VFD - Single vs. Three Phase Input

[Bob Stroman]

What is the difference between a Variable Frequency Drive using single phase input versus three phase input? I'm interested in the 1 - 7.5 hp range.

[Chris Rosenberger]

The single phase VFD would be used if you do not have 3 phase service available.

[Bob Stroman]

What if you are considering adding 3 Phase to the whole shop?

Then what is the cost and complexity differences between single and three phase input to the VFD?

For example, I have a 1-1/2 hp Delta 1642 with factory VFD that takes 110v input. However, hp is limited to about 2 hp for single phase 110v input. This is logical given the limits of wire size as hp climbs on 110v supply. On 220v, you can get larger VFDs, but do you pay a cost penalty for supplying single phase versus three phase? Do the units go from 1lb coffee bag sized to the size of a 15" CRT? Are the single phase 220v units limited to 3hp?

[Steve Stube]

Are the single phase 220v units limited to 3hp?

Yes for now but I expect this will change in the future. I have a 3 HP single phase input 115/230V with 0 - 230V 3 phase output.
BUT, most any 3 phase input VFD unit built after ~ 1996 is capable of operating on single phase input (be careful for the exceptions) with an appropriate "derating" of the drive. Rule of thumb - derate by 1/2. A 10 HP VFD fed by single phase power will be suitable for running a 5 HP 3 phase motor. Size? Depends on the manufacture but most offer a Micro line and they are small over a wide HP range. I remember the first VFD I saw in ~1976 had to be lifted with a forktruck and it was only 2 HP. I have a 3 HP unit I can easily hold in my hand and the packages will continue to get smaller I'm sure.

[Steve Stube]

What if you are considering adding 3 Phase to the whole shop?

Adding 3 phase power to your shop will make your selection of VFD's for variable speed applications that much easier and at a lower cost.

[Dev Emch]

VFDs are used to replace three phase service when only single phase is available.

But VFDs can do lots more than just get you three phase. There are cases where you will need three phase input to a VFD but these are super advanced and I will not get into these here.

On a simpler note, a VFD can be used to replace a starter/contactor in a three phase machine. So out with the old Allen Bradley 509 and in with the VFD. VFDs provide a soft start feature and also provide excellent motor protection circuits thereby negating the use of heaters and other more clunky overload relays.

Overall, VFDs are pretty nice and more folks are going to them. On the down side, they are not easy to work on and contains lots of specialized elctronics making service an issue. There are 60 or so year old phase converters still running today with nothing more than grease and a new set of bearings.

[Steve Stube]

Dev, you point out several advantages of the VFD's (being an old iron guy I'm sure you see where the Soft Start feature could be a real plus) let me say/add again variable speed and multi voltage w/o an additional transformer - two features not available with phase converters.

Oh ya, did I say they are quiet, small, nearly indestructible and very efficient.

[Bob Stroman]

So the longer I take to find those dreamy 3-phase monsters, the more options the electronics folks will offer in smaller, less costly packages.

Variable speed is great on the lathe, and I would think it would allow one to use a bandsaw for both wood and metal work.

Thanks everyone...

[Dev Emch]

Watch that variable speed thing. Its nice but you need to remember something here. A VFD can, under programming, run constant torque up to its original sync speed. Lets say the motor is a 2 pole so thats 3600 RPM. Throw in a little slip and were at about 3450 revs. As I take it up to twice speed or 7200 RPM, I go into constant horsepower mode. Here, torque output becomes a function of shaft RPM and it goes down the faster I go. But the horsepower remains contant. So you think your cranking out 5 ponies on a 5 pony ride. Not so fast. For a small machine with a high speed cutter like a bridgeport running a 1/4 inch endmill, no worries. But a large machine like a 20 inch jointer or planer is going to bog down due to the loss of available torque.