# How does a VFD work?



## HMF (Feb 11, 2011)

A VFD takes AC (alternating current) in, converts the power to DC (direct current, and then converts the DC back to a variable form of AC, such as three phase, variable frequency AC. The rotational speed of three phase motor fed by a VFD can be proportionally varied as a setting of the VFD output frequency (speed control).

Suppose you have a gear head lathe with a lowest selectable spindle speed of 70 rpm. You may find that too fast for threading when dealing in tight quarters. Varying the VFD output frequency offers slower speed to make the job easier. You may also need to accomplish a facing cut. The cutting speed (and finish) will vary as a function of the radius of the cut. With variable speed control you can continue to adjust spindle speed for that 'sweet spot". 

VFDs also offer electronically variable braking, variable start up acceleration and current/phase correction for "constant torque' at variable speeds. 

As for motor reversal, that can also done by the VFD. Motor reversal is accomplished by changing phase to the windings. It can also be done mechanically by switching power connections (solenoids).

When using a VFD, the lathe motor wiring is disconnected from the lathe control wiring and hooked directly to the VFD. This means that all control is through the VFD. A lot of folks make a custom control panel to interface with their VFD to make access to functions easier. I rewired my lathe to use the existing lathe controls to operate many of the VFD operations.


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## HMF (Mar 5, 2011)

I went to YouTube to learn about VFDs and three phase motor control. There isn't a lot there, which is unusual for YouTube, which usually has a lot on any topic. 

One set of videos I found was by CVMikeRay, who is on PM a lot. I am going to post the videos here, since our new video integration system allows it. Now I know I am stupid, but his videos - all 5 of them- left me in the dust, even though I watched them several times.

Part 1- [video=youtube;llTO2X-jOIA]http://www.youtube.com/watch?v=llTO2X-jOIA[/video]

Part 2- [video=youtube;FsR_VuPTB8k]http://www.youtube.com/watch?v=FsR_VuPTB8k[/video]

Part 3- [video=youtube;QZNg-4D4DFM]http://www.youtube.com/watch?v=QZNg-4D4DFM[/video]

Part 4- [video=youtube;58rGtwMgR4g]http://www.youtube.com/watch?v=58rGtwMgR4g[/video]

Part 5- [video=youtube;youzEZuLIuw]http://www.youtube.com/watch?v=youzEZuLIuw[/video]

He just went too fast for me, especially with respect to rewiring the motor.

Nelson


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