Rpc Or Vfd?

Milehimachine

Milehimachine

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Which is more efficient a rotary phase converter or a variable frequency drive? Since what I know about electricity is you dont mess with it. As I get closer to my shop getting set up I need to use the most effective system since the machines Im getting are 3 phase and I dont have 3phase in my home so I need to add one of these devices. My father had his shop setup with a rotary phase converter years ago and it seemed to be a simple set up only minor wiring. What I see having to do with a VFD and what some others have done far exceeds my abilities with electricity. The machines I will have will be a 2hp Series 1 variable speed Bridgeport and a 11" 1 hp logan lathe. Any insight would be appreciative.
 
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Good question! Given that the VFD is only used on the machine that it's attached to I would have to say that the VFD would be more efficient than a RPC which is on all the time when you are going to be using the machines.

The advantages to the VFD is the variable speed control, this allows you to make adjustments on the fly to optimize your cutting speeds, and the simple shop wiring.

Having a central RPC can be a bit of a complex wiring excersize, to do it correctly you would want to feed the output to a 3 phase distribution panel, then to your machines. You don't say how many machines you are planing on installing, but if more than three, then you need to look at the total cost of each system.

In general the cost of multiple VFDs could exceed the cost of a RPC, but the convenience of the VFDs are a pretty good offset in my book. If your machines are 3 HP or less, then I would definitely go with VFDs. The cost of VFDs greater than 3 HP starts rising pretty dramatically as the power goes up. This is because there are a lot of VFDs available that are rated for 3HP, with 240V, single phase input. Over that HP, most are rated for 3 phase input only, which means that you have to oversize the VFD if you want to run it on single phase. For instance, if you want to run a 5hp motor, then the minimum size VFD would be 7.5HP, or better a 10HP rated VFD.

Don't be afraid of the wiring issues of the VFD, the basic wiring is pretty simple, and there is a lot of help available here.
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I'd put a vote in for RPC for multiple machines,
as Jim said earlier.
VFD's are a great choice if you only have one or 2 machines that need them.
I have both VFD's and RPC's.
i use the VFD's for the low HP applications and use a 5hp, 7.5 hp and 10 hp as necessary on the larger equipment.
 
Thanks Jim if I went with a VFD how can I use that and keep using the existing system like fwd/rev and variable speed on a Bridgeport series 1 that already has variable speed control?
 
VFD is more efficient for the reason Jim stated above and because RPC'S have more in heat and friction losses, you are running an extra motor.
The VFD has heat loss and usually a small fan, but only when the motor its driving is running, when idle, only a tiny amount of current for the control circuit is drawn.
 
That is no problem. That is exactly the setup that I have on my mill. You could use the existing FOR/REV switch on the mill and simply wire it into the VFD control terminals. I have my VFD mounted in the location of the original switch, so I just use the front panel buttons for all of my control. I normally leave my VFD set at 60Hz, but if I need slower speed for light operations like reaming, countersinking, or small power tapping then I turn down the speed on the VFD rather than going into back gear.

Here's a pic.

counterbal2.jpg
 
Milehimachine said:
If I went with a VFD how can I use that and keep using the existing system like fwd/rev and variable speed on a Bridgeport series 1 that already has variable speed control?
Click to expand...

Yes and you can also mount the VFD on the wall near the machine. Here is a link to a thread I started on wiring a switch for my mill that wasn't immediately compatible with my VFD: http://www.hobby-machinist.com/thre...omentary-pushbutton-switch.39157/#post-336083
It doesn't need to be that complex, I only went that route because I wanted to use the switch I had on hand.
 
Hi,

What is the voltage you will use?

Vfd will generate a square wave that is basically a set of harmonic sinus shaped waves. I have not measured these waves but it is a risk that they can reach a voltage peak value that is above the specified isolation voltage typically for older motors.

This is mostly a concern for 380vac motors.

I have a cva lathe that is wired for 380/600vac and I selected to skip the vfd because of my concern that the high voltage spikes could damage my old motor. I did build a rpc and they are not overly complicated. Because of my need to have a TN ground network I changed from rpc to permanently installed 3 phase with an isolation transformer (IT to TN ground system).

I might be too precautios on this topic and I am happy to learn more.

Regards
Bjørn
 
Hi Milehi,


You have raised a good question (VFD vs. RPC). Many folks have worked through this before you, so you can get plenty of advise, ideas & opinions. Both of the solutions you are considering can work very well (see responses above). Another solutions is an electronic device, called "Phase Perfect" - more expensive, but can also provide a whole shop solution (gives very good quality 3 phase power - which the machines you spoke of do not need, but certain newer machines do).


If you follow the manufacturer's instructions and the appropriate electrical codes - all the above options will perform well. If you do a poor install - none of the above options will perform very well.


Jim and Ulma presented a good questions, with - how many machines will you be looking at? I started with one machine quite a few years ago. I splurged and bought an off the shelf RPC - from then on, I have mostly aquired 3 phase machines (I have 7 - 3 phase machines plugged in now). With whole shop 3 phase, when there are feed motors, or coolant pump motors - there is no issue powering them (just plug in the machine and everything works).


You said you were wanted the "most efficient" solution. What do you mean by efficient? If you are considering energy efficiency - the RPC is going to lose (as others have pointed out). If you are powering several machines - you only have to sort the issue out once, then connect everything up - the RPC is very efficient on your time.


I use an RPC and I am a home hobby guy (with a day job). I acknowledge that the RPC is not as efficient as other options, but whether I spend a little time in the shop, or a lot of time in the shop - my power bill is really not affected by the lower efficiency of the RPC. I'll bet that if I was running the RPC all day, every day (60 hours a week) - I doubt my power bill would be up by more than $20 (of course, the VFD does not run free either - so I don't see the difference in energy efficiency as a major aspect in the decision).


I have two variable speed machines (drill press with a VFD, and a mill with a Reeves drive) - I really have not found the variable speed (or lack thereof on both lathes and my original mill) as a big deal. Multi-speed machines have been around a long time, with a lot of very good work done on them. Certainly VFD is a nice to have. However, gears are simple, strong and you get full torque at the low speed (VFDs don't have great turn down - with much torque left). VFDs are nice, but they have short comings (supposedly hard on non-rated motors).


Let us know what you finally decide on.
 
I vote for the RPC you can run your whole shop on it for less cost . If you need a variable speed you can go DC drive or vfd. If you have a bunch of machines go RPC.
 
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