Variable speed drive

There are variable speed drives that take a DC input. Once again, we need a little more data to truly answer your question. What kind of variable speed drive are we talking about, really? What's the application? Do you actually mean a DC drive, running a DC motor, or do you mean a variable speed drive running a standard AC motor? Or is this a positioning application that requires a servo? What kind of power range are talking about?

Gozuk variable speed drives has some nice descriptions of variable speed drive types. Click on AC or DC Drives and choose "Category Overview."

Let's start with power requirements. Let's say you need a 1 horsepower (~750 Watt) motor. Let's say you're feeding it 24VDC. That's THIRTY AMPS. Where are you getting this 30A from? That's a pretty big power supply to run one motor. So double the voltage, to 48VDC, and now you only need 15A. Better, but still a lot, and 48VDC is generally less common than 24VDC. Going above 48VDC gets even more unusual. Picking a power supply company at random, consider the offerings from Siemens. Clearly 24VDC is the expected voltage. They have 48VDC, but it's not considered mainstream, and they don't go above that. Pretty typical of the automation industry. If you want big motors (and by "big" here we just mean about 0.5 kW or greater) you almost always want to use AC.

Incidentally, there's no particular reason that you can't use 600VDC. It exits. There's nothing that prevents your power company from sending it to you. But they don't. In the entire world, I don't think there's any power company that provides DC power. Similarly, AC could be 100Hz or 42.8Hz or whatever, but the whole world uses 50Hz or 60Hz, period. In the U.S. we have 3-phase 480VAC 60Hz as standard for industrial equipment. In other countries it's often 3-phase 360VAC, 50 Hz. So at some point in powering your motor you HAVE to deal with AC. There's just no way around it.

You can, by the way, run an "AC" variable speed drives with DC power. AC drives convert to DC anyway, so it's possible to design a system that takes in the AC provided by your power company and has ONE large converter to get DC power, which is then fed to all the variable speed drives. This is a rob Peter, pay Paul kind of thing. Yes, you can now use smaller variable speed drives that generate less heat because the variable speed drive no longer has the circuitry for converting AC to DC. But you have this large heat generating power supply, so it offsets. Servos often use this type of setup, but it's much less common in variable speed drive.