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What makes a high power to weight motor?
Eagle_85:
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.
Benta:
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.
filssavi:
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 mechanically
This 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 machines
To 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
oschonrock:
--- Quote from: filssavi on August 24, 2020, 12:05:19 pm ---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 mechanically
This 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 machines
To 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
--- End quote ---
Good 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.
Doctorandus_P:
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_268
But 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 is
--- Quote ---five times more than comparable drive systems.
--- End quote ---
There 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.
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