Why does a Tesla car use an AC motor?

As Tesla cars are electric vehicles and use rechargeable batteries for Power supply, they could have used DC motors which may run directly from the batteries. Use of DC motors would reduce a lot of circuitry (power inverter circuits that are required to run AC motors from DC supply). And reduced circuitry would mean reduced cost, weight etc. Then what are the advantages of using an AC motor here?


Best Answer

  • Accepted Answer

    Not only Tesla, but most of the electric vehicle manufacturers us AC motors for traction. Though there isn't one size that fits all. Let us start with different types of electric motors that can be used in an electric car.

    Types of electric motors used in electric cars

    1. Brushed DC motors (DC series or shunt wound motors): These motors are mechanically commutated, i.e. a commutator and brush assembly is used for the current commutation to the rotor. Although, DC series motors provide high starting torque and can be suitable for traction, the commutator and brushes tend to wear out quickly due to their continuous contact. Also, brushed DC motors are quite less efficient than BLDC motors or AC motors.
    2. Brushless DC (BLDC) Motors: Clearly,they are brushless, so the commutator-brushes assembly is absent. Instead, here permanent magnets are used on the rotor and the stator winding is electronically commutated. Along with high starting torque, these motors also have better efficiency. BLDC motors are used in many electric vehicles.
    3. Permanent Magnet Synchronous Motor (PMSM): A PMSM motor is quite similar to BLDC motors, so that sometimes both the motors are considered to be same. But the main difference in these two types is the electric signal that is used to drive them. BLDC motors are driven by trapezoidal waveform, while PMSM motors are driven by sinusoidal waveform. PMSM motors have high performance characteristics, and hence are preferred by many electric car manufacturers.
    4. Three phase AC induction motors: Although these motors do not have high starting torque under fixed voltage and frequency operation, the torque characteristic can be easily altered by V/f control method. Also, these motors cost effective than BLDC and PMSM motors.
    5. Switched Reluctance motors: These are also brushless motors in which rotor is usually made up from laminated steel. These motors are also finding their place in new generation electric cars.

    So on what basis a motor is selected for an electric car?

    Major factors deciding the selection of a motor for an electric car are -

    1. High output torque under low speed
    2. Easy speed control
    3. Reliable performance
    4. Efficiency
    5. Cost effectiveness etc.

    Considering the above factors, most automakers prefer BLDC, PMSM or three-phase induction motors. Brushed DC motors are almost out of scenario when it comes to electric cars due to these reasons - efficiency about 75 to 80 %, periodic maintenance required for commutator-brushes assembly, other commutation problems such as arcing and heat generation, etc. Whereas, BLDC, PMSM and induction motors can be designed to provide efficiency more than 90%.

    What kind of electric motor does the Tesla car use?

    Well, not one. For example, Tesla Model S uses AC Permanent Magnet Synchronous Motor at the front wheel and AC induction motor at the rear, both motors with variable frequency drive (according to the owner's manual which can be found on their website here).

    So, the specific answer for 'what kind of motor an automaker selects for their car' can be given by their Engineers only.

    Kiran Daware
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