Switched reluctance motors will save the planet?

[filssavi]

I see there is a lot of confusion about different kinds of machines and how they are driven, so I will try and clarify a bit...

First and foremost we are talking about synchronous machines where the mechanical and field rotation frequency are the same, there is thus no slip, such as in induction machines.

These machines can then be classified in:

• Wound rotor: These are the big turboalternators that are used in power plants, the rotor field is generated through an electromagnet, energized with either a set of brushes (exceedingly rare nowadays) or an exciter machine (basically a rotating transformer) with a rectifier on the shaft[/ li]
  
  • Permanent magnet machines: these use permanent magnets (usually but not necessarily rare earths based) to generate the rotor field
  • Reluctance machines: these machines don't use a rotor field at all to generate torque but instead they rely on a radially asymmetric reluctance profile
  
  The first class of machines will be ignored as it is competitive only at the MVA scale.
  
  Both other classes can be then further split into two sub-groups based on what is the back-emf waveform shaped (this is determined by how the rotor is physically designed).
  
  
  • Trapezoidal: These are BLDCs and switched reluctance machines, they are typically driven by turning on only the phase that is aligned with the rotor quadrature axis
  • Sinusoidal: These are PMSMs and synchronous reluctance (or synRel or synRM). they need to be driven with a sinusoidal current profile at the correct angle for each phase
  
  
  Now that the theory is clear, the real world is much messier than that a bldc can be driven with a sinusoidal waveform, and it will have less torque ripple (though not as low as a properly designed and driven PMSM), and a PMSM will spin if run like a BLDC (though with lowish torque).
  
  Reluctance machines have few advantages over magnet ones:
  
  • Much wider speed range due to better flux weakening  support
  • They are a lot more rugged, both thermally and mechanically since the rotor is basically a solid hunk of steel
  Disadvantages are
  
  • lower power density as reluctance is much worse at generating torque than magnets (basically you need to throw more current at it, thus more copper in the stator)
  • They have typically (though not necessarily) a somewhat  worse power factor (down to 0.5-0.6 lagging) requiring larger drives
  • They have typically (though not necessarily) have a worse torque ripple
  • They are typically harder to drive since the d and q axis inductances change widely with the rotor angular position
  
  For all machines listed above the control NEEDS to know the rotor angular position, either through a sensor or with some sort of observer (sensorless control) in order to work at all they CAN'T be run as stepper machines, they do not use microstepping, they can't be run open loop and they most certainly can't be run without a drive by just sticking them in a mains socket.
  
  Now stepper motors are the red headed bastard step-child of the synchronous machine family. they are fundamentally either BLDCs or Switched reluctance machines (or a mix of both) that have been specifically designed by maximising cogging torque (which is usually very undesirable). They sell efficiency, torque ripple, torque capability and maximum speed to the devil in exchange for the possibility of being run open loop as they now step consistenty with each current pulse at the winding and hold position if energised.
  
  They are great for low performance low power and low complexity tasks, where the lack of a proper drive can be made up by just overspeccing them. however as soon as you need reliable actuation under load they must be run in a closed loop configuration, and by that poin you are more than half way towards implementing a true machine drive and might as well toss that piece of trash where it belong and buy a proper electrical machine that will have higher performance in basically every metric you can think of
  
  As for the EVs at creep, my guess is that it is a thermal limit probably on the machine drive rather than the machine itself (their thermal time constants are measured in minutes not seconds)

[Picuino]

[Zero999]

Switched reluctance motors saving the planet? No, die cast copper rotor motors will. They're much more efficient than aluminium rotor motors and work with ordinary variable frequency drives, or directly off the mains, if speed control isn't required.

https://www.copper.org/environment/sustainable-energy/electric-motors/education/motor-rotor/pdf/Three-Phase_Induction.pdf
https://www.aceee.org/files/proceedings/2007/data/papers/11a_2_069.pdf

[tszaboo]

Switched reluctance motors saving the planet? No, die cast copper rotor motors will. They're much more efficient than aluminium rotor motors and work with ordinary variable frequency drives, or directly off the mains, if speed control isn't required.

https://www.copper.org/environment/sustainable-energy/electric-motors/education/motor-rotor/pdf/Three-Phase_Induction.pdf
https://www.aceee.org/files/proceedings/2007/data/papers/11a_2_069.pdf

So they concluded that copper conducts electricity more than aluminium?
I am more interested in Figure 2, the power vs efficiency curve. So the motors are most efficient at 50% their rated power. All cars today have grossly oversized engines, and spend 99% of their time at less than 25% of their rated power. You need maybe 15 HP to go on the highway.
So having a secondary 30 HP motor would make the car more efficient by at least 5%. And would you look at that: The Toyota Prius has a 27 hp electric engine.

[Zero999]

Of course it's been known for a long time that copper is a better conductor than aluminium. What's new is improvements in copper die casting, which makes motors with copper rotors easier and cheaper to make.
https://www.machinedesign.com/news/article/21829687/industry-could-take-a-shine-to-copperrotor-motors

[tszaboo]

Of course it's been known for a long time that copper is a better conductor than aluminium. What's new is improvements in copper die casting, which makes motors with copper rotors easier and cheaper to make.
https://www.machinedesign.com/news/article/21829687/industry-could-take-a-shine-to-copperrotor-motors

Yes, that was my point. If it would be cheaper to make, they would've switched to it, so something tells me that it is not cheaper. We are talking about companies that try to shave pennies off the BOM cost of ECUs.
Copper is getting stupidly expensive in general, and machining it is harder.
The other issue is the typical upfront cost of things. If switching to copper would add 500 EUR to the sales price of a car, and it would add 3% range improvement, who would buy that? That 500 EUR is very visible, while that 3% would be a range of 360Km instead of 350Km.
If people would think about long term running cost of cars, most people would drive a hybrid.

[Zero999]

Of course it's been known for a long time that copper is a better conductor than aluminium. What's new is improvements in copper die casting, which makes motors with copper rotors easier and cheaper to make.
https://www.machinedesign.com/news/article/21829687/industry-could-take-a-shine-to-copperrotor-motors

Yes, that was my point. If it would be cheaper to make, they would've switched to it, so something tells me that it is not cheaper. We are talking about companies that try to shave pennies off the BOM cost of ECUs.
Copper is getting stupidly expensive in general, and machining it is harder.
The other issue is the typical upfront cost of things. If switching to copper would add 500 EUR to the sales price of a car, and it would add 3% range improvement, who would buy that? That 500 EUR is very visible, while that 3% would be a range of 360Km instead of 350Km.
If people would think about long term running cost of cars, most people would drive a hybrid.

I'm not talking about for use in cars, but industry in general. If the motor needs replacing anyway, it might make sense to spend a little extra to save more in the long run. Changing to a switched reluctance motor would require new drive electronics, whilst a copper rotor motor would be a drop in replacement.