If you notice erratic changes in the fan speed of your air conditioner, then it may be there is a problem with the indoor fan on your air conditioner.
There are some settings on your air conditioner that could cause it to exhibit this behaviour - but if they are not on - then there may be a problem.
The fan (sometimes called the blower) is the thing that moves the air. Your air conditioner works by passing air over a coil (which has refrigerant flowing through it). As the air passes over the coil it is cooled (or heated) by the refrigerant in the coil. This cooled air is then blown out the supply air diffuser\s or grilles.
The fan in your air conditioner will either pull or push air through the grille, depending on the air conditioner's design.
Most air conditioners have four fan modes:
The low, medium \ medium \ and high settings are fan speeds and are constant. This means the air conditioner's fan will always run at the assigned speed, irrespective of whether the air conditioner is cooling or not.
The auto setting is different. In auto, the fan will only run when the air conditioner is calling for cooling, and the speed of the fan may vary depending on how far away the system is from the set temperature. When the air conditioner has the room temperature close to the set temperature, the fan will slow; when the room temperature is not close to the set temperature, the fan will speed up.
When an air conditioner is set to auto, the system will normally cycle off when a set temperature is achieved - this means the fan will stop running. When the room temperature rises above the set point, the air conditioner will start again, and the fan will start at the same time.
When an air conditioner is in auto mode, the air conditioner will vary the ac fan speed in an attempt to reach the ac units set temperature as quickly as possible. This means if the temperature is a lot lower than the room temperature, the ac fan will speed up. As the room temperature approaches the set temperature, the ac fan will slow down. When the set temperature is achieved, the fan will stop.
In auto mode, the fan will speed up and slow down as needed and stop altogether at times.
When your ac settings are on a single fan speed - and the air conditioner fan is changing speed - it is normally an indication of a problem with either the fan motor or the control board.
If you think there is a problem with your air conditioners fan, you should engage the services of a qualified air conditioner mechanic to carry out a diagnosis of the fault and make a recommendation for its repair.