VSD and VFD

VSD is a more inclusive term, encompassing DC variable voltage drives, for instance. When applied to AC, the terms often mean the same thing.

VFD, variable frequency drive does just what the term says: it varies the frequency of the power going to the motor. In the US, most motors are designed to operate on 60 Hz frequency. In Europe and other countries, the frequency of choice is 50 Hz. If you vary the frequency of the power you vary the speed in direct proportion. A motor operating at 60 Hz frequency at a speed of 1800 rpm, will operate at about 1500 rpm on 50 Hz, because 50 Hz. is 83.3% of 60 Hz so 83.3% of 1800 rpm would equal 1500 rpm. So, varying the frequency varies the speed. Hence the Variable Frequency Drive. It the follows that a VFD is also a VSD in that it does, indeed, vary the speed of the motor. So the two terms can almost be called synonomous. I say "almost" because there are other ways to vary the speed of the motor which do not vary the frequency, although they are not in common use. I hope this helps.

• The speed control in a modern battery-drill could be called a variable speed drive, as it controls directly the speed of the motor by regulating the current flowing to it. However, as the drill is supplied with direct current from a battery, it cannot be called a variable frequency drive.
• A variable frequency drive can be used to control the speed of an AC induction motor supplied with alternating current, which it does by altering the frequency of the current supplied to the motor. It can also be called a variable speed drive.

Thus, topologially, variable frequency drives are a subset of variable speed drives.

VFD is a variant of VSD. VSD is inclusive of other forms of electrical speed variation technologies, both AC and DC, some of which have no relation to frequency. Even in AC systems, some VSDs have no direct relation to frequency, such as eddy current drives and rheostats. VSD also is inclusive of non-electrical devices, such as variable pitch pulleys and fluid clutches.

All squares are rectangles, but not all rectangles are squares.

nothing, just the name, they are exactly the same in internal function.. they change the frequency of the supplied voltage to alter the rotational speed of the motor they're so you controlling so you can say "speed or frequency"...both are varied, it's just the word you choose to use. I personally always say "vfd"

VSD is generic & VFD is specific to AC Motors.