Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
(OP)
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!!!
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!!!





RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
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
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
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
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
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!
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
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.
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
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
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
No. It's this.
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.
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
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
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
Mea culpa.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
Keith Cress
kcress - http://www.flaminsystems.com
RE: Expertise & Recommendation on Variable Speed AC Motor 3HP - 5HP Required
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington