Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required 2

[Evolve]

We are designing a custom small extruding machine which needs to have a 3HP or 5HP AC motor with around 1750RPM imput 3 Phase 220V - 240V. The desired RPM output is to be a variable speed for the extruding feedscrew to be between 15RPM to 45RPM. We know we can achieve an RPM output of around 30RPM if we use a 60:1 ratio gearbox reducer or buy a motorgear motor with built-in 60:1 reducer. Our questions are as follows:

1. Are there any off-the-shelf 3HP - 5HP AC motors that are variable speed such as AC drive motors of VFD which are available to yield right out of the box the desired 15RPM to 45RPM speed. We have looked and did not find any but we are not AC motors experts and the reason we need motors experts from this forum.

2. The other thought we had would be to have an inverter duty type 1750RPM motor with the 60:1 ratio gearbox reducer and to add a variable frequency drive controller so we can set the motor output of the feedscrew speed to the desired 15RPM to 45RPM from the original 30RPM output from the 60:1 gearbox. This option seems to make sense to us but again we have no expertise with VFD motors and controllers.

3. Are there any other options that we do not know of or that we might have overlooked?

Any recommendations and suggestions regarding these options above or on motor, reducer & VFD controller brands, types, etc. would be greatly appreciated!

Thanking you all in advance for all your help!!!

 

[jraef]

A VFD can definitely make an AC induction motor run at that speed. Whether or not that is a good idea is a different part of the discussion. But first off, when you say "3HP or 5HP", how did you determine that? Is that 3-5HP AT 15-45RPM? The difference is in the amount of torque you need. You have not described the application, so we don't know.

Here is the "reader's Digest" version of what I mean. the term "HP" is essentially a shorthand expression of X torque at Y speed. So 5 lb.-ft. of torque at 15RPM is a very different amount of total HP than 5 lb.-ft. of torque at 1750RPM. So where you start from in that determination dictates how you will want to get there. Basically what the VFD will do for you is to MAINTAIN the motor's design torque at any* speed, the gear box route will MULTIPLY the torque at the working shaft by the inverse of the gear reduction. So per your example, a 60:1 reduction will result in the shaft torque at the low speed shaft being 60 times as much as at the motor shaft.

*The concept of "any" speed is dependent upon the technology of the VFD you pick and how accurate your speed needs to be, among many other mitigating factors to be determined later.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ScottyUK]

Well, there are old-school mechanical variators:

http://www.seweurodrive.com/produkt/varimot-variable-speed-gearmotor.htm

which do pretty much what you want, or you can buy integrated gearbox-motors (gearmotors) as a packaged unit from quite a few places including the company that makes the mechanical variators above. In my opinion a gearmotor with a VFD would be a more reliable solution than the mechanical variator.

 

[waross]

Further to jraef's post.
The VFD will maintain rated torque from rated speed (1760 RPM in, 30 RPM out) down to the lower speed (880 RPM in, 15 RPM out).
As you go over-speed, the torque will drop. When you go over-speed the HP will remain the same so as the speed increases the torque drops. At 150% speed, the torque will be 1/150% or 67% of rated.
(2640 RPM in, 60 RPM out).
There is a workaround if you need full torque at 150% speed. That is to apply 150% of the 240 Volt rated voltage or 360 Volts.
It is cheaper and easier to buy a larger motor.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Evolve]

Thank you all for all the feedback and comments as this is very much appreciated.

So regarding my application as mentioned in my original post this motor application is for a small extruder. The AC motor requirements will be 5HP around 1750RPM with 10 - 15 FLAmps output 3 Phase 220V - 240V with the desired RPM output to be a variable speed for the extruding feedscrew to be between 15RPM to 45RPM with constant torque at low speeds.

So after talking with a few motor and VFD specialists, they have suggested a 5HP around 1750RPM 230V AC inverter vector duty motor 1000:1 constant torque with a VFD vector drive controller 7.5HP or 10HP with 23 to 27amps (will be running VFD on single phase 220V). Because the motor will be running always at low speeds with constant torque they said to use a TEFC fan cooled type motor and to stay away from the TENV motors because of motor heat generation. They said that a standard induction motor would not work with the VFD because of the 60:1 speed ratio of the motor application with constant torque as the motor will overheat and the insulation and motor will not last and fail.

This seems to make a lot of sense to me. I hope you motor experts agree with the recommendations and any thoughts or feedback on this option would be great!

Thanking you in advance for all your help!

 

[imcjoek]

The AC motor requirements will be 5HP around 1750RPM

Because the motor will be running always at low speeds

Previous posters already said it, but I'll say it again. You need to be careful here.

Where did the 5HP number come from?

A 5HP Motor running @ 50RPM
vs.
5HP Motor running @ 1750RPM through a 35:1 gearbox = 50RPM output.

These are substantially different in the torque department...
Typical 5HP inductive motor is in the 15 lbf-ft F.L. torque ballpark. I have a hard time picturing that turning an extruder feed screw.

 

[jraef]

Don't go with the 7-1/2HP drive, whomever said that is a nincompoop and should be avoided.

For a VFD to work on a single phase input and give you full motor HP output, it must be a MINIMUM of 2x the motor current rating. Some people think that applying the current difference from single vs 3 phase (the sq. rt. of 3, 1.732x) is sufficient, because that addresses the increased current through the diode bridge rectifier on the front end (all of the motor current has to come from only 4 diodes instead of all 6). This however ignores what is actually MORE important, in that the rectification of single phase causes significantly more DC bus ripple in the mid section of the drive. So you need more capacitance in the drive to handle that safely. 2X the motor current is considered the MINIMUM sizing to get there, but even at that, the capacitors will run warmer, so you should re-rate the ambient temperature by about 25%. That means if the VFD size was based on ambient being at 50C max, you should only use it if you can maintain the ambient at 37C (but be careful, some drives are rated 40C, so you must get down to 30C operation, which is difficult). The alternative, if you do NOT want to give up the ambient temperature rating, is to de-rate by 65% current. So for your 5HP motor, use a 15HP VFD. that's never a popular idea, but it's important to know what you are getting in to. LOTs of sales People are ill educated on this subject.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Evolve]

Thank you incjoek for your feedback. That is true what you said... For the 5HP motor @ 1750RPM you would yield Full load 15lbs/ft torque and that same 5HP 1750RPM motor running at 30RPM would be 875lbs/ft torque. That is what we need around 875lbs/ft torque to run the extruder feedscrew. Can we not use the VFD vector controller to output a constant torque of 875lbs/ft while running the motor/feedscrew at 30RPM? Or do we still need to add a 60:1 ratio gear reducer in the system between the motor & feedscrew? I was told that the VFD vector controller with the inverter vector duty motor takes care of that by removing the gear reducer and adding the variable speed to the system... Any thoughts on that?

 

[Evolve]

Thank you jraef for your replies and very much appreciated. The 2X amps for the VFD is what I was told to have and we have a 5HP around 1750RPM with 10 - 15 FLAmps output 3 Phase 220V - 240V for a VFD vector drive controller 7.5HP or 10HP with 23 to 27amps. We will need to make sure that we have at least 2X FLAmps rating on the VFD we select so your suggestion of a 15HP VFD might be the way to go depending on final output FLAmps of the selected motor. Thank you for your feedback and great recommendation!

 

[waross]

Everything has been said but you don't seem to be listening. This may be a good time to re-read all the posts, maybe more than once.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

the 5HP motor @ 1750RPM you would yield Full load 15lbs/ft torque and that same 5HP 1750RPM motor running at 30RPM would be 875lbs/ft torque

No. It's this.

a 5HP around 1750RPM 230V AC inverter vector duty motor 1000:1 constant torque

A 1750rpm motor running directly at 30rpm and producing 875ft-lbs would be 300hp.


Also, don't try to buy cheap parts and expect to run a motor at 150% of rated speed. You have to be very careful doing that.

 

[jraef]

A 1750rpm motor running directly at 30rpm and producing 875ft-lbs would be 300hp.

That's not what "constant torque" means in this context. When the motor mfr says that, they mean it is suitable for being used at its rated torque output continuously down to the speed depicted by the ratio. So if the motor is 5HP at 1750RPM, that is 15 ft.-lbs. of torque, and 1000:1 means you can use it with a VFD to get 15 ft.-lbs. of torque all the way down to 1.75 RPM.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[LionelHutz]

If the OP wants to directly couple a 1750rpm motor and produce 875 ft-lbs of torque at 30 rpm then he WILL require a 300hp motor. That is EXACTLY how constant torque works!

 

[jraef]

Ah, I think I see where you were coming from, you were basing that on the torque he would have via a 60:1 gearbox after a 5HP 1750 RPM motor? Then yes, if he NEEDED all of that torque gained by the gearbox reduction but wanted to remove the gearbox, then yes, he would need a 300HP motor. I don't think that's what he meant though, I interpreted his statement about the gear reduction as an alternative to the VFD.

Lots of people use gear reduction for speed change only, not concerning themselves with the torque multiplication aspect. I can't count the number of times I've seen that, including one last week where someone had a 900:1 gear box on a 10HP motor to move a turn table that held plastic cups filled with pellets. The turn table was a heavy steel machined platen because they needed it to be flat and precise, but he determined the torque required to move it based on the motor shaft speed, then used the gearbox just to drop the speed. I pointed out that with that gear reduction he could have used a 1/4HP motor and it wasn't until I showed him the math on paper that he finally got it.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[djs]

Every extruder I have seen uses a gear reduction of some sort. For larger extruders, the gear reducer is a separate unit from the motor. For smaller units it is integral with the motor.

 

[ScottyUK]

I read it the same way that Lionel did - in the post of 21 Feb 17 22:31 the OP states that he needs 875ft-lbs, and seems to be debating whether a motor plus VFD can achieve the same results as a motor with gearbox. The answer to that is plainly 'no'.

 

[LionelHutz]

Yes, in that post the OP was clearly asking or pondering about using the VFD as an alternative to the gearbox, which means the motor is running at 30rpm. The first sentence I quoted shows the OP calculated the torque assuming the motor was constant HP and the second quote of the basic motor spec was to point out the constant torque, which 15 ft-lbs vs 875 ft-lbs is not.

 

[jraef]

Ah, I missed that little detail. I thought he meant WITH the gearbox, but he said "Do I still need it?", which means he was asking if he could get 875 WITHOUT the gearbox. The mind is a terrible thing to waste, and I must have spent too much time wasted...

Mea culpa.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

salami poisoning

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

It catches up to you eventually...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington