New motor install. very slow speed

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cmaki

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sheboygan, WI USA
So where I work we have an 1750RPM motor going into a variable speed pulley into a gear reducer to the final output shaft. I believe the motor is starting to go and I was tasked with looking into a different way to run everything. My first thought was a direct drive. Motor is 480 3ph. problem is the final drive is spinning between 50-70RPM. I was going to put a small freq drive on it so we can still adjust the speed . (and we need to) Can motors spin this slowly for an extended period of time? It would be running for about 8-10 hours per day 5 days a week in a very dusty woodshop.

My next thought is to run pulleys to the reducer so the motor can still spin at a higher RPM. Is there an advantage to either way?

(admin if this is in the wrong forum please move)
 
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You're in the right place. I'm not a mod but I think I can call that one.

Wait until Jraef wakes up and has his coffee out in San Fran. This smells like a good question which will yield a response from him that will blow your mind. He's the best!
 
As long as he can keep it kinda simple I'm good. I dable in electrical work. and every other type of work. In my maintenance shop its just me. I make the rules, I fix, and I order parts. regardless of what it is.
 
I think I owe an appology to others on here.

There are MANY very highly qualified people on here who can answer the question. Jraef just came to mind because (I believe) he works in industrial motors application engineering on really big stuff.

There are probably another 30 or so people on here highly qualified to chime in.
Sorry everyone :ashamed1:
 
Sorry I just went and looked at everything. The entire assembly line is all 480 except for the 2 motors on this one part. this is 240 grounded B. (really its grounded A but whatever).

Does this make any difference?
 
If that motor is loaded at that speed it will generate significant heat- any integral shaft mounted fan will not provide enough air flow for cooling and other cooling methods will be needed at the very least.

Unless you do vary the speed regularly you are still probably better off to use the gear reduction to get final output speed.
 
kwired said:
Unless you do vary the speed regularly you are still probably better off to use the gear reduction to get final output speed.
Click to expand...

The speed will be varied but not by much. we have a roller feeding another roller. both rollers are adjustable but as they wear out at different speeds they will have to be adjusted to match. Not enough to make any real huge difference. maybe 10 RPM total and not all at one time. Might look at leaving the gear reduction and still running a VFD
 
There is a huge downside to running motors direct drive at low speed: reduced power.

The _size_ of a motor is pretty much set by the torque you want to get out of the thing. For a given torque, the faster you go the more power you move through the motor. If you take your 1750 RPM motor and run it at 50RPM, you will only get 1/35 of the power.

You will be much better off using a gear reduction to get your approximate final speed, and then using a VFD to do the fine adjustment. In this way you will run the motor at near full speed, which is good for cooling, and you will get the torque multiplication effect of your gear reduction.

-Jon
 
You should definitely go with a gear reduction system to achieve the output RPM you need. With the addition of an adjustable speed drive on the motor, you would be able to fine tune your output RPM.
 
So there are two things that a mechanical vari-drive system provides: speed change at the final work shaft, and inversely proportional torque change at the final work shaft. The device essentially trades one for the other. So if you want to go from 1750RPM at the motor shaft down to 70RPM at the work shaft, that is a speed turn-down ratio of 25:1. That then means that if you have a 1HP motor (you never said), the torque at the motor shaft will be 3 ft-lb, and at the WORK shaft it will be 75 ft-lb.

If you remove the mechanical vari-drive and go with a direct drive and a VFD, you will have 3 ft-lb of torque at 1750RMP and if you turn it down to 70RPM, you will STILL have just 3 ft-lb. Conversely if you NEED to have 75 ft-lb of torque at the work shaft, then to go with a direct drive and VFD, the motor (and VFD) must be 25HP.

So before you decide how to approach this, you must investigate and thoroughly understand the nature of the machine, it makes a big difference.

If you have a machine that just needs slow speed and the excess torque was irrelevant all along, then the issue of using a VFD on a motor at a 25:1 turn down (i.e. running the motor at 2.4Hz), the only safe way to accomplish this is with (A) a good quality drive that is capable of Sensorless Vector Control and (B) an motor designed for inverter duty, which ALSO has a separately powered blower to keep it cool.
 
Thank you for that. I do apologize for not having enough information. It is a 1 hp motor that drives a set of chains to turn a series of rollers (5 rollers I think). These rollers feed large sheets of wood through them. Each board is 4-5 feet wide and up to 12 feet long. I can assume that I will need more than 3 ft-lb of torque to feed these in. I'm thinking the smartest way is going to be to disassemble my housing and remove the variable speed sheave on the driven side and mount a pulley. The pulley to the motor via pulley and belt. THe motor could be controlled via VFD. I could control the input speed to the gear box with the correct size pulleys to keep the motor running at say 75%. This would allow for proper cooling. I would need to figure out input RPM on the gearbox which is not too easy because of the access points.
 
cmaki said:
So where I work we have an 1750RPM motor going into a variable speed pulley into a gear reducer to the final output shaft. I believe the motor is starting to go and I was tasked with looking into a different way to run everything. My first thought was a direct drive. Motor is 480 3ph. problem is the final drive is spinning between 50-70RPM. I was going to put a small freq drive on it so we can still adjust the speed . (and we need to) Can motors spin this slowly for an extended period of time? It would be running for about 8-10 hours per day 5 days a week in a very dusty woodshop.

My next thought is to run pulleys to the reducer so the motor can still spin at a higher RPM. Is there an advantage to either way?

(admin if this is in the wrong forum please move)
Click to expand...
A couple or three points.
Yes, you can use a VFD to run the motor at that speed. We have applications where we run motors at below 1% of nameplate rating.
But, and there had to be some buts didn't there?........:)
Cooling is likely to be an issue. Standard cage motors usually have a shaft mounted fan at the NDE end.At low/very slow speeds this is ineffective so you'd need a separately driven fan.

But what may be for more important for your application is power output.
Run the motor at 10% of nameplate speed and you can't any more than 10% power.
Gearing or pulleys is probably the way to go.
 
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