What is the Difference Between SERVO Drives & VFD?

Generally speaking, If you want to control speed and tork only use a VFD. But if beside that, you want to control accurate position then you need a servo.

The biggest difference is the use.
VFD's you use to control the speed of a motor, which means that you are working in a single direction.
With servo motors you are controlling the direction of movement of a motor, forward or reverse, in in some cases the speed.
Servo motors are used to move equipment in a controlled manner forward, reverse, up or down depending the motors direction, and with a VFD you only control the speed.

Victor, above, has the most concise practical explanation. If you need to control position, use a servo.

Also, VFD setups usually require a gear box, because the motor does not provide sufficient torque at low speeds. ( see Warren's explanation above)

Following to be considered
1.Construction
Servos are available as a solution in the form of a set that consists of motor,matched gear box with position feed back sensor like encoder along with controller module.All the components are tightly coupled and designed to work as a system
In contrast, VSD (Variable Speed Drive) is available as product.
2.KW rating
Servos are available upto 5KW - Maximum
VSD are available at various Kw ratings at different voltage ratings like LV & MV
3.Application
Servos are used for precise positioning and CNC machines
VSDs are used in pumps,blowers,materials handling cranes,HVAC.......

VSD - Variable Speed Drive is based on different technologies like Eddy current, Variable Voltage and Variable Frequency.

Best Regards

Servo motors are used for getting a constant torque on all the speed ranges. Normal Induction motor torque varies with speed. Servos are normally used with machines for better torque characteristics. Servos are in normally closed loop controlled. Induction motors can be controlled with VFD in vector & vector less control.

Thank you so much all for sharing precious knowledge with us :)

There seems to be huge misunderstanding in this area. First of all, to the best of my understanding, the term 'servo' refers to an application rather than a motor type - usually positioning. DC motors have been used in servo applications, and there is no reason why AC induction motors cannot be used. As pointed out, most servo applications tend to involve synchronous motors these days, but steppers could also be used. most VFDs currently allow the use of either asynchronous or synchronous motors, selectable between permanent magnet or external excitation types. Most VFDs also have additional function cards which allow very precise positioning. It is not true to say that VFDs are only used for speed control. Another incorrect statement is that VFD/squirrel-cage motors have variable torque with speed. Since the introduction of vector drives it is possible to have constant torque down to about 1 Hz.
Eric Carter, turboTRAIN, South Africa

There seems to be huge misunderstanding in this area. First of all, to the best of my understanding, the term 'servo' refers to an application rather than a motor type - usually positioning. DC motors have been used in servo applications, and there is no reason why AC induction motors cannot be used. As pointed out, most servo applications tend to involve synchronous motors these days, but steppers could also be used. most VFDs currently allow the use of either asynchronous or synchronous motors, selectable between permanent magnet or external excitation types. Most VFDs also have additional function cards which allow very precise positioning. It is not true to say that VFDs are only used for speed control. Another incorrect statement is that VFD/squirrel-cage motors have variable torque with speed. Since the introduction of vector drives it is possible to have constant torque down to about 1 Hz.
Eric Carter, turboTRAIN, South Africa

I would agree it is possible to control an induction motor using a VFD in either open or closed loop control. If the motor and VFD are without encoder feedback, I.e. open loop then the motor speed will try to follow the pulse width modulated frequency from the VFD. However it will be subject to slip which will vary according to the load, also speed holding at slow speeds will be difficult. If an encoder is fitted to the motor and the VFD can use the encoder signal, then it is possible to get fairly accurate speed and position control from an induction motor, even full torque at zero speed should be possible. However because of the much higher inertia of an induction motor and limited peak currents available I.e. only typically 1.5 times nominal current is usual against 3 or 4 times from a servo. This means the dynamic performance wil be limited. Basically if you have a high dynamic performance, stop start type application then use a servo. If you require less performance, you may save a few pound going VSD.

Servos have a much higher bandwidth than VSDs and also can be controlled at much less than 1 rpm. They maintain the optimum current in the windings using an algorithm that calculates using information from a very high resolution positional feedback device (often a resolver) on the back of the motor. Their response times are much faster (as they have incredibly small inertia values) They can maintain accurate speed and, position if a position loop is provided by a motion controller, to very high accuracy. VSDs have, at best, an encoder on the motor and much lower bandwidth