Most motors seem to have the same proportions of length to diameter so there must be some theoretical principal there. However, I once operated a milling machine that had what was called a Pancake motor that drove the feeds. It was large diameter and only about 2 inches deep. Then there is the direct drive washing machine.
Any website I should read to clear this in my mind.
I designed, built and 'raced' an electric motorbike with the BVS and used a 24V DC motor that I believe was originally used in some aircraft application.
At the time I was racing, a young lad called Cedric Lynch designed, build and raced his own two wheeler but also designed and built his own 'pancake' motor, that performed way better than pretty well anything available commercially at the time.
Only one person ever beat him ... and that was just though (my) 'luck'. ;-)
Cheers, T i m
I don't know for sure, but it is easy to see how pancake motors might be a favourable geometry in some applications where space is tight for other reasons.
But in a pancake motor, I suspect that both the armature and stator windings might be more difficult to make (and perhaps need more copper) than in a conventional geometry. There is also a bit more scope for "flux leakage". I suspect the conventional geometry will have arisen from manufacturing considerations. It is also now to some extent "locked in" because there are standard frame sizes.
If you make a "long, thin" motor then the larger separation of bearings and thinner shaft give you more issues with vibration, especially at higher speeds.
Not really. There is no perfect size or shape for an electric motor - think linear motors!!!
On general a given weight of iron will produce a given peak force, before the iron saturates so you can get more *torque|* with a larger
*diameter* motor for the same amount of iron, BUT it wont rev so highly so its power won't be any higherLow RPM high torque - or high RPM low torque. If you dont want a gearbox you may well tend towards pancake.
long thin motors rev very highly and can be very high power to weight - but you may need a gearbox to get down from 100k plus RPM
Well it all depends on what sort of motor it is. All use magnetism in the end and how the poles that repel and attract are arranged makes many shapes possible. Some direct drive motors operate via locally generated dc or different frequency ac. Thos generally are used when speed differences are required, unless the device has more than on motor, or has switch windings to make it change speed and torque. Brian
"Brian Gaff" wrote in news:p64r78$g6p$ snipped-for-privacy@news.albasani.net:
I was thinking of the "leverage" effect of the magnetic field being greater at a bigger radius - or more of it if the armature was longer.
yes, but thats just torque. Bigger diameters can't rev as much so teh power generally is no better. It just avoids the need for a gearbox, thats all
I guess that can depend on the design of the motor.
For example, if you had a PM DC motor and were able to back off the power of the permanent magnets using a coil (coils), then you could allow the motor to rev higher once you had made best use of the lower rev torque.
It certainly seemed to work well from what I saw as it sailed past me ... stuck at max rpm on my conventional PM motor. ;-(
Cheers, T i m
Larger diameter motors have lower max speeds above which the forces on the armature will break it. Larger diameter motors also allow space for more poles giving lower speed higher torque characteristics. To increase torque with a given diameter, the motor body can be made longer.
Obviously, there may be physical constraints on fitting a motor into a given space too. Some mains tools/appliances with limited space will use a DC motor with a rectifier, because it enables replacing the field windings on what would have been a universal motor with a permanent magnet which can be made significantly smaller than a set of field windings, and reduce the outer diameter.
Most elecric induction motors run at near synchronous speed or half or one third. The speed they run at is determined by the number of (pairs of) poles. Either 1, 2 or 3, Low speeds are obtained by use of pulleys or gearboxes with these motors.
However"pancake" motors have many more poles and hence run much slower. It's become feasible to have slow speed motors with many permanent magnets/poles when neodymium magnets were developed. This can do away with the neccesity for gearboxes. They started off very small but now there are bigger ones.
Not only that but the use of the latest rare earth magnets means less copper can be used for a more efficient motor or else the space within the motor taken up by *both* rotor and stator windings can be concentrated to the stator (DC Brushless motor example) allowing even heavier gauge wire again than the reduced turns that the much stronger magnetic field produced by powerful rare earth magnets allows, resulting in a considerably more powerful motor for a given volume or mass.
It's the use of rare eath magnets in those tiny motors used in drones that allows them to achieve more useful endurance times out of their LiPo battery packs (much greater power to weigh ratios out of what generally makes up most of their mass).
Indeed, I'm surprised that they haven't replaced the complex mechanical transmissions that seem to still curse most electric cars today by being incorporated as part of each wheel where the transmission is entirely heavy duty cable (with a specially flexible section to feed the power past the suspension to the hub motors), reducing the 'gearbox' to nothing more than a sophisticated program controlled heavy duty switch mode converter. This might not suit the handling demands of a more extreme performance road car but it should do nicely for a typical family saloon or small runabout.
Given the huge environmental costs of rare earth element extraction it?s maybe no bad thing that they?re not being used in the booming electric car market. If Tesla can manage without them I reckon that?s good enough for most of us.
Additionally, ?motor in hub? adds hugely to unsprung weight (which isn?t supposed to be a good thing). I appreciate that this might be mitigated by the use of rare earth magnet motors but that brings us back to the environmental problems again.
As I?m sure you know, ?rare earth elements? aren?t actually rare, just fecking difficult, dirty and expensive to extract.
Tim
ht (which isn?t
?t actually rare, just
unsprung mass kills road holding. You could always use 2 inboard motors rather than 4 in wheels.
NT
If the motor mostly replaces the shaft and disc brake its a tossup whether its heavier or not
I deliberately made the caveat about their unsuitability for high performance road cars on account of the 'unsprung mass' issue but I feel that a fully integrated hub motor will weigh little more than the steel wheels currently used by most saloon cars today (and may possibly prove to be slightly lighter through the use of suitable materials).
They'd be a horrible compromise for a high performance car but most likely a more than acceptable compromise for a 'standard electric road' car, considering the elimination of the weight and expense of a klunky space consuming mechanical transmission system.
A fully developed active energy recovery suspension system could ultimately overcome this problem of 'unsprung mass'. It's also worth remembering that the suspension components themselves (spring, damper and drive shaft on each driven wheel) form a part of this 'unsprung mass'. Also, let's not forget the mass of the disk brake assemblies which, with regenerative braking, can be reduced in size thus offering yet a further reduction in 'unsprung mass'.
This latter weigh saving would require an emergency electric backup in the event that the normal regenerative braking system suffers a failure that could result in burning out the downsized disk brakes on a long and steep descent. I'm sure such risks can ultimately be addressed if given sufficient thought and development. :-)
a motor of such low weight would have little performance
NT
eight (which
en?t actually rare, just
the wheels are still required. A low speed motor must necessarily be large, and that means a lot more weight than the wheels. The resulting heaviness is unsuitable for ordinary road holding performance.
no, they're not be compatible with high performance at all
they'd increase unsprung weight greatly. Good enough for a low speed bus.
it can't
suspension can't be eliminated
not really, the friction brakes still need to stop the car from top speed.
you can't back up a safe braking system with an inherently unsafe one
that's easy to work around with electronics. Use regenerative braking. Also monitor brake temp, and warn then stop the car if too hot.
NT
DerbyBorn wrote in news:XnsA88ADCDEAE542TrainJPlantntlworldc@81.171.92.236:
Thanks all - very enlightening stuff
Yes - remember a pretty advanced design of 1/4" tape recorder. Had twin capstans with direct drive. And they stopped turning when the tape stopped running. They were pancake motors. Had a very fast start up time. That would explain it.
Tesla are using rare-earth magnet motors in (some of?) their Model 3.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.