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How does a variable rate inverter work in a Tesla car?

  1. Jun 1, 2017 #1
    So I recently learned that Teslas use AC induction motors rather than DC motors. Now I always assumed that DC motors would be easier to control the speeds of vs. induction motors, so it surprised me that they used an induction motor in the Teslas. Now in my old days I had heard that induction motors are limited to the speed at which their input power frequencies run at. So to change their speed, you need to change their input power frequency. I heard that they use a variable frequency inverter system to achieve the different frequencies.

    Now from my understanding of inverters (limited), they produce a very square-wave AC pattern vs. a smooth sine wave AC pattern. The square wave is apparently not well liked by delicate electronics, but am I right in assuming that an electric motor wouldn't care if it's a square wave vs. a sine wave? If so, then the inverter probably doesn't have to be too sophisticated and need circuitry to convert the square into a sine wave?

    Also how do they vary the frequency of the inverter?
  2. jcsd
  3. Jun 1, 2017 #2
    When looking at 3 phase BLDC type motors ( like a real servo motor) you do often see a square wave pattern at lower power, that is probably where you have seen the Square wave, however they can also be driven with PWM Sine Wave, as are induction motor Variable Frequency Drives.

    In a way you can think of a VFD as a synthesizer - allowing the controller to vary many aspects of the power fed to the motor - a basic inverter topology (circuit) can be used for ether square wave or PWM. Frequency, Voltage, and Rotation, can all be changed independently to drive the motor at the optimal operating point - and can be used to control dynamic braking - so bi-directional power flow. The most basic PWM method here is referred to and Volts per Hertz ( V/Hz) where the magnitude of the sine wave is proportional to the output frequency. Example a 480VAC 60Hz motor - will be run at 48V and 6 Hz... the current LIMIT ( max torque) is about the same across the operating range of the system.

    The motor DOES "care" about the square wave, it is hard on the insulation (Voltage Spikes/ dV/dt) and generates harmonics (= losses) especially at higher currents and voltages, also square wave at high current is not good for any of the systems involved. So a good PWM (sine wave) is better for the motor and other systems, however it is less efficient for the inverter. The motor as an inductor - resists changes in current.
  4. Jun 2, 2017 #3
    So am I to assume that even a cheap modified sine wave inverter would be bad for the motor, as it's not a pure sine wave?
  5. Jun 3, 2017 #4
    There are many factors that go into a commercal product, but you can build / modify an iinverter that would serve most purposes. In general the larger the motor the better an inverter you will want / need. In a Tesla - for example, the battery is the major investment, so many design desisions may put the care of the battery as one of the top considerations - ripple current, for example may be a major concern ( or not at all).

    So "Bad for the motor" will have a number of factors - it used to be inverter grade motors to deal wiht the the affects of the VFD, but as the drives have gotten better you see this type motor less and less. (Also - the ability of the average motor to deal with the drives noise, has increased as well).

    Building a 3 phase motor drive from scratch - is a pretty good project, if you have a good application ( e-vehicle for example.)
  6. Jun 3, 2017 #5

    jim hardy

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    You might enjoy this introductory article . Even though it applies to locomotives not Tesla roadsters the basic principle is the same.

    And yes, motor designers are aware how non-sinewaves affect their motors and can design to handle those effects by better insulation and heat removal .

    One of the effects most interesting to me was the phenomenon of "Shaft Currents".
    Small currents flow along the length of the motor shaft and return through the frame . On its way through the bearings it eats them up by arcing.. Before VFD's it was problematic only in big motors more than about 400 hp .
    When VFD's moved into household appliances they brought shaft currents along with them . The appliance industry quickly learned from the power industry to insulate the bearings.
    So your humble washing machine motor "Ain't what it used to be" .

    Try a search on "Inverter Duty Motors" .

    old jim
  7. Jun 4, 2017 #6


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