Why Tesla cars have AC Induction motors instead of DC

#1

Post by trolliloquy » Fri, Dec 22, 2017, 11:44 PM UTC

https://www.tesla.com/blog/induction-versus-dc-brushless-motors

The above link was found in the original discussion board, which is here: https://forums.tesla.com/forum/forums/ac-induction-motor-why

Some key observations:

Batteries can't store AC electricity. One requires power electronics circuitry to convert DC to AC which might involve a trade off. Yet why go for AC motors?

Below are the arguments

- DC motors require rare earth magnets which can add to the weight of the motor which can be expensive & can't be mass produced. They can only be extracted. AC motors require the induction coil which can be easily manufactured. There can be numerous counter arguments to it, but when measured, advantages of AC motors manufacturing will always weigh more than DC.

- AC generators have 3% more efficiency than DC (as mentioned in one comment) during re-generative braking

- High Traction: Max torque at 0 rpm - this is something that only electric vehicles with high traction electric motors can claim. 3 phase AC induction motors have been known to give 100% of their torque as soon as you start them.

Re: Why Tesla cars have AC Induction motors instead of DC

#2

Post by Gibbon1 » Sat, Dec 23, 2017, 6:25 AM UTC

An actual DC motor uses a commutator and brushes, which is a serious maintenance and safety issue. The motors can have permanent magnets or field windings. A common old school speed control for electric golf carts was to control the current in the field windings via a variable or tapped resistor

Thing with multiphase AC motors is there are a number of ways to skin the cat. They each have slight advantages and disadvantages. I think an AC induction motor is generally efficient over a wide range and has a higher output per pound then a permanent magnet brushless DC motor.

Far as I know there isn't much difference in power inverter. Note high voltage power inverters are hella efficient because your losses are due to a fixed voltage drop through the power transistors X current. For a high voltage inverter the current is low, voltage high[1]. Means the percent loss is low[2].

[1] Opposite is low voltage switching power supplies where the output voltage is low and currents are high. You see people use active diodes to avoid voltage drops despite the expense. Even then 90-95% efficiency is common.

[2] Imagine a 75kw inverter running at 300VDC. 75kw/300 = 250 amps. If the voltage drop in the power transistors is 6 volts then the power loss is, 6V X 250A = 1500W. 1.5kw/75kw = 0.02. So the inverter is 98% efficient.

Re: Why Tesla cars have AC Induction motors instead of DC

#3

Post by trolliloquy » Sat, Dec 23, 2017, 1:59 PM UTC

Gibbon1 wrote: post #2

An actual DC motor uses a commutator and brushes, which is a serious maintenance and safety issue. The motors can have permanent magnets or field windings. A common old school speed control for electric golf carts was to control the current in the field windings via a variable or tapped resistor Thing with multiphase AC motors is there are a number of ways to skin the cat. They each have slight advantages and disadva…

> [1] Opposite is low voltage switching power supplies where the output voltage is low and currents are high. You see people use active diodes to avoid voltage drops despite the expense. Even then 90-95% efficiency is common.

Thanks, I din't know about this one at all. I should have paid more attention during the lectures, missed this part totally.