I'm designing a linear actuator for a particular application, and the electric motor I'll be using will be a 1/8 HP three-phase motor. I need to decrease rotation of the motor, and my first idea was to design a reduction drive. But I know that this can also be done with a Variable Frequency Drive (VFD). I don't know how VFDs work and neither how to install and use them, but for this particular project, what do you think would be the best option? Would a VFD be cheaper than a reduction drive? And how VFD affects torque?
VFDs can easily get a motor to produce full torque at any speed from full speed down to zero. The difficulty is in motor cooling. An inexpensive standard motor can probably not operate continuously at much less than half speed without overheating. It is generally always better to use fixed gearing so that the motor operates at full speed for the highest machine speed that is required. Use adjustable speed only to reduce from the highest machine speed to lower speeds. Don't use a variable speed drive just to operate at a reduced constant speed.
For 1/8 Hp, a DC speed controller and a permanent-magnet DC motor or even a commutator DC motor will probably be less expensive than A VFD. DC motors can also have difficulty with continuous low-speed operation.
A gear system will reduce speed and INCREASE torque by the inverse ratio. A VFD will reduce speed and MAINTAIN torque. You need to start by evaluating your torque needs at the lower speeds.
VFDs also allows you to ADJUST speed, a gearbox will only CHANGE speed to a fixed value. If you don't need to ADJUST the speed, don't use a VFD.