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Motor Spins Backwards During Regen?

Discussion in 'Model S: Battery & Charging' started by gavine, Oct 17, 2014.

  1. gavine

    gavine Petrol Head turned EV Enthusiast

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    #1 gavine, Oct 17, 2014
    Last edited by a moderator: Mar 13, 2016
    There has been discussion about how the regen works and debates about whether the motor turns backwards during regen. That sounds counter-intuitive to me and my DS told me during delivery that it does. Maybe it's just easier to make people understand it by claiming this.

    Anyway, at 25:50 in the video below, they show a graphic of regen working and the motor continues to spin forward. Just thought I'd set the record straight for those who are in doubt or unsure -->

     
  2. ElSupreme

    ElSupreme Model S 03182

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    When people are saying the motor works backwards they are not commenting on the direction of the motor spin. As that is easily determinable as it is directly coupled with the wheels.

    What they mean is the motor normally converts electricity into torque and rotational motion. It can also work in reverse converting torque and rotational motion into electricity.
     
  3. doug

    doug Administrator / Head Moderator

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    It's a little bit technical and I'd guess the DS doesn't really understand it.

    I think maybe the simplest while still mostly accurate way to look at it is when accelerating the phase of the pulses from the inverter are ahead of the rotation of the motor, thus providing a positive torque. During regen the phase lags behind the rotation, providing a negative torque.


    These are AC induction motors. The reversing the polarity type pictures are perhaps more applicable to DC motors.
     
  4. jaanton

    jaanton Roadster NA #1026

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    It doesn't actually spin backwards, but magnetically it does. Link below to the Wikipedia page on slip in induction motors.

    The easiest way to think about this is that the three phases of power going into the motor set up a rotating magnetic field and the physical rotor wants to match the magnetic field. To the extent that it does not match up, it's slipping behind the field and that's called slip. Power draw and torque relate to slip. If the field and the rotor match exactly there is no slip and negligible power. But with a variable speed/frequency motor, the controller can slow the magnetic field rotation to be slower than the physical rotor, then you have slip being negative which magically again the rotor wants to match the field and power comes out of rotation and electrical power comes out of the motor. In the diagram about slip in the page below you can see the torque curve going negative with negative slip.

    That torque diagram is really worth thinking about. You can see there is a point of maximum torque which relates to a small percent of potential slip. Clearly the controller has to be very careful not to get the magnetic field rotation too far ahead of the physical rotation. To keep maximum torque as the motor speeds up, the controller must keep the slip at just the right ratio.

    Induction motor - Wikipedia, the free encyclopedia
     
  5. doug

    doug Administrator / Head Moderator

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    This explanation matches mine, but I wouldn't say it's "magnetically spinning backwards" either.

    Again it's the rotating magnetic field either leading the rotor (positive torque, pulling it to speed up) or lagging behind it (negative torque, pulling it to slow down).
     
  6. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    To have negative slip to turn the motor into a generator, the electrical (magnetic) "spinning" of the stator does not go negative or backwards, it only goes to a value less than the mechanical "spinning" of the rotor. Nothing goes backwards to regen or turn the motor into a generator; the electrical rotation of the stator just has to lag the mechanical rotation of the rotor, rather than the normal lead mode for a motor.


    From the Wiki article above:

    Slip, s, is defined as the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm or in percent or ratio of synchronous speed. Thus
    Slip.png
    where n[SUB]s[/SUB] is stator electrical speed, n[SUB]r[/SUB] is rotor mechanical speed.​


    To make s positive for a motor, n[SUB]s[/SUB] must be larger than n[SUB]r[/SUB], to make s negative for a generator, n[SUB]s[/SUB] must be smaller than n[SUB]r[/SUB], but does not have to be negative (backwards electrical rotation).
     
  7. tom66

    tom66 Member

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    One interesting characteristic of Model S drive train is the accelerator pedal is essentially a torque command. So although an AC induction motor does not typically maintain equal torque over its speed range, the controller compensates for this by keeping the ac drive current the same for any given frequency (until the point of maximum power where the torque rolls off and the accelerator pedal input becomes a power control.)
     

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