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1 Horsepower VFD Quandary


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  #1  
Old 03-13-2018, 01:12:41 PM
Pete Spaco Pete Spaco is offline
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Default 1 Horsepower VFD Quandary

I want to put a VFD on my old US Machine Tool vertical milling machine.
Years ago, when I bought it,, it had a 1 hp 3 phase 240 volt motor. I replaced it with a 1 1/4 hp single phase 120 volt motor and it works fine, except that this mill has no back gear, so I can't go below about 350 rpms. (Both motors are 1725 rpm).

For many years I have been drilling large holes (up to about an inch) and hole sawing up to about 2" in steel that is up to about 1 1/2" deep. Going too fast, but that's what I have. Lately I have been making anvils out of railroad rail. In my experience, the bigger the rail, the harder the material. Drilling a 3/4" hole for the hardy hole takes quite a bit of work.
If you want to see why it's a problem, look at:
https://www.youtube.com/watch?v=5aNUFc9_ki0


So, I want to put the 3 phase motor back and get a VFD so the mill will run slower.

Finally: Here's my problem:
I see several VFD's for sale that say they can power a 1 HP 3 phase motor on 120 volts. I have 20 amps of 120 right in back of the machine.
However, when I look deeply into the specs for THOSE VFD's, they all say "3/4 hp", even though they are advertised for 1 HP.
From what I have found so far, it seems that I have to get a bigger VFD to be sure it will work, AND I will have to put in a 240 volt line for it.
Is that correct thinking on my part?

Money isn't all that tight. I'd rather do it well than have to do it over.

Pete Stanaitis
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Old 03-13-2018, 02:05:55 PM
ehpower2 ehpower2 is offline
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Default Re: 1 horsepower VFD Quandary

just in my experience the 240 input is way better in many respects. As for VFD application most of the time i buy a 5hp vfd drive and be done with it assuming the specs will work. You can pick them up cheaply on ebay. As for your application i would suggest going no lower than 3ph VFD drives as this way you can have a bit of overload capacity and you wont work the device as hard.
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Old 03-13-2018, 07:14:59 PM
JFleury JFleury is offline
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Default Re: 1 Horsepower VFD Quandary

here is a link for a 115 volts in 230 3ph out rated at 1.5 hp. I looked at this on but decided to go with the 220 single phase in

http://dealerselectric.com/DRIVE-T205.asp
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Old 03-13-2018, 11:13:56 PM
slip knot slip knot is offline
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Default Re: 1 Horsepower VFD Quandary

I've bought a couple of the sets from Dealers electric. they are pretty reasonable.
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Old 03-13-2018, 11:19:55 PM
dkamp dkamp is offline
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Default Re: 1 Horsepower VFD Quandary

A 120v single-phase in, 240v three-phase OUT VFD is challenged with several tasks, so the 'fine print' must be very carefully considered.

The 120v/20A outlet is good for 2400w... which equates to 3.21hp. An electric motor of 3hp allows you the remaining .21 as waste energy (heat, noise, etc), which is being kinda optimistic... 2hp out is closer... but still, that's asking alot from a typical wall receptacle. While that's what it's RATED, and it very well may be WIRED for it, that circuit may have other loads.

It's for this fact, that pulling a dedicated 240v circuit is certainly worth it.

My VFD'd tools run on 240v single phase, but have either 240v or 480v three-phase inputs. I drive only 2 of the 3 input pins, either by feeding direct 240v, or by feeding a 480-240v dry transformer BACKWARDS. 240v secondary winding to the AC line, and the 480v PRIMARY connects to A and C of the VFD. Most 3-phase VFDs will run on single... some don't realize it, some are smart enough to realize that there's a 'lost phase', but that detection can be disabled in software. Others will be mad about it, but connecting one of the hot legs to the unused input with a resistor and capacitor will 'fool' the VFD into running fine. The 'gotcha' here, is that the VFD's power handling capacity on the INPUT is only 2/3rds of it's original, because you've left one input leg out of the game. The solution, is to use the next-size-up drive... so for a 2hp motor, use a 3hp VFD. In the case of my Monarch lathe, I used a 10hp VFD to drive a 7.5hp motor. I ALSO programmed the VFD's motor settings to 7hp, which limits the motor's output a wee bit, but it prevents the VFD from killing it's own input rectifier.

In the case of your mill, what I'd do, is find a 4-5hp VFD, and put a 240v circuit in your shop, but have two or three 20A/240v receptacles, so you can do same with your other machine tools. If you're a one-man shop, you probably won't be running more than two machines at once, so you won't overload one circuit.
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Old 03-15-2018, 12:35:09 AM
countryboy07 countryboy07 is offline
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Default Re: 1 Horsepower VFD Quandary

I am running my 1 horse powermatic mill with a 3 hp 230 1ph input 230 3ph output. It's always good practice to upsize a little on a drive. On our largest drives at work we upsize them 20% due to the amount of reverse current they generate when they dynamically brake the motor they run.

Something else to consider here is you lose motor torque as you reduce frequency (speed). That also generates alot of heat in the motor.
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Old 03-15-2018, 12:38:18 AM
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Jim McIntyre Jim McIntyre is offline
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Default Re: 1 Horsepower VFD Quandary

I'd oversize the drive. Go for one with 240V in, rated 2 or 3 HP.

You can always set it to limit the current so it won't smoke your motor, but if you get a drive that's too small, it'll stall...

example:

http://www.factorymation.com/CV-2002-H1
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Old 03-15-2018, 08:54:05 PM
s100 s100 is offline
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Default Re: 1 Horsepower VFD Quandary

"Something else to consider here is you lose motor torque as you reduce frequency (speed)."

WRONG! Torque is a function of volt turns. Torque does not change when you slow the motor down, but horsepower does as horsepower is a function of torque and speed. More voltage, more turns in the windings, more torque. This is true on a theoretical level but in practice things reach a point of diminishing returns.

VFD's are programmed to run in constant torque mode until they reach nameplate speed, then they go to constant horsepower to avoid overloading the motor. Constant horsepower means that as the motor speed increases the torque decreases, maintaining the same HP.


"That also generates alot of heat in the motor. "

The motor heats up because at lower speeds the fan is less effective. At high loads and very low speeds this becomes a real problem, and an external fan may be required to keep things cool.
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Old 03-15-2018, 09:58:58 PM
countryboy07 countryboy07 is offline
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Default Re: 1 Horsepower VFD Quandary

Ok you got me. My physics is a little rusty....
Moral of the story is as you slow down you lose power and the motor gets hot.
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Old 03-15-2018, 10:39:51 PM
Pete Spaco Pete Spaco is offline
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Default Re: 1 Horsepower VFD Quandary

Well, in my case, it's torque that turns my drill bit, it just takes longer than if I had more horsepower. Good point about the possible need for an external motor cooling fan.

I have heard that the peak pulse voltage of the VFD may be hard on older motors that don't have the insulation the newer motors do. I won't be totally surprised if something happens to this one. But, as many of you know, used 3 phase motors are about one dollar per horsepower, since many are replaced on a fixed time in service rotation.

We will see.

Pete Stanaitis
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