efficiency would not be as high as larger motors, 70% would be more realistic I suspect. Everything is being run at its limits….I have had an ESC catch fire when the prop touched the ground while at full takeoff power. When I say catch fire, I mean there was a fireball, and the ESC was still burning when I got to the plane.

Think of them as being like a hot rod engine.

John

[V8EngParticipant @v8eng]

Just as a general aside on this:- I seem to remember seeing an article about making your own Outrunner Motors, using old (small) mains motor parts, this was a few years ago, may have been in ME or MEW but not sure of that.

[RainbowsParticipant @rainbows]

Having gone through 20 pages of aliexpress brushless motors I found nothing that ticked the criteria of low Kv, ok price, thorough information and >1000w.

Would this motor be better/ useable?

Its the same line of motors as the go cart, lower power though. Way lower Kv for 8400RPM at full voltage. No worm gears involved

I googled it and found a video on youtube of it powering a mill

It isn't actually cutting for 90% as far as I can tell? Which makes it hard to gauge how well it performs. Atleast it must have cut some stuff out with out imediately melting.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ajohnw on 16/07/2016 23:05:12:

Thought I had posted a link for Michael that showed a motor that might well be used on the go cart – price over £300 and some makes can be more expensive than that.

.

< sigh >

John,

The motor on the go-cart was specified in the text on the YouTube page … That's why I provided the link for you; to avoid speculation.

MichaelG.

_____________

Quote:

Published on 17 Apr 2012Model: Turnigy 50CC
Watch my website for details and real tests and video outside 
http://www.masinaelectrica.com/simple-brushless-controller-explained/

• Category

  • Science & Technology

 

Edited By Michael Gilligan on 17/07/2016 05:46:58

[JasonBModerator @jasonb]

Posted by Neil Wyatt on 16/07/2016 21:26:59:

Posted by JasonB on 16/07/2016 16:41:54:

Here is a vid of one putting out more watts than it is rated at so sounds like they are rated about right

**LINK**

About 10% more input power than rated?

That is about the right sort of max power for a 5mm shaft. 380-odd watts rather than 1.5kW

Neil

Neil the one in the video only has a 3.17mm shaft so that would be nearer to the values you got in your first incorrect calculations but still well within the shafts limits of 1.23Kw when worked out correctly.

[John HaineParticipant @johnhaine32865]

John Rudd's suggestion is a good one if you really want a brushless motor. There is a lot of info via Google on converting car alternators into motors using a standard esc and driving the rotor with a low dc current. People like Allegro Microsystems also do control chips for escs if you want to go that far. A car alternator would probably be the right sort of size for a machine tool drive.

[RainbowsParticipant @rainbows]

Well I think before buying motors I'm gonna get a transformer. 240V 13A becomes 36V 87A.

I am gonna try and power an alternator and see how that goes (though there are still brushes connected to the rotor as far as I can tell?) since they can be £10 and if that fails I can use the ESC and power supply to power a store bought one or a homemade one. Really not sure how to make the stator though.

[Michael GilliganParticipant @michaelgilligan61133]

Sanity-Check, Rainbows …

The only "affordable" 3kvA transformer that you are likely to find is a 110v site transformer. This could be easily rewired to provide 0-55v instead of 55-0-55 but [continuously rated] they are very big and very heavy.

**LINK**

The other, non-trivial, issue is that you need three phases to drive a typical bldc motor !!

Think about it, before you waste too much effort and money.

MichaelG.

.

P.S. … If you haven't done so yet … Have a look at the go-kart man's site … He knows what he's doing.

Edited By Michael Gilligan on 17/07/2016 19:28:50

[John HaineParticipant @johnhaine32865]

The ESC generates 3 phase since that is what BLDC motors need anyway. Really, the alternator is just a BLDC on the stator side but has a dc excited rotor instead of a PM one. Yes it has brushes, but these run on slip rings rather than commutator segments, and the current is quite low. There was a gentleman who used to exhibit on one of the club stands at MEXs who demo'd alternators being used as traction motors, with home brew electronics. I pointed him towards the OTS chips from Allegro, not sure if he ever tried them, but people on the web have used them in alternator conversions I think.

In theory you don't need a transformer, you could run the 3 phase windings direct from rectified mains using chopper drives just as standard VFDs do. What is critical is not the voltage across them but the current through them, so you apply the high DC voltage for long enough to get the current you need in a winding through a MOSFET, then turn it off and allow a diode to commutate the current. Standard practice in VFDs and stepper drivers. This is in effect a buck converter, and will act the same as a transformer in giving a step down between the winding current and mains current.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Haine on 17/07/2016 19:34:21:

The ESC generates 3 phase since that is what BLDC motors need anyway.

.

Quite so, John … I was simply reminding Rainbows that he needed 3 phases; having seen no mention of him intending to source an ESC.

MichaelG.

[John HaineParticipant @johnhaine32865]

"Well I think before buying motors I'm gonna get a transformer. 240V 13A becomes 36V 87A.

I am gonna try and power an alternator and see how that goes (though there are still brushes connected to the rotor as far as I can tell?) since they can be £10 and if that fails I can use the ESC and power supply to power a store bought one or a homemade one."

[RainbowsParticipant @rainbows]

Posted by Rainbows on 17/07/2016 18:15:17:

if that fails I can use the ESC and power supply to power a store bought one or a homemade one.

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?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Haine on 17/07/2016 20:10:34:

"Well I think before buying motors I'm gonna get a transformer. 240V 13A becomes 36V 87A.

I am gonna try and power an alternator and see how that goes (though there are still brushes connected to the rotor as far as I can tell?) since they can be £10 and if that fails I can use the ESC and power supply to power a store bought one or a homemade one."

.

Sorry … My mistake

MichaelG.

[RainbowsParticipant @rainbows]

No harm done

[John OlsenParticipant @johnolsen79199]

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

[Andy AshParticipant @andyash24902]

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.

[John RuddParticipant @johnrudd16576]

Andy,

Nice conversion on a mini mill there….

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

[Andy AshParticipant @andyash24902]

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.

[MuzzerParticipant @muzzer]

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

[RainbowsParticipant @rainbows]

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.

[Author]

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