Assume that I have in front of the above referenced options. I can pick only one - a VFD, a (rotary) Phase Converter or straight 3-phase power for the 3-phase 16" Rockwell RAS I picked up for $300. In the long run, which will cost less to own and operate over time? Assume the 5hp VFD is $250 and the 5hp converter is $400. Am I likely to spend a lot more money on power if I have to spring for 3-phase direct? Manintenance issues with the VFD and converter relative to 3-phase power - and to each other?
Question: Do you actually have 3-phase power available in your shop? If no, then getting it installed is probably prohibitively expensive (call you local electric utility, and don't forget that you would have to replace the main breaker panel and meter), for example we were quoted about $30K by the utility to get 3-phase power (it would require stringing over a mile of overhead wire). This would eliminate the option of going to a 3-phase motor. On the other hand, if you do have 3-phase power available, it is the easiest thing to do. Probably ignore the rest of my post.
A VFD gives you the advantage that you can run the motor at different speeds. For production, this is silly to do, but it has a great advantage: you can ramp up the motor speed slowly from zero to full speed, say over 5 seconds. This means the motor doesn't have a huge turn-on surge, which greatly reduces the electrical and mechanical stress on all components. Flipside: Not all 3-phase motors are rated to run on a VFD. Common folklore says that it's OK to run pretty much any 3-phase motor on a VFD, as long as you run it at full rated speed (except for ramp-up and spin-down without load); the problems occur if you run the motor at slow speeds. I'm not sure I would rely on common folklore.
Also, I'm not sure you can find an affordable VFD with single-phase
240V input and 3-phase motor output for 5 HP. I've looked, and the single-phase VFDs all seem to top out at 3 HP (meaning at 10-11A
3-phase output). If you know a source for an affordable single-phase-input 5HP VFD, I (and probably many others) want to know about it.
Between a VFD and a phase c> Can you get 3 phase power in your shop? Three phase will be cheaper
Theoretically correct. In practice, it makes no difference. Why? First, power consumption of woodworking tools is small anyhow, because they are only run for short periods. The refrigerator runs for many hours a day (say 10 or 15 hours), at 600 or 800W. I would guess that in no hobbyists shop the RAS runs for more than 4 or 6 hours per week (not per day); even it it uses 2kW, that makes little difference. Even in a professional shop, where the RAS might run for 3 or 4 hours a day, the refrigerator in the corner is still even in power consumption. So if you are worried about efficiency, get a new and efficient refrigerator. Second, the efficiency of a modern VFD is in the high 90%'s. Think about it this way: the inefficiency of a VFD is turned into heat; if it were 50% inefficient, the wasted part would be a few kW, it would output as much heat as a large heater, and would certainly melt down. Admittedly, VFDs do get a little warm, but they don't melt down (hopefully).
In many cases, VFDs actually improve the efficiency of a system, because they allow the motor to run at the minimal current that can support the desired load at a constant RPM. For that reason, many very large industrial motors (like large air fans in mines, or large air-conditioning compressors) are run on VFDs, even though in those cases 3-phase power is certainly available. All these effects are too small to matter for amateur woodworking tools.
Where VFDs really shine is for variable-speed machines, like a drill press. Again, this is a case where a VFD would be used even if
3-phase power is already available. But this is an application where a VFD-rated motor is obviously required, as the system will be run at full torque and low speed regularly.
| Assume that I have in front of the above referenced options. I can | pick only one - a VFD, a (rotary) Phase Converter or straight | 3-phase power for the 3-phase 16" Rockwell RAS I picked up for | $300. In the long run, which will cost less to own and operate | over time? Assume the 5hp VFD is $250 and the 5hp converter is | $400. Am I likely to spend a lot more money on power if I have to | spring for 3-phase direct? Manintenance issues with the VFD and | converter relative to 3-phase power - and to each other?
Jay...
You might want to check out the Delta (not the same Delta that makes power tools) VFD-B. I paid a bit more than $250 for it; but (IMO) it's great. Mine is used to control a Colombo 5HP 0-24K RPM (in 1/10 RPM steps!) spindle on my ShopBot and it's been absolutely trouble-free since I got it 2+ years ago. It's fully programmable from its own keypad - so if you wanted to, for example, have automatic braking when you're done with a cut, you could.
The downside is that you'd want to re-wire your RAS switch so that it provides a control signal to the VFD.
The Delta guys in Atlanta have been extremely helpful by digging out the info and coaching me on how to control the VFD-B using an RS-485 serial port on my PC. Not sure that'd be terribly useful on a RAS, but is great on the CNC router.
I prefer a RPC over the VFD, but is mainly a personal preference unless you need the variable speed the VFD allows.
VFD's take longer to get the motor up to speed, I have noticed. I have also killed a static PC and a VFD on my Bridgeport mill but the RPC is still going strong after 5 years of use.
I made my 5Hp RPC for less than $30 with salvaged components, it is very easy to do.
Straight 3-phase is best, unless you need variable speed, but for only
ask your electric company. policies are very local- some places they'll refuse to hook it up to a residential building, other places there is a hookup fee, other places they will set you up for free.
If three phase power at the right voltage is available it will require a seperate wire run to the shop and meter. There is a monthly charge just like your electric bill. If you do not have a high demand it is not horribly expensive but I would not pay it for a hobby.
Is it an option to change the motor to a 220 volt single phase. ?
| Assume that I have in front of the above referenced options. I can | pick only one - a VFD, a (rotary) Phase Converter or straight | 3-phase power for the 3-phase 16" Rockwell RAS I picked up for | $300. In the long run, which will cost less to own and operate | over time? Assume the 5hp VFD is $250 and the 5hp converter is | $400. Am I likely to spend a lot more money on power if I have to | spring for 3-phase direct? Manintenance issues with the VFD and | converter relative to 3-phase power - and to each other?
Jay...
I got a sale flyer from Enco today. They're advertizing Phase-a-matic(tm) static phase converters with prices of US$123.95 for a 1-3HP converter and $150.95 for a 3-5HP converter. It might be worth checking out catalog numbers DN297-3715 and DN297-3725 at
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