Another dumb INVERTER question

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wally318
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Another dumb INVERTER question

Post by wally318 »

Not really.
Ok so here's the playing field.
I've got my chipmaster up and running. Chased leaks checked out the brake
Leveled etc. And just finished a taper attachment for it.
But as I use it more the sound of the variator and timing belts are overly
loud and annoying. The sound of the variator is not that of a worn or worn
out one that I've read about but louder and different sounding from other/
different lathes that I've run. Part of the problem is that I have an extremely
small shop which amplifies sound.
Tony @ Lathes.uk.com really pushes the idea that if the variator goes on
these lathes that they're an ideal candidate for an inverter.
Problem is they have a fairly wide speed range of 35 to 3000 rpm,
a 85.6-1 ratio. The headstock is responsible for a 10-1 ratio so that
leaves 8.6-1 for the variator.
Right now its running with a 2 Hp 1 ph motor, 1750 rpm 230 Vac.
If I were to put a 3 Hp 3 ph motor it could be mounted in place
of the variator and thus eliminating 2 timing pulleys and the longer
timing belt from motor to variator, driving a pair of V-belts direct from
motor to clutch pulleys and leaving a short timing belt from clutch shaft
to main spindle and a narrow timing belt in the gear train.
The problem or question as I see it is it possible to run an 1800 rpm 3 ph motor
and over speed it to 80+ Hz to get at least 2400 of the stock 3000 rpm,
and as slow as 350 rpm or 5-1 speed drop to get the 35 rpm low end?
What type of motor and drive would I need to accomplish that?
The problem I'm having a difficult time wrapping my head around is this:
If you take a 3 Hp motor and drop the speed to 1/5th then you would end up with
only 1/5 of the Hp am I on the right wave length so far?
And down at 35 rpm for example where one might be turning/boring something fairly large in diameter
or taking fairly heavy cuts is an area where you would least be willing to give up Hp.
I hoping some kind knowledgable souls might help me understand a bit more about this?
John Hasler
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Re: Another dumb INVERTER question

Post by John Hasler »

I'd put a DC motor on it.
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Bill Shields
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Re: Another dumb INVERTER question

Post by Bill Shields »

No matter what you are going to need some sort of reduction ratio for low speeds.
Too many things going on to bother listing them.
wally318
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Re: Another dumb INVERTER question

Post by wally318 »

#1-will a DC motor get full torque all the way down? Any users with first hand experience?

#2-Re: reduction ratio req'd
Whats all the hype about inverter duty 3 ph motors capable of 10-1 speed reduction and vector drives
that suposedly give full torque?
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Bill Shields
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Re: Another dumb INVERTER question

Post by Bill Shields »

It is not a question of full motor torque...it is a question of how much torque you need to run a lathe at very low speeds
Too many things going on to bother listing them.
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BadDog
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Re: Another dumb INVERTER question

Post by BadDog »

When I had a Rockwell 11x37 1hp lathe that topped out at 1500 rpm, I pondered removing the worn out Reeves drive and replacing it with a bigger motor using a VFD for speed control. Keeping the back gear and working through the math I realized that to keep almost as much torque at the lowest factory speed while giving me a bit more head room in the 1800 rpm range, I would need to mount a minimum 5HP motor. The motor itself wasn't an issue, but the cost of an VFD that will run a 5HP motor on 1ph 220V closed that door for good. All that's based on memory of something I looked into over a decade ago, but in the end rebuilding the Reeves drive wasn't that bad and provided great service without the cost or electronics. But that's a Reeves drive, not a complex electronic drive. I only include that as a reference for attempting to keep torque needed to avoid stalling when working larger diameters at low rpm.

General idea is that you can go below 60 Hz with constant torque down to the point that it's cooling issues and auxiliary fans that you have to worry about. And, as I recall, on most modern inverter rated motors, 200% with reduced duty factor is generally considered a reasonable upper limit. Again, that's my memory, so that would need some further supporting evidence before depending on it..

The only other thing I'll add is that there were a few people with 10EEs and problematic super complex motor-generator drives who replaced the whole mess with a servo setup and were very happy with the very functional results. You'll have to search for the details, but might be worth the effort.
Last edited by BadDog on Tue Feb 09, 2021 11:42 pm, edited 1 time in total.
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John Hasler
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Re: Another dumb INVERTER question

Post by John Hasler »

wally318 wrote: Tue Feb 09, 2021 11:06 pm #1-will a DC motor get full torque all the way down? Any users with first hand experience?

#2-Re: reduction ratio req'd
Whats all the hype about inverter duty 3 ph motors capable of 10-1 speed reduction and vector drives
that suposedly give full torque?
I replaced the induction motor on my Logan with a series wound DC motor from a treadmill. It's controlled by tachometer-feedback speed control. Using the backgear I get full torque down to 5 rpm. Full speed without the backgear is 2000 rpm.
Bob D.
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Re: Another dumb INVERTER question

Post by Bob D. »

You need to remember that the gear reduction of the lathe is not only giving you slow rpms but of most importance it is multiplying the torque of your drive motor. A VFD is varying rpms and at best, maintaining the torque output of the motor. It doesn't multiply the torque being delivered to the spindle. Take into consideration the loss of torque (power) you are willing to sacrifice for low speed work removing gear reduction. You'll need a substantially larger motor to regain capacity.
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wally318
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Re: Another dumb INVERTER question

Post by wally318 »

Maybe I didn't phrase my question quite correctly.
The plan was not to bypass or not use the headstock backgear,
but to eliminate the variator, which is esentially a mechanical version
of the Reeves variable speed contol.
<The variator has a 8.6-1 speed reduction/variation, but not a gear reduction
or torque increase.
The headstock backgear I would definetly still want to use it has a 10-1 reduction
or if I understand it correctly, beacause its more than the average standard of
about 6-1 would have corresponding increase in torque?
Would not running an 1800 rpm motor from 350-2400 rpm for lighter cutting-
smaller dia. work and from 35 to 240 rpm in the back gear for larger-heavier cutting
be enough?
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liveaboard
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Re: Another dumb INVERTER question

Post by liveaboard »

I have an old 3-phase motor lathe and a similar drillpress. I added a VFD for speed control.
Both of these motors are physically big for their ratings, and really old.

I allow up to double stock rpm, and down to 30% lowest speed.

I'm a bit careful with the lowest speed not to fully torque the machine for long, and high rpm makes me a little nervous but it works just fine.

This range (6:1) is just a little less than your requirements, so I think with a motor intended for VFD use you can do it.

I'm VERY happy with the vfd; my ancient machines are much more pleasurable to use now. With low gear on the lathe and low frequency, previously difficult threading work is now easy, the machine practically stops dead on command.

The VFD calculates the motor load and supposedly will prevent overheating, but for a couple of bucks you can add a remote sensor thermometer.

High rpm will only be a problem when the armature explodes. I'm not an engineer, but I think 200% is ok and it's working for me so far.
Larger frame is better for cooling (low rpm) but the larger diameter rotor will be under greater stress at high rpm.

In conclusion, I say go for it.
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Bill Shields
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Re: Another dumb INVERTER question

Post by Bill Shields »

if you think that any type of speed reduction (like the variator) is not a torque increaser...you should stop and think again.
Too many things going on to bother listing them.
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Steggy
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Re: Another dumb INVERTER question

Post by Steggy »

liveaboard wrote: Wed Feb 10, 2021 7:38 amI have an old 3-phase motor lathe and a similar drillpress. I added a VFD for speed control. Both of these motors are physically big for their ratings, and really old.
Motors have undergone several frame size revisions over the years. As the materials used to insulate the stator windings got better and could tolerate higher temperatures without breaking down, it became possible to produce smaller-diameter stators for the same power rating. It's gotten to where, for example, 2 HP, three-phase motors are now available in NEMA frame 56—same size as a typical 1/2 HP, single-phase motor, whereas they used to only be available in the larger frame 145.
I allow up to double stock rpm...high rpm makes me a little nervous but it works just fine...High rpm will only be a problem when the armature explodes.
The armatures in just about all induction motors can tolerate much higher speeds than one might think. In many cases, the armature of a manufacturer's 3450 RPM motor is the same part used in the 1725 RPM version. Commercial-grade motors, i.e., ones that would be commonly used to power machine tools, are conservatively designed. Overdriving such a motor is generally not a problem if the load can tolerate it.

That said, if you are going to use a VFD to power your machine it's best if the motor is inverter-rated. While a non-rated motor may work okay with a VFD and readily tolerate being overdriven, its insulation may not tolerate the high-frequency "hash" present in the output of all VFDs, resulting in the motor's smoky demise.
Larger frame is better for cooling (low rpm) but the larger diameter rotor will be under greater stress at high rpm.
Most of the bulk in the larger-framed motor of a given horsepower rating is in the stator, not the armature. My (ancient) bench grinder is powered by a 3/4 HP, frame 145 motor whose armature is no larger in diameter than that of a frame 56 equivalent. I will note, however, that even after prolonged operation with the big wire wheel mounted on the arbor and vigorously cleaning off rust, paint, etc., the motor barely gets warm to the touch.
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