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.
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.
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.
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 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.