# Difference between VVD and VFD

1. Sep 11, 2009

### b.shahvir

Hi all,

Can someone please tell me the basic difference between a Variable Voltage Drive (VVD) and a Variable Frequency Drive (VFD) in terms of industrial applications. Why can't VFD be used for all speed control applications or the likes in general?... i.e., why a requirement of a VVD?

Also, please ennumerate the industrial applications in which a VVD is preferred over a VFD and why.

Thanx & regards,
Shahvir

Last edited: Sep 12, 2009
2. Sep 12, 2009

### b.shahvir

3. Sep 16, 2009

### mustafa_2009

I have no lots of info. but searching the net i found this link which explains VFD.

http://www.joliettech.com/what_is_a_variable_frequency_drive.htm" [Broken]

Last edited by a moderator: May 4, 2017
4. Sep 17, 2009

### b.shahvir

Dear Mustafa,

Thanx for the link. I'm aware of the basic operation of a VFD, but then what is difference between VFD & VVD...as theoretically both carry out function of speed control.

Regards,
Shahvir

5. Sep 17, 2009

### KLoux

VFD is for driving AC motors. VVD (although I'm not familiar with this term) sounds like it is for driving DC motors, where the speed of the motor is roughly proportional to the voltage rather than being dependent on the frequency.

-Kerry

6. Sep 17, 2009

### b.shahvir

Thanx, Variable Voltage Drives (VVDs) are also used for speed control of AC motors, but I'm unsure of it's industrial application.

7. Sep 17, 2009

### uart

Hi b.shahvir. When you vary just the voltage to an AC motor (induction motor) you can only control the speed to a very limited extent. AC motors are relatively constant speed devices at any given excitation frequency. When you reduce the voltage on an AC motor you reduce the excitation and as a result the slip will increase a little. This mean the motor will slow a little, but not very much. Also the amount to which you can control the speed in this manner is greatest for inefficient motors with high rotor resistance and least for efficient motors with low rotor resistance. Variable voltage drives are often only used for soft starting.

If you are able to vary both voltage and frequency then it's a much more desirable situation. By keeping voltage in approximate proportion to frequency you can maintain full excitation (and hence full torque) over a wide range of speeds.

8. Sep 17, 2009

### b.shahvir

Thanx. I think this is a more technically relevant reply! However any special industrial applications where VVDs gain prominance over VFDs?

Regards,
Shahvir

9. Sep 17, 2009

### dlgoff

Varying the voltage can be a way to control the torque. I believe this is useful when needing to limit it to couple one machine to another in an assembly line type of system.

10. Sep 17, 2009

### vk6kro

Varying the voltage is also a way of controlling speed where the load increases with speed.

A good example of this is a fan. It starts readily, because it has no wind resistance to start with, but wind resistance builds up with speed.

So,the more torque the motor has, the faster it will run. So, you can vary the supply voltage (usually with a reactance in series with the fan) and hence control the speed of the fan.