VFDs

[bensonelectric]

My instructor and I were talking today about replacing a broken motor on a drill press, we settled on a nice 3-phase 1/3 horsepower motor. He then started talking about using a variable frequiency drive to control the speed of the motor. What exactly do 'VFD's do? The one we are using only takes 117v single phase in, but manages to output 208v 3-phase power. How is this possible?

 

[iwire]

Re: VFDs

The engineers here can get much more specific but in a nut shell they change the voltage and the 60 cycles from the utility to control the motor speed.

You can run the motor very slow or well beyond it's design speed.

I do not have a clue about the 3 phase from single phase trick.

[ October 04, 2005, 09:22 PM: Message edited by: iwire ]

 

[charlie tuna]

Re: VFDs

benson,
i have a 120 volt drive that operates a 480 volt three phase motor with extreamly accurate speed control! the drive creates the three phase output at the programed voltage and also the frequency or square wave that determines the motor's rpm........................

 

[jim dungar]

Re: VFDs

Very quick and dirty:

The input AC voltage to the VFD is converted into DC. The DC is then "pulsed" to simulated 3-phase AC output.

By the way, your VFD probably outputs 240V not 208V three phase.

 

[steve66]

Re: VFDs

To expand on Jim's post (but this is still very simplified), one transistor (or SCR) pulses one output line to get one phase. Two more transistors give the other two outputs.

The outputs are actually Pulse Wave Modulated outputs. That means if you look at one output, it is a series of very short pulses (maybe 100 for every cycle of output). The pulses get wider to make the peak of the sine wave output.

Steve

[ October 05, 2005, 09:19 AM: Message edited by: steve66 ]

 

[Duuuuug]

Re: VFDs

Here is some casual reading
http://www.kilowattclassroom.com/Archive/VFDarticle.pdf

 

[petersonra]

Re: VFDs

Originally posted by jim dungar:
Very quick and dirty:

The input AC voltage to the VFD is converted into DC. The DC is then "pulsed" to simulated 3-phase AC output.

By the way, your VFD probably outputs 240V not 208V three phase.

actually, it probably outputs whatever it was configured for. by default it might be 240, but there is nothing to prevent you from using the keyboard to change it to 208.

 

[bensonelectric]

Re: VFDs

http://www.telemecanique.com/en/_data_sheets/motor_altivar11/fiche.htm

This is the one I used. I wired it up today, and had it all configured within 20 minutes. Works great and was really easy to use. I looked at that article and it sort of clarified the inner workings. Thanks guys.