What makes a high power to weight motor? - Page 1
Forum section: electronics / projects

author: Eagle_85
author_rank: Contributor
timestamp: August 24, 2020, 09:14:09 am
content: Hi. I was wondering what factors drives a high power to weight ratio motor? I was thinking you wold need high RPM, strong permanent magnets, brushless, active liquid cooling, and maybe a ironless core. But then i see on wikipedia that Emrax 268 is apparently the highest power to weight motor out there and it is a relatively low RPM axial flux synchronous motor. I could not find how the core was constructed but i guess it is a traditional iron core.

author: Benta
author_rank: Super Contributor
timestamp: August 24, 2020, 10:48:29 am
content: Highest power to wieght ration is usually found in universal motors as used in power tools.EDIT: Strike that, I didn't realize that we're talking 200 kW motors.

author: filssavi
author_rank: Frequent Contributor
timestamp: August 24, 2020, 12:05:19 pm
content: I don’t have part numbers or such but the highest power densities are achieved with PMSMs and BLDCs (in that order).First of all power is speed*torque, in theory you can increase any one of them to increase power density, however torque is somewhat directly related to both volume and weight of the machine (the higher the torque the higher the current needed on the stator, and thus more iron to avoid saturation, same story for the copper, more current = more copper).Thus in order to increase the power density we need to increase the speed, while this needs higher voltages (complicating stator insulation system) it is not so strongly related to mass or volume (they still are though). Analogous considerations apply also to the rotor, both electrically and mechanicallyThis obviously excludes all machines with slip rings/ brushes as these greatly limit the speed of operation for anything but The tiniest machines (Like power tools)2) no conductors in the rotor to worry about having copper on the rotor is also major problem, the losses are extremely hard to cool, thus the resistance of the rotor windings must be much lower than what you would use typically on the stator. This is a challenge for anything but PM machinesTo be more specific however You must probably talk to aerospace companies, as that is basically the only field where power density has been a concern for a long time

author: oschonrock
author_rank: Regular Contributor
timestamp: August 24, 2020, 02:56:04 pm
content: Quote from: filssavi on August 24, 2020, 12:05:19 pmGood answer.I have worked with servo motors in high speed mechatronics / motion control / automation. These are typically a class of PMSMs which, in addition to having very high power to weight ratio, are optimised for low rotational inertia and hence high dynamic response. The PMs in servos are rare earth... think $$$ £££It really depends on your application. There is much more to a motor than power/weight.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 24, 2020, 04:03:29 pm
content: I am not very well versed in electric motors.I have never been able to tell the difference between a PMSM and a BLDC motor.Same for 3-phase stepper motors, They're just the same, just tend to have more poles.11kW per kg does look like a decent number.https://en.wikipedia.org/wiki/Emrax_268But don't believe every advertisement you read.Here, amongst a whole lot of other blabla and nice looking pictures they claim that 260/50 = 5.2kW/kg isQuoteThere are differences in motors. Old stepper motors sometimes are of the "variable reluctance" type. These do not have any permanent magnet in them, and rely on magnetisation of a chunk of iron in the rotor by external field from the stator coils. These tend to have low efficiency.In the "classical explanation" of a stepper motor, a permanent magnet is used in the model, but that's mostly for simplicity. In real life stepper motors tend to be of the "hybrid" variety. These have a relatively small permanent magnet, combined with a few chunks of (laminated) iron. Everyone and some more builds steppers like these, and it's probably the best power to cost ratio.Power to cost ratio is almost always far more important than power to weight ratio.

author: Benta
author_rank: Super Contributor
timestamp: August 24, 2020, 04:34:22 pm
content: Quote from: Doctorandus_P on August 24, 2020, 04:03:29 pmThere isn't any. The difference is the drive strategy.

author: filssavi
author_rank: Frequent Contributor
timestamp: August 24, 2020, 08:44:21 pm
content: There most definitely is a difference between true “BLDC” and PMSMA bldc I has a trapezoidal back EMF while a PMSM has a sinusoidal one, this is due mainly to how the stator is wound. That said there are a lot of second order effects that makes it so that most BLDCs nowadays are mostly crappy PMSMs.Of course If you drive a PMSM as a bldc and vice versa they will work ( not as well as they would if driven properly, however they will spin and produce torque)

author: Benta
author_rank: Super Contributor
timestamp: August 24, 2020, 10:50:33 pm
content: Quote from: filssavi on August 24, 2020, 08:44:21 pmCompletely true. The field can be optimized for one or the other according to the shape of the stator poles. That doesn't change the fact they are the same motors from a basic functional point of view.

author: paul8f
author_rank: Regular Contributor
timestamp: August 25, 2020, 09:55:00 am
content: Simply bring the motor to Mars or the moon. The lower gravity will mean the motor will weigh less, but still be capable of the same amount of work!Sorry, I couldn't resist..

author: Eagle_85
author_rank: Contributor
timestamp: August 25, 2020, 10:35:02 pm
content: Quote from: filssavi on August 24, 2020, 12:05:19 pmThat was my thinking too. But as i said the Emrax 268 is listed as the highest power to weight rated motor and it has a max speed of only 4500 RPM (or 5500 RPM for a few seconds). That is not a veary high speed after all. It is even less than most ICE engines.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2020, 11:36:02 pm
content: FWIW, the physical reason underlying it, is achievable flux density.We can achieve quite high flux density with or without core, if we deliver enough current. But dropping 10kA through even very cold silver, will make it not-cool very quickly. That is, we need a metric shitload of ampere-turns to do any work at high flux density.So we are mostly confined to work within the limits of core materials -- having a saturation flux density around 1.8T or less.The most thrust we can get, is to distribute that 1.8T over as large an area as possible. Flux density is also energy density in air (namely, in the air gap between rotor and stator), and energy density is also pressure. Anywhere we have a difference in pressure, we have net force.1.8T is equivalent to (1.8T)^2 / (2 * 1.257e-6 H/m) = 1.28MPa, or about 13 atm. Which is higher than, say, a blowgun powered by your average air compressor (~6 atm), but much, much less than a gun (~1000 atm).Which, speaking of guns, this is why gauss guns are so unimpressive.Now, the neat thing about the electromagnetic force is, it's not a scalar pressure -- it's a vector. Which means we can have this pressure applied tangentially as well as radially -- we aren't limited to pushing pistons in cylinders, we can spin them on axis, too! You can imagine if we have 1.8T in the 0° position (in the air gap of a radial flux machine, with cylindrical rotor and stator) and none at 90°, and the rotor has the opposite, then the rotor will be torqued so as to balance the fluxes. And the torque is proportional to the area of the rotor's cylindrical face, and the flux density.We can also subdivide the rotor and stator into smaller magnetic domains -- multiple poles. A single winding each gives two poles (N and S). A pair of windings, placed in quadrature and wired sequentially opposing, gives four poles (and divides the electrical frequency by half, hence 60Hz machines run synchronous at 1800 RPM -- this is the most common configuration). The field alternates more frequently around the circle, but it's the same flux density, and if anything there are more chances to waste it (flux leakage) because the distance between poles is smaller.(When the distance between poles is comparable to the air gap, the flux density drops off extremely quickly across that gap; very small gaps are required in that case, and even then, the available power may not be much. This isn't so much a problem with motors (because no one uses motors with this many poles), as it is for alternators, which have been used to generate higher frequencies for induction heating and even radio transmission purposes! Simply using more poles is feasible up to the low kHz, but they used a different strategy to push toover 100kHz.)I'm... pretty sure, but I can't quite convince myself that: torque depends on area period. It's tempting to think of multiple poles as teeth on a gear, but gear teeth are limited by the physical strength of materials and the the engagement width and depth; here, the magnetic "teeth" are weaker simply by the fact that they are smaller, i.e. they have less area each, at the same flux density. (But flux density will also decrease with more poles, due to leakage.)So, you can imagine constructing a machine, around the air gap where work is done. We use steel and copper, reaching 1.8T in the air gap. It is rotationally symmetric (so, a disc, cylinder, or other surface of revolution). We need as much steel going away from the gap, as the width of the poles -- that is, we must maintain core cross-section as the core wraps around the copper windings. We can potentially save on core material if we use more poles (the core can have less height above/below the air gap), so long as leakage doesn't compromise performance.The resistivity of copper, available cooling, and desired efficiency, sets how much copper is needed, and thus how much space the core must wrap around. (This doesn't much depend on number of poles, as we normally divide the core into many slots anyway, distributing the windings between them to get a smooth magnetic field that minimizes harmonics and torque pulsation.) We can pack copper more efficiently using square wire (or even more specialized shapes, I suppose), and we can dissipate more power (at lower efficiency) if we have liquid cooling, or less power (higher efficiency) if we cool it a lot (copper's resistance has a strong tempco; the core does too, so that we can potentially run a little higher flux density at very low temperatures).Though the power we'll spend onthat muchcooling, won't actually win us efficiency overall, plus the cryocooler takes up a lot of space, ruining our overall power density (though maybe that's still a win in some very special applications).Of course if we're going to that trouble, we might as well use superconductors. We can get nearly zero losses in the windings, but we still have to deal with core losses, which will be very expensive indeed to keep that cool. (It's probably not feasible to keep the windings that cold, separate from the core.) At least we can use much less wire, so the core can be somewhat smaller. We might even use a lower-loss material, even at the expense of flux density or permeability. But then again, what do we really need core for, at all? If we have almost no loss in the windings, let's just use all windings, and dump kiloamperes through it because we can! This has some consequences: we still need some structure to support the rotor and stator, and we don't have air gap so much, because it'sallleakage -- there's no core to confine flux, we need to get the windings in just the right places. Even then, we'll necessarily have poorer performance (less torque) because of that leakage. So we'll need to run it a bit higher to compensate. (Fortunately, 2-10T flux density is quite reasonable even for high-temperature superconductors!)This doesn't give much technical detail, but it seems they are indeed pursuing it:https://www.engineering.com/AdvancedManufacturing/ArticleID/19454/Fully-Superconducting-Motor-Prepares-for-Testing.aspxTim