×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

kW Savings From Replacing Oversized Motor

kW Savings From Replacing Oversized Motor

kW Savings From Replacing Oversized Motor

(OP)
Could anyone explain to me the kW savings from replacing an oversized motor.

I have calculated the kW Input from
      (V x I x 1.73 x P.F.)/1000
and calculated load from:
      Load = kw Input/(Rated H.P. x 0.746)/F.L. Efficiency.

If I have a 10hp motor running at 50% load, how do I justify replacing it with, for example a 6hp motor.

Thanks!!

RE: kW Savings From Replacing Oversized Motor

Efficiency of induction motors start falling down steeply generally below 50%. The no-load losses (which is constant) in a 10 hp motor is more than a 6 hp motor. So, a 6 hp motor running a 5 hp load will be less lossy as compared to a 10 hp motor.

On the other hand, a 10 hp motor will run lot cooler and hence will have a longer life.

RE: kW Savings From Replacing Oversized Motor

And of course, there is the issue of frame size, which would involve modifications to the old bed.

Make sure the 10 HP motor was not provided in the first place to overcome starting issues.

RE: kW Savings From Replacing Oversized Motor

(OP)
Thanks edison123.

What I am really looking for however is a method of calculating the kW savings after replacing the motor, if thats possible...

Thanks again!

RE: kW Savings From Replacing Oversized Motor

Put an energy meter and measure the KW-hours. ;)

Calculating the losses and efficiency of an induction motor is not a simple one.  

RE: kW Savings From Replacing Oversized Motor

Calculate kwatt-hr consume before and after the change and take the difference between the two. Multiply with the cost/kwatt-hr.

Only you have the data of load, motor nameplate and run time etc.

If not hire an engineer who knows.

RE: kW Savings From Replacing Oversized Motor

If you can get hold OEM's test certificates for both the motors, you can get some numbers (not all) like no-load and load losses and do the math. That would just be a starting point.

RE: kW Savings From Replacing Oversized Motor

Great point starkopete.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

The general trend: each motor has its own highest efficiency in the neighborhood of 75% (give or take). But for higher horsepower motors the curve is generally shifted up compared to lower horesepower motors.

Even if you should find a motor with marginally better efficiency, considering you have already invested in the large motor, you will need a lot of efficiency gains to justify purchasing a new motor.  In contrast if you have no motor and choosing between two motors, it is often easier to justify a small increase price for higher efficiency motor.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

last comment - as edision mentioned you have to match frame size for interchangeability.  These two particular motors have different frame size and wouldn't be mechanically interchangeable even if you wanted.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

Good catch starkopete.

RE: kW Savings From Replacing Oversized Motor

Don't compare efficiency percentages with different size motors.
As an example,
10 HP motor, 90% efficient, about 1 HP losses.
5 HP motor, 85% efficient, about .75 HP losses.
Which motor will be cheaper to run?
Calculate the actual losses in HP or kW before comparing the efficiency of different size motors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: kW Savings From Replacing Oversized Motor

Bill

Good point too.

Per starkopete's motor master catalog

10 HP motor at 50% load - Efficiency 92.3% - Loss - 0.77 HP

5 HP motor at 100% load - Efficiency 89.5% - Loss - 0.525 HP

hammer hammer hammer
 

RE: kW Savings From Replacing Oversized Motor

Another point that bugs me why the motor designers design for maximum efficiency at 75% and not at 100%. Is it because the end users tend to over spec the motor HP requirement ?

RE: kW Savings From Replacing Oversized Motor

They could do that by calling a 10 Hp motor a 7.5 Hp motor. That would put it at the most efficient operating point, but like so many design questions, the best answer is often a good compromise.
Now, consider that even though the motor is running at best efficiency at 75% loading, the efficiency of a 7.5 Hp motor fully loaded may not be much different. Also, many motors tend to operate at less than full load anyway. If you have an 8 Hp load, you will buy a 10 Hp motor and it will be running close to maximum efficiency.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: kW Savings From Replacing Oversized Motor

Quote:

Don't compare efficiency percentages with different size motors.
As an example,
10 HP motor, 90% efficient, about 1 HP losses.
5 HP motor, 85% efficient, about .75 HP losses.
Which motor will be cheaper to run?
Calculate the actual losses in HP or kW before comparing the efficiency of different size motors
I don't think you have captured starkopete's point.
10 hp motor carrying 5hp load - 92.3% efficienct
5 hp motor carrying 5hp load - 89.5% efficienct
Which is cheaper to run when carrying 5hp load?
Answer: The 10hp motor.  

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

Not to beat a dead horse, but I think an earlier comment was incorrect:

Quote:


10 HP motor at 50% load - Efficiency 92.3% - Loss - 0.77 HP
5 HP motor at 100% load - Efficiency 89.5% - Loss - 0.525 HP
should be:

Quote:


10 HP motor at 50% load - Efficiency 92.3% - Loss - 0.77/2 HP=0.385HP
5 HP motor at 100% load - Efficiency 89.5% - Loss - 0.525 HP

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

pete

If you are calculating 92.3% efficiency of the 10 HP motor on a 5 HP output basis, it's not right. Note the no-load loss (iron loss + windage + other mechanical losses) are higher in a 10 HP motor.

RE: kW Savings From Replacing Oversized Motor

It is 92.3 % efficient at 50% load (5hp).  That means the output is 5hp when the input is 5hp/0.923  (satisfies efficiency = output/input).   We don't use 10hp in the calculation of efficiency at the 5hp operating point of a 10 hp motor.

Let's do it a little more precisely:
10 HP motor at 50% load:  Efficiency= 92.3%: Losses=5*(1-0.923)/0.923 = 0.42 hp
5 HP motor at 100% load:  Efficiency 89.5%: Losses=5*(1-0.895)/0.895 = 0.59 hp

If the 10hp motor is more efficient at 5hp then the 5hp is at 5hp, then.... the 10hp motor is more efficient at 5hp then the 5hp is at 5hp! (i.e. the 10hp motor consumes less power driving this 5hp load).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

edison - I'm guessing maybe you didn't read starkopete's link.  It gives efficiency over a range of loads for each motor.   I agree I could not apply the full-load efficiency toward calculating losses at half load.  But I ceratinly can apply the half-load efficiency toward claculating the losses at half load.  That is what the 92.3 is... efficiency at 50% load of the 10 hp motor.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

Hi Pete;
I believe that I made a blunder. I will defer to you on this.
edison123;
The no load losses are less on the 5 HP than on the 10 HP, but the load losses follow a square law and the I2R is greater on the smaller motor for a given current.
Although the no load losses are less for the 5HP motor, the load losses for a 10 HP motor running 50% loaded will be about 1/4 of the full load losses. That more than compensates for the greater no load losses.
(Did I get it right this time Pete?)

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: kW Savings From Replacing Oversized Motor

I know this is great fun, and good practice, but why not just give the manufacturer your requirements, and ask "what do you have which will effiecently do this at the lowest operating cost?"     

RE: kW Savings From Replacing Oversized Motor

No reason at all why not. The best comparison uses the data from existing motor (which may or may not be available) against the curve of replacement motor.  And lower operating costs alone do not justify replacement... you've gotta pay back. And you've gott have mechanical interchangeability as edison said.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

pete

You're right. I forgot what efficiency meant. :)

RE: kW Savings From Replacing Oversized Motor

lol.  I've done that myself lately.

Bill I think you are right that in general going from 10hp motor driving 5hp load down to 5hp motor driving 5hp load, the no-load losses increase but the load losses decrease.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

I believe that in most normal installations, a 10 HP motor will pull a 7.5 HP load cheaper than a 7.5 HP motor.
Going from 10 HP to 5 HP, I would recommend running the numbers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: kW Savings From Replacing Oversized Motor

Well if that don't beat all!  I have been laboring under the understanding that a partially loaded motor was a power wasting inefficient situation.  Now we're saying run a motor a few hp bigger than needed for better efficiency..

Where's my Nola.

Keith Cress
kcress - http://www.flaminsystems.com

RE: kW Savings From Replacing Oversized Motor

Hi Keith, for a very lightly loaded motor you are right. but for a motor loaded over 75%, there is a sweet spot. Depending on the efficiency curves of the individual motors and the available sizes, you can't always take advantage of the sweet spot.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: kW Savings From Replacing Oversized Motor

Keith -   I came into the thread with the same assumption as you (a motor oversized by a factor of 2 would have to be wasting energy).   Starkopete showed something different for two motors that appear to be from the same product line, differeing only in horsepower.   So the lesson imo is it's tough to make generalizations.  As retred said, look at the invidivudal motor data whenver possible.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

Quote:

As retred said, look at the invidivudal motor data whenver possible.
Sorry - waross said that too... and a little more. Didn't mean to leave that out

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

Last comment  - Keith - I know I didn't say anything you didn't already know.  

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: kW Savings From Replacing Oversized Motor

[quote itsmoked]... Where's my Nola. [quote]Actually Keith, this is one of the reasons why the Nola circuit didn't pan out as the panacea of energy savings. People were ASSuming that a motor running unloaded was wasting a lot of energy and it really isn't true a lot of the time, especially since motor efficiencies have been regulated to increase.

But the scammers still try to capitalize on the basic instinct people have to believe that.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: kW Savings From Replacing Oversized Motor

Well. That's why I like associating with other engineers.  It's fun to have solid truths being smashed and replaced by actual reality.

This is right up there with wye-delta starters suck, VFDs can make three phases from a single phase, and centrifugal pumps work less with outlets restricted - not more.

Now I can add: over sized motors are not necessarily less efficient.

Thanks folks!
  

Keith Cress
kcress - http://www.flaminsystems.com

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources