# Can I Add A Vfd After The Disconnect?



## John Caven (May 22, 2015)

Ok so I purchased and had installed a 3 phase rotary converter. I'm thinking I'd like to control my mill only with a VFD. it's a 1/2 HP bridgeport M head. I have a 3 phase load center box that is wired to a disconnect and the disconnect has a plug on it for my milling machine. 

Could I some how go from the machine to a VFD to the plug? I don't need the phase converter features. I just want the ability to control my motor speed.  Do I just ignore the 1 phase input side and use only the 3 phase input and output on a VFD?


----------



## Karl_T (May 22, 2015)

VFDs can use either 3 phase or 1 phase input. Simpler is better, you could go from 1 phase to 3 phase converter to VFD, but i would not recommend it. That said, I've done just this on one of my mills. It had several 3 phase motors and only one for speed control.

I think it works best for the VFD to be a part of your machine. I generally mount them inside an electrical cabinet. This means needing a remote switch to start/stop the machine. And a pot to control speed.  You can just use the run/stop buttons and speed pot on the VFD itself, but they are not durable. Break them -> buy a new VFD.

Let us know what we can do to help.  You'll like a VFD upgrade.


----------



## mzayd3 (May 22, 2015)

Should be fine, but if you connect or disconnect the mother while the vfd is running, the drive will fault.


Sent from my iPhone using Tapatalk


----------



## CluelessNewB (May 22, 2015)

Both single phase and three phase input VFDs convert the input power to DC and then convert that back to AC.  Every VFD is a phase converter!  The main difference between 3 phase input VFDs and 1 phase input VFDs is the number of rectifiers used to convert AC to DC.  The 3 phase input VFDs would have more parts to fail .  How well balanced is your RPC?  I'm not sure how tolerant VFDs are to unbalanced input.    Personally if you want the VFD for speed control I would just go with a single phase VFD, I don't really see any benefit from using a 3 phase input VFD for such a small motor.


----------



## John Caven (May 22, 2015)

Ah this is making my brain hurt.       I'm not understanding the 1 phase vs 3 phase.    All I know is that my RPC turns the power to 3 phase and sends it to the 3 phase load center that sends 3 phase power to both my machines that are on a disconnect.  Every time go to look at getting a VFD I get totally confused and I drop the issue of doing anything with a VFD.    I feel like a toddler trying to run backwards.     

So instead of using single phase or 3 phase...  Can someone just tell me in terms a 3rd grader can understand please?    IE:   Does hooking up the power leg from the pigtail on my 3 phase disconnect that already has 3 phase power to a VFD work and in simple to understand English and realizing your most likely talking to an idiot ..   (haha)   how would one do it?


*Karl:  I know you said I could but it's a bad idea.. why?*    I might have made my bed if that's the case and will just have to stick with RPC only.  ugg


----------



## mzayd3 (May 22, 2015)

I must have misunderstood the question altogether.  In my eyes, just get rid of the rpc and put in a 120v (or 240v if that's what you have)1/2 hp vfd.  Simple and done.  Don't worry about the behind the scenes stuff.  Single phase in, three phase out.


Sent from my iPhone using Tapatalk


----------



## Karl_T (May 22, 2015)

John Caven said:


> ...
> 
> *Karl:  I know you said I could but it's a bad idea.. why?*    I might have made my bed if that's the case and will just have to stick with RPC only.  ugg



Its just wasteful. More componets to fail and give trouble. KISS is best (keep it simple stupid) It will work.


----------



## JimDawson (May 22, 2015)

+1 what mzayd3 said.  You don't really need both a RPC and a VFD for low power motors.


----------



## John Caven (May 22, 2015)

I just spent $900 on my RPC unit.  I don't think I'm getting rid of it.   I also use it to power my 5 HP lathe.  It's a 10HP american rotary RPC.         VFD's keep leaving me feeling very very frustrated .


----------



## John Hasler (May 22, 2015)

John Caven said:


> I just spent $900 on my RPC unit.  I don't think I'm getting rid of it.   I also use it to power my 5 HP lathe.  It's a 10HP american rotary RPC.         VFD's keep leaving me feeling very very frustrated .


I think that everyone was assuming that the mill was the only thing you were powering with the RPC.


----------



## John Caven (May 22, 2015)




----------



## JimDawson (May 22, 2015)

John Caven said:


> I just spent $900 on my RPC unit.  I don't think I'm getting rid of it.   I also use it to power my 5 HP lathe.  It's a 10HP american rotary RPC.         VFD's keep leaving me feeling very very frustrated .




That changes things a bit.  Since you have a RPC it's safe to assume that your shop is supplied with single phase power.  I would use the RPC to power the lathe and other equipment as needed and run another line from the single phase panel to the mill, then run the mill with the VFD.

The reason you are not getting a definitive answer to your original question is that nobody has run a VFD off of a RPC, so no experience with that arrangement.  Will it work?  Probably, but the RCP outputs a odd wave form so nobody is willing to say 'Yeah, that'll work''.  I'm not going to try to give a detailed technical explanation here, but the wave form of single phase is 2 legs 180 degrees, the wave form of 3 phase from the power company is 3 legs that are 120 degrees, but the wave form from a RCP is somewhere in between and is load dependent.  So not sure how the VFD will react.

On a 1/2 hp motor, I might be tempted to get a 120V input VFD (240V, 3 phase out), and just plug the mill into the nearest outlet. Here is an example of that type VFD
http://www.automationdirect.com/adc...ts_(115_-z-_230_-z-_460_-z-_575_VAC)/GS2-10P5

Or just run another 240V line from the single phase panel and run the mill with a single phase input VFD.
.
.


----------



## John Hasler (May 22, 2015)

JimDawson said:


> That changes things a bit.  Since you have a RPC it's safe to assume that your shop is supplied with single phase power.  I would use the RPC to power the lathe and other equipment as needed and run another line from the single phase panel to the mill, then run the mill with the VFD.
> 
> The reason you are not getting a definitive answer to your original question is that nobody has run a VFD off of a RPC, so no experience with that arrangement.  Will it work?  Probably, but the RCP outputs a odd wave form so nobody is willing to say 'Yeah, that'll work''.  I'm not going to try to give a detailed technical explanation here, but the wave form of single phase is 2 legs 180 degrees, the wave form of 3 phase from the power company is 3 legs that are 120 degrees, but the wave form from a RCP is somewhere in between and is load dependent.  So not sure how the VFD will react.
> 
> ...


An RPC (as opposed to a "static comverter") supplies true three-phase,  though the voltage of the generated phase may droop a little.  However, it is not balanced with respect to the neutral of the single-phase source.  I've never used a three-phase VFD but I'm guessing that the full-wave bridge in it would need the three-phase neutral.   A three-phase VFD might or might not work if you just hook it up to your RPC.  I second Jim's suggestion.


----------



## mksj (May 22, 2015)

I did a recent install that had single phase to an 10Hp RPC and a distribution panel to multiple machines (10+). Some used VFDs at the machine for variable speed and braking requirements. We equipped a plate  roller with a Hitachi WJ200 VFD (3 Phase input) running the plate roller 2Hp drive motor, the height motor ran directly off the 3 phase RPC, the electronics required another separate DC supply and used a step down transformer for the 120VAC hour meter. There were no issues or problems.  RPCs are used to run CNC machines that use VFDs.  Since a VFD rectifies the incoming AC to DC and then the output is regenerated back to 3 phase AC, I am not clear on the issue, but I am only going on posted manufacturer  information on the topic. There may be some derating factor of the VFD based on the quality of the 3 phase power and the phasing. Yes, KISS is fine, but with a shop full of machines with different power requirements, you try to do the most logical and practical approach. You also reach limitations of motor sizing when using a single phase input VFD. Contact American Rotary and see if there unit will work with a VFD and if their are any limitations or issues, or what they recommend.

http://www.kayind.com/tech_center/FAQ.htm
Can I operate a motor with a variable speed (variable frequency) drive (VFD)? (off of a RPC)
Absolutely. And don't believe anyone who says otherwise. Just be sure to make it clear that you will be powering a VFD because it may change the size recommendation.

So you need to decide what is the easiest option for you, you could run a separate single phase branch circuit to the mill, or you could use the current RPC 3 phase and power the VFD from that. Many VFD are rated based on single phase input or 3 phase input, so you should use a VFD that is matched to the input power and the mill motor rating. Some of the smaller VFD can be wired for 1 or 3 phase input (see VFD specs). So you would wire the incoming power to the VFD and the output of the VFD directly to the mills 3 phase motor. You could use a power disconnect switch or a plug before the VFD to break the power. You would use the VFD main panel or wire switches and a pot to the input terminals to control the direction and speed. You would probably use a 1HP VFD for the mill.


----------



## ChrisW (May 27, 2015)

My take on it is......why fire up the RPC  just to run a 1/2 HP VFD?  Seems like a waste of energy.  Do what JimDawson said....plug the VFD into a 120v outlet and simplify your life.  Save the RPC for when you need it for the lathe or anything else you get in the future.

Chris


----------



## bbutcher (Jun 2, 2015)

There is no need to run the 3 phase convertor. Just buy a suitable VFD that has adequate horsepower and connect it to either 110 V or 220V as specified in the documentation. One thing to keep in mind, your forward/reverse switch on your mill will not work with most VFD's . Just leave it in forward position and use the VFD to start (either FWD or REV) and stop the mill. At least on my setup when I start the VFD first and then try to connect a motor, I get an overload condition on the VFD, probably due to high start-up current of the motor.


----------

