A standard motor has a set speeds. Some have more than others. But basically, whichever speed tap power is applied to, that is the speed it is going to run. Red=low speed Black=high speed etc. A variable speed motor can run at whatever speed it needs to for optimum efficiency. Well that is not entirely true. I think Carrier uses motors that have 36 speeds. There are not 36 speed taps though. The control board of the furnace "talks" to a module connected to the motor and tells it what speed it should be running at and the module powers the motor. Variable speed motors are very efficient, and are designed for continuous fan operation. I have never heard of a variable speed motor being used as a single speed as that would defeat the purpose of having it. Try this: Turn a/c to off. Then turn fan switch from auto to on. You should hear the motor come on and run at a very low speed. Now turn on the a/c. You should hear the blower "ramp" up to a high speed. Turn the a/c off and the motor should ramp back down to a low speed. I this doesn't happen, you should call whoever installed it to see why the blower is not set up properly.