Brushless DC Motors

Greetings all.

These have been fitted to some of the Sieg lathes and milling machines for a while now and seem to be proving reliable.

I've got an application that would greatly benefit from using such a motor and associated power supply, due to the excellent power/size ratio, which would mean that I can dispense with the gearbox from my project, but I can't seem to find out much about them or where they can be bought.

Any information would be gratefully received that would help me find a 200-300 watt motor/driver combo that can have its speed varied by a potentiometer, all at a reasonable price of course.

Martin.

Edited By blowlamp on 27/05/2012 01:53:50

Look at the RC plane, heli and even car sites they all use brushless motors.

200 and 300 W brushless motors are the size used for electric bicycles, the come in various voltages, ranging up to 70+ V (I think). Ian S C

How much power do you want? Look at www.mtroniks.net. Not all their motors are large, if you have seen the gas turbine electric loco being built and shown at some exhibs you can see quite a difference.

Thanks for the replies chaps.

The motor pictured on the machine here would be an ideal candidate as far a physical size is concerned, although at 500 watts rating it's more powerful than I think I need. Let's just say that if I could buy that and its driver then I'd be sorted. It also looks like it might comply with some standard (NEMA?), which would help me with making the parts that will be bolted to it.

I have looked at the RC motors and they must surely have uses in the model engineering workshop environment, but I haven't seen one that I'd be able to use for my application, as the motor needs to be sealed from swarf and robust too, because it's going to get handled quite a lot.

Martin.

Martin

I have been playing around with brushless motors designed for the RC market, albeit at the low power end of the spectrum. These motors seem to be able to absorb high levels of power for their diminutive size and have quoted efficiencies of above 70%. Typically a motor of 28mm dia with a length of 35mm can absorb over 100 watts. Sounds good but the downside is the shaft speed which is quoted as KV or rpm/volts and with figures in the range of 1000 to 4000 KV then even at relatively low volts, say 15 to 30, the shaft speed can be well in excess of 15,000rpm .

They also come as out-runners and in-runners. With the out-runners most of the case revolves and is attached to the shaft with the mounting for the motor being on the remaining non-rotating part, often just the front face. In-runners are more like conventional brushed DC motors with the only exposed rotating part being the shaft.

The controllers are designed to be driven from a RC receiver, much like any model servo, and require a pulse width modulated signal of about 5V amplitude with a range of between 1ms to 2ms with a repetition rate of somewhere between 10 to 20ms. This can be derived easily using something like a LM555 timer IC and will drive the motor controller directly, some of the controllers have a BEC function, battery eliminator circuit, which can be used to power the timing circuits.

Some of the larger motors can absorb over 2kW of power but because all of these motors are intended to run on fairly low voltages the current required is very high. All this come at a cost, even going direct Chinese suppliers on Ebay, and of course the shaft speed tends to be fairly high, though the KV figures are lower. Some controllers and motors come with water cooling facilities built in to help with heat dissipation!!. The larger motors often have shaft diameters in excess of 5mm.

The brushless motors used on the mini lathes etc tend, I believe, to work on much higher voltages, probably directly rectified mains, and therefore can produce the power at much lower currents and hence need less robust electronic controllers. Also being designed for static use where size is not the prime concern, unlike in a model, the motors can be much bigger and the increase in rotor diameter will give them higher levels of torque which means they can be run at lower speeds while still achieving the desired shaft power.

Hope the above helps a bit but as I said I am only just beginning to get involved with this technology.

Regards

T'other Martin

PS

Out of interest here is one that is high power and totally sealed

Edited By Martin W on 27/05/2012 11:57:10

This thread might be of some use.

Russell.

Thanks for the information and links.

Because my project is a spindle, I will need a good range of speeds - say 100 to 3000 (ish) or thereabouts.

Do any of the RC type of motors have a range of shaft speeds like this, as a large concern form me is to make my device compact and without any kind of mechanical gearing?

The milling machine I gave a link to earlier has such a system and has more than enough torque even at its slowest speed setting.

Martin.

Edited By blowlamp on 27/05/2012 12:27:12

Martin

If you find a suitable RC brushless motor then another cost that hasn't been covered would be that of a suitable power supply. As I indicated the RC motors and controllers are designed to run from supplies with outputs in the range of approximately 10 to 40 volts therefore they require relatively high currents which can be in excess of 100 amps. The cost of a power supply that can supply 30-40 volts at around 60-80 amps plus is not going to be cheap .

That is why the variable speed motors commonly used in lathes and mills, both brushed and brushless are typically designed to run from much higher voltages. Effectively rectifying the supply voltage means that there is not a need for expensive power supplies, the electronics is only required to switch/control comparatively low currents and the I2R motor losses are reduced. A Win Win situation if there isn't a size restriction and there are relatively high voltages available to hand.

Have you tried to price the bits you want from Arc?

Cheers

T'other one

Yes, they are available with a wide range of speed ratings. The speed rating is quoted as kV which confused me for a while as that is the abbreviation for kilovolts, however for these motors it is the number of rpm per volt so for 3000 rpm maximum and running off a 12 V supply you need 250 kV.

They will work OK on 12 V so you could use a 12 V car battery connected to a charger.

Russell.

Martin (T'other one).

You might be right about trying Arceurotrade for the parts.

I'll phone them tomorrow and let you know how I get on.

Thanks again for your help.

Martin (blowlamp).

Martin,

PM sent

A gentleman has exhibited at various exhibitions the use of car alternators operating as motors, essentially as "brushless" dc motors except with a wound dc-excited rotor instead of permanent magnet. Because the alternator uses slip rings instead of a segmented commutator, and the brushes carry only the excitation current, brush life is a lot longer. Except for the excitation current, speed control is by applying 3-phase to the stator windings and this could be derived from one of the standard BLDC controllers. There's also quite a lot about this on the web - google "alternator as motor". As alternators are made in large numbers for cars this seems a nice approach

Ive hear about this chap before but wonder it he reads this forum and would like to post a few words on the subject? Certainly sounds interesting!

Peter

There was an artical in ME Vol172 no 3961 p103 by Geoff Bartlett in part of a long series called "Low Voltage DC Motor Drives. He tryed running it as a stepper motor, he got it to run but not successfully, and ended by sayingI will wait for someone to prove me wrong. He has stuff on car generators, Sinclair C5 motors, and more, and the control. Things have changed a bit since 1994. Ian S C

The chap I referred to above gave a lecture at MEX last year on behalf of SMEE - see

**LINK**

and scroll down to find the summary. You don't run them (alternators) as stepper motors but as polyphase motors where the stator phases are switched from rotation of the motor. Speed increases until the back emf equals the volts applied to the rotor, just like for example the outrunner motors used in R/C modelling. That's for a given rotor current - if you increase the rotor current then the motor runs slower but generates more torque.