Motors - running 24/7, any issues? 1

[iankent]

Hi all, new to the forum so please bear with me will get the hang of it eventually! Looks like a really useful site!

Just a quick question for now. The project I'm working on calls for a motor to be running 24/7 at relatively low speed (perhaps 40-80rpm max), with motor 'blades' which that will be sat in water (along the lines of a food mixer I guess). The setup of the blades would mean a lot of resistance against the motor.

My question is, what sort of motor would be suitable for this, and are there likely to be any issues around overheating or burning out? As the water will also be heated (to about 26C), is there a way I can capture the wasted heat from the motor? I was thinking I could put the motor below water level so the heat would naturally be absorbed by the colder water, but this adds complications around waterproofing unless theres a motor designed for this kind of job?

Also, presumably the power output from the motor will depend largely on the volume of water and the resistance created by the blades? Any quick easy way to estimate what sort of power is required, or should I resort to calculations or trial & error?

edit: in the initial stages of the project we'll be using relatively low volumes of water, around 50L per container.

That actually turned into a few questions lol
thanks in advance for any info you can give

 

[TurbineGen]

There is absolutely nothing wrong with running a motor 24/7 so long as it is run within the specifications on the nameplate. It is done all of the time.

For this application, I'd choose a TEFC (totally enclosed, fan cooled) inductive motor and gearbox. Use a VFD (variable frequency drive) is speed regulation will be needed. I would not worry about absorbing the heat from the motor. It's more trouble than it's worth.

Before you go any further, you need to calculate how much power is required from your motor. You also need to consider the environment (ie. is it corrsive?). From there you can spec a motor and gearbox.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[benta]

TEFC? Would the fan help at all at such low revs?

Benta.

 

[DRWeig]

Benta,

Motor will run fast, within its specs for fan cooling. The driven paddle will run slow -- he'll have to choose a gearbox as TurbineGen suggested.

Good on y'all,

Goober Dave

 

[iankent]

Thanks for the very quick replies!

Not a problem re the heating bit, I've got separate heating elements anyway but was just thinking of conserving a little extra energy, but as you say, probably more effort than its worth

The environment will possibly be a bit humid, but certainly no corrosive chemicals around and no water should be able to reach the motor, so I should be ok there.

Over time the liquid will become more resistive, so speed regulation would be required to maintain the RPM depending on resistance its applying, ideally with some kind of feedback system to auto-adjust it. Would a VFD be best for this? There's obviously no way to adjust gears in this way, so would I need both gears and a VFD? Sorry, a bit confused

Will have to look into calculating power requirements - any suggestions on where to start? Not too bad at physics or google, but any relevant keywords you can suggest to get me started would be great

Just had a quick look at RS Electronics website and its given me another question. Whats the difference between geared and non-geared motors? I assume with either of those I'd still need external gears/VFD to adjust the speed?

One last question, any reputable UK suppliers I should try other than RS?

Thanks

 

[dpc]

The motor is going to turn at nearly the same speed regardless of the load imposed on it. If you size the system properly, the motor will be sized to handle the maximum torque required (thicker), and will just draw less current when the torque requirements are less. This should work unless you need very precise speed control.

 

[itsmoked]

Just so you know, you're looking for something of this sort:

http://www.grainger.com/Grainger/items/4Z386?Pid=search

And as stated above you don't generally need a speed regulator for stirring a liquid. Ideally for something that runs 24/7 you would want the correct turning speed and stirring blades of the correct size and pitch to just run full out on a properly selected gear motor.

A VFD will add cost, efficiency loss, and an increase in failure modes.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Thedroid]

Running the motor 24/7 won't cause you any problems as long as it's sized properly. We have numerous motors that haven't shut down in years.

 

[TurbineGen]

It sounds to me like speed regulation is uneccessary. As Keith pointed out, inductive motors have a very narrow speed window in which they operate. Speed might change 5% between unloaded and fully loaded. Unless your process requires the changing of speed or very precise regulation forget the VFD. The inductive motor and gearbox will work fine.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[iankent]

Brilliant, exactly what I'm looking for, thanks everyone motors were a complete mystery to me but its starting to make a bit of sense now!!

Thanks again