Your other thread referred to controlling the speed of a motor using a PLC. I assume this is the same question.
Most PLCs do not directly control the speed of a motor. There must be hardware which performs that function. The PLC by various means can direct the controlling hardware as far as speed or other control parameters.
As requested in the other thread we must know the type of motor, the type of motor controlling hardware and the type of PLC. How is the PLC connected to the controlling hardware - if it is at all?