Final answer:
A Variable Frequency Drive (VFD) is used to control the speed of a condenser fan motor in response to changing outdoor temperatures, maintaining efficiency and preventing large head pressure swings in air conditioning systems.
Step-by-step explanation:
The device that is used to gradually change the speed of a condenser fan motor according to changing outdoor ambient temperatures without causing a large head pressure swing is known as a Variable Frequency Drive (VFD). The VFD controls the frequency of the electrical power supplied to the motor, which in turn controls the speed of the motor. This is essential in air conditioning systems where the condenser fan needs to adjust its speed with varying outdoor temperatures to maintain efficiency and prevent large swings in head pressure, which could damage the system.