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I know Tesla and others use AC motors. I read a lot and no one really provides a good answer. I know they are using an AC induction motor, and with Model 3 also an AC synchronous permanent magnet motor also referred to as brushless DC.

Basically my question is why does almost everyone opt to use frequency drive to control their motors?

I know they could use synchronous permanent magnet motors and run in from DC but they still run it on AC. What advantage are they gaining? Better control, cost, torque?

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  • \$\begingroup\$ All motors are AC inherently \$\endgroup\$ Commented Mar 10, 2020 at 1:21
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    \$\begingroup\$ AC motors are much, much more power efficient than DC motors. Also, the motor has much less maintenance required (almost no maintenance). \$\endgroup\$
    – user57037
    Commented Mar 10, 2020 at 1:27
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    \$\begingroup\$ Does this answer your question? Why does a Tesla car use an AC motor instead of a DC one? \$\endgroup\$
    – Marino
    Commented Mar 10, 2020 at 1:57
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    \$\begingroup\$ A synchronous permanent magnet motor cannot run from DC. It runs on AC and the name "synchronous" should be a clue. They call it "synchronous" because the motor rotational frequency is locked to the drive frequency. They are synchronous. \$\endgroup\$
    – user57037
    Commented Mar 10, 2020 at 2:38

3 Answers 3

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I know they could use synchronous permanent magnet motor and run in from DC

This is the problem in your understanding. You cannot feed DC into a motor with no brushes. It will lock up and not spin.

All spinning motors are AC at heart, even brushed DC motors. You need AC (i.e. currents that change direction) in some form at the lowest level to produce a rotating magnetic field to spin. It is just whether you use brushes or electronics to convert the current from your DC power source into the AC the motor needs, or whether you run it off AC directly. Think about what commutation really is, whether with brushes and sliprings or electronics, in any "DC" motor. It's just reversing DC currents to simulate AC.

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  • \$\begingroup\$ In that case there is no DC motor. Let me rephrase and ask why they input 3 phase with varriable frequency instead of DV with variable voltage. \$\endgroup\$ Commented Mar 10, 2020 at 1:34
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    \$\begingroup\$ @JiříJezdinský You still don't seem to understand. A motor can't accept straight DC unless it has brushes to commutate the DC into AC inside the motor. A motor cannot truly run off DC. If the motor has no brushes, you MUST give it AC in some form. If you only have batteries which are DC, then you need a frequency drive to turn that DC into AC. \$\endgroup\$
    – DKNguyen
    Commented Mar 10, 2020 at 1:36
  • \$\begingroup\$ @JiříJezdinský the synchronous permanent magnet motor cannot run from DC. It is a type of AC motor. A permanent magnet synchronous motor is largely the same as a brushless DC motor (BLDC motor). \$\endgroup\$
    – user57037
    Commented Mar 10, 2020 at 2:40
  • \$\begingroup\$ @JiříJezdinský DC motor are called that way because you can power them with true DC current, using simply a switch or a relay. It doesn't matter what happens inside them. Brushless motors (and AC motors which inherently are also brushless) cannot turn when fed with DC current, you must give them some modulated current. But with "AC motor" normally/often is intended "high voltage AC", i.e. the mains. A BLDC (B.rush L.ess DC) is not called, normally, an "AC motor". \$\endgroup\$ Commented Mar 10, 2020 at 6:44
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There are two main reasons for using AC rather than DC in an electric vehicle:-

  1. A DC motor has a commutator and brushes which switch power to each armature winding at the correct angular position to make the motor rotate (ie. it converts DC voltage to synchronous AC at the windings).

This causes extra loss due to friction and brush/contact resistance, and wears down the brushes and commutator which eventually have to be replaced. Wear is higher at high current and/or rpm, which limits the maximum power and speed the motor can produce without reducing its lifespan.

  1. A brushless motor can have its timing adjusted 'on the fly' by the controller rather than being fixed mechanically by the brush/commutator position. The BLDC/AC controller can optimize timing to suit what the motor is doing. This allows using a fixed gear ratio rather than having to change gears to keep the motor in its most efficient speed range.

The combination of lower maintenance, higher power, (slightly) higher efficiency, and lower cost due to simpler motor and gearbox construction makes a BLDC/AC motor more attractive. This is partially offset by higher controller costs, but since a sophisticated control system is desirable anyway it isn't that much more expensive than a similar DC controller.

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Before Tesla existed, enthusiasts were doing conversions on a variety of cars to turn them to an all-electric drivetrain. Most of these used traction motors that you'd find in fork trucks on under railway rolling stock. enter image description here

There are a couple of limitations with these motors. As other comments have noted, all motors need to have a current that rotates relative to one of its components, the armature, which in the case of a DC motor is the rotor, and this is achieved by the commutator - the cylindrical copper component on the left end.

enter image description here

The brushes (made of a carbon and metallic composite) that run on the commutator are subject to wear, and would require replacement, probably several times during the life of a typical vehicle, so this is the first disadvantage. The construction of the armature, and especially the commutator poses a maximum speed limit on the motor, before you risk the assembly bursting - windings or commutator bars flying out, which is fairly catastrophic, since it'll destroy pretty much all of the motor. Since the maximum torque you can get out of a motor is dictated by the rotor volume, and the flux density you can achieve, the way to get maximum power out of a given size of motor is to run it as fast as it will physically tolerate. BLDC motors, which have no windings on the rotor, can be made to run at much higher speeds. The windings on induction motor rotors are usually a squirrel cage made from bars either cast through the laminations or fabricated from rods, and again can be made to tolerate high speeds.

That leads to the inevitable issue with higher power density - the need to cool the motor. Since BLDC and induction motors have the windings on the stator (this is now the armature, since it is fed by currents alternating in a fashion to produce a rotating field), it'a far easier to fluid cool this - commutator motors are mostly air cooled, though a few low voltage types can run with the rotor submerged in non-conducting fluids - though generally at low speeds.

Overall, given the weight incentive to have higher power density, and the need for lowering maintenance, DC motors are not a good choice for EVs.

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