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Comment by bad_alloc | Hacker News Reader

Yeah but in most EVs you need to transform the battery DC current to AC for the motor.

I was under the impression the Model 3 uses permanent magnet DC motors, with the increased efficiency accounting for some of the range gains.

No electric motors are really DC driven. They do differ how the AC is derived, though, mechanically or electrically.

To say a word about the physics: The fundamental force at play is the electromagnetic force. If you run a current in a coil in the magnetic field, it will rotate so that the coil’s field comes into alignment with the outer field.

Once it’s there it is happy to stay. You’ve got to change something to get the motor running continuously. AC is quite clever about it because it just changes the field electrically and this is much more reliable than changing it with moving parts.

I've got an idea for a DC electric motor that has no apparent AC. Can anyone spot any hidden AC here?

Take a disk on a shaft. Use a ratcheting mechanism on the shaft so that it can only rotate in one direction. Attach permanent magnets around the rim of the disk.

Attach more permanent magnets to bimetallic strips positioned outside the rim of the disk. Near each bimetallic strip place a resistor.

The angular spacing of the bimetallic strips and magnets around the outside of the rim should not be that same as that of the magnets on the rim.

Turn the disk by moving one or more of the outer magnets closer to the rim to create asymmetrical torque on the disk. You move the outer magnets by using the resistors to heat the their bimetallic strips. You move an out magnet back to its original position by letting it cool back down.

Why would you use bimetallic strips instead of, say, a solenoid? The whole point of the discussion is to avoid thermal losses...

The comment I was replying to said DC motors actually use AC internally. My comment was an attempt to design a DC motor that definitely uses no AC.

Solenoids have an inductor with a changing magnetic field that stores energy and eventually gives it back. I want to avoid the possibility that that changing magnetic field will induce a current somewhere that is in the opposite direction of the normal current in that place thereby producing AC.

By just using the DC input to make heat, there is no chance of inadvertently getting AC.

> You move the outer magnets by using the resistors to heat the their bimetallic strips. You move an out magnet back to its original position by letting it cool back down.

You are switching the resistors on and off with a pulse pattern that will look a whole lot like you're driving a stepper motor. That's what we call AC.

DC means that the voltage stays constant over time. AC means it changes. Switching DC on and off creates AC.

DC is any current that flows consistently in one direction. AC is current whose flow direction switches back and forth.

That sounds like a https://en.wikipedia.org/wiki/Stepper_motor with extra steps.

You could argue that a stepper motor is also just pulsed DC, though it starts looking more like AC as you increase the drive efficiency.

When the resistors are heating, there must be more power going through them then when they’re cooling, right?,

The voltage driving the resistors? Just terribly slow, matching the terribly slow speed of the motor.

That would be variable DC, not AC.

Your resistors would still need a varying voltage applied to them in order to spin the rotor. If you view your "pulsed DC" as not being AC then a simple brushless DC motor would meet the same definition. I believe if you want a truly DC motor without a commutator generating AC like a brushed DC motor you would need a homopolar motor.

https://youtu.be/TzsUhiOej-s

Model 3 uses two types of motors in the dual motor model and performance model. AC in the front and DC in the rear.

I think you're confusing induction versus synchronous reluctance permanent magnet motors with AC versus DC.

The motors Tesla uses are all AC, but you could also call them DC if you're referring to the whole motor/inverter assembly, which inputs DC from the battery.

DC motors aren't used much in EVs because the brushes require periodic maintenance and doing regenerative braking or running in reverse are both rather difficult whereas with a motor that runs off of 3-phase power, those features can be implemented in the software of the motor controller. They tend to be rather low efficiency, too. That said, there are some really powerful and cheap series wound DC motors that have been used to great effect in vehicles like the White Zombie.

Articles refer to the rear motors as DC similar to the DC motor used in the leaf. Here's an example:

https://www.motortrend.com/news/electric-cars-explained-gear...

I'm pretty sure the author of that article is just confused. The synchronous reluctance motors in the Model 3 run on AC just like an induction motor, but can be thought of as a DC motor if you consider the whole inverter/motor assembly, just like an induction motor.

I think Tesla has been migrating from induction motors to permanent magnet motors (which usually have higher efficiency and produce less heat), but they all run off of 3-phase AC from the inverter.

Sometimes the inverter/motor combination is referred to as a brushless DC motor because it takes DC as input before converting it to AC for the motor to use.

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