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Comment by tzs | Hacker News Reader

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.

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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.

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

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

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.

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

That would be variable DC, not AC.

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