What to look for in a VFD?

[Aaron_W]

Yes another VFD question, it seems there are dozens of these but none directly answer my questions.

Most important there are hundreds of VFDs out there and they range from a low of $50-75 up to several hundred dollars for what appears to be the same device. What are the critical features / specs to look for to distinguish one VFD from another? What do you get for paying $300 instead of $75 when they are both rated for the same load?


I'm getting a mill with a 220v 3 phase 3/4hp motor, so I'm going to need a VFD to run it. Looking at VFDs there are relatively few options at 1hp or less, but lots of options in the 3-5hp range. and these larger VFDs in many cases are actually cheaper than the small ones.

I understand that the hp rating is just max load, and a larger VFD can be programmed to control a lower rated motor, so other than being in many cases physically larger there doesn't really seem to be any drawback to getting a higher rated VFD than the minimum required size.


Last question 3phase vs VFD. It appears I do have 3 phase power available at the pole. I'm sure there would be added cost to install a 220v 3 phase outlet vs 220v 1 phase, but would I see any benefit to doing this? The VFD offers me additional speed control so even if I ran 3 phase power it seems I would still want a VFD.

 

[Winegrower]

I would agree with all your points, Aaron_W. VFD features don't matter that much to me so the mechanical style and accessibility of control is more important for selection. Honestly, after a long career as an EE in computers, printers, robots, disk drives and internet, I for the most part have a hard time correlating price with quality. Most competitive products will use very similar active semiconductors, with similar reliability specs. There is such a wide range of VFD prices that you could get one of the cheapest, and if it breaks, buy another and still come out ahead of some AB or other name brand device.

I just wish I could get 3 phase power into the shop! At some high power level, VFDs use 3 phase input.

 

[Bob Korves]

Hi Aaron. What kind of mill are you getting?

For just running a mill and for using the VFD to also deliver variable speeds, all you need is a basic VFD. For my surface grinder I bought a cheap 3 HP (2.2 kW) Chinese VFD off eBay for well under $100. One with half that capacity would have powered the grinder, but would only have cost $10 less. The larger VFD will likely draw about the same power as the smaller one, so it won't cost more to run, and will be versatile for moving it to a larger machine, and I think it will also make it more valuable and easier to sell if I want to sell it someday. On a lathe, where I might want more functions like rapid deceleration and others, then I might be more fussy about which VFD I would purchase.

 

[kb58]

I wanted one that came from a reputable manufacturer, has good support, one that others use, with a warranty, and most importantly, a well written manual. The Hitachi WJ200 fit the bill. Others may meet the same requirements but I'm happy with it.

 

[Bob Korves]

I do have 3 phase power available at the pole. I'm sure there would be added cost to install a 220v 3 phase outlet vs 220v 1 phase, but would I see any benefit to doing this?

You would need to ask your local utility whether you can get three phase, even if it is for sure at the pole. Sometimes they only deliver to commercial customers, and sometimes will have a fairly large minimum usage per month ($$$). The cost of bringing it to your house could also be too high to consider. However, it probably does not cost you anything to ask about it...

 

[Aaron_W]

Bob the mill is a Clausing 8520, but the original motor was tired and replaced with a 3 phase of the same 3/4hp.

I wanted one that came from a reputable manufacturer, has good support, one that others use, with a warranty, and most importantly, a well written manual. The Hitachi WJ200 fit the bill. Others may meet the same requirements but I'm happy with it.

That has crossed my mind, although more concerned with the seller than the maker. Even the cheap ebay VFDs have 2 year warranties, but I understand it can be difficult to actually use the warranty with some based outside of the USA.

I was looking at a Westinghouse / TECO 1hp VFD for $140, or some ebay no name 3-5hp VFDs for $73-115. Half the cost for more capacity kind of seems too good to be true, but with electronics often times you do pay a lot for the name and the components are about the same between the no name and the name brand.

You would need to ask your local utility whether you can get three phase, even if it is for sure at the pole. Sometimes they only deliver to commercial customers, and sometimes will have a fairly large minimum usage per month ($$$). The cost of bringing it to your house could also be too high to consider. However, it probably does not cost you anything to ask about it...

This winter has apparently been the year to work on the utilities for my street. We've had sewer upgrades, new gas lines, new LED street lights installed (very bright, I can now do yard work at night) and power line work done over the past few months. Out of curiosity I asked the utility guys if the lines running past the house could supply 3 phase power and they said yes. Of course they were only answering a technical question, not policy on who can actually get it.

I was really just wondering what if any benefit true 3 phase power might provide over a VFD for a home shop. I'm sure there is a substantial benefit for a shop running lots big equipment, but I'm not.

I am going to be having an electrician install a couple 220v outlets, so I can ask about 3 phase when he comes, but I suspect even if allowed, it would be cost prohibitive.

 

[JimDawson]

What do you get for paying $300 instead of $75 when they are both rated for the same load?

Quality capacitors and overall quality, readable documentation, and factory support.

 

[mksj]

Basically the VFD gives you 3 phase power for motors, along with numerous ways to control the VFD functioons. Pulling in 3 phase would be impractical if you are looking at the costs to put in a main service entrance panel, pull new wires, outlets, etc. Let alone the additional cost of the service is substantial. It would be practical "maybe" if you had a big shop with a lot of large 3 phase machinery. Even then, many small shop owners opt for something like a Phase Perfect VFD which generates the 3rd phase. An RPC is also practical for multiple 3 phase machines where the option of adding individual VFDs is impractical.

You can buy anything cheap, and you typically get what you pay for. A lot of this is the quality of the components, warranty and technical support. Also in order for VFDs to be optimized for a particular motor/machine you want to have adequate programming parameters (the software) to control the VFD. The cheaper VFDs are often lacking these features, or may be missing items like the braking circuitry even though it has the braking terminals. Most of the VFDs are made in China, but it is how it is built that matters. I participate in a few other forums where they have examined the internals of these cheap VFDs, and according to those that work on them they are pretty dismal as far as design and components. The one I installed about 6 years ago, the terminals were way too small for the supposed current rating, it was also missing the braking circuitry. I haven't touched one since.

When matching a VFD to a motor, the VFD needs to be sized according to the amperage and Hp/kW rating of the motor, it needs to meet both specs. You can buy a larger VFD if you want, but it is not necessary. VFDs in the 1-3 Hp range are quite common and reasonably priced, above 3 Hp there are only a few mainstream VFDs that are available at 5 Hp single phase input, beyond 5 Hp you need to purchase a 3 phase input VFD and derate it for use with single phase. The typically derating is to divide the output by 1.7. The install also gets a lot more expensive due to the sizing parameters for the electrical circuit powering the VFD is specific to the VFD input not it's output.

On a lathe application you need a bit more control/programming functions and an external braking resistor if you are going to use the VFD for electronic braking. Most of the cheaper VFDs do not support the use of an external braking resistor, they also are limited as to programming parameters. You cannot just buy a VFD and directly connect it to the machine and use the machine contactors. The VFD needs to be directly wired to the motor and low level signal inputs are used to control the VFD functions. On a mill you probably would not need to use a braking resistor on a smaller mill/motor (under 2 Hp), so mane people use the Teco L510, this comes in a 1 Hp model. On a larger mill it probably pays to get a better VFD. But all depends on what you want to spend and how often you use it. I have seen the cheaper generic VFDs last for years and also be DOA. Forget technical support and if you need to send it back to China under warranty, it isn't going to happen.

 

[Briney Eye]

I have a Fuji Frenic Mini 1/2hp on my 8520, with an external braking resistor, and I'm completely happy with it. It's mounted in an enclosure on the wall next to the mill, and I use the speed control knob on the VFD all the time. I think the 1hp model is about twice the size to accommodate the larger capacitors required for the 120V-220V conversion. The 1hp Teco that I put on my drill press is that way. If you run from a 220V input the VFD will be smaller for the same horsepower rating. I like the Teco just fine too, but it doesn't support a braking resistor. That's not such a big deal on a drill press or mill, but I would consider it a necessity if I put one on my lathe, with all the mass of a chuck to bring to a stop.

 

[machPete99]

If there is any possibility of upgrading your motor buy a VFD for the biggest motor you might put in. I upgraded my 3/4HP mill to 1HP, as an example. I happen to like the Hitachi WJ200 series.