servo is an automatic device which uses error-sensing
feedback to correct the performance of a mechanism. The term
correctly applies only to systems where the feedback or
error-correction signals help control mechanical position or
other parameters. For example an automotive power window
control is not a servomechanism, as there is no automatic
feedback which controls position—the operator does this by
observation. By contrast the car's cruise control uses
closed loop feedback, which classifies it as a servomechanism.

A servomechanism, or servo is an automatic device which 
uses error-sensing feedback to correct the performance of a 
mechanism. The term correctly applies only to systems where 
the feedback or error-correction signals help control 
mechanical position or other parameters. For example an 
automotive power window control is not a servomechanism, as 
there is no automatic feedback which controls position—the 
operator does this by observation. By contrast the car's 
cruise control uses closed loop feedback, which classifies 
it as a servomechanism.

Servomechanisms may or may not use a servomotor. For 
example a household furnace controlled by thermostat is a 
servomechanism, yet there is no closed-loop control of a 
servomotor.

trasformer is a static device which have no rotating part 
and a device which used to step up and step down the 
voltage from high value to the low value.