Need a Bit of Help Choosing a VFD

[Papa Charlie]

@Papa Charlie I have never herd of this as a possibility. I would check the TECO/Westinghouse support folks before trying it.

That is what I thought, and good advice. The second post above made comment about setting up the VFD for single phase motors, which is what generated my question.

 

[MikeInOr]

Something that has not been mentioned about running a VFD on a lathe is that you should not have any switches between the VFD and the motor, i.e. the reversing switch. The VFD should be wired DIRECTLY to the motor taking the forward - stop - reverse switch OUT OF THE CIRCUIT. THE START STOP REVERSE SHOULD BE HANDLED BY THE VFD!

With a RPC you can just plug your lathe in and go. With a VFD there is going to be some rewiring involved! The VFD will give you speed control and possibly braking and jog BUT a VFD on a LATHE is not plug and play! Even on my saw the VFD isn't plug and play but the wiring was very quick and straight forward because there is no need to reverse a saw like you do a lathe.

 

[Dabbler]

you should not have any switches between the VFD and the motor

That is great to point out - especially in a thread where new-to-VFD people will be reading!

 

[vocatexas]

Thanks for that info, Mike. I had intended to use the drum switch on the lathe, but I'll make sure we remove it and wire the VFD directly to the motor. It will probably be Greek to me, but do you know why you can't use the drum switch? Just curious...

I doubt I will use any dynamic braking or reverse on the Southbend. It uses spin-on chucks. I don't care to eat one!

 

[rabler]

I'm not Mike, but -
Any switch between a VFD and motor (drum switch or other) will cause a problem with the VFD. A switch causes voltage transients that the VFD doesn't expect. It is a bit like water pipes that knock when a soleniod on your clothes washer kicks on/off (or any other fast valve). That extra force can rapidly degrade the electrical components in the VFD.

 

[jwmelvin]

It’s more accurate to say that operating a switch between the VFD and motor while the motor is energized is likely to cause a problem for the VFD. Basically, when energized, the VFD relies on having a reliable connection to the motor.

I use a drum switch between the VFD and motor to switch the motor between low- and high-speed configuration (4-pole vs. 2-pole). It works fine; I do not switch while the motor is energized. The drum switch also informs the VFD to change motor parameters. It’s true that simply having the switch where I do is a potential point of intermittent connectivity, which would be a problem for the VFD, but that has not been an issue in my experience.

 

[jmkasunich]

Not to hijack this thread, but there was a comment above regarding using a VFD on a single phase motor, utilizing only two of the three available legs from VFD's output, presumably to control the speed.

Does this really provide a speed control or other advantages?

Using a VFD with a single phase motor is in general a bad idea. Starting torque suffers, speed range is very limited. If the motor uses capacitors (start or run caps) they may not be happy with VFD power and the VFD may not be happy with them. If there is a centrifugal start switch the motor won't run right below the speed at which the switch trips. The electrical transient when the switch opens might make the VFD unhappy. You are paying for three phases worth of expensive transistors but only using two (in other words, to run a 2HP single phase motor you would need at LEAST a 3HP VFD).

Basically, three-phase motors are the ideal electro-mechanical machine. Single phase motors make a lot of compromises in order to run on single-phase power. Why would you want to accept those compromises when you have a VFD that is making perfectly good three-phase power? Put a three phase motor on the machine and never look back.

 

[Papa Charlie]

Using a VFD with a single phase motor is in general a bad idea. Starting torque suffers, speed range is very limited. If the motor uses capacitors (start or run caps) they may not be happy with VFD power and the VFD may not be happy with them. If there is a centrifugal start switch the motor won't run right below the speed at which the switch trips. The electrical transient when the switch opens might make the VFD unhappy. You are paying for three phases worth of expensive transistors but only using two (in other words, to run a 2HP single phase motor you would need at LEAST a 3HP VFD).

Basically, three-phase motors are the ideal electro-mechanical machine. Single phase motors make a lot of compromises in order to run on single-phase power. Why would you want to accept those compromises when you have a VFD that is making perfectly good three-phase power? Put a three phase motor on the machine and never look back.

All very good points. After @Dabbler comment, the thought of replacing my single phase motor with a three phase did come to mind. There are a lot of advantages to three phase over single. Something to consider.

 

[MikeInOr]

The posts above do a good job of explaining why you do not want any switching between a VFD and motor (lathe motor).

I really like the feel of the original big drum reversing switch to control my lathe and do not have confidence in myself to always remember to power the motor down via VFD BEFORE switching the drum switch. My solution was to remove the drum switch from the high voltage wiring and repurpose it as a low voltage control switch for the VFD. The following thread was my starting point for my VFD control of my South Bend lathe.

VFD'ing Around

I was not sure where to post this thread so it is here. Purpose is a little bit of documenting, bragging (can I do that?) and sharing what I learned. When I purchased my little Asian lathe (PM1340GT) I knew nothing about turning metal so I did not want anything standing between me and getting...

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