Why would stepper motors have high resistance in the coils? (20 ohms)

I've read about 10 different guides on stepper motors, and can't quite find the answer to this question.

Why exactly would a stepper motor have high resistance in the coils? Like these ones that I'm having problems with, which have about 20 ohms of resistance in the coils. I realize there are other factors at play like inductance, which contribute to overall impedence, but it just seems like you would want your copper coils to have as low of resistance as possible right?

Isn't having higher resistance just going to contribute to excessive heat, and thus be less efficient due to heat losses, as well as require much higher voltage to drive it just to overcome the resistance? I also don't quite understand how the manufacturer is even able to get a copper coil up to such a high resistance, unless it is extremely fine gauge and very long wire, or else if they add some resistors in there or something.

Alternately, my other stepper motors have around 2.2 ohms per coil, and I don't seem to be having any problems with them.

Edited 1 time(s). Last edit at 11/20/2014 04:17PM by scubaguy.

To get more torque out of a motor, you need more turns in each coil. To get those coils into a given volume, you need to use thin wire. So, if you need a NEMA 17 motor with a lot of torque, you don't have much choice but to let the resistance rise.

Actually, using thinner wires doesn't necessarily give you more torque. The wire size gives you a voltage/current trade-off for the motor. For a given volume motor, more windings with thinner wire increases the rated voltage and decreases the rated current. Higher voltage motors can't go as fast if you are limited to, say, a 12v supply, but for something like a direct-drive extruder that doesn't need to go fast, you can get a lot of torque with a small amount of current.

To get more torque, you generally need a bigger motor or stronger magnets in the motor.