when i did some research on ebay to buy motors, AC motors were usually rated for 120 volts, or 240 volts, while most DC motors were only rated for like, 24 or 50 volts. Why? Is AC better?
I don't think you can justify any global conclusion about the quality of motors based on the observation that 'most' AC motors are rated 120 or 240 v, while most DC motors are rated at lower voltages. In fact, I might hypothesize a totally different global conclusion. First observation, most DC motors are intended for application where they are integrated into a piece of electronic equipment, for example, a computer. Therefore, the motor must be designed to be powered from the DC power supply associated with that equipment. AC motors, on the other hand, may be intended for use in either stand alone applications or applications in equipment that are fundamentally AC systems, for example, HVAC systems. And in these applications, the controls and motors typically operate on the input line voltage, usually 120 or 240 v, and not on some lower voltage derived within the equipment. Therefore, one might speculate that the reason that you see more DC motors listed on e-Bay is that there are more folks out there tinkering with electronic equipment that uses DC power supplies, and where the motors therefore have to be DC.
You need some sort of power source to power the motor. The most common AC voltages (in households at least) are 120V and 240V, so it makes sense that many AC motors will be designed to run off of those voltages. Most DC sources are batteries, so not surprisingly, DC motors will be designed to run off of typical battery voltages (i.e. 9V, 12V, 24V, 48V).
