If your compressor is not running while the condensing unit is running, a number of things could be causing the problem. Some of the voltage problems that could cause it are a bad capacitor, contactor, burnt terminal, or burnt wire. One thing that can occur is the compressor going out on heat overload. That can be caused by a dirty coil or incorrect freon pressures. A service technician needs to be called to troubleshoot your condensing unit.