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VFD Vs. Rotary Conversion

5K views 31 replies 9 participants last post by  JRaef 
#1 ·
So i am kind of torn between getting a vfd or rotary converter for 3 phase in my home shop. I need something in the 20-30 hp range. All Advice welcome.
 
#2 ·
It sounds like you're wanting to operate more than one motor, maybe several at once.

If so, a VFD would be a very poor choice. They do not do well with motors coming on and off while it's running.

If a motor is switched on or off while the VFD is producing power, it's very likely to burn up.

If a VFD operates one motor, the starting and breakdown torque will be less than if it were on POCO power, but it'll produce its rated HP once it's running.

A rotary phase converter doesn't care though. It's perfectly happy to run just about any 3Ø load within its rating. Figure about 80% of rated HP for any motor. And less starting torque.
 
#3 ·
There are other options, depending on your price range, and what you intend to run in your shop.

Digital phase converters, for example. They can run multiple motors if need be, in the range of hp you are giving.

Never used them, just found them doing a search a few days ago, when someone else asked about running a three phase compressor on single phase.

Borgi
 
#5 ·
Is your service sized to handle it? Most of the shops I've seen use a rotary phase converter. The biggest issue I've run into is you have to make sure none of the controls are connected to the manufactured leg. Some of the cheaper ones, or the homemade ones, seem to have a higher starting current than the high end ones. How many motors are you going to be running at the same time and what size?
 
#9 ·
I am an electrical engineer, just not in the complete sense. I never finished college for financial reasons so i could not be certified, but i do know alot.

Anyway, I can spend about 2g. It would be located in my home shop(a pole barn on my property). i have 200 amp service.
 
#12 ·
Probably oversized for your application. The converter must be sized to the largest motors you're going operate and other factors. An oversized converter can cause problems. A number of manufacturer's web sites have sizing charts etc.
Be sure to study the appropriate code section to find out what the requirements are.

http://www.phaseconverter.com/rotary-converter.sizing/
 
#21 ·
VFDs are much better at running A motor, not A shop, for the reasons' micromind already said.

But here's the rub on something like your 15HP compressor as well. Yes, a VFD can accept single phase input and run a 3 phase motor, but above 3HP at 230V, you must at least double the size of the VFD with respect to the motor. So your 15HP compressor will require you to buy at least a 30HP VFD. Then the NEC now requires that the conductors feeding a VFD are to be sized based upon the VFD maximum amps, not the motor. A 30HP 230V VFD is going to be rated for 85A, which would mean conductors rated for 106A minimum, so 1ga. building wire, even though the load is actually only going to be around 45A. Then even though you could use a 60A breaker because it really only has to cover the 15HP motor, that size cable is unlikely to fit in the lugs of a 60A breaker, etc. etc. etc.

Bottom line it's rarely practical to power large 3 phase motors from a single phase service through a VFD. Were it me, I would maybe build my own RPC, but for that much power, you might want to inquire about getting a 3 phase service.
 
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#24 ·
This is the start of the thread.



This is the end. After telling us there are multiple motors, drill presses, two compressors, and that you have a 200 amp single phase service in your shop.

That makes sense. I think its coming down to either a huge rotary on its own service line or running 3 phase straigh from the pole.
Now your telling us three phase is available at the pole! :eek:

Did you buy all the three phase equipment before or after you built or moved into the shop?

Not to mention you think you're an Electrical Engineer! :mad:

Borgi
 
#25 · (Edited)
This is the start of the thread.



This is the end. After telling us there are multiple motors, drill presses, two compressors, and that you have a 200 amp single phase service in your shop.



Now your telling us three phase is available at the pole! :eek:

Did you buy all the three phase equipment before or after you built or moved into the shop?

Not to mention you think you're an Electrical Engineer! :mad:

Borgi
Im going to clear all this up. I have plans to build a shop on my property in the spring. I already have the ability to have 200 amp service when i build the shop. I am planning on having 3 phase equipment instead of single phase. I I already know what equipment i need.

When I said its available at the pole, i meant have them run 3 phase service from the local mains, not from my property, or any kind of converter.

I am sorry for any problems i have caused.
 
#29 ·
Now that all the hugging and back patting is done, here's another little dose of reality for you.

The cost of getting 3 phase service will depend GREATLY on whether or not there really are all 3 phases on your pole or not. In far flung rural areas, it's not unusual for there to be only 2 wires, maybe even one (SWER), which means single phase service only. So if the nearest 3 phase tie in point is 5 or 15 miles away from you, they will make you pay for running the extra cables the entire distance, and it gets REALLY expensive. A quick way to tell is if any of your neighbors have 3 phase services. If so, you are OK.

Then assuming this is OK, the next issue is that with 3 phase service, you also often get another nasty little surprise you may not be used to now, a Demand Meter. Public utilities can't charge residential users for Peak Demand, but they can for commercial and industrial users, and depending on the utility, that definition is often tied to 3 phase service drops. So just be very thorough in your investigation, don't assume anything and you shouldn't have any surprises.
 
#32 ·
John Valdes said:
My bet says "Marathon" :laughing:
Why, because of the Marathon "Blue Max" color?

I've always thought that was an interesting choice of marketing names. People from the WWI era would not have liked it, it would be kind of like calling your product the "Jihad Series" now. But by the time Marathon started using it most of the WWI era people were either dead or at least out of the workforce, so almost no backlash I guess.

For those who don't know, the Blue Max was a German medal of honor given to WWI pilots for getting 20 confirmed kills in air to air combat. There was a movie about that in the 60s, my grandfather about blew his stack when it was released. He was a cook in the rear lines in WWI, but had been strafed many times by German planes, which he though was cowardly because they were unarmed. Plus his childhood friend was a pilot who was killed in the air by the German pilots. He hated them to his dying days.
 
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