Can a VFD be used to just provide 3ph Power?

[Assiniboine_Iron]

More to the subject - yes, I know a VFD can convert 1ph to 3ph - this is my goal however I'm curious, is there more to the "Type" of 3ph power being supplied? Can I not just set my VFD at 60hz and bring 3ph 220v power into the back of my lathe to run the internal electrics as the MFG intended? all the mag switches and transformers working as planned? or is there more to this and I need to essentially bypass all the factory switches.

I have VFD's on a couple other machines - a 3hp mill and a 2hp belt grinder - both of these are wired to give me speed variation on the dial. I don't "Really" need the complication in the Lathe - just need the 3ph power. The VFD was cheaper than a digital phase converter - so I went with the VFD. Can it be dumbed down to just sit there and send me power, fixed at 60hz?

 

[macardoso]

Edit: Brief answer removed

 

[macardoso]

Short answer, No. The VFD output should only be wired to the motor.

Long answer, VFD's provide variable frequency AC current, but rapidly switched DC voltage which approximates an AC waveform but is not truly one (see image below).



This rapid pulsing of DC voltage should be assumed to be harmful to electronics, power supplies, etc. It is permissible to pass this waveform through switches, contactors, and to power motors. Do NOT feed this voltage into a transformer, you will damage it very quickly. This voltage also has very high voltage transients (dV/dt) which causes a lot of electrical noise in your shop. Short runs and shielded cable between the VFD and motor are recommended.

Couple of other comments, single phase input, three phase output VFDs are really only available up to 3HP (sometimes 5HP). Beyond that you must use a three phase input VFD with a single phase input derating. This is typically 50% of the drive rated power, consult the drive user manual.

Sine filters are available to put on the output of a VFD and can generate very clean low harmonic sinusoidal AC voltage, but they are quite large and expensive and typically reserved for very long cable runs (up to several miles!) where reflected waves in the cable (transmission line) can generate peak voltages beyond the insulation strength of the motor windings or the dielectric breakdown threshold in the IGBTs on the output of the drive.

It is typical to mount the VFD on the machine and then wire all the IO (switches, lamps, etc.) into the 24V IO points on the drive. The functions of these IO (Start, Stop, etc.) can be set up in the drive configuration. I know this might seem intimidating to someone who is new to VFDs, but you would not see it done any other way on a professional machine.

Tip: Do NOT place a switch between the VFD and motor which can open while the motor is running (unless it is for machine/life safety application). The magnetic field breakdown that occurs when this switch opens will cause a massive voltage spike which can damage the motor or more likely the IGBTs in the drive. Use the drive IO to command it to stop.

Many VFDs today have a Safe Torque Off (STO) input. This is a safety rated circuit that immediately cuts power to the motor in a highly reliable manner. You should look for a VFD with this feature and you should use it to wire up your ESTOP.

 

[Eddyde]

Can I not just set my VFD at 60hz and bring 3ph 220v power into the back of my lathe to run the internal electrics as the MFG intended?

Basically No, The VFD must be wired directly to the motor, no switches, transformers, lights or anything else can be in the circuit.
While VFDs can convert AC power from single to three phase, it isn't their primary purpose, they are motor control devices.

If you want plug-n-play, consider buying or building a Rotary Phase Converter.

Here is my journey: https://www.hobby-machinist.com/threads/rotary-phase-converter-rebuild.73303/

 

[Assiniboine_Iron]

Basically No, The VFD must be wired directly to the motor, no switches, transformers, lights or anything else can be in the circuit.
While VFDs can convert AC power from single to three phase, it isn't their primary purpose, they are motor control devices.

If you want plug-n-play, consider buying or building a Rotary Phase Converter.

Here is my journey: https://www.hobby-machinist.com/threads/rotary-phase-converter-rebuild.73303/

Thanks for the insight. Given the cost, size and complexity of the rotary converters (in my case primarily the cost), and I this is purely a hobby for me at the moment, would a digital or static phase converter and take the HP reduction be a reasonable middle ground and still provide the 3ph feed the Lathe needs?

 

[macardoso]

Honest question, are you planning on powering multiple 3 phase machines off the rotary or static converter? If not, VFD conversion on the lathe might be the better option and adds the adjustable speed feature to your machine

 

[markba633csi]

A static phase converter might be a good temporary solution- it's basically a large capacitor in a box. You get 2/3 the horsepower
from any given motor but you can't beat it for simplicity and ruggedness
Essentially it converts a 3-phase motor to a permanent split-cap (also called capacitor run) motor
-Mark

 

[Assiniboine_Iron]

Honest question, are you planning on powering multiple 3 phase machines off the rotary or static converter? If not, VFD conversion on the lathe might be the better option and adds the adjustable speed feature to your machine

I'm only concerned with the Lathe. The other 2 machines are on small VFD's as I want the variable speed and do not have all the start/stop needs of the lathe and it's wiring. I was contemplating a re-wire of the lathe but believe I will make my life hell trying to wire in all the braking and direction changes needed in typical machining. The hand switch on the carriage also scares me - that I'd lose that function.

The relays and switches internal to the lathe presumably work already - feeding "simple" 3 phase and using the existing wiring seems to be the most straight forward solution. At this point it looks like a rotary or (most likely) static or digital converter is what I should have gone with (already have the VFD on order, so I think it'll go back).

In hindsight, if I had seem my collection of 3ph tools growing like it did - I probably would have invested in a rotary converter in the first place.

 

[pdentrem]

I would be surprised that the internal controls require 3ph. Usually they are using only a single hot and then that feeds a transformer for the controls. I know that some machines use 3ph coolant pumps but none here at work do nor at home. I would check the wiring diagrams.
Pierre

 

[Assiniboine_Iron]

A static phase converter might be a good temporary solution- it's basically a large capacitor in a box. You get 2/3 the horsepower
from any given motor but you can't beat it for simplicity and ruggedness
Essentially it converts a 3-phase motor to a permanent split-cap (also called capacitor run) motor
-Mark

Thanks Mark. Yes I think I'll give this method a go. If I power out, I'll have to look at a rotary. Seems like a "cheap" experiment.