direct-drive electric motors

[Frances IoMParticipant @francesiom58905]

[Frances IoMParticipant @francesiom58905]

there is an interesting description of a direct drive legged robot – http://spectrum.ieee.org/automaton/robotics/robotics-hardware/ghost-robotics-minitaur-quadruped

a comment elsewhere is that such direct drive motors have v high torque at low speeds and are used in washing machines – as my local amenity centre (IoM) allows dismantling of such, has anyone knowledge of which machines (UK based) I should look for to extract the motor ? – possibly ideal for lathe drives or similar? or are the drive electronics too complicated

[MWParticipant @mw27036]

Hi Frances,

I don't know about taking one out of a machine

but the main reason why lathes don't do direct drive is because of varying load, for one size material or diameter you wont be wanting the same speed and therefore a different torque set up with pulleys or gears.

For example, you could get away with a one setup scenario, where full motor power gave you 2000rpm on the lathe, this would work for most things, but then on others you'd need to slow down, and even if it managed to turn on this setup you'd be turning the motor fan quite slowly, so it could over heat.

The other reason would be forgiveness if the lathe jams, when a belt or a sacrificial drive gear train gives way, you'll either lose a tooth or the belt will slip, but this could take the damage back to the motor instead, even stepper motors used oldham couplings so that the coupling could give way first.

I Hope this gives you a few answers,

Michael Walters

[JAParticipant @ja]

Posted by Michael Walters on 20/09/2016 11:24:15:

The other reason would be forgiveness if the lathe jams, when a belt or a sacrificial drive gear train gives way, you'll either lose a tooth or the belt will slip, but this could take the damage back to the motor instead, even stepper motors used oldham couplings so that the coupling could give way first.

Michael Walters

Or even forward, damaging the lathe headstock, spindle, chuck or worse.

JA

[Frances IoMParticipant @francesiom58905]

all the washing machines I know have variable speeds for different parts of the washing cycle – the beauty of the direct drive as succinctly put in one posting re the robot is that it is difficult to strip teeth from a magnetic field! – also the smaller motors in the linked robot definitely run at wide range of speeds

[Clive FosterParticipant @clivefoster55965]

LG are, I think, the folk who started the direct drive thing for washing machines and are probably the only breed that has been about long enough for there to be a reasonable chance of finding one in the disposal yards. I think Samsung also do some but finding out which ones seems an impossible task especially as their terminology about motors is at best non standard and more than bit obscure.

Direct drive motors look hard to re-purpose as they are built into the back of the drum with no independent shaft or bearings. Also appear to be of inconveniently large diameter. You will need all the controls too, as these are a brushless design, and the skills to excise the motor control gubbins from the rest of the controller chip program.

Looks to be a lot of effort for not too much return with a significant chance of getting nowhere. In my view repurposing washing machine motors stopped being sensible around about the time Hoover stopped using their nice square single phase motors on simple washing machines and the robust two speed semi automatic sliding gear gearbox integrated to the motor on more complex ones. At 62 I'm hardly old enough to remember that! Should be a salvaged 1/3 rd hp integrated sliding gear gearbox one under the bench waiting for a good use to arrive. Probably been there approaching 40 years now but I live in hope!

Clive.

[Martin WParticipant @martinw]

Hi

There's a description of direct drive washing machine motors here. As can be seen from the pictures the motor case rotates and it is quite large. Not something I would try to drive a lathe with anyhow.

Martin

Edited By Martin W on 20/09/2016 19:05:12

[Ex contributorParticipant @mgnbuk]

lathes don't do direct drive

Not correct.

Most drive manufacturers produce "kit" motors for direct integration into machine tool spindle arrangments. These comprise a stator unit with the windingsto fit into the headstock casting & a hollow rotor that is pressed or shrunk onto the machine spindle. The stator units can be liquid or forced air cooled. Characteristics are the same as self-contained asynchronous AC motors – constant torque up to base speed (usually around 1500 rpm) and constant power up to maximum (6000 rpm upwards). The associated drives usually have the ability to run 2 sets of parameters (switchable via an input) so that a constant torque C axis positioning mode can be selected in addition to normal spindle operation

First one I recall reading about in the brochures was a liquid cooled Indramat system around 1985. Assembled a couple of Italian (IIRC) motors at the last employer for a customer who wanted to use them as a contra-rotating mini submarine drive. We subbed out the fitting of the shrink-on rotor to a motor rebuild shop & they got the first one stuck half way on – needed a new spindle and rotor, as bothe were damaged beyond use getting them apart again. We subsequently used BOC Cryospeed to cool the spindle in liquid nitrogen for the sucessfull assemblies.

My current employer bought a Gildemeister CTX400 slant bed CNC lathe incomplete from an auction for very little. I noticed the sale & suggested getting the machine for control & drive spares for other machines, as the complete headstock was missing. My MD did a bit of digging & found that the built-in motor headstock had been sent away for refurbing & the company went bust before it was returned – the refurb company were happy to sell us the missing bits (refurbished) for the original refurb cost. Amazingly all the parts were present (to the last small guard & screw) and the machine worked fine when reassembled. Unfortunately (or maybe fortunately !) during conversation with DMG about the machine it was suggested that the forced air cooled spindle didn't play nicely with graphite & all we machine is graphite – so the machine was sold on without ever turning a part for us.

Advantages to using a built-in motor include lower rotating masses for faster acceleration & deceleration and easier (i.e cheaper) headstock machining (no gears, drive pulleys and belts, lubrication systems etc).

While driect drive headstocks have yet to appear on hobby lathes, it is probably only a matter of time before they do.

Nigel B

[MWParticipant @mw27036]

Posted by Nigel B on 20/09/2016 20:45:13:

lathes don't do direct drive

Not correct.

While driect drive headstocks have yet to appear on hobby lathes, it is probably only a matter of time before they do.

Nigel B

You aren't doing away with lubrication systems entirely, you still have a headstock to contend with. I don't doubt what you say but it's certainly a case of the exception rather than the rule (the evidence is enormous), what happens when or if you overload the machine?

These must be machines that use enormous power motors in order to justify using 10% of power to deliver say 200rpm and full torque, if that wasn't liquid cooled or an independent fan drive overheating would be the inevitable result. Can you quote the wattage of that drive?  

PS. i just looked at the info plate on the Gildemeister CTX400, the motor current is 64 A (which says alot for a 3 phase too, if you adjusted that for a single phase equivilent power you're probably looking closer to 100A)…so i don't think that direct drives will be a hobby machine any time soon..

25,600W my machine is more like 750W, 3 phase

I obviously can't look into the italian motor but for a mini submarine sounds a bit heftier than a bench lathe.

If you wanted to make the case for direct drive lathes, none of which i've heard the like of outside the CNC example, looks like the smaller wheel to drive a bigger wheel principle still holds water for the mean time. 

 

Edited By Michael Walters on 20/09/2016 21:51:49

[MuzzerParticipant @muzzer]

Most of the machines we see in our workshops have fixed-speed motors. Some have switchable 2-speed windings but fundamentally they are designed to work directly on-line ie from 50Hz or 60Hz mains. Some of us improve the operation by using VFDs to provide some speed variation but they are still the same motors, designed for 1500rpm (4 pole) or 3000rpm (2 pole) base speed.

"Proper" grown-up machine tools, (sorry, "machining centres&quot have motors that are optimised to operate over a wide range of speeds. If you think about it, the alternative would be to use fixed speed motors and (messy) automatic or variable ratio gearboxes. Generally, as noted above, motors have a constant torque region (at speeds up to base speed) which is limited by the magnitude of the magnetic flux and electric currents; and a constant(ish) power region between base speed and some upper limit (usually mechanical eg burst speed).

Motors that are intended for driving machine spindles usually have a fairly wide range of operating speeds eg 0-10000rpm or higher, with a constant power speed range of 5 or 6 to 1. They can then drive the spindle through a fixed ratio or possibly even directly, coaxial with the spindle.

Interestingly, the operating characteristics of these machines are actually very similar to those required for vehicle traction (eg hybrid and electric vehicle drives) and of course the result of coupling an IC engine to a multi ratio gearbox actually results in a very similar overall speed-torque curve – namely constant power. So my earliest experience of electric vehicle development involved the use of spindle drive motors directly coupled to the prop shaft. We used the Indramat motors (rated at 35kW "S1" continuous duty) to develop peak power of 75kW and over 800Nm of torque (more than the Dodge Viper) in our applications in buses and vans.

"Frameless" motors are simply supplied without the housings, bearings etc that make them self-contained, stand-alone motors. But if you are integrating them into a complex machine, you can save a lot of space, mass and cost by providing those elements yourself. It just takes a bit more engineering to do it.

There's nothing magical about the direct drive machines in these Korean washing machines. However, they get rid of extra bearings, belts etc and presumably are quieter and suffer less wear as a result. I have a Samsung direct drive washing machine (with heat pump etc) that claims a 10 year warranty on the drive system. Generally speaking, if you want reliable white goods you are going to be buying either Korean or German.

Murray

[Neil WyattModerator @neilwyatt]

Hobby direct drive isn't around the next corner, it's been left behind at the last one!

http://www.arceurotrade.co.uk/Catalogue/High-Speed-Spindles-Motors-Inverters/ER20-High-Speed-Spindle-11kW-148HP-24000rpm

Neil

[Author]

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