AC MOTOR (REPLACE OR NOT) 1

[mobex]

There is a 5,5kw motor for ventilation fan (2ways air blowing)- having a 7.3A consuption. What should be considered for having replace or not this motor with a smaler one? Reason:energy saving

 

[Skogsgurra]

What voltage? Difficult to say anything about power consumption without having the complete picture.

Gunnar Englund

http://www.gke.org

 

[mobex]

3 phase motor 380V/50hz in star connection

 

[ozmosis]

energy saving is not just a reason to replace, it should be your return on investment(ROI). Energy saving forms a part of the ROI but you still have to look at the investment necessary: new motor, new switchgear, new cables (maybe), new couplings (maybe), time for installation, downtime etc. Basically, the change from a 5.5kW motor to (maybe) a 4kW could actually cost you a lot more than the relatively minor savings you get by fitting a smaller motor.
Also, to use the well used saying, if it's not broke then don't try and fix it. Mybe if you had a problem with the motor then yes, look at it but I would still have my doubts personally if it would really bring savings going smaller on such a (relatively) small motor.

 

[Skogsgurra]

It seems to be the right motor for the job. Putting a smaller motor there will probably mean that it will be running very close to its limits - or overloaded.

Full load or overloaded means worse efficiency (usually optimum at 3/4 load). So no reason to change.

If you need to reduce flow, then you can save energy. Yes. But that's another story.

Gunnar Englund

http://www.gke.org

 

[waross]

This topic has been discussed recently.
You pay the power company for the power that you use. This is the power to turn the fan plus the losses. If the smaller motor is turning the fan at the same speed, then the power required will be the same. The losses of a smaller motor may be higher than the losses of a larger motor running at part load.
A caveat; If you are considering changing from a low efficiency motor to a high efficiency motor, you may realise a saving.
However, I have found that the cost to change a motor or other equipment to a more economical unit invariably has such a long payback time as to be unrealistic.
As skogsgurra points out, this motor looks like it is well loaded. The next smaller size motor will probably be overloaded.
If you want to show a quick saving and you can live with slightly less air volume, then slow the fan down. This will give you an energy saving without changing the motor. I strongly suggest changing to a variable pitch pulley on the motor. Changing to the next smaller size of fixed pitch pulley may be too much reduction.
It depends.
respectfully

 

[jraef]

Good comments all, and waross' argument sums it up nicely.

The only thing I'll add is that if reduced flow is a possibility in your ventilator, then rather than replacing the motor you might also consider adding a VFD to it and a system to match flow to demand. That is a well proven energy saver, most VFD manufacturers will supply you with software to calculate your ROI.

Tangent alert! (based on waross' next-to-last comment)
The the people who gain the most by replacing working motors with energy efficient versions are the supplying utility. That is why many of them here in the US are offering rebates to users so they can better recover their investment in doing so. The users get a faster ROI on the change costs so the bean counters will approve it, the utilities get what they call an "avoided cost of delivery", a complex algorithm derived from not just the generating cost, but also the litigation expenses from needing to expand transmission capacity in a NIMBY environment. Not that the users don't save money, it's just that total Dollars saved actually favor the utilities more than the users. If a motor needs to be replaced for other reasons anyway, then by all means it favors the user.

For you non-native English speakers, NIMBY is short for "Not In My Back Yard" and describes the tendency for residents to oppose any additions of even changes to power distribution and transmission facilities based on fear of "radiation" and/or declining property values. It has become a big deal to utilities now.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

Thanks. Needed that explanation. It is difficult to understand - and even more difficult to accept - that people are so afraid of things they don't understand. The dark ages are over us again...

Gunnar Englund

http://www.gke.org

 

[jraef]

No problemo Gunnar, although I must add that NIMBY-ism is not exclusive to power utilities.

Might I also point out that the worst of the damning "studies" on cancer from power line EM radiation have been propagated by your fellow countrymen? What's with you Swedes anyway? Mucking it up for the rest of us while you sit around in your wood-fired saunas I suppose ;-)

 

[Skogsgurra]

I fight those fellow countrymen incessantly. I have even been told that I am a cynic brute that will not let them tear down our national 400 kV back bone.

I am a cynic brute, yes. But they are calling me a brute because I will not let them - not for the valid reasons.

Gunnar Englund

http://www.gke.org

 

[jraef]

You may not recall, but my house is almost directly under a 110kV power line. No cancer here or anywhere along these lines, but I am contemplating suing the utility for making me eat too much. That MUST be the reason!

Rotten deviants... it's all their fault.

 

[DickDV]

Regarding NIMBYism, it can get worse! I read about a utility project in upstate New York a couple of years ago that was finally abandoned after 10+ years of litigation. The newspaper reporter said it was stopped by the local
BANANA's.

That would be: B-uild A-bsolutely N-othing A-nywhere
N-ear A-nyone

(sigh)

 

[Marke]

We have the green BANANA problem here which is causing a power shortage.
Govt is getting stick for pending shortage of elect supply but can not build/expand current infrastructure because it might hurt a worm or affect the ecology. - cant have it both ways!!

Mark Empson

http://www.lmphotonics.com