# Phase converter vs VFD



## Bob Korves (Nov 19, 2018)

You can use a 10 hp or even a 7.5 hp VFD with your 7.5 hp motor.  They are rated for that power.  We usually go higher, mostly just "to be safe" or in case we get a bigger machine down the road.

The other question is if you care about the additional abilities of a VFD, like being able to change speeds easily with the turn of a knob.  The RPC can handle all the machines, but no variable speed, or other perks. 

The rotary phase converter is easier to set up.  

Fuses and breakers are there to protect the wiring.  The VFD needs to power the machine.  If all is workable, then you are good.  You did not mention voltage, wire length or sizes, or other details, and the devil can be in the details.

In a one man shop, only one machine will be running at a time, typically.


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## Eddyde (Nov 19, 2018)

I disagree with Bob on the VFD sizing. Unless they are specifically rated for given horsepower at single phase input, you should indeed derate the VFD by ⅓, if you are using it to convert single phase to 3 phase (actually not their primary purpose). None of the Chinese units I have seen are, they say to derate for single phase, check the fine print in the ad or look in the manual.  A VFD rated for a 7.5 hp 3 phase motor with 3 phase input, should only be used to power a 5 hp motor on single phase input. So a 10 hp VFD will also be light since it should be only used on a max 6.6 hp motor. Since the next size up is 15 hp, that's what you'll need to run 7.5 hp and be in spec. Of course, you may get away with a lesser powered unit but I wouldn't recommend it. 

I have traveled down a similar road. I have VFD's on my mill, drill press, bandsaw and air compressor. I just added a RPC for a new lathe and soon will use it to power my surface grinder, I'll likely switch the compressor over as well. I like having both systems. VFD's are fine for machines you really need the speed control, reversing etc. In the case of my lathe it has 16 easy to change speeds so no need for the VFD.


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## 8mpg (Nov 19, 2018)

Thanks for the info both of you. The lathe is technically 5.2kw which is closer to 7hp. I doubt I will ever use the 7hp of the lathe. It also runs the coolant pump which I will no longer use. So Im thinking the thing will pull a max of say 6-6.5hp. This will be the biggest motor I have to run and it should soft start essentially. I think Im going to do a 10hp RPC. The surface grinder is actually 5hp spindle 2hp hydraulic pump. Again, I doubt those will run at max load ever. I almost might be better off putting a 5hp motor in the lathe if I need to.

After reading and reading, it seems that a single VFD for the two isnt the best idea. The motors while total close to the same hp, they are not probably the same (different motor speeds). So two VFD's would be in order. I was thinking that the VFD's (two of them) would be close to $500. A larger 15hp one (lots of people say its better to derate) is over $1000. So... RPC it is.


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## ThinWoodsman (Nov 19, 2018)

I had a similar problem and went with the rotary phase converter. Both lathe and mill are variable speed (though not as minutely controlled as a VFD would make possible), and using the machine's gearing ensures a bit more power for those heavy cuts. 

If you use only one machine at a time, size the RPC for the largest machine, and give it a couple extra HP for the startup surge. This puts you into the 10 HP range, as you expected. The American Rotary people will tell you to double the HP (see Quick Sizing Guide section: ADX RPC) wihch would put you into 15 HP territory, but that seems a bit excessive.

I ran a 50-amp breaker to a (60-amp) disconnect switch, then to the RPC, then to a 3-phase subpanel with room for 4 breakers (i.e. twelve terminals. 3 per beaker). I think it was about $150 for the electrical components. The disconnect is because the RPC draws single-phase power even when it is turned off, so a master off switch is nice to have. Can give you the make and model for these if you're interested.


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## 8mpg (Nov 20, 2018)

Well... I was going to go with American Rotary but pulled the trigger on North American instead. For $10 more you get a lifetime warranty vs 5 year warranty on the American Rotary. Also picked up wome 10/4 SOOW wire and a non-fusible switch from Amazon. Sometime next week or the week after, it should be up and running.

Trying to find a 3phase panel and breakers cheaper. Call me cheap but breakers are $40 each!


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## ThinWoodsman (Nov 20, 2018)

Yeah the Q340 breakers ran me forty bucks a pop. Forgot about those, that would bump the electrics to over the 200 mark. 
Panel was a Siemens P1224L3125CU (rolls right off the tongue, dunnit?) at something like eighty or ninety bucks.

I used 6/3 from the breaker through the disconnect to the converter, and converter to idler. 10/3 from the converter to the panel and from the 3-phase breaker to each machine. The 6/3 is a bit of overkill but it was a short distance (maybe 10' total) and I really don't want that are getting hot.


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## ThinWoodsman (Nov 20, 2018)

Heh internet went out due to the snow right when I was editing that post for typos. Hello, half-hour edit timeout.

BTW I used a fusbile safety disconnect, fifty something bucks. A neighbor got his entire house fried by a surge from the power company (no, not lightning) so I figured a bit of protection was in order.


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## 8mpg (Nov 20, 2018)

Thanks for the part numbers. I ended up ordering the box and two 40 amp breakers. This whole project is getting more expensive than everything I have in my machines. LOL


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## ThinWoodsman (Nov 20, 2018)

Eh, it's a one-time cost - and you'll sleep better for having done it right to begin with


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## warrjon (Nov 28, 2018)

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.


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## Bob Korves (Nov 28, 2018)

warrjon said:


> 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.


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## alloy (Nov 28, 2018)

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/


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## warrjon (Nov 29, 2018)

Bob Korves said:


> 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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