Phase converter vs VFD

I know these questions come up but I need some input. So far I have the Bridgeport mill on a VFD. Now I need to power the lathe and the surface grinder. The question is to keep going VFD or to buy a rotary phase converter. Literature says I need a 15hp VFD for a 7.5hp lathe. I was hoping to get a 10hp RPC and use the existing 40amp circuit its on. If I go RPC, I need to pull another wire and it would be worth it to run a 3phase panel and distribute power.

OR I can just buy 2 more VFD's. Total price for VFD's (Im running 2 chinese ones now) its about $500. The RPC is $900 (10hp...$1300 15hp) and need a panel and misc electrical supplies. If I run VFD's, I can skip the new power pull, panel, etc. I actually may be able to get away with a single VFD for both the lathe and the surface grinder because they are both 7.5hp units.

Any input?

 

VFD's have over current protection and ramp up/down settings. In ALL cases the inrush current when you turn the motor on WILL be higher than the max current the motor will draw at full power. Because my house only has 20amps to the meter box I have my 5hp Mill VFD set to slowly ramp the motor so it does not trip the main house breaker.

If you set the VFD parameters correctly you can run a 7.5hp motor on a 5hp VFD, the VFD will limit current to the motor so the motor will not make 7.5hp.

You will need a VFD for each different motor, as the motor parameters (RPM, HP etc) need to be entered into the VFD so changing motors requires re-programming the VFD.

 

VFD's have over current protection and ramp up/down settings. In ALL cases the inrush current when you turn the motor on WILL be higher than the max current the motor will draw at full power. Because my house only has 20amps to the meter box I have my 5hp Mill VFD set to slowly ramp the motor so it does not trip the main house breaker.

Circuit breakers already have some tolerance for inrush current.
https://www.electronicdesign.com/el...ng-wrong-circuit-breaker-waste-money-or-worse
Electrical circuits often have spurious spikes, and must be able to accommodate them. Circuit protection is to protect the wiring, not the connected devices.

 

I just bought a VMC and needed a phase converter to run it.

I picked up a brand new 25hp Siemens motor for $500 shipped and got a converter panel for $275 from Crowman Converters. I was amazed how easy it was to wire it all up and how balanced the power is. For $775 I'm pretty happy with it.

From what I can see the larger motors go for less than smaller ones on Ebay. I'm guessing there isn't much of a market for them.



http://www.cromanconverters.com/

 

Circuit breakers already have some tolerance for inrush current.
https://www.electronicdesign.com/el...ng-wrong-circuit-breaker-waste-money-or-worse
Electrical circuits often have spurious spikes, and must be able to accommodate them. Circuit protection is to protect the wiring, not the connected devices.

Yes but those spurious spikes are usually a few ms. Where as inrush current can be a second. Breakers here in Australia must trip in 23ms. Before I retired I worked in the repair centre of a Calibration Laboratory and have repaired and calibrated loads of Megger circuit breaker test equipment.

 

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