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Voltage of Model S Motors

Discussion in 'Model S' started by MsElectric, Jul 21, 2015.

  1. MsElectric

    MsElectric Active Member

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    So the remark that the P90DL consumes 1,500 Amps at peak acceleration got me thinking about the voltage... Do all Tesla motors consume the same voltage regardless of 70D, 85D, P90D etc., and is the torque controlled by the Amps fed into the motor? Is the voltage the same for the front and rear as well?
     
  2. apacheguy

    apacheguy Sig 255, VIN 320

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    Is the motor voltage not controlled in part by HV pack voltage?
     
  3. MsElectric

    MsElectric Active Member

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    Isn't the battery voltage around 375-400? So does this mean the electric motors gets whatever the voltage is of the battery pack without any step up or step down? For the 90kwh (and eventually higher capacity batteries) pack does that mean the motor will receive a higher voltage? I assume the torque of the motor is controlled by the amount of Amps of the current? Just trying to understand the science behind the battery pack electricity and the motors.
     
  4. Yggdrasill

    Yggdrasill Active Member

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    #4 Yggdrasill, Jul 21, 2015
    Last edited: Jul 21, 2015
    The motors are fed with three phase AC power, whereas the battery outputs DC power. The inverter electronics (found right next to the motor) takes the DC and turns it into three phase AC. And I'm 90% sure that the torque is controlled by varying the voltage (and consequently amperage) of the three phase AC.

    Of course, if the inverter only receives 400 kW of DC power from the battery, it can only output 400 kW of three phase AC (less when you consider losses). Upgrading the fuses and main contactors means you can feed more DC to the inverter, and then the inverter can take this power and crank up the voltage of the three phase AC.
     
  5. mspohr

    mspohr Active Member

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    I always assumed that the voltage of the battery packs were all the same with a nominal voltage of 370 volts. When charging, I have seen the battery voltage goes from about 350 to 400 volts. I don't think it would make sense to have a voltage converter stage. I assume that the motor controller just takes whatever voltage the battery is delivering (range 350 to 400 volts) and adjusts the amperage to control the power.

    Has anyone noticed that their battery pack has a voltage different than nominal 370 (350 to 400) during charging?
     
  6. Yggdrasill

    Yggdrasill Active Member

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    The voltage of the battery pack will vary with state of charge. Here's a graph showing how the voltage changes for a generic cell:
    18650-2200mAh-discharge-curve.jpg

    The battery pack has a bunch of these cells in series, thus boosting the voltage to a much higher level, but each cell will act pretty much like this, and when the battery is almost empty, you will have a much lower voltage on the battery pack. When charging, the voltage one would measure on the battery would be higher than if you are not charging, this is because you need to apply a higher voltage to the battery to get the current to flow into the battery and thus charge it.
     
  7. mspohr

    mspohr Active Member

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    I assumed that the Tesla battery packs were made up of sets of 100x 3.7 volt batteries in series giving a nominal 370 volts for each set. If each battery has a 2000 to 3000 mah capacity, then 100 of these cells would have a capacity of about 7 to 10 kwh. "Larger" battery packs would have more of these sets connected in parallel. I don't think they change the number of cells in series to change the voltage since that would complicate charging and the motor control electronics.
    I was wondering if anyone had noticed that their Model S battery pack had a voltage different than the nominal 370 volts that I have observed.
     
  8. Yggdrasill

    Yggdrasill Active Member

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    I don't recall the exact number of cells in series, but it may be close to 100. As the graph shows, the battery pack voltage would with 100 cells in series vary between ~350V and ~400V depending on state of charge. (The far left and the far right of the graph would be hidden in the inaccessable margins at the top and bottom of the battery.)
     
  9. MsElectric

    MsElectric Active Member

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    So basically as the SOC changes the voltage is more or less around 350-400V to the inverter and the inverter converts DC to AC for the motor. The torque is then managed by adjusting the Amps to the motor by the inverter?
     
  10. tom66

    tom66 Member

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    #10 tom66, Jul 21, 2015
    Last edited: Jul 21, 2015
    It's 96 series cells for the 85 and 90kWh battery and about 84 series (don't quote me) for the 60 and 70kWh packs

    Torque is varied by motor current which is defined by voltage amplitude and frequency. Frequency is varied in software. Voltage is also varied in software - by varying the width of the ON pulses. (Pulse-Width Modulation.) The drive inverter will attempt to approximate a sine wave, using the motor's parasitic inductance to reduce ripple current.

    In the case of Tesla, the waveform probably does not saturate near the peaks until the battery voltage gets low. This will be one factor in the power limit near low SOC. The other factor will be limiting voltage droop, and also reducing heating near low SOC which may impact battery lifetime.

    905ecmMFfig2.jpg
     
  11. spottyq

    spottyq Member

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    You can see the pack voltage and the corresponding SOC when supercharging.

    Supercharging Tesla Model S 60 kWh vs 85 kWh - YouTube

    To my knowledge, there's no such video for the 70kWh battery. But we've had screenshots which proves they have the same number of cells in series as the 60, so the same voltage. (It has more capacity as the 70kWh pack uses the same modules as the 85kWh pack, whereas some cells in the modules for the 60kWh were replaced with ballast.)
     
  12. djp

    djp Roadster 2.0 VIN939

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    This old Roadster blog has a good explanation on how torque is generated in electric motors - the same principles apply to the Model S. Torque is controlled by changing the frequency of the 3-phase AC waveform. The AC waveform creates a spinning magnetic field in the motor. If this field is spinning faster than the motor (higher frequency) then you get positive torque, if the field is spinning slower than the motor (lower frequency) you get negative torque (ie regen braking).

    Induction Versus DC Brushless Motors | Tesla Motors Canada
     

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