looking to get something like a variable frequency drive, but don't need something that costs $850

[CamTheNewby]

I want to get a 5 hp, 1800 rpm electric motor, and run it from 0 to 1800 at a steady rate. Variable frequency drives are all huge, very expensive units that have multiple applications. Does anyone know if there is something simpler? I literally just want a knob that I can turn from low to high frequency. Is this possible?

 

[jraef]

Define "huge" and "very expensive". I will sell you a basic model 5HP 480V VFD for $850 dollars right now in a package that is roughly 7"H x 4"W x 5"D.

I will of course buy it off of my local distributor for a LOT less than that and make a very tidy profit selling it to you for that price by the way...

"Will work for (the memory of) salami"

 

[CamTheNewby]

well, notice that my name includes Newby in it. Could you recommend something that is affordable and small that would suit my requirements? Also, (and again, my name has newby in it) can you get a VFD and motor in a single package? All I need is 2 things.

1 - it should be 3 or 5 hp (I think for its application, the more torque the better)
2 - i want to be able to run it from 0 rpm to its max rpm

What kinds of products should I be looking at for this?

Please and thank you!

 

[jraef]

What voltage? Do you have 3 phase available at the site?


"Will work for (the memory of) salami"

 

[CamTheNewby]

crap, ok, so I think I need a 10 hp, 230-460 AC, and I have access to 3 phase

 

[jraef]

You would either have 230 or 460V 3 phase available, it would be highly unlikely that you have both.

Big difference between 5HP and 10HP. What happened?

Maybe you should elucidate the entire concept here.

"Will work for (the memory of) salami"

 

[mikekilroy]

even bigger difference between the 3hp and 10hp.... quite a range to be winging it....

http://www.KilroyWasHere<dot>com

 

[MikeHalloran]

Start with a junkyard Chebby, and measure your actual requirements before trying to select an electric motor.


Mike Halloran
Pembroke Pines, FL, USA

 

[LionelHutz]

You have to vary the frequency to vary the speed of a typical 3-phase induction motor.

You haven't search hard enough. Try Ebay or Automation Direct. Under half your price, or less.

 

[jraef]

That was my point really. Your initial comment seems to be based on outdated information, VFDs are much smaller and less expensive than they used to be. Look around more.

Just be very careful with FleaBay drives old drives that have been sitting on a shelf somewhere can often suffer from an early demise because capacitors inside of them degrade if not powered up. There are things you can do about it if you have the right tools, but from the nature of your question, you would be better served by steering clear of them.

There are also a lot of VFDs being sold on FleaBay now that are being spewed out of a factory in China often with no brand name on them or you can see the name Huanyang in the photo, but never mentioned in the ad and sold at impossibly low prices. Those are absolute trash and are of such poor quality that people have to go through 3 or 4 replacements to get one that works out of the box. In many cases, the vendors are front companies of surplus junk dealers who buy them in bulk, sell them dirt cheap, then disappear before they have to replace anything. The mfr in China will refuse to honor any warranties, they will tell you to take it up with your vendor, who is long gone (or has changed names).

"Will work for (the memory of) salami"

 

[CamTheNewby]

ok, basically, I am looking to run a test on the performance capabilities of a rotary valve, and inside the rotary valve there are 4 seals. What I want to do is attach a stationary arm to the rotary valve and then spin the rotary valve with the electric motor at different speeds to see how the seals hold up and prevent leakage from occurring while they spin, and the outer housing remains stationary. Therefore, I need a motor with enough torque to be able to overcome the drag created between the rotating seal and the stationary outer housing. I have "20% compression" between the seals and the housing, and I have a coefficient of friction between the surface area of the seal and the housing. So it is difficult to quantify the total drag force created between them, but I was told that, using a ball park estimate, a 10 hp motor should be sufficient, and I really don't even know how to make that calculation, so I'm going with 10 hp. If anybody knows, the dimensions of the seal are 0.4" thickness, OD 5", ID 4.5".

That doesn't really matter all that much though. I've basically been told to just source the necessary components, and I'm looking for my best priced most suitable options. I believe that it is a 10hp, 3 phase, 230V motor. As well as a VFD that is capable of running linearly through a minimum to maximum RPM range so that I can do testing. Someone had suggested I use a gear box, and just switch the RPMs, but I want to run the full range.

 

[John2025]

Maybe put a torque wrench on the valves and turn them manually to get a ballpark. From that you can size the motor and drive. You'll want to consider what the lowest speed you want to go as that could be a problem with cooling, etc. 10HP sounds like a lot to me, but the valves may be much bigger than I imagine.

 

[jraef]

Also consider the maximum speed that you want to turn that valve. Is it really 1800RPM? Surely you have some sort of gearbox driving your mechanism already, no? So if the estimated torque you need at the valve surface was at the FINAL shaft of the valve axis, which is the OUTPUT shaft of the gearbox, the motor torque will need to be determined at the INPUT shaft of that gearbox, so that valve torque will be divided by the drive gear ratio to get the motor torque. Is that how you came up with a 10HP estimate? Because a 10HP 4 pole motor spinning at 1750RPM is going to produce about 30 ft-lbs of torque. Is that what you need BEFORE the gearbox multiplication?



"Will work for (the memory of) salami"

 

[ScottyUK]

Without knowing the valve this may be an option or not, but could the test piece be mounted to the cross-slide of a lathe? Most will give you a decent range of speeds from a handful or RPM in back gear to maybe 2000 rpm at top. For $850 you could pick up an older but half-decent machine with all the speed changes and the like taken care of, or you might even have one in the works already.

 

[CamTheNewby]

using a 3 : 1 ratio gear box reduction, and spinning the rotary valve with a max RPM of 600. The 10 HP will be overkill, but we don't really have a way of quantifying the seal drag. Lathe option was used previously, and doesn't work like we need it to.

 

[ScottyUK]

" Lathe option was used previously, and doesn't work like we need it to. "

That's potentially useful information: if you expand a bit on the reasons why then someone might be able to offer some ideas on a solution.

FWIW, 10HP sounds like a massive motor for these tests, especially with a 3:1 torque multiplication. Is there any credible service scenario that would see the valve rotate at 600 rpm? I work with a lot of valve actuators and I have never seen any valve operate above 100 rpm, and most are a fraction of that. I'm not saying you're wrong, I'm just curious.

 

[MikeHalloran]

If the rotary valve doesn't rotate continuously in service, then the proposed test will not give representative life values for the seals.

You need an indexer that runs at representative speed, with representative dwell times. Actually, pseudo- random dwell times would probably be a better choice.


Mike Halloran
Pembroke Pines, FL, USA

 

[CamTheNewby]

All that I have been asked to do is set up a design using Inventor, and source the materials required to perform the testing. Reasons as to why the lathe option is no longer viable isn't something I know. The motor is being purchased, and, being that it will be NEMA standards motor, it is likely to be used for a variety of applications, hence the excessive hp. We just require something that is sufficient to overcome the seal drag. One potentially useful piece of information would be if anyone new how to calculate the seal drag based on my previously stated dimensions and specs (i believe the housing is bronze). My original question wasn't really about how to go about conducting the experiment itself, and all i was asking was whether or not I had any options for alternatives to a VFD for increasing/decreasing the motor RPM. I do appreciate the help though, but I am still actually poking around for some VFD alternative. I'm stepping out of my element here, but could a possible solution have something to do with some sort of a pulse width modulation device maybe?

 

[ScottyUK]

The speed of AC motors (let's exclude universal types for now) is governed by the frequency of the supply. The change the speed you need to change the frequency of the supply, hence the rise of VFDs. The speed of DC motors can be controlled by varying the armature voltage, and to some extent also by controlling the field current. This may be where you've picked up 'PWM' from, although pulse width modulation is also used in VFDs so perhaps best to forget you heard about it.

DC motors are expensive relative to AC types, especially relatively large ones like 10HP. I think you're on the right track with an AC motor plus VFD, because while you could solve this using a DC motor and drive you'd almost certainly spend a lot more money doing so.

 

[jraef]

Yep. So the over arching answer is, there is nothing "like" a VFD that isn't a VFD. The expensive part of a VFD is the part you are after. The things you might think are "extras" are essentially freebies or really cheap add-ons because once you overcome the technological hurdle of being able to vary the voltage and frequency together, the rest is child's play.

Your basic premise is akin to saying "I need something like a car, but it doesn't need to have a stereo or a cigarette lighter, it just needs to move down the road when I step on the throttle and stop when I step on the brake."

"Will work for (the memory of) salami"