Mechta
HN39's response is correct. On most two spool engines, fan speed (N1) is controlled setting the power (thrust) to achieve desired speed at any altitude. Thrust is more accurately controlled because all the air passes through the fan with only a portion passing into the core. For various reasons, core speed is not an optimum manipulated variable to control fan speed. However, in three spool engines, the control for thrust seems to be N2, or the speed of the IP spool. Either way, since the spools are coupled, the speed of the fan remains within the desired power/thrust settings.