Servo motor

HI to all ,
i just want to know why we use servo drive insteed of ac drive with machnical braking....

Becours a servo motor and controller is much much much more precise then ordinary motors and VFD.
And just FYI there are both ac and dc servomotors.

A servo drive can be brought to a precise location under power down to zero speed. The servo will then HOLD the position using power.

The AC drive is dependent on the current values of decelleration, friction and the holding power of the brake against vibration.

If a precise end stopping point is not critical and if holding that point in all cases is not critical then an AC solution may be more appropriate especially with regard to cost.

Servo motors are used when timing is critical; like Bernie said, they can be brought to a precise location under power to zero speed. An example of this is a slicing and stacking machine that both use servo motors. Our slicing machine has a blade that turns around 900 rpm, where blocks of meat are fed to it from the top. Under the blade we have a small belt with flights on it which is hooked to a servo motor. The operator sets the machine to how many pieces of meat he wants in a stack. The belt will stop for a split second (enough time to catch the proper number of pieces) then moves forward to kick the stack out on to another machine belt. The stack has to be straight while it goes in to the paper stacking machine (a piece of paper is placed between each stack of meat) One roll of paper continuously feeds the machine; it has a pull roller, knife roller and tear roller which all are controlled by servo motors to keep paper the same length and feed the piece of paper out at the proper time as the meat comes up the belt. The stacks of meat are then dropped on a long belt with flights that moves one position foreword when the proper number of stacks is on it. We run at a high speed which we could not do without the servo motors stopping in the same exact location at the same exact time. Just like ojz0r said they are much, much, much more precise. When we have a servo motor go bad, we no instantly, because we have a big mess. I hope my example helps you understand why servo motors are used.
Sincerely:
Maintenance Man