As I explain in question ask, about speed regulation and speed control of motor, if I am not wrong, the VFD, is for variable frequency control, applicable to induction motor, as the speed can be controlled by changing the speed of rotating field speed(Ns) which depends on frequency.(Ns=120f/p).

The VVVF applicable to D.C motor speed control, variable voltage, variable field control, as explain, below rated speed, voltage control(Speed is praposnal to Voltage applied to armature), and VF the variable field control, speed above rated can be controlled by field control(Speed inversely praposnal to field applied i.e by weaknning the field the speed above rated can be achieved).

VFD- Variable Frequency drives means we can control the speed by varying the frequency alone. So that during low speed more flux will be there in the airgap. So that the flux may be saturate and the core loss will be more.

Where as in VVVF- Variable voltage variable frequency control is nothing but V/f control. Where in flux is always constant by maintaining V/f as constant. So core loss will be reduced and only the sufficient flux produced in the air gap.

I was using, VVVF by keeping V/f constant, for transformer to seprate hysteresis and eddy current loss, from total core loss, but not knowing the use of same method in speed control.

In my opinion, VFD is the same as VVVF. In both of them ,the frequency and the voltage are controlled simutanously to maintai V/F=Constant, which is the basic requirments of induction motor control to maintain the flux is constant.

Bhupendra Desai and Rajin If VFD and VVVF are not same then how is it possible for Vfd to control the induction motor without changing the voltage whereas it the requirement of induction motor to keep the V/f ratio constant so that the air gap flux remains constant .

In Drives, torque produced by motor is of prime importance. Hence in VFD speed control the air gap flux will changed with rated(same) applied voltage,hence torque produced will changed,, also the motor can run safely with 10 to 15 % change of voltage, hence with increased speed (frequency) the voltage can also be increased, and with decreased speed the voltage can be decreased and the torque produced for mechanical load, can be made constant(as air gap flux is maintained constant), and quality of work will not suffer. Or we can say, in VVVF more quality controlled, speed control is achieved,than VFD(Though work done(HP produced) by motor at different speed, at constant torque will not be same(more HP at higher and less HP at lower speed, as torque remains constant). In induction motor speed of rotating magnetic field , decides the speed of motor (Ns=120f/p),so with increase of frequency, speed can be increased and by lowering frequency,speed can be lowered.

Iron loss in induction machine/transformer consist of hysteresis loss and eddy current loss {(Wi=Wh+We), Wh=A(f) and We=B(f)(f)},hysteresis loss is praposenal to frequency and eddy current loss is praposnal to frequency square, which is the relation of frequency and iron loss, A and B are constant depends machine configuration parameters(material use, stemping thickness etc.).hence for VVVF drive, loss at lower speed is not the major criteria.

I agree with @Yanxia and @Karrar. There is no difference.

Many years ago (

My opinion is the motor speed Ns=120f/p. So one can control the speed of an induction motor by keeping V as fixed and varying frequency. Whereas in VVVF drive both voltage and frequency are varying to maintain V/f ratio as constant. If i am wrong please correct me

Within voltage limit of +/- 15 to20% , over of the rated voltage is possible with current(Iload) 25% over of rated full load..

The formula for synchronous speed is well known and had nothing to with voltage. Rajin your argument only applies to open loop v/f control. Other control techniques like vector control, FOC and DTC or current control techniques do not rely on this relationship. I do not know of any AC drive that keeps V constant and varies frequency only in the constant torque region of operation. This may apply in the constant power region of operation above synchronous speed. You need to do some background reading into the history of drives.

VFD

A VFD is a variable frequency drive. The term refers specifically to electronic drives controlling the speed of AC motors – by adjusting the frequency and voltage delivered to the motor.

Most commonly, the terms VFD and VSD are interchangeable.

VVVF

The term VVVF is often used in the mining industry – it refers to a variable voltage variable frequency drive.

It describes exactly the same AC drive as the term VFD – any AC drive will automatically control voltage to suit the frequency it is running at.

A VFD is an electrical device used to control the rotation speed of an alternating current (AC) electric motor by adjusting the frequency of the electrical power supplied to the motor. It is also called pulse-width-modulated drive (PWM drive), adjustable-speed drive (ASD), adjustable-frequency drive (AFD), AC drive, inverter drive, or variable voltage variable frequency (VVVF) drive.

https://www.intechopen.com/books/new-applications-of-electric-drives/variable-frequency-drive-applications-in-hvac-systems

The V/f control method is, in principle a control method for maintaining the air-gap flux constant by controlling voltage V of stator and stator frequency f so to keep ratio V/f constant.

The advantage of using V/F for speed control of three phase induction motor is as follows:

● The current required at the starting is reduced.

● The motor runs more in stable region. Instead of simply running at its base rated speed the motor can be run typically from 5% of the synchronous speed up to the base speed. The torque generated by the motor can be kept constant throughout this region.

Does V/f ratio still remain constant in vector controlled VFDs ? i.e. if a motor which is rated as 460V/60Hz is made to run at 30 Hz, does the voltage still remain 460V ?

Ranjan Pal

Although they are two different methods of control both in effect try to maintain a constant air-gap flux, and in fact, all AC control techniques do the same.

V/F is just a simple scalar method of control that uses PMW to maintain the correct V/F ratio whereas vector control uses current control loops and slip control to do the same and maintain the correct voltage and current required at any given frequency depending on the magnetizing and load current required through appropriate PWM control of the voltage.

Recall for any AC variable frequency drive the only two physical things that may be controlled are the voltage and frequency both of which are done via the PWM modulator control of the inverter.

Thanks a lot Gregory Diana for your valuable feedback. I could now understand the concept.