If your condensing fan motor is not running, but compressor is it could be a couple different things causing this problem. Most of the time the run capacitor or the motor itself is bad. Although it could also be a burnt wire going to the motor or bad motor relay/control board. In some cases the condensing motor will run a while making a grinding or whining noise. If this is happening the bearings in the motor are most likely going bad. Alot of times in this situation the motor will run for a period then will eventually stop. The motor will most likely need to be replaced. If your condensing motor is not running it will also cause the compressor to go out on heat overload. Basically the compressor will overheat and lock itself out until it cools off. A service tech should be called to come and check out your system.