I'm looking for a high-torque DC motor (able to lift 20 pounds) which, when turned off, will spin freely (meaning I can turn it very easily with my hand). I have looked around on google and have found stuff about slotless and brushless motors, but they don't seem to have very high torque.
Can anyone shed some light on this for me?
Ungeared dc motors do not tend to have very high torque. And the torque has (almost) nothing to do with the lifting ability. The lifting force will be equal to M/r where M = torque(Nm) and r=drum radius(m). So an as small as possible drum will give the highest force possible. Of course there are limitations to this since the lifting tether has a minimum bending radius. If you approach this radius friction between the tether and the drum will diminish and finally cease.
For maximum torque at a given power your motor should have a as low as possible kv constant. This can be achieved in two ways:
The motors that have the lowest kv are to my knowledge brushless outrunners
Torque is a mainly function of motor volume and cooling, not windings(*) or pole count...
For this sort of application you will need a brushless, switched-reluctance
or induction motor. Without gearing it will have to be large diamater motor. With
gearing you'll need low-friction gearing (which isn't easy to achieve).
1/Kv is a measure of torque per unit current, not of total torque, which is the same for
all winding arrangements when driven at the same copper-loss.