"Why Tacho Feedback is Necessary?"

Feed back is needed to maintain a steady output verses input reference which is called Tach/Reference feed back otherwise , its called armature feed back which really does nothing more than maintain the output voltage to the input voltage reference ratio. in the case of the second, this is not a steady reliable way to maintain speed because the motor may require more voltage on to maintain speed depending on shaft load&load Swing variations. Simply put, armature feed back can maintain the output voltage but not the actual shaft speed on load.. etc... there are the cases where you have a feed back from some other reference point inputting to a Trim controller which then can regulate the input reference to the drive, this would then be regulated from the Trim control by simply biasing the input reference of the drive which is in armature mode. this all applies to DC PM/Shunts and the like motors.

Yeah - it'll run - but not at any speed you can control ! That's what the tacho is for.

Graham

Armature feedback can make a pretty fair job of it, if you use good IR compensation. What it won't do is allow you to run the motor above base speed using field weakening.

Thanks to you all guys...

The function of the tach depends on the kind of servo it is. In its simplest case, the tach provides velocity or speed feedback and it is the speed that is controlled. The tach element provides a voltage or count frequency that represents the actual speed of the motor or system it is connected to.

In a position servo where a physical position is the controlled element, a tach is often used to feedback the differential of the position, dx/dt. This is a velocity or rate term and helps to stabilize the servo by controlling the motion.

Thirdly, tach outputs can be integrated or counted to provide position information. Line tacks of this kind are used on all kinds of numerically controlled machine tools. Even the lowly mouse on your computer works this way.