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Switched Reluctance motors

Discussion in 'Technical' started by ggr, Jul 12, 2015.

  1. ggr

    ggr Roadster R80 537, SigS P85 29

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    I recently heard something that made me do a little research into Switched Reluctance motors. Here's an article. It sounds like they'd be perfect for something like electric cars, since they are slightly more efficient than AC induction, and can be cooled more easily. But I really don't know much about this area. Can anyone comment on their applicability or otherwise for Tesla?
     
  2. wart

    wart Member

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    Another article here that goes into some pluses and minuses from an EV perspective. It does seem like we're likely to see someone put an SR motor in an EV before too much longer. I'm curious about the handheld Dyson vacuum that was mentioned in this article; specifically I wonder how loud it is, and if the noise is the reason Dyson didn't continue with it.
     
  3. rolosrevenge

    rolosrevenge Dr. EVS

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    I'd be reluctant to switch to those motors.
     
  4. pbceng

    pbceng Member

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    I work with SR motors as part of my job and it is true they do have a reputation for NVH issues. The problem is not specifically that they're noisy, but that they produce pure tone noise which is very noticeable and considerable design effort is require to eliminate the problem. Having said that it is quite feasible to produce a motor that is about the same at low speed and much quieter at high speed than an induction motor.

    The losses in an SR machine tend to be concentrated in the stator which makes them easier to cool, also the rotor is lower inertia. Constant power speed range is typical twice that of an equivalent induction design and while the peak efficiency is lower, the high efficiency speed band is much wider.

    Mechanical construction is simpler, but the achilles heel has always been in the electronics. Because the power is switched rapidly from one set of coils to the next about double the mean current flows through the electronics pack making it more expensive, and the competing requirements can be a lot higher.

    Like everything there are advantages and disadvantages but in theory SR machines could offer a lot to an EV. Induction motors however are a commodity item these days and hence dirt cheap!
     
  5. pbceng

    pbceng Member

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    I should have also added that they're DC motors - they don't require AC!
     
  6. jscholl

    jscholl Member

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    Does this mean that you could just drop the AC inverter, or would you likely need some sort of DC-DC converter between the battery and an SR motor?
     
  7. Ingineer

    Ingineer Electrical Engineer

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    They are not AC (do not require a full reversal) but most definitely do require inverters, they cannot run off pure DC. In fact, The semiconductor requirements are about the same as for BLDC or induction.

    SR is ideal for an EV and can reduce the need for as much gear reduction as needed for AC induction. This could lead to more reliable drive units in Tesla's case.
     
  8. pbceng

    pbceng Member

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    SR machines do require switching electronics but not inverters. They do also do work directly off DC - the electronics switch the DC across each of the coils in turn to attract or repel the rotor poles. There are several variants of electronics and winding patterns proposed or in use which have different cost/performance attributes but typically the electronics cost and the software are the biggest issues.
     
  9. okashira

    okashira Member

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    Inverter control will still be preferred. Switching DC will bring you back to a cheap BLDC motor and it would be noisy like a cheap hobby motor.
    Field oriented control should still apply to SR motors and Ti is working on their solution.

    I guess technically, you could even make a brushed SR motor. that would be interesting.

    I dont fully understand the performance benefit of SR other then reduced core losses like an induction motor and no $$ magnets.
    But I think a motor that combines permanent magnets and reluctance will offer better torque.
     
  10. kennybobby

    kennybobby Member

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    i've always equated SR motors to stepper motors, which make their max torque when not moving (all windings energized to hold position), and make little to no torque when actually moving from one position to the next.

    Nearly useless for anything that requires torque and motion.

    Lowest cost to produce but a sorry excuse for a motor... Look up the tethered satellite first mission for an example of a $B experiment failed by a crappy SR/stepper motor.
     

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